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Week 5 - Atmospheric Moisture and Precipitation - Coggle Diagram
Week 5 - Atmospheric Moisture and Precipitation
Atmosphere stores 13x10^12 tonnes water. = 2.5cm depth water.
Need hydrological cycle = constant great inputs and outputs of water to sustain global climate.
Moisture also influences air motion, especially vertically, through lapse rate.
WATER VAPOUR
96% of atmospheric moisture exists as water vapour.
Proportion of water vapour various from 0-7%.
Absolute humidity (g m-3) = mass of water held by particular volume of air.
Air holding max amount of WV is saturated.
Air temperature determines max amount of WV that can be held, e.g. air may hold 15g wv at 20ºc but only 1.5g wv at -10ºc.
Relative humidity (%) = ratio of airs actual wv holding to max that it could hold at that temperature.
CLOUD
Cloud (dew) formed when air cools below dew-point temp (=100% RH), causing wv to condense.
Returns energy to environment, so warms surroundings. (Energy was initially taken up as latent heat during evaporation from surface, which cooled surface).
This process tends to reduce adiabatic lapse rate i.e. fall of temperature with height of a rising air parcel.
SALR (sat air) = -4 to -9 K km-1, lower at higher temperatures (more moisture).
For comparison, DALR (dry air) = -9.8 K km-1.
LAPSE RATE
Adiabatic process = no energy is added to or taken away from system.
As air parcel ascends, its temperature decreases adiabatically due to parcel expansion
Rising air parcels cool at either:
dry adiabatic lapse rate (DALR, for parcel of air that has not yet reached saturation conditions)
saturated (moist) adiabatic lapse rate (for parcel of air that is saturated)
As air parcel descends, its temperature increases adiabatically due to parcel usually warms DALR because typically some moisture has been lost due to some precipitation.
TYPES OF AIR STABILITY
ELR (Environmental lapse rate) = decreases of temperature with height in free atmosphere = -6.5 K km-1.
If ELR<DALR: air parcel is cooler and denser than the surrounding air, and therefore tends to sink, so is stable.
If ELR>DALR : air parcel readily becomes warmer and less dense than its surroundings and tends to rise, so is unstable.
Absolutely Stable Air (DIAGRAM IN SLIDE 9 WEEK 5)
Absolutely Unstable Air (DIAGRAM IN SLIDE 9 WEEK 5)
SPECIAL CASE - air can also be conditionally unstable if its displaced above a certain height, reaches saturation and then cools at the SALR (ELR<DALR but ELR>SALR) this requires air to be lifted initially.
HOW TO LIFT AIR
Forced ascent = orographic lifting over mountain.
Forced ascent/upglide over mass of cold air = frontal uplift.
Convergence and uplift at centre of surface low pressure system
Convectional lifting e.g. caused by heating of warm summer land surface + INTERNAL ADIABATIC AIR BUOYANCY
CONDENSATION NUCLEI
To be formed precipitation needs saturated air, lifting mechanism and condentsation (hygroscopic) nuclei.
Condensation nuclei have d<0.4um; 1l of air may have 10^9-10^10 of them.
Sources: windblown soil dust, volcanic ash, salt from ocean spray, carbonaceous or sulphate products from fossil fuel combustion or from dimethly sulphide over oceans.
PRECIPITATION FORMATION IN WARM/COLD CLOUDS
WARM
Precipitation occurs when cloud droplets (d. -20um) are sufficiently heavy to have terminal velocities > any updraught or lifting mechanism.
For temps >0º rising and sinking air causes droplets to collide, with preventing coalescence of larger droplets. This process enhanced in thick clouds with sustained strong updraught.
In colder clouds supercooled liquid water droplets can still be found. These form ice round suitable solid particles
Ice crystals can selectively grow in mixed water/ice clouds due to pressure difference between water and ice surfaces at same temperature (more wv molecules above water surfaces migrate towards ice surfaces, where they are absorbed)
BERGERON-FINDEISEN PROCESS
Ice crystals collide and splinter, instigate chain reaction and aggregate into snowflakes, which then collide and coalesce.
PRECIPITATION TYPES: RAIN/SNOW/SLEET/HAIL
RAIN AND SNOW - snow falls in cold conditions with freezing level at or near (up to -300m>) surface.
Longer passage through warmer air layers melts out snowflake -> raindrop
large raindrops fall with terminal velocity -9m s-1, so may be held in air or evaporate if updraught of this magnitude.
Stratiform cloud tends to have gentler vertical motion - longer-lasting less intense rainfall than cumuliform clouds.
SLEET AND HAIL - sleet comprises refrozen raindrops that have encountered freezing layer close to surface, or more loosely known as mix of snow and rain.
Hail comprises ice pellets formed by large-scale violent updraughts that sweep nuclei ‘embryos’ high into subzero zone of Cb cloud where they accumulate icy coatings through repeated rising and falling motions - can give opaque and clear ice layer strucure.
Hailstone diameters have reached 14cm.
GLOBAL STORES OF WATER
Oceans = 97.2%
Ice sheets, glaciers = 2.15%
groundwater = 0.63%
lakes/freshwater stores = 0.009%
saline lakes 0.008%
soil moisture 0.005%
atmospheric moisture 0.001%
rivers 0.0001%