Review for Midterm 2

What we have discussed after midterm I

• The global water cycle• Dew, frost and fogs• How do the clouds form?• Why does it rain on us?• Formation of snow and hails• Lightning• Thunderstorms• Twisters• Mesoscale convective systems• Downbursts and dust storms

The most common atmospheric circulation

structure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

Radiation Convection

Latent/Sensible

Conduction

Standard units of measurement

SI (System International)Quantity Name Units SymbolLength meter m mMass kilogram kg kgTime second s sTemperature Kelvin K KDensity kilogram kg/m3 kg/m3

per cubic meterSpeed meter per m/s m/s

secondForce newton m.kg/s2

NPressure pascal N/m2 PaEnergy joule N.m JPower watt J/s W

Components of global water cycle

• Ocean water• Land soil moisture, rivers, snow cover,

ice sheet and glaciers• Sea ice• Atmosphere water vapor, clouds,

precipitation• Water in biosphere (including human

beings)

The magic of water on earth

A significant fraction of the human body is water (~75%)

Every 16 days nearly 100% of the water in a human body is exchanged.

The remaining: fat, protein, carbonhydrate, other solids

• Water Vapor Basics (names of different phase changes, associated latent heat release or consumption)

• Humidity indices (there are 6 total). Saturation vapor pressure increases with temperature, i.e., warmer air can hold more water vapor

• Two methods of achieving saturation and condensation (diabatic vs. adiabatic processes). Different types of condensation - dew, frost, fog (radiation, advection, upslope, precipitation, steam), clouds.

Dew, frost and fogs

Water (H2O ) is unique on earth because it can exist in all 3 states

(phases)• An H2O molecule

• 3 states (gas, liquid, solid) depending on how the molecules are connected together

• Can change from any state to any other state. Latent heat is consumed or released in a phase change

e.g. Evaporation -> liberation of water molecules, requires energy

• Saturation: equilibrium between evaporation and condensation

• Saturation vapor pressure depends only on temperature and increases non-linearly with temperature

• Diabatic processes – add/remove heat– Conduction (e.g. movement of air mass over a cold

surface): dew, frost, advection fog– Radiation (e.g. cooling of boundary layer air by longwave

radiation): radiation fog

• Adiabatic processes - no addition/removal of heat– Add water vapor to air (precipitation fog)– Cooling of air parcel when it rises (because air parcel

expands when it rises, like a balloon): upslope fog, clouds

Methods to achieve saturation and condensation:Cooling down the temperature or add water vapor

Different types of fog found throughout the U.S.

3 cloud properties, 9 ISCCP cloud types

Why do clouds constitute a wildcard for climate change? Competition between greenhouse effect and albedo effect

Formation by convection: 3 types of stability. Two factors limiting the height of clouds

Clouds

ISCCP Cloud Classification

3 cloud properties:

Cloud top height/ pressure

Cloud thickness

Cloud amount

• Clouds are both good reflectors of solar radiation (cooling effect) and good absorbers of earth emitted longwave radiation (warming effect).

• The net effect (cooling or warming) depends on the type of cloud

• In a changing climate, increases in some types of clouds would promote warming, while increases in others would cause cooling

• Climate models have difficulties in simulating clouds

• Conclusion: Clouds cause the largest uncertainty in model simulations of future climate.

Why do clouds constitute a wildcard for climate change?

Strongerwarming effect

Stronger cooling effect

When comparing the parcel temperature and environmental temperature, there are 3 possible outcomes:1)absolutely unstable air2)absolutely stable air3)conditionally unstable air

Lifting by local convection Most clouds form as air parcels are lifted and cooled to saturation.

The air parcels could be lifted by mountains, meeting of different air masses, surface convergence, and local convection

•Static stability – refers to atmosphere’s susceptibility to being displaced

•Stability related to buoyancy function of temperature

•When an air parcel rises, the cooling rate of the parcel (adiabatic lapse rate or ALR) relative to the cooling rate of surrounding atmosphere (environmental lapse rate or ELR) determines the “stability” of a parcel.

