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?

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

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

Vertical structure and mechanisms Polar Cell (thermal): Driven by heating at 50 degree latitude and cooling at the poles Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells 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

General circulation of the oceans Ocean surface currents – horizontal water motions Transfer energy and influence overlying atmosphere Surface currents result from frictional drag caused by wind - Ekman Spiral Water moves at a 45o angle (right) in N.H. to prevailing wind direction Due to influence of Coriolis effect Greater angle at depth

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

Tropical mean state: Sea surface temperature (SST) Indo-Pacific warm pool Eastern Pacific cold tongue 2 basic regions

El Nino/Southern Oscillation (ENSO): The 4-year oscillation 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 Southern Oscillation: The atmospheric oscillation associated with the El Nino-La Nina cycle. The whole phenomena is now called El Nino /Southern Oscillation (ENSO)

Land-Sea Contrast I: Seasonal “Monsoon” 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 II: Diurnal Sea/Land Breeze 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

The magic of water on earth

How do the clouds form? 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.

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

Methods to achieve saturation and condensation 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)

Different types of fog found throughout the U.S.

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

Why does it rain on us? 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

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

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

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?

Necessary environmental conditions for tropical cyclone formation SST > 27 oC (Poleward of about 20o SST too cold for formation. Highest frequency in late summer to early autumn when water is warmest.) Warm ocean mixed layer is thick enough to supply energy (this is why they weaken quickly upon landfall) Unstable atmosphere with a moist lower/middle troposphere (central and western ocean basins) Low vertical windshear (Otherwise upward transfer of latent heat disrupted) Coriolis force (hurricanes do not form between 5N-5S) Pre-existing low-level rotating circulations (tropical waves and other disturbances)

Structure of tropical cyclones 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.

How do the blizzards form? Definition of airmasses? Bergeron classification of air masses Fronts: 6 types. What is a cold front? A warm front? An occluded front? Surface weather analysis: Station model, wind speed code, present weather The developmental stages and vertical structure of middle latitude cyclones (boundary between northern cold air and southern warm air, upper level low to the west of surface low) How upper level longwaves and shortwaves may enhance cyclonic development at the surface (upper level low to the west of surface low) The three regions of cyclogenesis and typical tracks

Fronts A weather front is a boundary separating two air masses Types: cold front, warm front, occluded front, stationary front, dry line, squall line

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.

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