review for midterm 2

27
Review for Midterm 2

Upload: olga-sawyer

Post on 30-Dec-2015

27 views

Category:

Documents


2 download

DESCRIPTION

Review for Midterm 2. What we have discussed after midterm I. How does air move around the globe? What are the El Nino and La Nina? How do the clouds form? Why does it rain on us? Where do the hurricanes come from? How do the blizzards form?. - PowerPoint PPT Presentation

TRANSCRIPT

Page 1: Review for Midterm 2

Review for Midterm 2

Page 2: Review for Midterm 2

What we have discussed after midterm I

• How does air move around the globe?• What are the El Nino and La Nina?• How do the clouds form?• Why does it rain on us?• Where do the hurricanes come from?• How do the blizzards form?

Page 3: Review for Midterm 2

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

Page 4: Review for Midterm 2

How does air move around the globe?

• Distribution of precipitation (heating)

• Primary high and lows

• Three-cell model. Mechanism for each cell

• Two characteristics of temperature structure

• Two characteristics of wind structure. Why does westerly winds prevail in the extratropical troposphere? What cause the jet streams?

• What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind? Two types of ocean upwelling

Page 5: Review for Midterm 2

Vertical structure and mechanisms

Hadley Cell (thermal): Heating in tropics - forms surface low and upper level high - air converges equatorward at surface, rises, and diverges poleward aloft - descends in the subtropics

Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells

Polar Cell (thermal): Driven by heating at 50 degree latitude and cooling at the poles

Page 6: Review for Midterm 2

• Ocean surface currents – horizontal water motions

• Transfer energy and influence overlying atmosphere

• Surface currents result from frictional drag caused by wind - Ekman Spiral

General circulation of the oceans

• Water moves at a 45o angle (right) in N.H. to prevailing wind direction

• Due to influence of Coriolis effect

• Greater angle at depth

Page 7: Review for Midterm 2

What are the El Nino and La Nina?

Tropical climate:

• Mean state: The two basic regions of SST? Which region has stronger rainfall? What is the Walker circulation?

• El Nino and La Nina: Which region has very warm sea surface temperature during El Nino? 4-year period.

• Land-sea contrasts: seasonal monsoon, diurnal sea breeze and land breeze

Extratropical climate:

• Mean state: westerly winds, polar vortex

• What is the primary way El Nino affect extratropics? (PNA)

• The oscillations associated with strengthening/weakening of polar vortex: AO, AAO

Page 8: Review for Midterm 2

Tropical mean state: Sea surface temperature (SST)

Indo-Pacific warm pool

Eastern Pacific cold tongue

2 basic regions

Page 9: Review for Midterm 2

El Nino/Southern Oscillation (ENSO): The 4-year oscillation

• Southern Oscillation: The atmospheric oscillation associated with the El Nino-La Nina cycle. • The whole phenomena is now called El Nino /Southern Oscillation (ENSO)

• Normal conditions: Walker circulation, which interacts with underlying ocean and causes equatorial upwelling• El Nino: Very warm sea surface temperature over central and eastern tropical Pacific, which occurs every 3-7 years. The Walker Circulation becomes disrupted during El Niño events, which weakens upwelling in eastern Pacific.• La Nina: the opposite condition to El Nino

Page 10: Review for Midterm 2

• A seasonal reversal of wind due to seasonal thermal differences between landmasses and large water bodies

• Orographic lifting often enhances precipitation totals

Land-Sea Contrast I: Seasonal “Monsoon”

Page 11: Review for Midterm 2

• Daily reversal of winds resulting from differences in thermal properties of land and sea

• During the day (night) land (water) surfaces are hotter (cooler) than large water (land) surfaces

• Thermal low develops over warmer region - air converges into the low, ascends, and produces clouds

Land-Sea Contrast II: Diurnal Sea/Land Breeze

Page 12: Review for Midterm 2

The magic of water on earth

Page 13: Review for Midterm 2

• Global water (hydrological) cycle. Why is water unique on Earth? What percentage of human body is water? • Water Vapor Basics (names of different phase changes, latent heat)• Two methods of achieving saturation and condensation (diabatic vs. adiabatic processes). Different types of condensation - dew, frost, fog (radiation, advection, upslope, precipitation, steam), clouds. Different types of fog found throughout the U.S.• Clouds: 3 types of stability (absolutely stable, absolutely unstable, conditionally unstable). Two factors limiting the height of clouds (entrainment and temperature inversion). 3 cloud properties. 9 ISCCP cloud types.

