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Weather Exam 1 Atmospheric Concepts and Processes - Coggle Diagram
Weather Exam 1 Atmospheric Concepts and Processes
Topic 3: North Atlantic Hurricanes
(Tropics)
These occur in the tropics, but can effect extratropic regions by migrating northwards
Understanding Weather in the Tropics
What are the tropics?
Between the tropics of Cancer and Capricorn
23 degrees North and South of Equator
Hurricanes originate in this region, but can effect extratropical zones
Weather doesn't nicely conform to these tropic boundaries
We will be interested in longitude (east and west) and seasonality
What
factors influence weather
in the tropics?
Weak
PFG and near-zero Coriolis
parameter = streamlines
These two dominant forces in the mid latitudes become much less important
Weak temperature gradients -
moisture variability in space and time is important
Much more important in the tropics, and seasonality (wet and dry season) have a large effect on tropical weather
Local factors -
diurnal land-sea breezes
Due to many of the areas will have water surrounding them with regular patterns of convection
Seasonality (wet and dry) due to
changes in insolation
and suns angle
The Hadley Circulation and Trade Winds
We have the Coriolis force which deflects surface level flow to the west Trade Winds (makes these winds curve to the right)
The same thing happens towards the equator, but the effects of this force gets weaker (why our winds are
North-easterly
Due to the circulation in the Hadley cell, air is convected,
rising at the equator in a poleward direction
We can see winds curving to the left, approaching the equator
This is part of the Hadley circulation which spans either side of the
Intertropical Convergence Zone
(low pressure belt where trade winds meet)
This is the largest circulatory system and this drives these winds for the formation of these hurricanes
Pressure cells and Intertropical Convergence
High pressure pattern that exists across the equatorial Atlantic called the
Bermuda-Azores High
H in the middle is a reliable high pressure system that sits over the Atlantic during the Summer Months
This further strengthens the dominant flow that steers the development of Hurricanes in the Atlantic
We know that high pressure systems have a clockwise movement in the NH of air as it flows out
High pressure system helps to guide any systems that develop in the Atlantic towards the Caribbean and the Gulf of Mexico
But we know that high pressure is not conducive to the development of severe weather systems
Because we are putting a lid on the development of convective processes, stopping air rising through the troposphere
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This causes a
temperature inversion
('lid')
This results in
The Trade Wind Inversion and Convection
How does the high pressure over this region affect the growth of weather systems?
How warm is our sea water?
Potential for development of deep cloud high in the troposphere
Transect across the Atlantic from East Atlantic - Off the West Coast of Africa (right) to West Atlantic - Caribbean and Gulf of Mexico (left)
Shaded orange section shows trade wind inversion (cool dry air putting a lid on the atmosphere)
If you combine lower sea surface temperature with the fact that there is a lower evaporative potential here, we only get
shallow cloud growth over the West African Coast
as there is a lack of instability
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What does the ocean surface look like in different parts of the Atlantic?
In the East Atlantic, the temperature off of
West Africa is fairly cool
due to upwelling of cool water from the Canaries (
canary current)
Sea surface temperature are lower than we would expect at this latitude
As we go westwards across the Atlantic, the
sea surface temperatures increase
, as well as the
depth of the warm water
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High sea surface temperature are good for hurricane
development
Where the cool air is coming out of the high pressure system around the West Coast of Africa
The formation of hurricanes
Tropical Disturbances: the origins of hurricanes
2.) Where the
trade winds meet and converge
, it causes a broken line of thunderstorms to develop (the
ITCZ
)
Disorganised thunderstorms with weak PFGs and minimal/no rotation in the Equatorial Zone
Disorganised thunderstorms in the ITCZ
Air moving towards each other at the bottom (converging)
Large pillars of cumulonimbus clouds with outflow regions between them
Strong outdrafts and indrafts
These pillars can grow substantially higher as the
troposphere is higher in the tropics
1.)
Mid-latitude troughs
(low pressure) are capable of migrating into tropical air masses - creating thunderstorms
3.)
Easterly waves
- Large ripples in the normal trade wind pattern (results in larger hurricanes)
Circulation pattern of air flow from
East
(Ethiopian Highlands) to
West Africa
and becomes entrained in the trade wind flow
Ethiopian Highlands are around 4000m high, which causes air travelling over to become disturbed and form ripples
These ripples (waves) are carried west
These
disturbances can grow
as they pass over Africa, into large clusters of thunderstorms
These are 2-3km in length and as they pass, they
draw their energy from warm surface temperature
in this areas
But, they reach a new cool environment of the West Coast of Africa,
the Canary Current
and its warm fuel supply is shut off
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In order for Easterly waves to turn into tropical systems, depressions, hurricanes -
they need to pass the hurdle of the Canary Current
(TWI and cool water)
They need to be
big, powerful and fast
enough to travel over cool sea temperatures to get back to warm temps where they can regain their fuel supply
These three factors can all create
tropical disturbances
which need to occur for the formation of hurricanes
Why don't all tropical disturbances grow into tropical depressions?
A tropical depression is defined as
a system that has it's own zone of low pressure (single isobar) beneath it
Established low pressure centre
We need a number of things to be in place for this disturbance to grow into a tropical depression:
High Sea Surface Temperatures
Our systems need to have passed the canary current, or have originated in the Gulf Of Mexico
This is important as we need evaporative potential to fuel the growth of a depression
High Levels of Humidity
High relative humidity
These two factors are fundamental as they are the fuel of our system
Correct amounts of
wind shear
Horizontal wind shear
to contribute to
cyclonic vorticity
(rotation) in base flow
Horizontal wind shear
to contribute to
anticyclonic vorticity
base flow
We need changes in wind speed and direction in the lower part of the atmosphere so our
depression doesn't fill
(air doesn't circulate properly)
We need warm air to feed in at the surface and rise up through the troposphere and removed at the top - this occurs in hurricanes too
Once systems turn from tropical disturbances to tropical depressions, they have much greater potential to grow
The likelihood of hurricane development
Tropical Depression
:arrow_right:
Tropical Storm
:arrow_right:
Require
sustained wind speeds of 39mph
to be classed as a tropical storm
Definitions of tropical systems are based on velocity
Hurricane
:tornado:
Deep Instability
Warm sea water (27 degrees)
Importance of
sea surface temperature
During summer, sea surface temperatures increase from the east to west Atlantic
Mid September is peak hurricane season in the Atlantic
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High Relative Humidity
Rotation (horizontal vorticity)
Sufficient coriolis force
Explains why there are no hurricanes on the equator
Difference between a tropical storm and a hurricane comes from
dynamics in the eye
and
wind speeds
in the central part
Wind speed exceeding 74 mph
Absence of strong vertical wind shear (maintain latent heat transport)
Genesis Region and Season
Where do hurricanes begin?
