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Title: Thunderstorms


1
Thunderstorms
2
Chapter 13 review
  • A persistence forecast is a prediction that
    future weather will be the same as the present
    weather, whereas a climatological forecast is
    based on the climatology of a particular region.
    A steady-state forecast assumes the weather
    systems will continue to move in the same
    direction and at the same speed as they have been
    moving so far.
  • Weather forecasts for up to a few hours are
    called very short-range forecasts those that
    range from about 6 hours to a few days are called
    short-range forecasts medium-range forecasts
    extend from about 3-5 days into the future,
    whereas long-range forecasts extend beyond, to
    about 8.5 days.
  • Seasonal outlooks provide an overview of how
    temperature and precipitation patterns may
    compare with normal conditions.
  • For a forecast to show skill, it must be better
    than a persistence forecast or a climatological
    forecast.
  • Predicting the movement of weather systems
  • Steady-state method
  • Lows (Highs) towards the greatest pressure drop
    (rise)
  • In the direction of the winds aloft (5500 m)

3
Examples
4
Thunderstorms
  • Storm with lightning and thunder. Also heavy
    precipitation, sometimes hail, gusty surface
    winds.
  • Favorable conditions
  • Unstable atmosphere
  • Strong convection (convective storm)
  • Buoyant force the force on a less-dense
  • object immersed in a denser environment
  • Trigger for the convection
  • Weather fronts - fast uplift of warm air
  • Unequal surface heating
  • Surface convergence and divergence aloft
  • Topographic barriers
  • Arrival of cold air aloft

5
Ordinary (Cell) Thunderstorms
  • Also Air mass thunderstorms, pop-up
    thunderstorms.
  • Typically they form where the surface converging
    winds have no significant change of either
    strength or direction with height (limited wind
    shear). What goes up comes back down in the same
    place.
  • Typical summer afternoon thunderstorms
  • Last no more than an hour
  • Extend no more than a kilometer
  • Rarely produce strong winds or large hail

6
Stages of Development - I
  • Cumulus (growth) stage
  • Updraft warm and moist air is uplifted, expands,
    condenses and forms towering clouds.
  • Latent heat is released -the temperature in the
    cloud is higher than outside the cloud, air keeps
    rising.
  • The updrafts are strong so that the cloud
    droplets remain suspended in the cloud.
  • Usually no precipitation and lightning during
    this stage.

7
Stages of Development - II
  • Mature stage
  • Precipitation starts
  • Entrainment dry air from around the cloud is
    drawn into the cloud. Some of the cloud droplets
    evaporate and chill the air.
  • Downdraft the cold and heavy air is sinking
  • The updraft and the downdraft form a storm cell.
  • Gust front the boundary between the cold and the
    warm air at the surface
  • Lightning and thunder

8
Stages of Development - III
  • Dissipating stage
  • the updraft weakens
  • the gust fronts move away
  • the precipitation is light
  • the downdrafts dominate cut off the fuel supply
    of the storm

9
Multicell Storms
  • The downdraft from the dissipating storm fuels
    the formation of next storm cell.

10
Multicell storm complex example
11
Severe Thunderstorms
  • At least one of the following
  • Large hail precipitation
  • strong wind gusts
  • tornadoes..
  • The longer a storm lasts the grater the chance
    for it to become a severe storm. Often forms
    along a weather front.
  • They typically form if strong vertical wind shear
    (large changes of the wind with height) is
    present. The warm air updraft is not suppressed
    by the downdraft and the precipitation.

12
Shelf cloud
13
Roll Cloud
14
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18
Supercell thunderstorms
  • Supercell storm consists of a single violently
    rotating updraft
  • Favorable conditions when the speed and
    direction of the winds aloft change with height
  • formation of a rotating updraft
  • Updraft and downdraft do not cross -gt the storm
    lives on for hours
  • Often produce large hail, damaging surface winds,
    tornadoes

A supercell thunderstorm with a tornado sweeps
over Texas
Skip the discussion around Figs. 14.6 and 14.7
19
Squall Lines and MCCs
  • Squall line A line system of thunderstorms. It
    shows as a line of storms on the radar images.
  • It forms along or in front of an advancing cold
    front.
  • Pre-frontal squall lines may be due to gravity
    waves
  • Mesoscale convective systems a large circular
    cluster of storms

20
Thunderstorm Movement
  • The storms typically move in the direction of the
    winds aloft (middle troposphere).
  • Some move at 30 deg to the right of the winds
    aloft. (Effect of surface friction).
  • In general the multicell storms move in the
    direction where the humid and unstable conditions
    in the atmosphere prevail.
  • The squall line storms move in the direction of
    the front movement.

21
Flooding
  • Flash floods floods which rise rapidly with
    little or no advance warning
  • Conditions the influx of water into an area is
    more than the amount of water that can drain away
    from the region.
  • The ground is saturated from a prolonged rainy
    season
  • Very strong precipitation over a short time
    period
  • Not so strong precipitation but for an extended
    time- storms reoccur at the same location
    (training).
  • Stationary weather fronts can result in a series
    of thunderstorms over the same region.

22
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23
Thunderstorm Climatology
  • Most thunderstorms occur in Florida
  • Unstable atmospheric conditions (warm and moist
    surface air) prevail throughout the year.
  • Summertime afternoon air mass thunderstorms.
  • Most severe thunderstorms occur in the Great
    Planes.
  • The warm (unstable) air is dry and shallow and
    the ice crystals do not have time to melt.

Stormy days / year
Days with hail / year
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