Advances in the treatment of ice clouds in general circulation models (GCM) will require not only increased horizontal and vertical model resolution but also a better description of the microphysical and radiative properties of ice clouds. The important role that ice crystal sedimentation (fallout) plays in the Earth's radiative balance has recently become more apparent. The fall velocities directly affect the buildup or decay of cloud ice mass and cloud lifetime. GCM sensitivity studies have shown that relatively small changes in the fallspeed lead to significant changes in model estimations of the Earths� net radiative flux divergence.
We propose to derive characteristic fall velocities of ice particle ensembles (Vm) geographically and as a function of height within cloud layers in terms of the ice water content (IWC). The velocities will be derived from CloudSat radar reflectivity measurements and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) derived extinction estimates. Representations of Vm will be developed for radar/lidar observations together and separately, depending upon the cloud conditions. The effort will draw upon in-situ particle size distribution measurements in artic through tropical ice clouds, and radar and lidar observations from several DOE ARM (Atmospheric Radiation Measurement) sites and from the NASA ER-2 to yield ensemble mass-weighted fall velocities. Preliminary evaluations of the approach using in-situ/Doppler radar/lidar observations taken at the Southern Great Plains (SGP) ARM site are presented. The primary goal of this effort is to produce a global database of ice cloud particle ensemble fall velocities as a function of IWC and to make this information available in a form useful for GCM and cloud resolving modeling studies. A secondary goal is to use CALIPSO extinction and infrared radiometer data if available to characterize the vertical distribution of mean particle fallspeed in tropical tropopause ice cloud layers, a topic of considerable current interest to the scientific community. A product output for subvisual cirrus will be the vertical distributions of ice water content.
The IWC will be obtained from archived products at the CloudSat Data Processing Center (http://c3vp.org/data/DPC/DPC.html). CloudSat standard products provide IWC profiles with two different versions. The radar-only vision IWC retrieval is mainly based IWC-Ze relationships. Meanwhile, CloudSat provides combined Radar-MODIS IWC retrieval, which uses ice optical depth retrieved from MODIS to constrain radar profile and has better accuracy. On the other hand, CALIPSO provides its IWC retrieval based on lidar measurements. To avoid potential inconsistent issues between lidar- and radar- based IWC retrievals and to provide more complete and consistent ice cloud properties, Co-PI Dr. Wang proposes to combine lidar and radar measurements to produce ice cloud microphysical profiles globally in a separate proposal. If that proposal is funded, we will augment the standard products with these products.