The
disturbances have seasonality
too - with extratropical mid-latitude depressions being more likely to affect hurricanes at the shoulder ends of the season
This is when we are most likely to see
mid-latitude low pressure systems
(troughs) migrate into the tropical air mass
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Convergence around the
ITCZ
happens through the hurricane season, but Coriolis force is not sufficient enough in the equatorial region so they
must migrate north in order to stop these systems from failing
Easterly waves
form Cape Verde hurricanes off the West Coast of Africa
These tend to be mid-season because that is when
Easterly waves are most prominent
and
SSTs are at their warmest
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Seasonality of Hurricanes
Peak in mid-september
But, that doesn't mean you can't have strong hurricanes/storms on the shoulder end of this season
Increased organisation
and circulation
Convection and shear, and a degree of cyclonic circulation
Change of fuel source to evaporation from ocean surface
If these conditions stay consistent, then this depression will turn into a tropical storm
As we move from a tropical disturbance to a tropical depression, our
system changes its source of energy
No longer relies on energy from the atmosphere but
now relies on energy from evaporation from the ocean surface
The Structure and characteristics of hurricanes
Characteristics
Sustained
wind speeds that exceed 74mph
Strongest occur around the eye (lowest pressure)
Central pressure that is typically 950mb (990-870mb)
Measured on the Saffir-simpson scale (1-5)
On average, 600km across - highly extensive
Size does not equal Power
Hurricane Structure
Hurricane Eye and Eye Wall
The eye is comparatively calm
A lot of research has been done to understand the eye
An important part of maintaining the eye is the inversion within it
Maintained by cool dry air from the upper troposphere coming down and
filling the eye
In the lower troposphere, we get a temperature inversion (lid on the atmosphere)
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Warm moist air flowing cyclonically (anticlockwise) around the eye wall
The eye itself has cool dry air descending from the upper troposphere
All of the heat that is being used up through the air rising through the system
Clear skies, no wind
At sea, the eye is the worst place to be (pushes water towards eye)
Eye wall contains the strongest wind speeds flowing around the central band of low pressure
Inside the hurricane, surrounding the eye wall, there is a
complex honeycomb structure of cumulonimbus clouds
Within this there are zones of updraft and downdraft (called the hurricane bands)
Cirrostratus clouds (ice crystals)
Exist at the outflow region, marked by cold air flowing from the centre of the storm
These make up our system with a bulk of air spiralling towards the centre to the eye wall, up the centre
As the air gets to the top of the system, the
cyclonic flow becomes anticyclonic
so the air can exit the system
This happens high up in the system starting at the transition zones
Spiral rain bands
Significant convection, arrow columns on diagram
Hurricanes: Temperature and Pressure
As air moves towards the eyewall, it undergoes adiabatic expansion
This means that the
temperature along the base of the hurricane remains remarkably consistent
Because air is moving towards a zone of lower pressure and expanding adiabatically keeps the temp consistant
Its only when we go towards the eye wall zone that we see these big contrasts in temperature
Rossby Number
The Rossby number illustrates the dominant forces acting across a hurricane
Outcome is either high/low for tornadoes/cyclones
But this is different for hurricanes
Rossby number depends on
where we are
in the system
Hurricane eye
(U > Lf) as cyclostrophic forces dominate =
Large Ro (Ro > 1)
Region of gale force winds
(U = Lf) in gradient region due to balance of both forces =
Ro ~ 1
Outer storm
(U < Lf) as geostrophic forces dominate = small Ro (Ro < 1)
Entropy
In thermodynamics, the
unavailability of a system’s thermal energy
for conversion into work
Inverse to energy (unavailability of thermal energy to do work)
Everything in the world is moving from order to chaos - tea cools down and entropy increases as no energy is added
Change in entropy
depending on where we are in a hurricane
(heat transferred into a system/temperature)
Internal energy
of the system is represented by the
heat transferred into the systems minus by work done by the system
This is important because it
links where our system derives its energy from and what happens to that energy
as the system develops
Entropy and the Carnot cycle
Hurricanes are a good example of a near Carnot cycle
Carnot cycle looks at the way that entropy changes throughout a cycle
Can be applied to hurricanes, but also other systems too
The zone between A and B (bottom line) is a zone of near constant temperature - but also the zone that is fuelling our system
Gaining that energy through evaporation of ocean surface
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As we travel up the system, we have a fast cyclonic flow through the
eye wall region - we see our entropy go from low to high
This reflects the face that as air is flowing through our system, more
work is being done
Highest entropy occurs at the outflow shield at the top of the hurricane
What are they and why are they important?
Social
Very talked about affect a lot of people
Hurricane Melissa (cat 5)
Billions of £ of damage to Caribbean and further north
Very destructive and powerful
Spatial and Temporal
Last for days and develop over days
Some are more rapid
Very large systems, hundreds of km across
No relationship between diameter of hurricane and its strength
Hurricanes are
formed over water
Nomenclature
Typhoon (East Asia), Cyclones (SW Asia), Hurricanes (N and S America)
We are looking at NA so we must refer to them as hurricanes
Saffir-Simpson Hurricane Wind Scale
Scale at looking for hurricane wind speed against damage
Audio Lectures
How the Atmosphere Works
Key Concepts
Pressure
- force exerted by the atmosphere at a specific point
The result of gravitational attraction on the column of air lying directly above a specific point
Volume
- the amount of space that a body of air occupies
Density
- the ratio of the mass of air to the volume occupied by it
(Mass/Volume)
Temperature
- Average internal kinetic energy of air molecules
When thinking about atmospheric pressure, we must think about how this changes with temperature and volume
Boyle's Law
At a
constant temperature
, the
volume of a sample of air is inverse to its pressure
Essentially, as pressure decreases, volume increases
When
M and T
are fixed =
Boyle's Law
P = k^1/V
Two examples of this are:
Pressurisation in aeroplane travel above 10,000ft to protect crew and passengers from low pressure
In a typical flight, the cabin air is programmed to rise gradually to the height of the airport to around 8000ft - and then slowly reduced during landing
If you were to open a bottle mid flight, it would hiss at you as its pressured is lower than that of the cabin and vice versa if you were to land and then open it off of the plane
Scuba diving, at sea level air pressure is equal to 1 bar, if you dive to ten metres, this pressure is doubled
For every additional ten metres you dive, the pressure on your body increases by 1 bar
(4 bar at 30m)
Charles's Law
At
constant pressure
, the volume of a body of air
varies with absolute temperature
(K) - direct proportion
Experimental gas law to best describe how gases behave when heated
Best example would be boiling a pan of water, as water is heated under constant temperature - the volume increases
This would then behave in the same way for air molecules
When
M and P
are fixed =
Charles's Law
V = K^2T
Ideal Gas Law
- equation of state
Pressure = Temperature x Density x Constant
Pressure x Volume = Temperature x Constant
All qualities are interdependent
This law describes the relationship
Air pressure is determined by the
temperature of the air and density of the air molecules
Mass of the Atmosphere
Unlike water,
air is highly compressible
(if we squeeze it, it's volume decreases)
A good example of this is a scuba tank, which can hold around 2000L of air at an extremely high pressure
Hydrostatic Balance
: what holds the sky up?
In the vertical dimension,
gravity is the most significant external force
acting upon the atmosphere
It explains why the atmosphere (an envelope of air) exists around our planet
The reason the atmosphere does not collapse under the force of gravity is because the energy embedded in the movement of the air molecules
This
movement of air molecules creates the force of pressure which counters that of the gravitational pull
on the atmosphere
And the
balance between the pull of gravity and the pressure force is known as
hydrostatic balance
(we are in an equilibrium)
Mass and Altitude
We can also see that the majority of the mass of the atmosphere occurs within the
troposphere
(lower)
This (troposphere) is where the air pressure is highest and air is most dense
If we went to the Netherlands, much of which is at
sea level
, we would have the entire mass of the atmosphere above us. The
air would be dense
and the weight of the air on the ground surface would be around 10,000kg per square metre
However, if we travelled to a higher elevation community (La Rinconada, Peru), which is over
5000m above sea level
. Here, only half of the mass of the atmosphere would be above us, the
air would be much thinner and therefore less dense
and the atmospheric pressure exerted on us would be much lower than at sea level
Measurement of Atmospheric Pressure
Mass of atmosphere decreases
as we
increase in altitude
Therefore as we gain in altitude, air pressure (measured as a force per/unit area) decreases
Mercury Barometers
Air pressure readings can be taken from a mercury barometer
Measures the height of a column of mercury that the atmosphere is able to support (in a vertical glass tube under a vaccum)
Sea level pressure globally can support 760mm of mercury
This occurs because the mass of the atmosphere pushes down on the surface of the mercury in the barometer
More commonly used units of pressure are mba (millibars) and hectopascals
A force of 10^5 newtons acting upon 1m^2 corresponds to one pascal (Pa)
However, in meteorology, it is very common to see millibars used as a measure of pressure
Conveniently, these numbers are the same
This is the international systems of units for pressure (si)
You will see lines mapped out on synoptic weather charts called
'isobars'
These are
lines of equal pressure,
with numbers on map representing surface pressure
Isobars that are very
close together
means that there is a
big difference in air pressure over a short horizontal distance
(windy conditions)
Mass, Density and Air Pressure
Pressure (related to number of air molecules per unit volume) decreases exponentially with increasing altitude
These three concepts are interlinked
At, or close to the surface, gravity causes the air to be compressed under the weight of the atmosphere
As you increases in altitude, air pressure and density decrease
As air pressure decreases the higher up you go, air molecules are more free to move farther apart from each other and the air is less dense
Density does not decrease linearly with altitude, but exponentially
This is why small increases in altitude near sea level result in significant changes in pressure and density, about 1mb decrease in pressure for every 8m gained in elevation
What does all of this mean for bodies of air moving through the atmosphere?
Unlike elements of interest in other parts of geography, meteorologists are faced with a big problem:
air is invisible
, therefore understanding atmospheric motion requires us to think about air using a hypothetic unit of space called an
'air parcel'
This denotes a body of air with specific characteristics, and there is no 'right size'; but it is usually thought up as around a cubic foot per se
Air is a really poor conductor of heat
because molecules are not in contact with each other as they are in a solid
Therefore, the atmosphere contains bodies of air that sit next to each other, which all have
different pressures, temperature and water contents
The parcel of air can be any size, and we can assign it to all of the basic dynamic and thermodynamic properties of air
A parcel of air has to be large enough to contain a very great number of molecules, but small enough so that the properties assigned to it are approximately uniform
This can be visualised as an object such as a baloon
Adiabatic Expansion and Contraction
If we take our balloon as a representative for the parcel of air, we can explore what happens as it moves through the atmosphere (can also be compared to a lava lamp)
The parcel of air starts out at the surface and has
1000mb pressure and is warm 25 degrees
This is a process known as
free convection
The air has been warmed through ground surface conduction. The warmer air is less dense than its surroundings so it rises up through the atmosphere using its own positive buoyancy.
