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UCSD SIO 217A - Recent Advances in Quantifying Cloud Forcings

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Recent Advances in Quantifying Cloud Forcings Between IPCC2001 and 2007GABE KOOPERMANSan Diego, CABENJAMIN WELLEDepartment of Civil and Environmental Engineering, Stanford University, Stanford, CAALEXANDER G. WINBOW∗Department of Physics, University of California - San Diego, La Jolla, CaliforniaDecember 4, 2007AbstractBetween the Third and Fourth Assessment Reports of the IPCC in 2001 and 2007, new progress has beenmade in understanding the subtleties of cloud behavior and their contribution to radiative forcing. Advancesin modeling and methodology have increased accuracy and improved predictions of the physical processes.It is now understood that the modeling of cloud feedback is the greatest source of discrepancy among globalclimate models, and that low lying clouds are the most significant source of this discrepancy. Cloud covermeasurements from satellite observation are better developed and compared with surface observations. Sur-face observation evaluations generally support earlier analysis for measurements over land, but show somedifferences between the two Assessment Reports for measurements over oceans. Aviation-induced cloudi-ness makes only a small contribution to cloud radiative forcing, but possible correlations with observedsurface warming and the diurnal temperature range have also been examined. Uncertainties are handledmore systematically and more quantitatively in many areas.∗Corresponding author address: Alex Winbow, Department of Physics, #0354, University of California, San Diego, La Jolla, CA 92093.E-mail: [email protected] Reviews of Atmospheric Science Topics1. IntroductionThe Fourth Assessment Report of the IPCC in 2007 statesbaldly, “The response of cloud cover to increasing green-house gases currently represents the largest uncertainty inmodel predictions of climate sensitivity” (§3.4.3). Con-tinued work since the Third Assessment Report in 2001has brought new understanding of the subtleties of cloudbehavior and their contribution to radiative forcing.†We consider developments in several directions. Un-derstanding of the physical processes of clouds, espe-cially feedback cycles, has improved, although critical un-certainties remain. The observation and measurement ofcloud cover has benefited from a great deal more satellitedata from new instruments. Faster computers permit moresophisticated modeling techniques, and the models havehighlighted discrepancies in our understanding of cloudfeedback. Finally, two interesting theories on aviation-induced cloudiness (contrails) were proposed in the liter-ature.The IPCC’s handling of uncertainty has been histori-cally inconsistent and is often an area of concern. In manycases, a range of values is provided which encompass thefindings of the most widely accepted studies. The IPCChas since made an increased effort to quantify uncertaintyin two directions: level of confidence or scientific under-standing, and likelihood of occurrence, and we remark onthese improvements.2. Cloud Radiative Forcings andCloud Feedback Processes (Back-ground)Clouds reflect shortwave visible solar radiation back tospace, and trap longwave infrared radiation emitted fromthe Earth’s surface and lower troposphere. These two op-posing effects are the primary components of cloud radia-tive forcing (CRF); the balance depends on many factors,and currently clouds exert a cooling effect on climate,yielding a negative CRF. Global climate change may in-†In this paper, “TAR” refers to the Contribution of Working Group Ito the Third Assessment Report of the Intergovernmental Panel on Cli-mate Change (Houghton et al. 2001), and similarly “AR4” to the Fourthsuch report (Solomon et al. 2007).crease or decrease the CRF. Prediction is difficult due tothe impact of complex oceanic, atmospheric, and surfaceinterrelationships. Cloud type, height (low-level bound-ary layer clouds to deep convective clouds and anvils),radiative properties, as well as the relative distribution ofthese variables all impact the CRF.The Earth’s CRF is considered separately from otherradiant forcings (RF) from anthropogenic sources, suchas the effect of aerosols and changes in surface albedo.This is because under normal global conditions a baselineCRF would exist, as opposed to the other RFs which aredefined as the consequence of perturbations to the normalglobal climate processes. RFs have an impact on cloudprocesses, and the cloud response to these RFs is con-sidered one component of the concept of cloud feedback.Changes in the CRF are caused by changes in the cloudfeedback, one source of which is the effect of RFs.Climate sensitivity is a metric used to characterize theresponse of the global climate system to a given forcing.Climate sensitivity analyses for clouds generally considerthe impact of a doubling of CO2on the cloud/global cli-mate interaction. These impacts may be considered inde-pendent of RFs. If CO2is doubled (holding all other an-thropogenic RFs constant), cloud feedback processes re-spond to this change. This is the second contributor to theconcept of cloud feedback, which can change the CRF.However, a change in cloud physics and dynamics dueto this doubling in CO2does not necessarily mean that aresulting change in CRF is due completely to the cloudfeedback process. Clear sky radiation, i.e. the impactof water vapor, surface albedo and temperature changes,also affects the CRF, yet that impact is sometimes con-sidered as a separate component of the cloud feedbackeffect, and sometimes not. For example, the Partial Ra-diative Perturbation (PRF) analysis method (Soden andHeld 2006) excludes clear sky impacts, and only consid-ers cloud feedback. This can be confusing, for changes intemperature are inherently a part of the clear sky and non-clear sky effects. Sometimes, climate sensitivity is alsoevaluated by incorporating a change in an anthropogenicRF.One could say that changes in CRF are determined bycloud feedback processes, which can be evaluated in fourdifferent ways: changes in anthropogenic RFs, changes inthe doubling of CO2concentration (holding RFs constant)and neglecting clear sky conditions, changes in clear skyKooperman, Welle, and Winbow 3conditions due to increased CO2, and changes in a par-ticular anthropogenic RF in a doubled CO2environment.For the purposes of this paper, cloud feedback is consid-ered to include all four of these effects and climate sen-sitivity is considered to include only the second and


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