Environment

ParcelEnvironment

ParcelEnvironment

Parcel

The three types of stability

AbsolutelyUnstable

ConditionallyUnstable

AbsolutelyStable

1) Entrainment• Turbulent mixing of ambient air into parcel • Leads to evaporation along cloud boundaries• Evaporation uses latent heat, cooling the cloud

reduces buoyancy

Courtesy Russ Dickerson, U. Maryland

What stops ‘unstable’ air masses from rising indefinitely ?

2) Encountering a layer of stable air (inversion)• a rising parcel may reach a stable upper air environment • the parcel cooling rate will exceed that of the ambient air• the parcel will slowly cease ascension and come to rest at some equal temperature level• three types: radiation, frontal, subsidence

Precipitation• Forces acting on a cloud/rain droplet. How does the

terminal velocity change with cloud drop radius?• Growth mechanisms for rain and snow (Warm

clouds, cool clouds, cold clouds)• Formation of rain: coalescence process (the

collector is larger than the cloud droplets but not too large)

• Bergeron process: happens with coexistence of ice and super-cooled water. Key: Saturation vapor pressure of ice < that of super-cooled water at the same temperature.

• Further growth of ice crystals (riming and aggregation)

Precipitation formation - cloud drop growth

• Not all clouds precipitate due to their small sizes and slow fall rates• Balance between gravity and

frictional drag eventually become equal to achieve terminal velocity VT, which is proportional to the square root of cloud drop radius VT=c r0.5 ,where r is drop radius and c is a constant.

• For a cloud drop to fall, its terminal velocity must exceed the vertical velocity of the upward-moving air parcel. Otherwise it will be carried up.

• Cloud drop growth is required for precipitation to form

Fgravity

Fdrag

1. Collision Coalescence (warm clouds, T > 0 C, form rain)

2. Bergeron Process (cool/cold clouds, T < 0 C, form snow)

Mechanisms for cloud drops to grow larger

Cold Clouds Cool Clouds

Summary of Precipitation processes:

Condensation

Collision-coalescence

Bergeron Process

Warm

clouds

Cool/cold clouds

Rain Snow

(can change to rain, sleet, or any other type of precipitation depending on underlying atmosphere

Riming/Aggregation

Riming = liquid water freezing onto ice crystals

Aggregation = the joining of ice crystals through the bonding of surface water

Collector is larger than other droplets but not too large

Different precipitation

Snow Rain Sleet Freezing rain Graupel/hail

LightningLightning

1.1. Two types of lightning (Two types of lightning (cloud-to-cloud 80%, cloud-to-cloud-to-cloud 80%, cloud-to-ground 20%ground 20%))

2.2. 4 steps 4 steps of lightning development. of lightning development.

3.3. How fast does thunder travel? Use the time lag between How fast does thunder travel? Use the time lag between lightning and thunder to lightning and thunder to estimate the distance estimate the distance of the stormof the storm

4.4. Climate impacts of lightning: nitrogen cycle, ozone, Climate impacts of lightning: nitrogen cycle, ozone, wildfirewildfire

5.5. Lightning safetyLightning safety

1. 1. Charge separationCharge separation. Charge layers in the . Charge layers in the cloud are formed by the transfer of positive cloud are formed by the transfer of positive ions from warmer graupel to colder ice crystal ions from warmer graupel to colder ice crystal when they collide with each other.when they collide with each other.

2. 2. Stepped leaderStepped leader. When the negative charge . When the negative charge near the bottom of the cloud is large enough to near the bottom of the cloud is large enough to overcome the air's resistance, a stepped overcome the air's resistance, a stepped leader forms.leader forms.

3. 3. Return strokeReturn stroke. A region of positive ions . A region of positive ions move from the ground toward this charge, move from the ground toward this charge, which then forms a return stroke into the cloud.which then forms a return stroke into the cloud.