As the parcel of air rises, the pressure of the surrounding air decreases. Our parcel has to expand to equalise to the pressure of its surroundings.
As it expands, it is pushing on the air surrounding it and therefore doing what we call
'work'
. The parcel does 'work', but does not gain any heat
Therefore it loses internal energy as its temperature decreases. This process of expansion and cool as the surface air rises is called
adiabatic expansion
A good example of this is letting air out of something that is inflated, releasing air from high to low pressure, it rapidly expands and and is forced to cool adiabatically
Conversely, if a parcel of air descends from a high to a low altitude, the reverse occurs. Pressure of the parcel increases and work is again acquired to contract the parcel, making it smaller in size
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Thermodynamics
Adiabatic processes refer to the
first law of thermodynamics
, which tells us how to account for the molecular energy in the atmosphere
Air parcels contain molecules that have internal energy, and this is made up of kinetic energy and potential energy
Doing work on the parcel requires expansion or contraction to equalise the parcel to its ambient air pressure
Energy is not created or destroyed, but simply conserved
Adiabatic processes are near reversible
Heat input (contact with ground)
=
Specific heat capacity of air at constant volume
x
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Why does air get colder as we go higher?
The farther we get from the Earth, the thinner the atmosphere gets
The total heat content of a system is directly related to the amount of matter present (cooler at higher elevations)
The heating of the earth itself plays a significant role, with the planet itself being heated by incoming solar energy
Some of this heat
bounces off the atmosphere and never reaches the lower atmosphere
In addition, the atmosphere acts as a greenhouse, reflecting this energy back to the Earths surface
At higher altitudes, its relatively
harder to retain this energy as more heat is lost to space
Pressure surfaces
- temperature and density on a larger scale
At a larger scale, changes in temperature and density can also impact air pressure
Think about a warm day, incoming solar radiation heats the ground surface
Some of that heat conducts into the overlying air. This surface air soon becomes warmer and less dense than ambient air
This causes the parcel of air to ride under its own positive buoyancy. It will then rise and cool due to adiabatic expansion.
But because the column of warm air is less dense than the cooler air, the decrease in pressure as you go higher is slowed
THis means that the average associations of pressure with elevation are
subject to change
due to different temperature and densities of larger bodies of air
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Water and the Atmosphere
Atmospheric Moisture
: why is it important?
There are
three main reasons
for the importance of understanding atmospheric moisture:
The amount of water can
determine the types of weather systems
we see in these areas
The amount of water determines the
degree to which energy can be transferred
throughout the atmosphere in the form of
latent heat
Available water
varies both spatially and temporally
, and this depends on air temperature and surface type
The Global Hydrosphere
The global hydrosphere is made up of
reservoirs
and these have
residence times
- and the movement through these is called
'cycling'
Reservoirs and Residence Times
Oceans
97% of water is held in oceans
Ice Sheets
70% of fresh water is stored in ice sheets
Ground water
30% of fresh water
Glaciers
Biosphere
Atmosphere
0.04% of fresh water
10 day residence time
Rivers and Lakes
3% of fresh water
The
size and nature
of the reservoirs
determines the time taken
for cycling between them
Cycling involves evaporation, atmospheric transport of water, condensation, precipitation and runoff
This knowledge helps us to understand the atmospheric water budget/reserve
Can be calculated using equation: ΔQ = E - P + D^Q
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The total mean water content of atmosphere (global) = 25mm
This is all water that is
precipitable
Although
significant transportation of moisture can lead to much
higher rainfall totals
over a short period of time
For example, the record for the most rain in one hour is 305mm - USA, missouri 1947
Most in one year is 27,470mm, India 1861
The global hydrological cycle - reservoirs and residence times
If we look at this diagram, it shows estimates of movement and amounts of water flowing through each type of reservoir
Humidity
: water in the air
When we think about water in the atmosphere,
it can present in many forms
: water vapour, water droplets and ice crystals in clouds
The amount of moisture in a column of air is
determined by evaporation in a locality
The temperature of the air in that area and the horizontal transport of moisture
We can express the moisture content of the atmosphere in several different ways
Most common being
'Relative Humidity'
: total amount of water vapour (water as gas) that a body of air can hold is determined by the temperature of the air
Warmer air can hold more water vapour than cooler air
Warmer air contains more water vapour because at higher temperatures, the molecules are more likely to go into the vapour phase
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Gases
exert pressure, and water
does too.
The more water that is held in gaseous form,
the higher the vapour pressure
in the atmosphere
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Simplest being
'Absolute humidity'
: total mass of water in a given volume of air (measured in: g of water / per cubic metre of air)
Phase changes of water: key processes
Condensation
Phase change of
water vapour back into liquid water
Sitting in a sauna, when water is poured onto hot coals, you feel heat
Latent heat is released when water vapour condenses on your skin, which is cooler than the surrounding air, raising temperature locally on your skin
This is also forced to happen when the air cannot hold any more water vapour - when air has reached
saturation point
If water vapour is transformed into a sold (ice), the process is known as deposition
When condensation occurs, the latent heat carried off with the water vapour during evaporation is released
This is seen as an
increase in air temperature
4 potential triggers:
Volume
Air is cooled due to adiabitc expansion
Parcel of air rises and is exposed to lower surrounding pressure, causes it to expand
Energy is consumed for this 'work' - temp decrease leads to saturation
Temperature and volume
Overall decrease in parcel temperature beyond water holding capacity
Condensation occurs
Temperature
Air can be cooled to dew point but it's volume remains constant
Forced condensation in foothills of valleys
Humidity
Evaporation adds moisture to parcel, pushed air beyond its moisture capacity
Condensation will occur
Evaporation
Phase
change of liquid water into water vapour (gas)
It is is called
Sublimation
when ice turns into water vapour
In order for evaporation to take place, there
must be:
The air must be
below the saturation vapour pressure
Air motion
is required to move the moisture that has evaporated to the surface layer
An
energy source
at the surface
Global Moisture Transport
Looking at moisture transport at the global scale, we see there are significant horizontal movements of moisture
Net transport of moisture from oceans to land areas, thinking about latitude, we see (evaporation - precipitation),
precipitation dominates over evaporation in the low and mid-latitudes.
While the dry,
sub-tropics see the reverse
Regional imbalances are maintained through large scale movements of air through processes known as:
convergence and divergence
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Evaporation provides moisture to the atmosphere, the majority of which coming from the Ocean
A huge amount of energy is required to cause a change in state from liquid to vapour because of the intermolecular attraction between water molecules
The energy required for this process comes from the removal of heat from the immediate surroundings
This causes an apparent loss of heat, known as
latent heat
and a subsequent drop in air temperature
Water vapour is invisible, steam that we can see is liquid condensing
Annual Moisture Budgets
Annual moisture budgets for different places can vary significantly
The budgets are made up of: surplus, deficiently, sm utilisation and sm recharge
Wetter west coast (cardiff) has a yearly surplus, but southend (london) has water deficiency for most of the year
Can show the complexity of regional differences
Atmospheric Stability and Instability
Why is this important?
How stable is air in a spatial and temporal context?
This directly impacts the formation of weather systems
Air parcels properties can change as it moves and the importance of moisture and temperature all relate to stability
Energy Transfer: Convection
3 forms of heat transfer: radiation, conduction and
convection
Conduction is the transfer of energy of particles coming into contact
Whereas convection is the movement of particles through a fluid, and they take their energy with them
Convective cells can become established (think of a lava lamp)
Convection is the most important mechanism
when looking at the movement of air parcels
It is key as a process that
redistributes energy
away from hotter areas to cooler ones
Without it, Earth would experience massive temperature extremes
Convection is the movement within a liquid or gas that's
driven by differences in temperature
It occurs when areas of liquid or gas are heated or cooled to a greater extent than their surroundings
Hot less dense areas of a fluid rise, and cool denser areas sink
Convection: Free or forced?
Two types of convection:
Free Convection
(Gravitational/Buoyant)
Motions that are predominantly
vertical
in nature and are
driven by buoyancy forces
Differences in
density, temp
etc (driven by gradient)
Forced Convection
Occurs when motion is induced by
mechanical forces
Topography
- mountain barrier or hill, with air moving towards it are forced to rise
Frontal Boundary
- air is forced to rise over the boundary that separates two air masses with different properties (cold/accluded fronts in the UK forming deep cumulonimbus clouds)
Turbulent Flow
- at the boundary of a fluid which causes significant forcing (turbulent
eddies
)
Convergent Air flow
- winds meeting each other that force air to rise
Adiabatic Lapse Rates
(on a parcel of air)
How is the rise and fall of air affected by temperature? How does it cool? How is this effected by temp and moisture?
