Snow and wintertime blizzards

Review of last lecture Formation of clouds: 3 types of stability. Two factors limiting the height of clouds. 3 cloud properties. 9 ISCCP cloud types. Why do clouds constitute a wildcard for climate change? Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius? Growth mechanisms for rain and snow Formation of rain: coalescence process (the collector is larger than the cloud droplets but not too large)

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

Coexistence of ice and super-cooled water (exists down to T= -40C !!) is critical to the creation of cool/cold cloud precipitation - the Bergeron Process Key: Saturation vapor pressure of ice < that of super-cooled water at the same temperature. When air is in saturation wrt super- cooled water, it ’ s over-saturated wrt ice - deposition of water vapor over ice. When air is in saturation wrt ice, it ’ s sub-saturated wrt super-cooled water - evaporation of super-cooled water into water vapor. In this way, ice crystals grow rapidly at the expense of super- cooled drops Bergeron Process

Bergeron Process usually not enough to produce large enough crystals for preciptation Further growth is due to collisions between falling crystals and drops  riming and aggregation Riming (or Accretion) = liquid water freezing onto ice crystals Aggregation = the joining of ice crystals through the bonding of surface water builds ice crystals, producing snowflakes Collision combined with riming and aggregation allow formation of crystals large enough to precipitate within 1/2 hour of initial formation Further growth: Riming and Aggregation

Graupel – ice crystals that undergo extensive riming –Lose six sided shape and smooth out –Either falls to the ground or provides a nucleus for hail Hail – concentric layers of ice build around graupel –graupel carried aloft in updrafts  high altitudes freezing temperatures –water accreting to graupel freezes, forming a layer –Hail begins to fall, carried aloft again by updrafts, process repeats –Hailstones are very heavy – high density –Capable of tremendous amounts of damage –Great Plains = highest frequency of hail events Extensive riming in severe storms

Video: Weather fronts _F0VKhM

Air masses An airmass is a large (usually thousands of km across) volume of air that has horizontally uniform properties of temperature and moisture. Airmasses acquire their properties from spending days to weeks over the same part of the Earth. “Polar” airmasses are colder than “tropical” airmasses “Maritime” airmasses are wetter than "continental" airmass Other specific airmass types include "arctic", "equatorial", and “monsoon”

Bergeron classification of air masses 3 letters: e.g. mTk, cPw 3 letters: e.g. mTk, cPw 1st letter for moisture properties: c - continental, m - maritime 1st letter for moisture properties: c - continental, m - maritime 2nd letter for thermal characteristics: T - tropical, P -polar, A - Artitic/Antarctic, M - monsoon, E - equatorial, S -superior air (dry air formed by significant downward motion in the atmosphere) 2nd letter for thermal characteristics: T - tropical, P -polar, A - Artitic/Antarctic, M - monsoon, E - equatorial, S -superior air (dry air formed by significant downward motion in the atmosphere) 3rd letter for stability: k/w - air colder/warmer than ground 3rd letter for stability: k/w - air colder/warmer than ground

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

Cold Fronts A cold front is a mass of cold air advancing towards warm air. Typically associated with heavy precipitation, rain or snow, combined with rapid temperature drops. Since friction decreases with height, winds move faster at higher altitude. Then the surface of cold front becomes more steeper through time, leading to a narrow belt of precipitation. Moving speed up to 30mph

Warm Fronts Warm fronts are warm air moving towards cold air. Friction decreases with height, so winds move faster at higher altitude. This causes the surface of the front to become less steep through time. Then clouds will be spread to a wider region. Shallow stratus clouds dominate and bring light precipitation. Frontal fogs may occur as rain evaporates in the colder air near the surface. Moving speed about 12 mph

Winter blizzards: mid-latitude cyclone The mid-latitude cyclone is a synoptic scale low pressure system that has cyclonic (counter- clockwise in northern hemisphere) flow that is found in the middle latitudes (30N-55N, 30S- 55S). The mid-latitude cyclone is a synoptic scale low pressure system that has cyclonic (counter- clockwise in northern hemisphere) flow that is found in the middle latitudes (30N-55N, 30S- 55S). It has a larger size than a tropical cyclone It has a larger size than a tropical cyclone

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 coastGulf of Mexico, east coast Alberta Clipper from eastern side of Canadian RockiesAlberta Clipper from eastern side of Canadian Rockies Colorado Low from eastern slope of American RockiesColorado Low from eastern slope of American Rockies Lee-side lows, lee cyclogenesis Lee-side lows, lee cyclogenesis

Summary 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.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)Extensive riming in strong updrafts (graupel, hail)Further growth of ice crystals (riming and aggregation)Extensive riming in strong updrafts (graupel, hail) Definition of airmasses. Bergeron classification of air masses (3 letters).Definition of airmasses. Bergeron classification of air masses (3 letters). Fronts: 6 types (cold, warm, stationary, occluded, dry line, squall line)Fronts: 6 types (cold, warm, stationary, occluded, dry line, squall line) Cold front (narrow, fast, heavy precipitation), Warm front (wide, slow, light precipitation)Cold front (narrow, fast, heavy precipitation), Warm front (wide, slow, light precipitation) 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)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) The three regions of cyclogenesis and typical tracksThe three regions of cyclogenesis and typical tracks

Works cited cloud-computing-is-run-on-actual-clouds/ cloud-computing-is-run-on-actual-clouds/ astr.gsu.edu/hbase/electric/diph2o.htmlhttp://hyperphysics.phy- astr.gsu.edu/hbase/electric/diph2o.html hotos.htmhttp:// hotos.htm