4. 4. Dart leaderDart leader. Not all of the first stroke . Not all of the first stroke neutralizes the negatively charged ions and neutralizes the negatively charged ions and results in another leader in 1/10 of a secondresults in another leader in 1/10 of a second

Processes of Lightning Formation

Thunder• Charge differences between the thunderstorm and ground can Charge differences between the thunderstorm and ground can

cause lightning strokes of 30,000°C, and this rapid heating of air cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called thunder.will creates an explosive shock wave called thunder.

• It takes about It takes about 3 seconds 3 seconds for thunder to travel for thunder to travel 1 kilometer 1 kilometer ((5 sec 5 sec per mileper mile).). A lag in lightning strike and thunder occurs due to sound traveling slower than light.

• When thunder is farther away, the echoing of sound waves off of objects (like buildings and hills) causes thunder to sound rumbling.

ThunderstormsThunderstorms

1.1. The The general size and lifetime general size and lifetime of mesoscale convective of mesoscale convective systems, thunderstorms and tornadoessystems, thunderstorms and tornadoes

2.2. 3 types 3 types of thunderstorms. of thunderstorms.

3.3. 3 stages of the ordinary thunderstorms. 3 stages of the ordinary thunderstorms. Downdraft and falling precipitation cut off the updraft.

4.4. Formation of multi-cell thunderstorms. Formation of multi-cell thunderstorms. Downdrafts Downdrafts initiate new thunderstorms in nearby regionsinitiate new thunderstorms in nearby regions..

1.1. 3 stages of the supercell thunderstorms. 3 stages of the supercell thunderstorms. Winds aloft push downdraft/precipitation away and the updraft is not weakened.

Convective systems

• Tornadoes: about 100-600 m, last 1 minute to 1 hour

• Thunderstorms: about 10 Km, last 10 minutes to a couple of hours. 3 types: ordinary, multicell, supercell

• Mesoscale convective systems (MCSs): A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 Km or more in at least one direction, and often last for several hours to a couple of days.

Three stages have been identified in ordinary thunderstorms:Three stages have been identified in ordinary thunderstorms:a)a) DEVELOPINGDEVELOPING: unstable atmosphere, vertical updrafts keep precipitation : unstable atmosphere, vertical updrafts keep precipitation

suspendedsuspendedb)b) MATUREMATURE:: entrainment of dry air that causes cooler air from evaporation, triggering entrainment of dry air that causes cooler air from evaporation, triggering

downdrafts and falling precipitation and gust frontsdowndrafts and falling precipitation and gust frontsc)c) DISSIPATINGDISSIPATING:: weakening updrafts and loss of the fuel source after 15-30 minutes.weakening updrafts and loss of the fuel source after 15-30 minutes.

Thunderstorms I. Ordinary Storms

Cool downdrafts leaving a Cool downdrafts leaving a mature and dissipating storm mature and dissipating storm may offer relief from summer may offer relief from summer heat, but they may also force heat, but they may also force surrounding, low-level moist air surrounding, low-level moist air upward.upward.

Hence, dying storms often Hence, dying storms often trigger new storms, and the trigger new storms, and the successive stages may be successive stages may be viewed in the sky. viewed in the sky.

Thunderstorms II. Multicell Storm

Formation of supercell thunderstormsFormation of supercell thunderstorms

1. Horizontal vortex tube1. Horizontal vortex tube. . Before thunderstorms Before thunderstorms develop, a change in wind direction and an develop, a change in wind direction and an increase in wind speed with increasing height increase in wind speed with increasing height creates an invisible, horizontal spinning effect creates an invisible, horizontal spinning effect in the lower atmosphere. in the lower atmosphere.

2. Updraft and Mesocyclone2. Updraft and Mesocyclone. Spinning . Spinning horizontal vortex tubes created by surface horizontal vortex tubes created by surface wind shear may be tilted and forced in a wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn and often stretching rotating air may then turn into a mesocyclone. into a mesocyclone. Winds aloft push the rain and downdraft away and the updraft is not weakened

3. Tornado3. Tornado. Most strong and violent . Most strong and violent tornadoes form within this area of strong tornadoes form within this area of strong rotation.rotation.