If our parcel of air is warmer than surroundings, it will
rise under free convection
, and if it is cooler, it will descend
As a parcel of air rises, it
expands and cools adiabatically
, and as air sinks, it warms and condenses (reversible process)
How does the amount of moisture in a parcel of air effect how quickly it cools?
If
all
of the moisture in an air parcel is in a
gaseous form
- we call it
dry
Said to be
unsaturated
as it could technically hold more water as a liquid (air has not yet reached saturation)
Dry and wet air
cool at different rates
as air rises through convection
Dry air cools at the
dry adiabatic lapse rate (DALR).
THis is the consistent 9.8 degrees per Km gained in altitude
This is also reversible
Wet/saturated air cools at a
slower
rate, at the
saturated adiabatic lapse rate (SALR)
. Can be as low as 4 degrees per Km climbed
Determined by the temperature of the air
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Rising air and cooling shows
increasing relative humidity
as it ascends
Wet/saturated
air is air where the
saturation vapour pressure
has been reached and
condensation is forced
to occur
When condensation occurs, clouds can form
Can occur for a variety of reasons
Lifting Condensation Level (LCL)
As soon as a dry parcel of air becomes saturated, its
rate of cooling slows from the DALR to the SALR
Condensation occurs and latent heat is released
The
point at which saturation occurs is marked by the base of a cloud layer
- known as the LCL
The altitude at which air can no longer hold moisture in gaseous form
3 different lapse rates are needed to understand how stable the atmosphere is (at a place and time)
Dynamic
SALR
DALR
Depend on the characteristics of an individual parcel of air
The balloon properties is the dynamic lapse rate
Static
Environmental (ELR)
Represents the
actual decrease in temperature
on a given occasion around our parcel of air
Air surrounding the balloon
The relationship between the
air parcel temperature
and its
surrounding environments
determines
where it can move
Stable
The
ELR is less than the SALR
, then when stable air is forced to move up or down, it tends to
return to its original position
once the displacing force has stopped
Unstable
The
ELR is greater than the DALR
, having a tendency to move away from its former position. Adiabatic cooling keeps air parcel warmer than its surroundings, keeping it rising
(Ball on the peak rolling away)
Neutral
Transitions between stable and unstable states are termed
'neutral'
-
no difference between ELR and DALR/SALR
Conditional Instability
Where SALR is less than the ELR, which is in turn less than the DALR
Simply, air is conditionally unstable, as in order for it to be lifted to become unstable it must:
1.) Become saturated
2.) Undergo forced convection
Needs to rise to a point where it becomes unstable
Termed the
level of free convection
The Radiosonde
From the 19th century, scientists experimented with meteorological instruments on balloons
By 1940, US Bureau of Meteorology developed a weather balloon and a 'radiosonde'
Economically viable enough, and could emit radio signals to the surface
There are many of these released every day - despite satellite technology, as these provide an unparalleled vertical snapshot of atmosphere state
1.5m balloon takes readings on the radiosonde of pressure, temp, rel humid and GPS every second (windspeed and direction aloft)
Could reach an altitude of 35km, and expand around 8m until they pop
Data from these Radiosondes are used commonly for weather forecasting, as well as implemented onto
tephigrams
Tephigrams
In the exam, there will be at least 3 questions on tephigrams
Helps us to interpret temperature and humidity structure of the atmosphere
Equal areas represent equal
amounts of energy
, so it can be used as a visual representation of atmospheric processes
Allows us to plot the changing properties of rising parcels as a '
path curve'
What 5 quantities are represented on a tephigram:
Saturation Mixing Ratio
Potential Temperature/dry adiabat (dashed lines that run diagonally running south east to north west)
Air runs parallel to these lines as long as it is dry (unsaturated)
Red
Air Temperature (straight lines diagonally south west - north east)
Blue
Saturated adiabats/ WB temps (heavy dashed lines on the left)
Green
Atmospheric Pressure (near horizontal lines)
When we plan the ascent of an air parcel - we start at a higher pressure
Orange
On a plotted tephigram, there are two plotted lines: the
path curve
(following our parcel of air) and the
environment curve
(surrounding air) -
ELR
If our path curve is to the left of the environment curve, then our parcel of air is
cooler than its surroundings
This tells us that the uplift that our parcel is experiencing must be
forced convection
When the path curve follows the DALR, it is rising unsaturated with all of its moisture is in gaseous form
However, this line turns blue, showing this air has cooled to the point of condensation
(Reaching the lifting condensation level)
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Interpreting Stability
Absolute Stability
Our parcel of air will always be cooler than its surroundings
Always
to the left
of the ELR
Moderate to small environmental lapse rate enhances atmospheric stability
On this graph, we can see the parcel of air rising along the DALR until it reaches the LCL where it begins to follow the SALR
This is because cooler air holds less moisture, and now the parcel has become saturated
If the parcel is moving through convection, this air will return to the point of origin
But, if it was moved by mechanical means, it will spread out more and clouds will be horizontal and thin (stratus cloud)
Absolute Instability
The path curve is to the
right of the ELR
, telling us our air is warmer and more buoyant than surrounding air
Therefore, it can rise under
free convection
(its own buoyancy) along the dry adiabat line
When the air cools to the LCL, our parcel remains warmer than its surroundings (still on the right)
Deep, rapid convection continues
(cumulus clouds forming)
When the path
curve crosses the ELR
and is now on its right, the air is now no longer warmer and the environment is
stable
Although it has gained enough
buoyant energy in its ascent to still push
slightly higher
Theoretically, cloud will continue to form until area above dash line and between ELR and path curve is equal to that of the unstable region
Conditional Instability
Instability is
conditional on the air parcel becoming saturated
These are common becuase
ELR oten sits between the DALR and SALR
We can see the air is forced to rise along the DALR (in a
stable environment
) and reaches the LCL, then following the SALR
And then, it
passes over the ELC
, and the air parcel becomes warmer as it then reaches the level of free convection, it can rise under its own buoyancy - and deep clouds can form
As before, if the zone of instability is shallow, we will get stratus clouds, but if it is deep we may see cumulo clouds forming
These are associated with severe weather
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Clouds and Precipitation
Cloud Formation: Condensation Nuclei
A
erosols are required
in air for condensation to take place
Clean air makes it harder for condensation to occur as there is a
lack of surfaces on which moisture can condense
Condensation occurs on microscopic nuclei, small particles to which water can adhere
These include: dust, salt, smoke and other compounds
They come in different shapes and sizes, and therefore have
different residence times
Concentration of 5-6mil per litre of air
Polluted atmospheres can have 10-100x more nuclei than oceanic environments
Aerosols have a significant
influence on cloud properties, so affect precipitation
They directly impact radiation reaching the surface so also affect
evaporation and condensation
Also
effect the number of cloud droplets and their transformation into rain droplets
Influenced by the
type and the depth of cloud
Cloud Types
Cloud types are classified internationally based on shape, structure, vertical extent and altitude
Stratiform Clouds
Form in layers, flat and smooth
Cumulo Clouds
Heaped, cotton wool, cauliflower like structure
Detached and individual and occur in fair weather conditions
Usually white in colour although the base is dark
Form as air heated at the surface is lifted through convection, cooling and condensing at the LCL to produce cloud
If conditions allow and there is sufficient energy, these clouds can grow to larger cumulus clouds
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Cirrus Clouds
Form high up, fibrous, wispy and composed of ice crystals
Cirrostratus Clouds
Transparent high clouds which cover large areas, produce white coloured rings, spots, arcs of light (known as halo)
As a result of slow rising air, generated at the forefront of frontal weather system
Movement of these clouds can be used to predict what the weather will do in the next 24 hours
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Can also form through aeroplane contrails
alto___ clouds
Mid level clouds
Altostratus
Sheets of thin clouds, usually composed of water droplets and ice crystals. They are thin enough in parts to see the sun through them
Often spread over a large area and usually featureless in nature, forming when both water and ice descends from cirrostratus clouds
Associated with a warm or occluded front, as the front passes, the altostratus layer deepens - it forms nimbostratus
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nimbus ___ clouds
Thick, low level, rain baring clouds
UK
Because the heating of the atmosphere and height of the tropopause vary with latitude, the same cloud types can be seen at different altitudes depending on location
Low latitude high cloud can be around 18,000m but at high latitudes its only as high as 8,000m
Global Cloud Fraction
Satellite observations help us to determine cloud type and amount, but both have been hampered by limitations in defining cloud structure and amount
Example: Indian Summer Monsoon (2017)
Starkest change in cloud cover, from april-may, blistering heat and drought
From July onwards, large cloud movement and monsoon rainfall
Cloud variability occurs with latitude, season and geography. The NH summer has a high % of cloud cover over west africa, south america and south east asia - and minimal cloud over the SH
Persistent cloud cover over the Southern ocean storm belt and the NA and pacific oceans
Formation of Precipitation
Types of Precipitation
Drizzle - emerged from fog
Rain - Must be around between 1-3mm
Shape results from water surface tension and air pressure
When the drop is small, surface tension is dominant and they become circular
With increasing size, velocity increase causing the raindrop to flatten
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Hailstones
