1. Review for Midterm 2

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

3. The most common atmospheric circulation structure
Radiation Convection
Cooling
or No Heating
Heating
Latent/Sensible
Conduction H L
Imbalance of heating
Imbalance of temperature
Imbalance of pressure
 Wind

4. Standard units of measurement SI (System International)
Quantity Name Units Symbol
Length meter m m
Mass kilogram kg kg
Time second s s
Temperature Kelvin K K
Density kilogram kg/m3 kg/m3
per cubic meter
Speed meter per m/s m/s
second
Force newton m.kg/s2 N
Pressure pascal N/m2 Pa
Energy joule N.m J
Power watt J/s W

5. 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)

6. The magic of water on earth

7. 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

8. Dew, frost and fogs
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.

9. 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

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

11. Different types of fog found throughout the U.S.

12. Clouds 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

13. ISCCP Cloud Classification
3 cloud properties:
Cloud top height/ pressure
Cloud thickness
Cloud amount

14. Why do clouds constitute a wildcard for climate change?
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.
Stronger
warming effect
Stronger cooling effect

15. 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.
When comparing the parcel temperature and environmental temperature, there are 3 possible outcomes:
absolutely unstable air
absolutely stable air
conditionally unstable air

16. The three types of stability
Environment
Parcel
Parcel
Parcel
Environment
Environment
Absolutely
Unstable
Absolutely
Stable
Conditionally
Unstable

17. What stops ‘unstable’ air masses from rising indefinitely ?
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
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

18. 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)

19. 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

20. Mechanisms for cloud drops to grow larger
Collision Coalescence (warm clouds, T > 0 C, form rain)
Bergeron Process (cool/cold clouds, T < 0 C, form snow)
Cold Clouds
Cool Clouds

21. Summary of Precipitation processes:
Condensation
Warm
clouds
Cool/cold
clouds
Collision-
coalescence
Bergeron
Process
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
Rain
Snow
(can change to rain, sleet, or any other type of precipitation depending on underlying atmosphere

22. Different precipitation
Snow Rain Sleet Freezing rain Graupel/hail

23. Lightning
Two types of lightning (cloud-to-cloud 80%, cloud-to-ground 20%)
4 steps of lightning development.
How fast does thunder travel? Use the time lag between lightning and thunder to estimate the distance of the storm
Climate impacts of lightning: nitrogen cycle, ozone, wildfire
Lightning safety

24. Processes of Lightning Formation
1. Charge separation. Charge layers in the cloud are formed by the transfer of positive ions from warmer graupel to colder ice crystal when they collide with each other.
2. Stepped leader. When the negative charge near the bottom of the cloud is large enough to overcome the air's resistance, a stepped leader forms.
3. Return stroke. A region of positive ions move from the ground toward this charge, which then forms a return stroke into the cloud.
4. Dart leader. Not all of the first stroke neutralizes the negatively charged ions and results in another leader in 1/10 of a second

25. Thunder
Charge differences between the thunderstorm and ground can cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called thunder.
It takes about 3 seconds for thunder to travel 1 kilometer (5 sec per 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.

26. Thunderstorms
The general size and lifetime of mesoscale convective systems, thunderstorms and tornadoes
3 types of thunderstorms.
3 stages of the ordinary thunderstorms. Downdraft and falling precipitation cut off the updraft.
Formation of multi-cell thunderstorms. Downdrafts initiate new thunderstorms in nearby regions.
3 stages of the supercell thunderstorms. Winds aloft push downdraft/precipitation away and the updraft is not weakened.

27. 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.

28. Thunderstorms I. Ordinary Storms
Three stages have been identified in ordinary thunderstorms:
DEVELOPING: unstable atmosphere, vertical updrafts keep precipitation suspended
MATURE: entrainment of dry air that causes cooler air from evaporation, triggering downdrafts and falling precipitation and gust fronts
DISSIPATING: weakening updrafts and loss of the fuel source after minutes.

29. Thunderstorms II. Multicell Storm
Cool downdrafts leaving a mature and dissipating storm may offer relief from summer heat, but they may also force surrounding, low-level moist air upward.
Hence, dying storms often trigger new storms, and the successive stages may be viewed in the sky.

30. Formation of supercell thunderstorms
1. Horizontal vortex tube. Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.
2. Updraft and Mesocyclone. Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into a mesocyclone. Winds aloft push the rain and downdraft away and the updraft is not weakened
3. Tornado. Most strong and violent tornadoes form within this area of strong rotation.

31. Tornadoes 3 stages of supercell tornado formation.
Tornado outbreak (number>6)
Tornado damage: Enhanced Fujita Scale (EF mph, EF-5 >200 mph)
Tornado occurrence: Global and U.S.. Which country has the largest number of tornadoes in the world? Which state has the largest number of tornadoes per unit area in U.S.? Tornado season in U.S. (March-July)

32. Tornado Occurrence (global)

33. Tornado Occurrence (U.S.)
Tornadoes from T-storms in hurricanes
Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes annually, but the highest frequency is observed in tornado alley of the Central Plains. Great setting for potent mixing of air masses.

34. Mesoscale convective systems
2 types of mesoscale convective systems (mesoscale convective complex, squall line)
Structure of MCCs
Structure of squall lines: four components
Derechos. Definition (swath wider than 240 miles, wind speed>57 mph). 4 types (serial, progressive, hybrid, low dewpoint). Which state has the largest annual number of derecho events? Most derechos happen in the three months of May, June and July

35. 2 types of Mesoscale Convective Systems
I. Mesoscale Convective Complex
An organized mass, or collection, of thunderstorms that extends across a large region up to 1000 x larger than individual storms.
Last for upwards of 12 hours and may bring hail, tornadoes, and flash floods.
II. Squall line
May contain several severe thunderstorms, some possibly supercells, extending for more than 1000 kilometers.
Always contains a convective precipitation region and a trailing stratiform precipitation region.

36. Vertical structure of a squall line
Convective updrafts (controlled by lower troposphere temperature and moisture)
Mesoscale updrafts
Mesoscale downdrafts
Convective downdrafts
Zipser (1977), modified by Houze (1993)

37. Downbursts and dust storms
3 types of downbursts (derechos, haboobs, microbursts)
3 types of microbursts (wet, dry, hybrid).
4 causes of atmospheric turbulence.
Haboobs (dust storms). Global desertification. Drying of global soil moisture

38. 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 ( km)
Microbursts (1 km)