Investigation of the Aerosol Indirect Effect on Ice Clouds

1. Investigation of the Aerosol Indirect Effect on Ice Clouds and its Climatic Impact Using A-Train Satellite Data and a GCM Yu Gu1, Jonathan H. Jiang2, Hui Su2, and K. N. Liou1 1Department of Atmospheric and Oceanic Sciences 2Jet Propulsion Laboratory, California Institute of Technology and Joint Institute for Regional Earth System Science and Engineering 4800 Oak Grove Drive, Pasadena, California, 91109 University of California, Los Angeles, Los Angeles, California Satellite Observations GCM Simulation Results Aerosol First Indirect Effect • Polluted Clouds (Aerosol 1st Indirect Effect): • Less OLR • Less solar radiation reaching the surface • More reflected solar radiation at TOA • Increased cloud cover • Reduced precipitation • Colder surface air temperature • Change in cloud droplet/ice particle numbers associated with increase in aerosol number concentrations • Cloud particle mean effective size • Cloud radiative forcing Global July Mean • Left Fig: Satellite observations indicate Re increases with IWC but decreases with AOT • Right: an empirical model derived from fitting of satellite data, as described below. OLR Parameterizations for Ice Number • Relate ice nucleation and number to aerosol concentration on the basis of explicit microphysics modeling, laboratory studies, as well as theoretical considerations (e.g., Diehl and Mitra 1998; karcher and Lohmann, 2002, 2003; Riemer et al. 2004; Liu and Penner 2005; Karcher et al. 2006) • Mean effective ice crystal size calculated from ice mass and number for radiation calculations Empirical Re, IWC, and AOT Relation Using least-squares fitting, we obtained an empirical formula for Re as a function of IWC and AOT. This function broadly captures the variation of Re with IWC and AOT. Precipitation large uncertainties in the parameterization of ice microphysics processes and requirement of significant computational efforts =1.436, =0.5858, =0.282, =8.09, Re0=56.0293, IWC0=1.3838, =1.11 Cloud Cover Surface Solar Flux Parameterizations for Re This empirical relationship of Re with IWC and AOT can serve as a first-order parameterization of the first indirect effect of aerosols on ice clouds for application to climate models. • A conventional approach has been to prescribe a mean effective ice crystal size in GCMs (e.g., Köhler 1999; Ho et al. 1998; Gu et al. 2003) • Use IWC and/or temperature produced from GCMs to determine a mean effective ice crystal size (Kristjánsson et al. 2005; Gu and Liou 2006; Ou and Liou 1995; Ou et al. 1995; McFarquhar et al. 2003; Liou et al. 2008). Model Description Surface Air Temperature None of the De parameterizations accounted for the distinction between “polluted” and “clean” clouds Use of A-Train Satellite Data Summary • Investigation of the aerosol indirect effect on ice clouds have been limited due primarily to the lack of accurate global-scale observations. • New data from the NASA’s A-Train constellation makes it possible to examine the aerosol-cloud interaction in a more comprehensive way that can lead to improved physical understanding of this interaction. • The A-Train is a constellation of 6 satellites spaced a few minutes apart from each other and so their collective observations can be used to construct high-definition three dimensional images of the earth’s atmosphere and surface. • GCM simulations show that the global averaged OLR and net solar flux at TOA are smaller in polluted case, illustrating more reflected solar flux and trapped OLR due to smaller De. Global radiative forcing produced by the aerosol 1st indirect effect is about 0.94 W/m2 for IR and -0.98 W/m2 for solar radiation. Surface solar flux is also reduced, resulting in colder surface air temperature. • Reduced precipitation and increased cloud cover are found globally in response to the aerosol 1st indirect effect. • Changes in the precipitation pattern show that, due to the aerosol 1st indirect effect, reduced precipitation is found in the regions where polluted clouds mostly occur.