Formed in thunderstorms in deep cumulonimbus clouds
Start as water droplets, and become supercooled and freeze on contact with nuclei
Growth rate is dependent of humidity and concentration of supercooled droplets
Snow
Form of solid, minute ice crystals formed when ice crystals in clouds adhere together and become heavy enough to fall to earth under gravity
If snowflakes fall in moist air, they will form together to make larger ice crystals
But, if snowflakes fall into dry air, they will be very small and make powdery snow
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Cyclonic
(frontal)
Precipitation formed when
cold air meets warm air at a weather front
The warm air rises above the cold air, and cools as it rises, condensing and forming cloud
Leading to precipitation, common in the UK winter months and large extensive rainfall
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Orographic
Affected by the
topography
of the land surface (particularly sharp reliefs)
Forces air to rise and cool and moisture condenses, with precipitation on the upward side
This also creates a rain shadow on the leeward side of a mountain/hillside
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Convective
Evolve from the heating of the Earths surface and the resulting convection and condensation
Small showers tend to be hard to predict as surface type has a localised impact on convection
These small showers also move very quickly, and can show why one place can be dry and a neighbouring place is raining
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Bergeron-Findeisen theory
(ice process) in mixed cloud
Because the equilibrium water vapour pressure with respect to ice is less than that of water at the same sub-freezing temperature, providing there is sufficient water content,
ice crystals will gain mass via deposition at the expense of liquid water droplets that will lose mass by evaporation
Ice crystals either fall as
snow
or are
modified
on their descent through accretion, melting or evaporation
This processes works in clouds with a temperature between 0 and -20 degrees Celsius with enough s
upercool water droplets and ice crystals
Highly effective at
seeding
, providing ice crystals to lower clouds
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Coalescence
Refers to the collision of two water drops in a cloud and their merging into a single larger droplet
Droplet size becomes very important here, as they must meet 19 micrometres to coalles with other droplets
Leads to rapid growth of droplets, enhanced by the turbulence seen within cumulus clouds
Important in warm clouds in the tropics and the mid latitudes
The Principles of Atmospheric Motion
Frame of References
Need a coordinate system: XYZ - a place with an origin on earths surface
Y - North, South
Positive Northward
X - Vertical (up and down)
Z - East, West Direction
Fundamental Atmospheric Forces
1.) Pressure Force Gradient
PGF is the
primary cause of air movement
; it is the pressure gradient (difference) that is caused by
spatial differences in surface heating
and the consequent changes in
density and air pressure
(Or by mechanical factors, such as mountain barriers affecting an air mass)
It has both vertical and horizontal components
The vertical components is in
balance with gravity (hydrostatic balance)
Air always wants to move to lower pressure, which we feel as wind
Lines of equal pressure are represented as isobars
, and anywhere along one represents the same surface air pressure
And the
closer the isobars
are to each other, the stronger the pressure, and therefore the
stronger the wind
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Sea Breeze Model
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2.) Coriolis Force
Inertial atmospheric force
, which arises from the fact that air masses over the surface of the earth are referenced to a m
oving coordinate system
(lat and lon), which move with the rotation of the Earth
Effects on winds?
Causes a deflection of air to the right of the direction of travel in the NH (right in SH).
This explains why we see the curved paths in wind and pressure belts
This model is complicated by many other things, and
coriolis force is negligible at the equator
and gets stronger the closer you get to the poles
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This explains why the
trade winds and the westerlies are curve
shaped towards the poles
3.) Centripetal acceleration
Centrifugal force is an apparent force in a rotating force,
deflecting masses radially outward on an axis of rotation
, acting upon
Earth
and its
atmosphere
due to the rotation of the Earth
This is incorporated into the Earths central gravity and is
equal and opposite to centripetal acceleration
Any particle following a curved path inwards towards a centre of rotation undergoes acceleration
This happens when you go around a loop on a rollercoaster - although it is usually small in magnitude, it is still important
In low pressure situations with a high PGF and low coriolis force, there is anticlock wise velocity with the gradient wind
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4.) Frictional Forces
The lowest part of the atmosphere, where friction affects the movement of air, is called the
'planetary boundary layer'
(PBL). This varies in vertical extent, depending on the roughness of the surface -
increased roughness = more disturbances
By 2km altitude,
frictional forces no longer play a role in atmospheric motion
. For example, over Dartmoor, frictional force will be much higher than over a smooth extent such as the Somerset levels
Where friction does play a role,
wind velocity is reduced below its geostrophic value.
This modifies the deflective power of the Coriolis force (which is dependent on velocity)
Therefore, the friction causes a decrease in the deflective force close to the surface. Causing friction to act to the right of surface wind - which can cause wind to blow obliquely to the isobars and against the pressure gradient
Surface wind represents a balance between the Coriolis force and the PGF. Where Coriolis is small, fiction may balance this out, leading to wind flowing down the pressure gradient.
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Types of Horizontal Flow
Vorticity
Relates to the
rotation of parcels within a fluid
. It has three elements;
magnitude
(which relates to angular velocity),
direction
(vertical or horizontal axis around which the rotation occurs) and a sense of
rotation
(Earth's rotation).
Vorticity acts in the
same direction
as the Earth's rotation (cyclonic in the NH), defined as
positive vorticity
Cyclonic vorticity can occur from cyclonic
curvature of stream lines or cyclonic shear
(or both?)
Divergence
Mass uplift/descent of air responds mostly to dynamic factors related to horizontal flow
Encompasses a range of components where the compressibility of air becomes important.
Divergence involves the expansion or spreading out of air and diffluence the rate of which air flow diverges along an axis in the direction of flow
Imagine cars on a motorway, closed lanes and cars must converge together and squeeze into less lanes
When the lanes open again and everyone can spread out, symbolises divergence
The opposite of this flow is convergence, which is the contraction of air and confluence which is the rate at which air is converging along an axis in the direction of flow
Vertical Motion
If air moves,
it has to be replaced
, any inflow or outflow of air must be
compensated by vertical motion
if a pressure system is to persist
When cool air converges and descends, divergence occurs (subsidence). The opposite happens with convergence and ascent causing divergence aloft
Such mass
ascent and descent of air is much slower than in convective systems
- may move at rates of 5-10cm per second
Topic 1: North Atlantic Extratropical Cyclones
(Mid latitude - Extratropical)
Understanding air masses
Air mass movement is mainly driven by radiation from the sun (
insolation
)
There is higher insolation at the Equator, so it is warmer than it is at the poles
Due to the rotation of the Earth, there are
three difference cells of air movement
(Polar, Ferrell and Hadley)
The Hadley cell is most distinct, rises at the Equator, moves northward and southward.
This cell movement has created multiple different large wind circulations (Westerlies caused by divergence of air from the Ferrell cell and the Trade winds caused by converge from the Hadley cell (as well as the polar easterlies)
These winds also spiral due to the Coriolis force
The mid latitude regions are low pressure regions
Air masses and source regions
Air masses are a large body of air
with uniform characteristics
(temperature, pressure, humidity), that originate in specific geographic areas (source regions)
When trying to figure out what UK weather conditions will be like – we can look at the
wind direction and what air is coming towards us
Tropical, polar, maritime continental etc
The characteristics of these air masses are shaped by the area that they came from
Maritime is high in moisture for example
Secondary Air Masses
Barotropic
Surfaces
Surfaces of uniform pressure and temperature (density). Pressure and temperature surfaces run parallel to each other
Baroclinic
Surfaces
Pressure and temperature surfaces do not line up - not parallel. a surface where a temperature (density) gradient exists across and a constant pressure surface. The surfaces do not line up.
The Zone of Baroclinicity
An area characterised by
low pressure
and
high temperature contrast
(Ferrell and Polar cell boundary)
Warmer wet air in Ferrell sell and cold dry air in the Polar cell
The
ZOB is the boundary between the two cells
Included in this zone is the Polar Front Jet (PFJ) stream, higher in the atmosphere
The cold air and warm air mix together because of eddies (analogous to storm patterns)
Eddies are affected by temperature conditions (turbulence is disturbed)
Role of the Polar Front Jet (PFJ) stream
What is it?
The PFJ is a
strong thermal wind
(11-13km altitude) several kilometres deep and several hundred kilometres wide
Located just below the tropopause
It has stronger and weaker parts
Rossby
(planetary)
waves
Rossby waves are mechanical (inertial) waves that form naturally in a rotating fluid (Earth)
They help to mark out the jet stream (wobbly bits)
PFJ Dynamics
Coriolis Force
Air being pushed away from the Earth's equator due to
Earth's rotation
, pushes the air until the speed has become balanced. Looking at isobars (lines of constant pressure), the
wind moves parallel to these lines of pressure
(rather than moving from high pressure to low pressure)
West to East flow
Direct balance of the PGF and the Coriolis force
PFG (pressure gradient force)
Area with
high pressure is pushed to an area of low pressure
These are often in balance
What can effect the jet stream?
