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ATOC215 Oceans, Weather and Climate
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Entrainment of drier air = some raindrops evaporate = cools air. Cooler (denser) air tends to fall; in parallel, more air is dragged down by falling drops ... – PowerPoint PPT presentation
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Title: ATOC215 Oceans, Weather and Climate
1
ATOC-215 Oceans, Weather and Climate
- Convective Storms and Thunderstorms, Part 1
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Overview
- Introduction
- Thunderstorms in Canada
- Main Thunderstorm Types
- Thunderstorms Formation
- Winter Thunderstorms
- Ordinary or Airmass Thunderstorms
- Multi-cell Thunderstorms
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Introduction
- Thunderstorm Definition
- any storm which contains lightning and thunder
- Significant natural hazard with potential for
- high gusty winds,
- intense rainfall (showers),
- flash floods,
- damaging hail,
- tornadoes,
- damaging lightning
- About 40,000 thunderstorms/day
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Thunderstorms in the World
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Thunderstorms in Canada
Average number of days per year with thunderstorm
(1951-1980)
0
S
Al
M
On
25
30
6
Taxonomy of Thunderstorms
- Thunderstorms are composed of one or multiple
cells. - Single-cell (cloud) storm
- 10 km horizontal
- Up to 15 km vertical
- Multi-cell storm (cluster)
- Line of Thunderstorms
- 50 x 100 km horizontal
- Up to the tropopause
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Single-cell Thunderstorm
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Multi-cell Storm
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Lines of Thunderstorms
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Taxonomy of Thunderstorms
- The way thunderstorms organize depend on
characteristics of their environment such as
atmospheric instability and wind profiles with
height.
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Thunderstorm Formation
- Warm humid air rising in conditionally to very
unstable atmosphere - Then a trigger is needed to overcome the capping
lid (preferably only at a limited number of
places to concentrate the deep convection in a
few spots) - Differential heating of the earth surface
- Low level convergence
- Upper level divergence
- Lifting mechanisms (terrain or front)
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Thunderstorm Formation (2)
- Differential surface heating will generate
converging zones of low level airflow (thermal
circulation see local winds part of course) - Thunderstorms tend to form along convergence
zones or lines to force air up other lifting
mechanisms (topography) will work well too - Most favourable conditions Diverging upper level
winds, with converging surface winds and rising
air. A low-pressure systems cold front, for
example, has all the right ingredients.
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Winter Thunderstorms
- Most thunderstorm occur in summer
- The warmer and more humid it is near the surface,
the easier it is to have (conditionally) unstable
conditions - Some may form in winter
- Low-level temperatures above freezing and cold
air advected aloft - Cold air aloft will destabilize the atmosphere
- The generated instability can be sufficient to
generate thunderstorms in wintertime snowstorms.
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Radar Data from Wednesdays Storm
Snow and ice pellets here
Convection there
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Ordinary (Airmass) Thunderstorms
- Characteristics
- Usually short-lived (20 min ? 1 hour)
- Rarely produce strong winds or large hail
- Region with limited wind shear
- Form along shallow zones of surface convergence
(topography, sea breeze fronts).
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Ordinary Thunderstorm Life Cycle
- Life cycle of an ordinary thunderstorm cell
- The Growing (or Cumulus) Stage
- The Mature Stage
- The Dissipating Stage
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Ordinary Thunderstorm Growth Stages
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Growing (Cumulus) Stage
- Warm moist air flowing into the storm (inflow)
rises and creates strong upward currents
(updrafts) - Dissipation due to evaporation and mixing with
dryer air aloft - Ambient air becoming more humid
- Instability increases
- Cumulus cloud growing taller
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Development and Mature Stage
- Cloud droplets grow to raindrops and start
falling as precipitation - Entrainment of drier air gt some raindrops
evaporate gt cools air - Cooler (denser) air tends to fall in parallel,
more air is dragged down by falling drops
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Mature Stage
- The rain-cooled air falls towards the ground as a
downward moving current of cold air (downdraft) - The leading edge of the outflow (outflow
boundary) is called a gust front (large gradient
in T, P and shift of wind direction)
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Mature Stage
- Most intense stage
- Much precipitation falls against updraft
- Plenty of turbulence created by up and downdrafts
- Ice crystals near the top (anvil cloud)
- Heavy rain (small hail)
- Thunder and lightning
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Dissipating Stage
- The outflow from the downdraft cuts the heat and
moisture supply - The storm starts to dissipate
- Dissipation occurs in a relatively short time
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Ordinary (single cell) Thunderstorm
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Thunderstorm Cells vs Time
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