TornadoesTornadoes

1.1. 3 stages 3 stages of supercell tornado formation. of supercell tornado formation.

1.1. Tornado outbreak (Tornado outbreak (number>6number>6) )

2.2. Tornado damage: Enhanced Fujita Scale (Tornado damage: Enhanced Fujita Scale (EF-0 65-85 mphEF-0 65-85 mph, , EF-5 >200 mphEF-5 >200 mph))

3.3. Tornado occurrence: Global and U.S.. Tornado occurrence: Global and U.S.. Which country Which country has has the largest number of tornadoes in the world? the largest number of tornadoes in the world? Which stateWhich state has the largest number of tornadoes per unit area in U.S.? has the largest number of tornadoes per unit area in U.S.? Tornado season in U.S. (Tornado season in U.S. (March-JulyMarch-July))

Tornado Occurrence (global)

Tornado Occurrence (U.S.)Tornado Occurrence (U.S.)

Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes annually, but the highest frequency is observed in annually, but the highest frequency is observed in tornado alley of the Central tornado alley of the Central PlainsPlains. Great setting for potent mixing of air masses.. Great setting for potent mixing of air masses.

Tornadoes from T-storms in hurricanes

Mesoscale convective systemsMesoscale convective systems

1.1. 2 types 2 types of mesoscale convective systems (mesoscale of mesoscale convective systems (mesoscale convective complex, squall line)convective complex, squall line)

2.2. Structure of MCCs Structure of MCCs

3.3. Structure of squall lines: Structure of squall lines: four componentsfour components

1.1. Derechos. Definition (swath wider than 240 miles, wind Derechos. Definition (swath wider than 240 miles, wind speed>57 mph). 4 types (serial, progressive, hybrid, low speed>57 mph). 4 types (serial, progressive, hybrid, low dewpoint). dewpoint). Which state Which state has the largest annual number has the largest annual number of derecho events? Most derechos happen in the three of derecho events? Most derechos happen in the three months of months of May, June and JulyMay, June and July

I. Mesoscale Convective ComplexI. Mesoscale Convective ComplexAn organized mass, or collection, of An organized mass, or collection, of thunderstorms that extends across a thunderstorms that extends across a large region up to 1000 x larger than large region up to 1000 x larger than individual storms. individual storms. Last for upwards of 12 hours and may Last for upwards of 12 hours and may bring hail, tornadoes, and flash floods.bring hail, tornadoes, and flash floods.

II. Squall lineMay contain several severe thunderstorms, May contain several severe thunderstorms, some possibly supercells, extending for some possibly supercells, extending for more than 1000 kilometers.more than 1000 kilometers.Always contains a convective precipitation Always contains a convective precipitation region and a trailing stratiform precipitation region and a trailing stratiform precipitation region.region.

2 types of Mesoscale Convective Systems 2 types of Mesoscale Convective Systems

Vertical structure of a squall line

Zipser (1977), modified by Houze (1993)

Convective updrafts (controlled by lower troposphere temperature and moisture)

Mesoscale updrafts

Convective downdrafts

Mesoscale downdrafts

Downbursts and dust stormsDownbursts and dust storms

1.1. 3 types 3 types of downbursts (derechos, haboobs, of downbursts (derechos, haboobs, microbursts)microbursts)

2.2. 3 types 3 types of microbursts (wet, dry, hybrid). of microbursts (wet, dry, hybrid).

3.3. 4 causes of atmospheric turbulence.4 causes of atmospheric turbulence.

4.4. Haboobs (dust storms). Haboobs (dust storms). Global desertificationGlobal desertification. . Drying of global soil moistureDrying of global soil moisture

Downbursts: Introduction

• Downbursts are gusts of wind that can reach speeds in excess of 270km/hr (165mph), and are potentially deadly.

• Three common types:• Derechos (1000 km)• Haboobs (10-100 km)• Microbursts (1 km)