Because it is a thermal wind, it relies on
temperature difference
So strength of this contrast
determines strength of the jet stream
Seasonality
- Moves further South in the Winter
Higher temperature contrast in winter
also provides more energy for storms
Sinuosity and ETCs
Shape of the Jetstream is 'wiggly' (has troughs and ridges)
How does the PFJ
influence the development or strengthening of surface low pressure systems?
We can see the jet stream at higher levels in the atmosphere that interacts with the storm we see on the surface.
Thinking through the troposphere
ETCs and
Convergence and Divergence
ETC are formed where
convergences happens at the surface
(low pressure), drawing air to ascend and diverge in the troposphere
Mean level of non divergence at around 600mb
Reverse of this for anticyclones
Convergence
The net inflow of air (increase in mass) to an area. The contraction or squashing of air
Confluence
When two air flows join together and then flow as one
Divergence
The net outflow of air (decrease in mass) from an area. The spreading out or stretching of air. Occurs either:
a) stronger wind moves away from weaker wind, or
b) when air moves in opposite directions
Diffluence
When a flow of air separates into to flows and they move apart
This diagram shows divergence and convergence of the PFJ
It has peaks and troughs, with high pressure below and low pressure above
The lines on this diagram are pressure, and we see the PFJ in the middle
On the left, after the first peak, we see
upper-level convergence, with downward mass motion
This process usually results in clear skies and calm weather
However, on the right, after the trough, we see upper level divergence, with upward mass motion
This process results in clouds and precipitation
Idealised Process
We see convergence towards C as the air slows down and is forced around a bend, divergence can be seen towards D as air able to stretch out and speed up
At the ridge, there is a vorticity minimum, and at the trough there is a maximum
Less air is flowing from C-D (up the trough) than D-E (back towards the ridge
This results in '
speed divergence'
Diverging air is forcing ascent – generally cloud/rain below
More air is flowing from A-B (down the ridge/peak) than from B-C (into and through the trough)
This results in '
speed convergence'
Some of the converging air is forced to descend – generally fine weather below
Vertical Vorticity
Local spinning motion or rotation within a fluid, like water or air, often visualised as whirlpools, eddies, or vortices
Negative Shear Vorticity
Clockwise - Wind speed decrease from low
Negative Curvature Vorticity
Clockwise Spin
Negative Earth Vorticity
North to South motion
Positive Shear Vorticity
Anticlockwise - Wind speed increase from low
Positive Curvature Vorticity
Anticlockwise Spin
Positive Earth Vorticity
South to North motion
Potential Velocity
A
spinning column of air is spread out, which slows it down
(similar to spinning on a roundabout and then pulling your legs in, which makes you quicker)
Conversely, if it is made thinner, it spins faster. This is to conserve angular momentum.
There is an i
mportant link between convergence and divergence
and vorticity. Vorticity can be calculated so is better to look at ETCs through that way.
Geostrophic and ageostrophic flow
Geostrophic Wind
In the upper troposphere (including within parts of the PFJ), winds are in near geostrophic balance. This is where
PGF and CF are in balance
Jet Streaks
Within the jet stream, there are faster parts called ‘jet streaks’.
These are a segment of the PFJ with relatively high velocity winds
These can accentuate trough and ridge patterns.
But why is this important for the formation of ETCs?
Jet Streak: Right Entrance Region
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2.)
This causes an imbalance, the
PGF is more dominant than CF.
This causes air to diverge on one side and converge on the other.
1.)
The geostrophic wind, running parallel to the isobars, is
faster when the isobars are closer together.
These conditions are favourable for ETC formation
The formation of ETCs
Cold front - stays below, approaches from the North or East and goes underneath the warm front
Warm Front - this front rises above the dry and cold fronts (as it is less dense) and splits, and part of it sinks
Dry Front - passes through between the warm and cold front, gives it the comma like appearance on satellite imagery
Cold and warm air meet at a
low-pressure centre
; sharp boundaries (fronts) form where warm air is forced upward over cold air.
The Frontal Boundary: Warm Front
Warm air front is moving towards to cold air - and rise above it
Causes high clouds, but the closer it gets, the lower the clouds get
Cold Front
Cold air front is moving towards to warm air - when it meets, it forces it to rise as they have different densities
Forms thick layers of strato clouds, heavy rain or hail with cumulonimbus
Occluded Front
Cold air is moving faster than the warm air
Cold front catches up with warm front and pushes warm front above
Triangle of warm air gets smaller
Interpreting air masses and fronts on synoptic charts
Warm air = Circles
Cold air = Triangles
Classification
based on simple conceptual models
Noweigan Model
o Meridonially-oriented cyclone
o Relatively strong cold front
o Occlusion through narrowing of warm sector
Classic, go to model
Shapri-Keyser model
o Zonally-oriented cyclone
o Relatively strong warm front
o Frontal fracture – weaker baroclinicity along poleward end of cold front
o High impact storms – fast forming and intense
(T-shaped)
Storm Ciaran (2nd November 2023)
Mean sea level pressure (MSLP) of 953mb at 1000mb pressure level (sea level) on 2nd November 2023.
The UK experienced very high winds and heavy rain from an explosive ETC (Storm Ciarán)
An example of a system that deepens more than 24mb in 24 hours (a weather bomb).
People say it was t-shaped, and sting jets were thought to be present
Streaks coming out the end of a storm
Extratropical cyclones are
not the same
as 'tropical cyclones'
ETCs are
Driven by temperature difference
Mid latitudes (higher than tropical cyclones)
Have a cold core
So
what are ETCs
and why are they
important?
Low pressure systems
that occur in the
mid-high latitudes
.
Extratropical cyclones (ETCs) are
large scale (synoptic
- typically 1000-1500km in length)
Societal impacts?
Affects us here in the
UK
Causes
storm surges, flooding, strong
(damaging)
winds
Important that we can understand these systems in order to forecast them
And understand how their behaviour may change in the future
These storms are 'named' when forecasters believe they will have societal impacts
Effect on surface?: feeding an ETC
Blue Arrows - Coriolis, Red Arrows - Pressure Gradient Force
Lower pressure in the middle and counterclockwise airflow
This is an example of cyclonic flow
Once the arrows are inside these concentric isobars, there is a force balance
Rossby Number
The Rossby number
illustrates the dominant forces acting in the weather system.
Synoptic-scale systems have a
long length scale
and
Coriolis force and pressure gradient force dominate
in these systems.
Therefore, Lf is greater than U and the
Rossby number is small (Ro ≤ 1).
This means that PGF and CF are the factors that are dominating. Just a way of what should be most worries about based on scale and speed.
Topic 2: North American Supercell Thunderstorms and Supercell Tornadoes
(Mid latitudes - extratropical)
Why are particular parts of the United States more susceptible to supercell tornadoes?
Understanding types of thunderstorm
What are they and why are they important?
