1Lecture Ch. 8a• Review of Ch.7 Concepts– Homework Ch. 7, Prob. 3• Cloud Classification• Precipitation ProcessesCurry and Webster, Ch. 8For Tuesday: Finish reading Ch. 8Atmospheric Structure• !Structure of the atmosphere– Decreasing temperature with altitude– Decreasing pressure with altitude– Changes in water vapor with altitude• Temperatures in meteorology– Potential temperature (meteorologists’entropy)– Virtual (potential) temperature– Equivalent (potential) temperature• Describing the atmosphericstructure– Example: Skew-T log P plot– Example: TephigramInversions• Inversion: A condition of strong stabilitycharacterized by a positive temperaturegradient.2Subsidence Inversions• Subsidence Inversion– Cause: adiabatic compression and warming of alarge layer of earth as it sinks to lower altitude.• dT/dP = 1/ (Cp ρ) , where Cp is essentially constantover T.Because of a change in P with z, what can be saidabout ρtop versus ρbottom ?Which layer is warmer, top or bottom?Radiation Inversions• Radiation InversionCause:• radiation of heat by the ground at night• air adjacent to the the surface has a T < layer athigher elevationsWater Vapor in the Atmosphere• the Earth’s surface is the primary source of water vapor for the atmosphere• the amount of water vapor in the atmosphere depends on– (1) the amount which enters the atmosphere through evaporation and sublimation,– (2) its transport by motions of various scales throughout the troposphere and the lower stratosphere,– (3) the amount which leaves the atmosphere intermittently as rain, hail and snow• wv,sat, the amount of water vapor in air at saturation on a mass-per-mass basis, decreases with decreasingtemperature– the amount of water in an air parcel consequently decreases with altitude, reaching a minimum in the lowerstratosphere a few kilometers above the tropopause– stratosphere has very low water content• clouds and fogs form by cooling of moist air– cloud formation is driven by the rise of moist air due to thermally-driven updrafts, which result in simultaneouscooling and expansion; in many cases this expansion is close to adiabatic– fog formation and some stratus cloud formation can occur by isobaric cooling, caused by surface cooling– after a sufficient amount of cooling wv=wv,sat and a liquid condensate is formed; this process occurs when the dew-point temperature (Td) is reachedLifting Condensation Level• Liftingcondensation levelvaries with initialrelative humidityand is a weakfunction of initialtemperature10.1Seinfeld and Pandis, Fig. 15.113Cloud ClassificationCumulus CloudsSwelling CumulusActive heaped-up cloudwith flat bottom andgrowing cauliflower top.[http://www.fox8wghp.com/spacious.htm]10.2Cumulonimbus CloudsCumulonimbusMassive cloud systemproducing heavy showers,sometimes with hail. Mostactive clouds may havelightning and thunder. Afew spawn tornadoes.[http://www.fox8wghp.com/spacious.htm]10.2Stratus CloudsStratusLow lying layer of cloud(called fog if on theground) with no structure.[http://www.fox8wghp.com/spacious.htm]10.2Cirrus CloudsCirrus An ice crystal cloud, wispyin appearance. Mayproduce ice crystal snow inwinter or in mountains.[http://www.fox8wghp.com/spacious.htm]10.2Altostratus CloudsAltostratusThickly layered waterdroplet cloud. Sun seen asthrough ground glass.[http://www.fox8wghp.com/spacious.htm]10.24Nimbostratus CloudsNimbostratusThick layered cloud -usually dark gray.Produces continuous rainor snow over large area.[http://www.fox8wghp.com/spacious.htm]10.2FogGlobal Cloud Distribution• Zonallyaveragedclimatologyof cloud type10.2Cloud Types and Drop Sizes• Frequencydistributions of themean cloud dropletsize for variouscloud typesPrecipitation Processes• Warm clouds (liquid water droplets only)• Cold clouds (ice particles)5Drop Growth and Size• Bigger particles (~25 micron) grow faster6Precipitation and Drop Size• Terminal velocity increases with drop size• Precipitation occurs when – terminal velocity exceeds updraft velocityPrecipitation and Cloud Type• Precipitation depends on– Condensed water (water and temperature)– Updraft velocity (dynamics)– Temperature (cold or warm processes)– Drop size (aerosol effects)Liquid Water Path7Precipitation EfficiencyRamanathan et al., 2001Current Research: Cloud Drops• Additional particles alsoreduce droplet size– Slowing growth toprecipitation-size droplets“These aerosol effectscan lead to a weakerhydrological cycle,which connectsdirectly to theavailability andquality of freshwater...”Erlick, Ramaswamy, and Russell, 2005Current Research: Global Models• Aerosolimpacts onrain are notlocalChange in JJA meanprecipitation (mm day-1) between the 6-yearperturbation and the 12-year control.What are the characteristics of“ship tracks”?Ship Track Observations Remote/OpticalIn situ/AerosolConover 1966⇑ albedo = 20%Coakley, Bernstein, Durkee 1987⇑ R(3.7µm) = 3.9%⇑ R(0.63µm) = 1.6%R(11µm) = 0.0%Radke, Coakley, King 1989⇑R(0.63µm) = 13.6%⇑τ =260%⇓re = 21%King, Radke, Hobbs 1993⇑I(τ,-1)(0.74µm) = 220%⇓I(τ,-1)(2.20µm) = 87%⇑Ndrop = 220%⇑CN = 250%⇑LWC = 250%Processes Governing Ship TracksHypothesisRadke, Coakley, King 1989ship stacks ⇒ ⇑CCNAlbrecht 1989⇓Nprecip ⇒ ⇑LWC Hudson 1991⇓CCN ⇒ ⇑NprecipAckerman, Toon, Hobbs 1994⇓CCN ⇒ ⇓h,
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