ATOC215 Oceans, Weather and Climate

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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
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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