Supercell thunderstorms are a particular type of thunderstorm, representing the most sophisticated type of storms
The way in which they form is
complex
They revolve around a central column of air (
mesocyclone
)
They are very powerful in terms of air velocity
Tornadoes that they can spawn
And rainfall and giant hail that can emerge from them
They lead to a the most powerful tornado that we see in the world: the
supercell tornado
Societal impacts
Damage across different parts of the US
Property damage, neighbourhoods destroyed in paths
Accompanying systems (the thunderstorms) can also bring really damaging winds and rainfall
Storm chases are common in the US, with people who chase these giant systems
Storm Warnings and false alarms
The UK does experience thunderstorms, but not this violent sort seen in the US
The National Weather service provide information on the severity of thunderstorm risk: individual catergories
The way that these are events are viewed in societies is different, and the way people view their risk
US Tornado distribution in 2024
There is a distinctive geography to tornado appearance, predominantly found in the
Midwest
all the way over to the eastern seaboard
This is because the rocky mountains run just to the east to the Midwest - and
there are very few tornadoes that occur on the western side of the Rockies
This is important because the formation of supercell tornado processes are dependent on the areas geography
Prevalent in Oklahoma (named 'Tornado Valley')
These systems are typically 20 - 50km across, with a time scale of hours
But tornadoes themselves can last from seconds - hours or metres to miles across
Development of thunderstorms
Key Conditions for Formation
Instability
Conditional Instability
– parcel of air starts off cooler than its surrounding, forced upwards by one of the types of mechanisms, reaches the level of free convection, crosses over the ELR and can rise under its own positive buoyancy
This buoyancy makes this parcel of air travel very quickly
Moisture
in the atmosphere
Moisture effects LCL and the level of free convection
We need an atmosphere that is holding a reasonable amount of moisture for a thunderstorm to form
When latent heat is released from condensation as parcel of air becomes saturated, it is used to fuel the thunderstorm
Moisture is also needed for the development of thunder clouds
Wind Shear
Shear is simply
change in wind speed or direction with altitude
Lifting Mechanism
Means of forced convection, getting our parcel of air up high enough so it reaches the level of free convection (becomes unstable
Moisture
We need air that has a
high humidity,
and is close to the
dew point
(air holding a lot of moisture as a gas) but is close to reaching condensation
We need a
low LCL
, meaning this saturated air doesn't have to gain that much altitude to condensate (low level cloud formation)
We need a
constant source of fuel - moisture
, as it is a good transferrer of energy (release of latent heat)
Therefore, we require a
rate of moisture advection
(constant supply of moist air)
Dry mid-troposphere
which promotes convective instability
Conditional Instability
(CAPE and CIN)
Convective Available Potential Energy (CAPE)
Red lines shows us our parcel of air along the dry adiabat, with it switch at the LCL along the moist adiabat
When it crosses over the ELR (goes from being cooler to warmer)
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The maximum buoyancy of an undiluted air parcel, related to the potential updraft strength of thunderstorms
Means that our parcel of air, above the LFC becomes warmer and less dense than its surroundings - so the
CAPE is the energy that that parcel of air is gaining as it ascends through the atmosphere
Convective Inhibition (CIN)
The energy needed to life an air parcel upwards, adiabatically (heat-less exchange) to the LCL and then past the LFC
What is trying to stop our thunderstorm from forming
For supercell thunderstorms - we need LOTS OF CAPE AND LITTLE CIN
Wind Shear
Can be vertical
speed shear
- Significant increase of wind speed with height
Or combinations of the two
Or can be vertical
directional shear
- Significant change of wind direction with height
Lifting Mechanism
For conditional instability, we need to get our parcel of air to the LFC
There are multiples way we can get a parcel of air to rise despite it being cooler and denser than its surroundings
Moisture Differences
Dry air meets moist area creating a natural barrier, forcing air to rise (seen often in the US)
Converging air masses
Seen in England, air streams moving towards each other, they have to rise up
Cold Fronts
Cold air frontal boundaries can cause warm air to be forcued up (seen with storm amy)
Topography
Flow of air is forcing it to rise up to higher altitudes, then it can become a similar temperature to the ambient surrounding
Types of thunderstorm
Air mass thunderstorms
Smallest and shortest lived
, tend to see most in this country
Isolated
thunderstorms, no more than a couple of km across, and weak in nature relatively and short in duration
But why?
The fuel supply for these thunderstorms occurs in exactly the same region as the exit for the rainfall/hail
Short lived
because updrafts and downdrafts are NOT separated from each other
Three stages in formation:
1.) Cumulus: Air close to the ground surface heating up, lots of evaporation, conditional instability, cloud growth and a fuel supply
2.) Mature: Deep anvil top (made of ice) cumulus cloud through the troposphere, holding lots of moisture, as we start to see the emergence of precipitation
3.) Dissipating: Air that fuels system is adjacent to air forced down by precipitation (cold air), more powerful than the fuel, cause the system to die - lack of warm air
Mesoscale Convective System
This is where these systems become more sophisticated, with a degree of organisation meaning they can last longer
Seen during the Boscastle floods (
squall lines of self propagating thunderstorms
)
This is because the
fuel supply is very separate from the downdraft
This resemble a 'shoe'. We can see that these systems rely on
warm air flowing above our
gust front
(over the toe of the boot) and heads into the troposphere to form thunderclouds
The descending rainfall creating the cold draft, happens in the rear of the system - fuel supply (warm moist air) does not meet the cold gust front
This system propagates the boot moving forward, sucking up more warm air over the toe
The anvil is over the front of the system and it moves its way forward (
organised convection
)
Meaning
these systems can become bigger in extent and duration
Supercell thunderstorms
These thunderstorms have the potential to cause the most severe tornadoes
This has a high precipitation core, and are extremely organised in terms of convection
Has a fine balance between convective available energy and wind shear
An often dangerous convective storm that consists primarily of a single, quasi-steady rotating updraft, which persists for a period of time much longer than it takes an air parcel to rise from the base of the updraft to its summit (often much longer than 10–20 min).
Only 30% of supercell thunderstorms lead to tornado formation
Understanding U.S supercell thunderstorms
Plan View
Centre Section
There is a
central core
to this system - rotating and
fuelling the system
Transporting warm moist air at the surface and transporting it up through the system
Usually goes through the troposphere and pokes through the stratosphere
Vortex (mesocyclone) working its way up through the system
Mid Section
Downdrafts
happen in
two
different zones
Forward Flank Downdraft
blue cold front on graph that has reached the surface and spread out
BOTH DOWNDRAFTS ARE COLD
Rear Flank Downdraft
brings air from the middle of the troposphere down to the surface
Side-on view
Green air is
air flowing into mesocyclone
and our system is moving from left to right close to the surface, and into the 'over shooting top
We can see the core (mesocyclone) vortex that is driving this system
The
forward flank downdraft
is bringing down the cold air (with rain and hail) at the front of the storm
The
rear flank downdraft
is bringing air from the middle of the troposphere, down to the surface
Vital for tornado formation
How do we distinguish if it is a
supercell or not?
Bulk Richardson Number (BRN or Ri)
Measures how buoyant air is against how much it is changing in speed or direction
Where
shear is very strong
(relative to buoyancy) =
low BRN
number and supercells are not likely to form
BUT a
balance of the two factors
with a value of 10-50 is
most favourable for sustaining supercells
Why?
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Where we have a
high BRN
number, it is likely we will see multiple cells or
clusters of thunderstorms emerging
Useful for supercell thunderstorms, but limited for identify potential for supercell tornadoes
This BRN number is useful in identifying
areas most susceptible to supercell thunderstorms
Cloud forms
Under what conditions can supercells form?
High CAPE
Strong and deep
vertical wind shear
The amount of wind shear can make a
big difference between our inflow and outflow regions being aligned/separated
These systems are so powerful, there needs to be an enormous amount of
instability
and energy transferred through the atmosphere for them to form
These two conditions are essential in providing atmospheric instability
Lots of warm moist air inflow, so peak in evening
Geographical area and time of year?
Why do these systems form where they do in the U.S?
These systems are at their
most prevalent in Spring and Early Summer
in the US - but can happen at any time of year
Why?
What is going on across the US during
spring time
?
Mid latitudes, with a prevalence of storm systems that are in the dying phase (similar to UK)
Lots of atmospheric
opportunities for lifting mechanisms
along frontal boundary
These fronts act in the same way as mountain changes, meaning we can attribute the passing of extratropical cyclones with the formation of supercell thunderstorms
Ground is heating up, increased solar radiation and therefore increased lapse rates
This is vital for creating
conditionally unstable
atmospheres
Jet Stream
Wind shear
is really important at this time of year when the polar equator temperature gradient is significant
Springtime sees a
large temperature contrast
between the poles and warm tropical air coming from the South
The
Baroclinic zone
, where we see tropical cyclones forming, creates
a lot of instability
This seems to increase the amount of shear going on in the atmosphere
'Great Plains'
(midwest)
Orange: Instability
Two air masses with different characteristics
Green: Moisture
Two different air masses
At the same time, we have very
cold, dry air
sweeping over from the West (from Asia) from the High Rocky mountains plateau
This air
sweeps over the top of the warm moist front
(from GOM)
These two do not happily mix, but do create the
perfect conditions
for supercell thunderstorm creation (
high wind shear and high instability
)
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During springtime, we have very
warm, (low-level) moist air moving north from the Gulf of Mexico
(red arrow on map)
This air is pushed below the cold, dry front (from HRMP)
Supercell Tornado Formation
Three Steps of Supercell Tornado Formation
Step 1.) Development of a mesocyclone aloft
Step 2.) Development of near-ground rotation
Need to bring down air from the middle of the troposphere and closer to the surface
Need a mechanism of differentiating air that's close to the surface that doesn't have much spin in it and air coming down from the troposphere that does have spin in it
This is where the
Rear Flank Downdraft
is important as it brings down air from the middle of the troposphere,
blocking air at the surface and bringing down air that has lots of angular momentum
This is a baroclinic process as it is initiated by the rear flank downdraft
Step 3.) Air flow towards the circulation zone
For this stage, we require a disruption of
cyclostrophic balance
Usually PGF is balanced by centrifugal force in most weather systems
This can happen because of the friction that is caused by the roughness of the land surface
If we have air coming down from the mesocyclone with lots of momentum, it is in the lower part of the troposphere and a rough land surface, there is the potential for imbalance of the cyclostrophic force
This can cause the boundary layer to
erupt upwards in a spiral jet
, and this has air moving over a tigher area,
forcing it into a smaller space
(bernoulli effect), pressure drops even lower and tornado forms
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Rossby Number
Tornadoes have a small length scale and a large velocity. Inertial and centrifugal forces dominate in these systems.
