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Lecture 18 - Hurricanes & tornadoes (Tropical cyclones = storms with…

- - - - Storm surge
        
        Killed 134 people
        
        2.5 billion dollars
        
        High winds gradually slowed down
    - - Gulf of Mexico & landed Florida (after 4 days)
        
        60 killed
        
        Strongest hazard = strong wind; Secondary hazard = limited amount of drinking water & electricity
        
        Storm surge up to 14 feet
        
        Fourth strongest hurricane ever landed on USA
    - - Typhoon Freda
        
        ended in the Columbus Day storm of 12th October 1962
        
        killed 46 people in the US and 7 people Canada
        
        1/3 tree fell from the strong winds
        
        Took two weeks to travel 8,000 km across the Pacific
  - - - The S Atlantic due to high winds speed
        
        SE Pacific due to insufficient water temperature, not warm enough (Hurricane require very warm sea water temperature)
        
        Red Sea & Persian Gulf due to insufficient surface area, lower the likelihood of forming cyclones
        
        The Equator due to insufficient turbulence
        • Cyclones require rotation triggered by Coriolis effect, but the air is too stable in the equator
    - - driven by summer heating of tropical oceans
      - Warm shallow water (> 27℃ to >50 m) to enhance/promote evaporation to the atmosphere
      - Unstable air for thunderclouds to form (start cyclonic rotation, Coriolis effect)
      - Weak vertical wind shear to stop the storm from breaking up (provide stable conditions for the storm to develop)
        
        Vertical wind shear - different velocity at different depths (lower the weaker)
    - - 2. Tropical Depression
        
        Winds strengthen (37-63 km/h) as pulled towards the low pressure core & then upwards
        
        Not named, but are numbered
      - 1. Tropical disturbance
        
        Low pressure draws in surrounding thunderstorms with weak winds (< 36 km/h).
        
        Too common to be named or numbered.
        
        Warm air draw in from individual local thunderstorms to bigger ones, relatively modest
      - 3. Tropical storm
        
        Stronger winds (64-119 km/h) form from winds moving/flowing in cyclonic (rotational, inward spiral) pattern, draw inwards to the eye (center) of the storm
        
        Significant/deadly enough to be given a name
      - 4. Tropical cyclone
        
        Winds >120 km/h
        
        No winds reach the core due to strong rotational (cyclonic) flow, which form the eye of the storm
        
        Identified by name
  - - - Based on...
        
        wind speed
        
        barometric pressure (the intensity of the low pressure eye)
        
        storm damage
      - Category
        
        Category 1: Minimal damage (74-95 mph)
        
        winds damage & unanchored mobile homes
        
        Category 2: Moderate damage (96-110 mph)
        
        some trees blown down
        
        major damage to mobile homes & some roofs
        
        Category 3: Extensive damage (111-129 mph)
        
        large trees blown down, foliage striped
        
        mobile homes destroyed
        
        structural damage to small buildings
        
        Category 4: Extreme damage (130-156 mph)
        
        All signs blown down
        
        Heavy damage to windows, doors, roofs
        
        Coastal buildings suffer major damage
        
        Flooding kilometers inland
        
        Category 5: Catastrophic damage (>157 mph)
        
        Severe damage to windows, doors, roofs
        
        Small buildings overturned & blown away
        
        Major damage to all buildings less than 500 m from the shoreline & 5 m above sea level
      - Trend: pressure dropping, winds speed increase, damage more catastrophic, max 250
        Historically, coastal flooding from storm surges has accounted for the majority of fatalities (up to ∼500,000) in the deadliest tropical cyclones.
  - - - Land Use Planning
        o Avoid population centres in low-lying coastal lands
        o Hurricane Harvey is the second time flooding the 500 year flood plain
      - Building Codes
        o Hurricanes often destroy buildings by first lifting off the roof, so securing this is a good first step
        o Eliminates or strengthen any eaves projecting from the roof
        o Strap the roof to walls inside attic
        o No thin asphalt roofs stapled by plywood
        o Stronger building codes for mobile homes
    - - Before
        o Listen to hurricane forecasts and warnings
        o Evacuate from exposed coastal areas & areas prone to flooding to higher ground as required
        o Park vehicles in safe place (not under trees)
        o Take cover in well-built home
        o Close & shutter/board all windows
        o Remove or secure all items around house (lawn furniture, tools, bicycles)
        o Stock food/water, have flashlights & battery-powered radio & extra batteries
      - During
        o Keep informed of storm progression via radio
        o Stay in secure inner room without windows
        o Use phone only for emergencies
        o Stay inside during passage of the eye - high winds will resume suddenly
      - After
        o Inspect your property
        o Report any downed power lines & stay away
        o Clean up debris
        o Take photos of damage for insurance
- - - - Single-cell thunderstorm cannot form tornadoes
        
        Warm, moist air/moisture (condense from cooling air) rises vertically in a single updraft
        
        Falling rain/hail causes a cool downdraft, surpassing the updraft
        
        This limits thunderstorm intensity & reduces the chance of a tornado
        
        *Rain is falling vertically, reduce the strength of updraft
      - Supercell thunderstorm can form tornadoes
        
        Wind shear shifts the updraft, tilting the thundercloud into an anvil shape & cause precipitation
        
        require High vertical wind shear (different speed at different altitude)
        
        This separates the warm updraft from the cool downdraft
        
        Tilting allows to intensify thunderstorms and a higher chance of a tornado forming
        *Hurricane require low vertical wind shear
        
        Example: tornado outbreak in May 2013, El Reno, OK, the widest tornado on record
        
        Higher vertical wind shear, higher from the ground
        
        Falling rain is not down but to the side
  - - - High-altitude, high-speed jet stream winds from the West
        
        Mid-altitude cool dry air from the Rocky Mountains to the Northwest
        
        Low-altitude warm moist air from the Gulf of Mexico to the South
        
        SW warm dry air from Arizona, New Mexico