How do the clouds form?

Page 14: Review for Midterm 2

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

Page 15: Review for Midterm 2

• Diabatic processes – add/remove heat– Conduction (dew, frost, frozen dew, advection fog)– Radiation (radiation fog)

• Adiabatic processes - no addition/removal of heat– Add water vapor to air (precipitation fog)– Mix warm air with cold air (steam 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

Page 16: Review for Midterm 2

Different types of fog found throughout the U.S.

Page 17: Review for Midterm 2

ISCCP Cloud Classification

3 cloud properties:

Cloud top height/ pressure

Cloud thickness

Cloud amount

Page 18: Review for Midterm 2

• Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius?

• Growth mechanisms for rain

• Growth mechanisms for snow

Why does it rain on us?

Page 19: Review for Midterm 2

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

Page 20: Review for Midterm 2

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

Page 21: Review for Midterm 2

Where do the hurricanes come from?

• Tropical cyclone genesis: 6 necessary conditions, 4 stages

• Tropical cyclone tracks• Tropical cyclone structure: 3 major

components, rotation direction of inflow and outflow, 3 feedbacks

• Tropical cyclone destruction: 4 reasons? Which side has the most intense destruction?

• Tropical cyclone forecast: track and intensity Currently which skill is better?

Page 22: Review for Midterm 2

1. SST > 27 oC (Poleward of about 20o SST too cold for formation. Highest frequency in late summer to early autumn when water is warmest.)

2. Warm ocean mixed layer is thick enough to supply energy (this is why they weaken quickly upon landfall)

3. Unstable atmosphere with a moist lower/middle troposphere (central and western ocean basins)

4. Low vertical windshear (Otherwise upward transfer of latent heat disrupted)

5. Coriolis force (hurricanes do not form between 5N-5S)

6. Pre-existing low-level rotating circulations (tropical waves and other disturbances)

Necessary environmental conditions for tropical cyclone formation

Page 23: Review for Midterm 2

• Size and lifetime: about 600km, last up to a week or more• Make up: many thunderstorms arranged in pinwheel formation• Three components: 1. Central eye - clear skies, light winds (25 km diameter) 2. Eye wall - maximum rainfall and wind speed. 3. Spiral rainbands • Cylonic inflow, anticyclonic outflow.

Structure of tropical cyclones

Page 24: Review for Midterm 2

How do the blizzards form?How do the blizzards form?1.1. Definition of airmasses? Bergeron classification of Definition of airmasses? Bergeron classification of

air massesair masses2.2. Fronts: 6 types. What is a cold front? A warm front? Fronts: 6 types. What is a cold front? A warm front?

An occluded front?An occluded front?3.3. Surface weather analysis: Station model, wind speed Surface weather analysis: Station model, wind speed

code, present weathercode, present weather4.4. The developmental stages and vertical structure of The developmental stages and vertical structure of

middle latitude cyclones (middle latitude cyclones (boundary between boundary between northern cold air and southern warm air, upper level northern cold air and southern warm air, upper level low to the west of surface lowlow to the west of surface low))

5.5. How upper level longwaves and shortwaves may How upper level longwaves and shortwaves may enhance cyclonic development at the surface (upper enhance cyclonic development at the surface (upper level low to the west of surface low)level low to the west of surface low)

6.6. The three regions of cyclogenesis and typical tracksThe three regions of cyclogenesis and typical tracks

Page 25: Review for Midterm 2

FrontsFronts• A A weather frontweather front is a is a boundary separating two air massesboundary separating two air masses

• Types: cold front, warm front, occluded front, stationary front, dry Types: cold front, warm front, occluded front, stationary front, dry line, squall lineline, squall line

Page 26: Review for Midterm 2

How does a mid-latitude cyclone form?

In mid-latitude there is a boundary between northern cold air and southern warm air

In the boundary a initial cyclone can advect warm air northward and cold air southward

If the upper level low is to the west of surface low, the cyclone will amplify and precipitation will form.

Mature stage. Cold air begins to catch up with warm air (occluded).

Cold air cools down the cyclone. Dissipation.

Page 27: Review for Midterm 2

Regions of cyclogenesis and typical tracks

– Gulf of Mexico, east coast– Alberta Clipper from eastern side of Canadian Rockies– Colorado Low from eastern slope of American Rockies

• Lee-side lows, lee cyclogenesis