Therefore, U (characteristic velocity) is greater than Lf (Coriolis force) and the Rossby number is large (Ro ≥ 1)
This allows us to see which force is dominant in supercell tornadoes
Tornadoes form over LAND
Topic 4: Indian Summer Monsoon
The Global Monsoon
What is it and why is it important?
Large societal impacts, with extreme events occurring before and during the monsoon
Pre-monsoon phase brings extreme heat and drought (April-May), worsening due to climate change
This heat and drought it worse around the Ganges river
During the monsoon, the area experiences high rainfall rates, huge volumes of rainfall over a short period of time
This extreme rainfall brings geological events: landslides, flooding etc
It is important we understand the monsoon as it is large in terms of duration and extent
We need to start with a global perspective, as monsoon activity is not disconnected from other regional and global phenomena
Whilst the ISM gets the most media, there are multiple monsoon systems (
seasonal changes in rainfall occurrence associated with the movement north and south of the ITCZ
)
This change (migration of the ITCZ) happens due to
changes in solar radiation
This brings
zones of maximum rainfall with it
- so these monsoons can occur in America, Africa and SE Asia
The ITCZ and energy transportation
Water associated with the cloud growth that we see around the ITCZ is an important transfer of energy
When evaporation takes place, energy is transferred (stored up) in a latent sense within that water
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The ITCZ is the central meeting point of the two Hadley cells
The troposphere is much higher in the tropics than the mid-latitudes
Deep convection along the ITCZ
Pressure systems and wind
Pressure around the globe
During the Boreal winter, there are particular parts of the world where there are 'source regions'
L for low pressure and H for high pressure, production of regional pressure patterns
Around Asia we can see a dominant high pressure source region during the Winter
During the Boreal Summer, this
region has now changed to low pressure
due to the migrating of the ITCZ
Seasonal movement, with a lag, follows maximum solar insolation North and South as we go through the season
Importantly, we will be looking at how far North that summer position of the ITCZ is over India
Rainfall
During the Boreal winter, there is high rainfall around the tropics
During the Boreal Summer, we can see the entire content of
Asia has high rainfall
Seasonal Migration of the ITCZ
The ISM is marked by abrupt and sharp shifts in the ITCZ between (65ºE -95ºE)
The South Asian position demonstrates that the ITCZ does not simply follow a) an insolation maximum or b) the interhemispheric temperature contrast
The zone of maximum insolation, combined with topography (roughness and height) have an enormous bearing on circulation patterns
We will see that this is why the ITCZ low pressure belt pushes much farther North into India than it does in many other places
Planetary Scale Phenomena
Terrestrial-oceanic pressure and wind patterns
Land-sea breeze model
The role of continentality and significant topography
Role of the Himalayas and Tibetan plateau
The Seasonal Migration of the ITCZ
The role of atmospheric teleconnections
Impacts that atmospheric processes happening in one part of the world have on weather experienced in another part
Effects regionally on the Indian ocean? Wider effects on the Pacific, specifically near the Equator. What is the relationship between ENSO and the Indian Ocean?
The Indian Monsoon
The South Asian monsoon covers a large geographical area and impacts different regions at different times of the year.
We are only focusing on the area of the monsoon affecting the
Indian Subcontinent
Monsoon is
simply a reversal in seasonal winds
and weather patterns
These winds are impacting the Indian subcontinent, affected by the Arabian Sea and the Bay of Bengal
This event provides 4/5 of India's annual rainfall
Large societal impacts, as it has large destructive impacts, but also gives life, crops, water
The Western Ghats along the Western Coast of the Indian Peninsula is greatly effected by/affects the monsoon
The Physical Geography of India
In the Northern Part of India we have the
Tibetan Plateau
(near the East of China)
This is the largest and highest region in the world
To the South of the Plateau, we have
the Himalayas
(very high mountains)
South of the Himalayas, we have the
Foothills in Northern India
Most of Northern India is the plains, around the Ganges - affected by the foothill of the Himalayas but also highly populated (Delhi)
Significant topography on both sides of the Peninsula with the
Western Ghats and the Eastern Ghats
The topography of India plays a significant role in determining when and where rain falls
Monsoon Seasonal Changes
(Timeline)
Winter Season: The Northeast Monsoon
The wintertime over India is a period of relative stability, with gentle, offshore winds due to high pressure and bifurcating jet with Westerly Flow
Winter is from late autumn to around March
At this time of year in the mid to upper troposphere,
westerly flow dominates
(brown arrows coming from the left)
This is our
subtropical jet stream
, weaker than the PFJ, but powerful jet occurring over India in the winter
This westerly flow goes above and below the Tibetan Plateau
It is so high, wind cannot go over it and the jet
bifurcates
Once branch sits over the plains of Northern India and the Himalayas
The other branch sits to the North of the plateau
So the plateau has a great effect on high atmospheric winds
2 more items...
Winter Season: Temperature Inversion and Pollution
A lot of cities in Northern India have bad pollution
This is, in part, caused by the cold air tumbling off of the higher ground (plateau) trapping the pollution in the Atmosphere in a temperature inversion
The Indian Monsoon: January
The spikey section shows the impact of the plateau on atmospheric flow
Distribution of wind velocity (km/hr) (orange line) and temperature (°C) (dashed yellow line) along the 90°E meridian for January.
Jw represent the westerly jet streams
and the purple lines show the different heights of the tropopause.
Additionally, during January and February extratropical systems from the Mediterranean can
deliver intense snowfall or heavy rain
, depending on elevation and synoptic conditions
Pre-monsoon Season: April and May
Subsiding air dominates over India. Hot and dry conditions prevail over northern India
Land surface is heated and increase insolation during the
transitional period
In this period, we see two things battling against each other
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Pre-monsoon Season: Transitional Period (May)
Low pressure from convection (over the Indian Landmass)
initiates onshore winds from the Arabian Sea and Bay of Bengal
This air replaces air displaced in low pressure convection from surface
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Monsoon Onset (Late May)
In late May, the
southern branch of the upper level jet breaks down and jumps north of the plateau
. Below 500mb, the plateau acts as a barrier and the jet axis jumps to the northern side.
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The Southwest Monsoon: June - September
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Indian Monsoon variability: regional and global influences
External and Internal drivers of the monsoons
External (to the atmospheric system, natural)
Natural
Volcanic Eruptions
Solar Insolation
Anthropogenic
Emissions
Aerosols
Deforestation
Internal
Processes that are due primarily to interactions within the atmosphere as well as those that involve coupling of the atmosphere with various components of the climate system
Atmospheric teleconnections
A linkage between weather changes occurring in widely separated regions of the globe.
Seen through a significant positive or negative correlation in the fluctuations of a field at widely separated points.
Changes that occur within SSTs in the Pacific have a bearing on dominant zones of convergence and convection
These dominant zone changes in zones of convection shift circulation patterns around the equator, so that impacts where air is rising or falling elsewhere
We can see this occurring at different scales, we will be looking at 2
El Nino Southern Oscillation
ENSO-Monsoon Relationship
A change in the SSTs of the equatorial pacific and associated zones of convection and subsidence abovie it
It has three phases, a positive phase (El Nino, Neutral, La Nina)
These phases refer to what the sea surface temperature is at a particular point around the equatorial pacific
La Nina
Body of cold water over the eastern equatorial pacific, causes upwelling over cold water at the surface at high pressure regions
At the western Equatorial pacific, we have much higher sea surface temperatures and dominant zones of deep convection
La Nina is an exacerbation of these normal conditions
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El Nino
Periodically the equatorial pacific shifts and we see changes in El Nino causing changes in the geographical distribution of sea surface temperatures
During El Nino, the eastern part of the EP warms, with dominant convection and a cooler WP drying out areas like Indonesia
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The Indian Ocean Dipole
Sustained changes in the
difference between sea surface temperatures of the tropical western and eastern Indian Ocean
are known as the Indian Ocean Dipole or IOD. The IOD is one of the drivers of the Indian Monsoon and it has three phases: neutral, positive and negative.
Pressure and temperature seesaw (regional to India)
Neutral
Phase where we see sustained average conditions, air that is dominantly subsiding over dry regions and convection to the East
Positive
Equatorial westerlies weaken and
higher moisture in the west, increasing rainfall during the SW monsoon
due to Arabian sea being warmer than usual
Negative
Equatorial
westerlies strengthen and higher moisture in the east, decreasing rainfall during the SW monsoon
due to Arabian sea being cooler than usual
Teleconnections: the walker circulation
The Walker circulation – the
circulation cell induced by temperature contrast between warm waters of the western equatorial Pacific
and the cooler water of the eastern equatorial Pacific.
The variability in the cell is
associated with the Southern Oscillation and often used to describe the entire (east-west) equatorial circulation
around the globe.
Looked at when looking at pressure differences
