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Thermodynamics M. D. Eastin

Atmospheric Vertical Structure &

Thunderstorms

Forecast Question:

Will a severe thunderstorm develop

today? Or not?

Having a solid understanding of

atmospheric thermodynamics

helps forecasters answer this

question every day…



Thunderstorms

Outline:

Vertical Structure

Thunderstorms

Single Cell

Multicell

Supercell

Importance of Thermodynamics


Standard Atmosphere:

• The vertical variability in pressure

and density are much larger than

the horizontal and temporal

variability in these quantities

• Earth’s atmosphere extends

>150 km above sea level (ASL)

• 50% of the atmospheric mass

lies within the lowest ~6 km

above mean sea level (MSL)

• 99% of the mass lies within the

lowest 30 km above MSL

Atmospheric Vertical Structure



• Vertical distribution of temperature

can be divided into 4 distinct layers

• Troposphere

• Stratosphere

• Mesosphere

• Thermosphere

• The troposphere accounts for

~80% of the mass and nearly

all water vapor, clouds,

and precipitation




• Very little mixing occurs between

the dry, ozone-rich stratosphere

and the moist, ozone-poor

troposphere

• Together, the troposphere and

stratosphere account for 99%

of all atmospheric mass




• Mesosphere is the region where

aurora borealis (Northern Lights)

are sometimes observed




• Temperatures in the thermosphere

are highly dependent on solar

activity (e.g. sunspots and flares)

and can reach very high values

• The International Space Station

maintains a stable orbit within this

atmospheric layer (at ~350 km)




• Changes in temperature with

height are very important for

atmospheric vertical motions

• Below the tropopause, pressure

and density decrease with height

more rapidly in cold layers than in

warm layers

• We will soon learn why

• These differences between cold

and warm layers are critical for

the development of thunderstorms



Why do forecasters worry about thunderstorms?

• Flash Floods

• Large Hail

• Strong winds

• Lightning

• Tornadoes

Thunderstorms

• Change people’s daily plans

• Damage homes and towns

• Influence local economies


What are the important ingredients for thunderstorm development?

Thunderstorms


Why do thunderstorms differ in size, shape, and longevity?

Thunderstorms


Three Primary Types:

1. Single Cell (or “air mass”)

2. Multicell (e.g., “squall lines”)

3. Supercells

Thunderstorms


Thunderstorms

Single Cell Thunderstorms:

• Cumulus Stage (cloud dominated by updrafts)

• Mature Stage (both updrafts and downdrafts present)

• Dissipating stage (cloud dominated by downdrafts)


Thunderstorms

Multi-Cell Thunderstorms:

• Composed of multiple single cells

• Updrafts and downdrafts work in concert to regularly develop new single

cell storms on a preferred flank of the storm

• Warm updrafts rise rapidly

• Cold downdrafts act as a wedge forcing warm moist air upward


Thunderstorms

Multi-Cell Thunderstorms:

• Warm air can rise spontaneously above certain levels

• Enormous quantities of moisture are condensed out of the air

• Modify the large-scale environment by transporting heat and moisture

• Often produce large hail and flash floods


Thunderstorms

Supercell Thunderstorms:

• Strongest and most dangerous

• Large Hail

• Lightning

• Strong Tornadoes

• Single rotating updraft

• Two prominent downdrafts

• Occur when very warm air

resides beneath very cold air


Thunderstorms

Supercell Thunderstorms:


Importance of Thermodynamics

• All thunderstorms are “powered” by strong updrafts and downdrafts

• The magnitude and location of all vertical motions are the result of

vertical temperature differences

• All thunderstorms “condense out” large quantities of water vapor,

producing clouds and precipitation

• All thunderstorms impact society (and they do so on a daily basis…)

• Successful forecasts of thunderstorms requires a solid understanding

of atmospheric thermodynamics…



Thunderstorms

Summary:

• Vertical Structure (variations in density, pressure, and temperature)

• Thunderstorms (produce severe weather and impact daily life)

• Single-cells

• Multi-cells

• Supercells

• Importance of Thermodynamics (critical to forecasts)


References

Bluestein, H. B, 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Volume II: Observations and Theory of Weather

Systems. Oxford University Press, New York, 594 pp.

Byers, H. R., and R. R. Braham, Jr., 1949: The Thunderstorm. Supt. Of Documents, U.S. Government Printing Office,

Washington, D.C., 287 pp.

Houze, R. A. Jr., 1993: Cloud Dynamics, Academic Press, New York, 573 pp.

Lemon, L. R. , and C. A. Doswell, 1979: Severe thunderstorm evolution and mesocyclone structure as related to

tornadogenesis., Mon. Wea. Rev., 107, 1184–1197.

Markowski, P. M., and Y. Richardson, 2010: Mesoscale Meteorology in Midlatitudes, Wiley Publishing, 397 pp.

Petty, G. W., 2008: A First Course in Atmospheric Thermodynamics, Sundog Publishing, 336 pp.

Tsonis, A. A., 2007: An Introduction to Atmospheric Thermodynamics, Cambridge Press, 197 pp.

Wallace, J. M., and P. V. Hobbs, 1977: Atmospheric Science: An Introductory Survey, Academic Press, New York, 467 pp.

Weisman, M. L. , and J. B. Klemp, 1986: Characteristics of Isolated Convective Storms. Mesoscale Meteorology and

Forecasting, Ed: Peter S. Ray, American Meteorological Society, Boston, 331-358.