Stability and Thunderstorms ESS 111 – Climate and Global Change

Atmospheric Stability & Clouds StabilityStability –Parcel Theory Parcel of air expands and contracts freelyParcel of air expands and contracts freely Remains as a single unitRemains as a single unit No heat exchange with the outside air (Adiabatic Process)No heat exchange with the outside air (Adiabatic Process)

Atmospheric Stability & Clouds StabilityStability –Adiabatic Process Parcel expands when lifted due to lessening pressureParcel expands when lifted due to lessening pressure Molecular action slows = cools at a steady rateMolecular action slows = cools at a steady rate Compresses when forced down = Warms at a steady rateCompresses when forced down = Warms at a steady rate

Atmospheric Stability & Clouds Lapse Rate: Rate of cooling (or warming) of the atmosphere:Lapse Rate: Rate of cooling (or warming) of the atmosphere: Environmental Laps Rate = actual temperature as measured by a radiosonde (weather balloon)Environmental Laps Rate = actual temperature as measured by a radiosonde (weather balloon) Dry Adiabatic Lapse Rate (DALR) – Unsaturated parcel of air = 10°C per kmDry Adiabatic Lapse Rate (DALR) – Unsaturated parcel of air = 10°C per km Moist Adiabatic Lapse Rate (MALR) – Saturated parcel of air = 6°C per km.Moist Adiabatic Lapse Rate (MALR) – Saturated 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 Triggers

Convection & Clouds Pockets of warm air rise as thermals with invisible water vapor, and at the dew point temperature condensation creates the cloud base. Rising air from below is replaced by sinking air from above, creating areas of blue sky.

Thunderstorms A cumulonimbus cloud that produces lightning (and hence thunder) Very common (globally) Transport heat and moisture vertically in the atmosphere Most 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 4 th 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 12

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 stages of development

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).