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UW ATMS 535 - Microphysical Macrophysical Interactions

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Microphysical-macrophysical interactionsorWhy microphysics mattersOptical properties of liquid cloudsCloud albedo /(+7)= 3LWP/reLWP is macro physical property- determined largely by meteorological contextre is a micro physical property- determined largely by properties of cloud condensation nucleiOptical properties of liquid cloudsWhat determines re ?re  (3LWC/4N)1/3for warm layer clouds N dominates (Ncan vary from 10-2000 cm-3while LWCvaries from 0.2-1 g m-3)MODIS, data courtesy of NASAMODIS, data courtesy of NASAMODIS, data courtesy of NASANet cloud radiative effect from ERBE-90 -70 -50 -40 -30 -20 -10 0 10 20 30 40Error in TOA net radiation in assuming no microphysical variability (constant N) -90 -80 -70-90 -80 -70-30 -20 -10-30-20-10 8 9 10 11 12 13 14 15 16 17 18 19 20Cloud droplet effective radius [m] -12-8-8-4-4-4-40004416881212-120 -110 -100 -90 -80 -70-110 -100 -90 -80-20 -10 0 10-20-10010Microphysics and radiation budgetWhat determines N in warm clouds?Aerosol concentration (r>0.1 micron) [cm-3]Cloud droplet concentration [cm-3]The Twomey Effecta.k.a first aerosol indirect effectIncrease N  decrease re increase  (for constant LWP)  increased albedo/(+7)= 3LWP/reAerosol loading and cloud dropleteffective radiusfrom Breon et al. 2002, Science, 295, 834-8386 8 10 12 14Cloud droplet radius [micron]0.0 0.1 0.2 0.3 0.4 Aerosol indexWilliams et al. (2001)IPCC, 2007Cloud droplet size also impacts precipitation formation• Terminal speed of falling droplets increases strongly with size• Collection “efficiency” also increases with drop size • smaller drops tend to follow flow field around impinging larger ones, whereas larger ones are collected• Initial production of precipitation is more efficient in clouds with larger droplets (lower N, for a given LWC)Summary of drizzle observations in stratiform warm clouds from field programsHowever, what about feedbacks?Increase N  decrease re decreasing collision coalescence  decrease precipitation reduced LWC sink  increase cloud LWC increase  increased albedoAlbrecht effecta.k.a “second aerosol indirect effect”Increase precip.  decrease cloud coverAlbrecht 1989Increasing N in a more sophisticated modelAckerman et al. (2004)reduced precip. decreasesLWP !….sometimesClouds containing the ice phaseImportance of microphysics for orographic precipitation distributionwind directionIce concentration:1 liter-125100Homogeneous freezing sensitivity to aerosol concentrationPolar stratospheric clouds (PSC) and ozone depletionHamill and Toon 1991Indirect effect ratio RIE1st AIE 2nd AIEDefine RIE = 2ndAIE / 1stAIERelative strength of the Albrecht effect compared with TwomeyFor adiabatic cloud layers, Nd1/3 LWP5/6short timescaleslong timescalesAlbrecht effect can enhance or cancel the Twomey


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