Stability and Thunderstorms

ESS 111 – Climate and Global Change

Atmospheric Stability & CloudsAtmospheric Stability & Clouds• StabilityStability

– Parcel TheoryParcel Theory• Parcel of air Parcel of air

expands and expands and contracts freelycontracts freely

• Remains as a Remains as a single unitsingle unit

• No heat No heat exchange with exchange with the outside air the outside air (Adiabatic (Adiabatic Process)Process)

Atmospheric Stability & CloudsAtmospheric Stability & Clouds• StabilityStability

– Adiabatic ProcessAdiabatic Process• Parcel expands Parcel expands

when lifted due when lifted due to lessening to lessening pressurepressure

• Molecular action Molecular action slows = cools at slows = cools at a steady ratea steady rate

• Compresses Compresses when forced when forced down = Warms down = Warms at a steady rateat a steady rate

Atmospheric Stability & CloudsAtmospheric Stability & Clouds• Lapse Rate: Rate of cooling (or warming) Lapse Rate: Rate of cooling (or warming)

of the atmosphere:of the atmosphere:• Environmental Laps Rate = actual temperature as Environmental Laps Rate = actual temperature as

measured by a radiosonde (weather balloon)measured by a radiosonde (weather balloon)

• Dry Adiabatic Lapse Rate (DALR) – Unsaturated Dry Adiabatic Lapse Rate (DALR) – Unsaturated parcel of air = parcel of air = 10°C per km10°C per km

• Moist Adiabatic Lapse Rate (MALR) – Saturated Moist Adiabatic Lapse Rate (MALR) – Saturated parcel of air = 6°C per km.parcel of air = 6°C per km.

Cloud & Storm Updraft Development– Mechanisms

• Surface Heating – Free Convection

• Topography

• Convergence of Surface Air

• Fronts

– Free Convection: Ground heated by radiation• Thermal Forms (Convective Updraft)

– Air in contact with warmest ground is warmer and therefore less dense than surrounding air

– Air rises…..it cools as it rises….it expands due to lessening pressure acting on it (Adiabatic Processes)

– If the rising (and cooling) parcel that started at the surface continues to be warmer than the surrounding air, then the parcel is unstable and will continue to rise.

– Numerous unstable parcels over time (tens of minutes) can eventually form a thunderstorm updraft.

Cloud Development TriggersCloud Development Triggers

Convection & CloudsConvection & CloudsPockets of warm Pockets of warm air rise as air rise as thermals with thermals with invisible water invisible water vapor, and at the vapor, and at the dew point dew point temperature temperature condensation condensation creates the cloud creates the cloud base.base.

Rising air from Rising air from below is replaced below is replaced by sinking air by sinking air from above, from above, creating areas of creating areas of blue sky.blue sky.

ThunderstormsA cumulonimbus cloud that produces lightning (and hence thunder)Very common (globally) Transport heat and moisture vertically in the atmosphereMost are not severe

Severe thunderstorm:a) Hail > 1 inch diameter (¾ inch prior)b) Wind gust >50 kt (58 mph)c) Tornado

Ingredients for thunderstorm initiation:

a) Water vapor

b) Instability

c) Lifting mechanism

For a severe thunderstorm, a 4th ingredient is:

d) Strong vertical wind shear

(change in wind speed / direction) 8

Thunderstorm Facts & Climatology

• At any given time there are 2000 thunderstorms in progress

• About 45,000 thunderstorms take place each day

• Annually, The U.S. experiences about 100,000 thunderstorms.

• About 16 million thunderstorms occur annually around the world!

• The lightning from these storms strikes Earth about 100 times each second

Thunderstorm Frequency Map

Global Lightning Frequency

Upscale development of thunderstorms

Individual thunderstorms•Single cell thunderstorm •Multicell thunderstorm•Supercell thunderstorm

Larger-scale systems with embedded showers and/or thunderstorms:•Mesoscale Convective System (MCS)•Squall line (quasi-linear convective system, QLCS)•Hurricane

– will examine this first

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Single-Cell Thunderstorm

Single-Cell Thunderstorm

• Life span of about 20 to 30 minutes

• Usually not strong enough to produce severe weather

• Single Cell Pulse Storms often produce severe weather– Usually strong winds as it collapses– Sometimes small hail

Life cycle of a single cell (air mass) thunderstorm

Vertical shear is weak (as in the SE during summer)

Form along weak boundaries or convergence zones.

Small size: <10 km diameter

Short lifetime: <1 h

Microbursts (strong downdrafts and their outflows) may form during first part of the mature stage.

Fig. 18.3

3 stages of development

1 2 3

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Towering Cumulus (Cumulus Congestus)

Is the Cumulus Cloud still growing?Cauliflower-type hard top =

Strong updraft & growing storm

Wispy soft top =

Weaker updraft & dying storm

Life Cycle – Mature Stage

Life Cycle – Dissipating (Orphan Anvil)

Air Mass Thunderstorms Summary

• Usually weak (but can produce heavy rain in a short period of time).

• Usually not severe• Usually move slowly (weak winds aloft)• Often develop and dissipate in less than one hour• Form in a weakly sheared environment and thus have

a BUILT-IN SELF-DESTRUCT MECHANISM that guarantees a short lifetime (downdraft collapsing down on the updraft).