A comparison among four different retrieval methods for ice-cloud properties using data from CloudSat, CALIPSO, and MODIS

Thorwald H. M. Stein (1) , Julien Delanoë (1, 2) , Robin J. Hogan (1)

1 Department of Meteorology [Reading]
2 SPACE - LATMOS

The A-Train constellation of satellites provides a new capability to measure vertical cloud profiles leading to more detailed information on ice-cloud microphysical properties than has been possible up to now. A variational radar-lidar ice-cloud retrieval algorithm, VarCloud, takes advantage of the complementary nature of the CloudSat radar and CALIPSO lidar to provide a seamless retrieval of ice water content, effective radius and extinction coefficient from the thinnest cirrus (seen only by the lidar) to the thickest ice cloud (penetrated only by the radar). In this paper, several versions of the VarCloud retrieval are compared with the CloudSat standard ice-only retrieval of ice water content, two empirical formulas that derive ice water content from radar reflectivity and temperature, and retrievals of vertically integrated properties from the MODIS radiometer. Typically the retrieved variables agree within a factor of 2, on average, and most of the differences can be explained by the different microphysical assumptions. For example, the ice water content comparison illustrates the sensitivity of the retrievals to assumed ice particle shape. If ice particles are modeled as oblate spheroids rather than spheres for radar scattering then the retrieved ice water content is reduced by on average 50% in clouds with a reflectivity factor larger than 0 dBZ. VarCloud retrieves optical depth on average a factor of 2 lower than MODIS, which can be explained by the different assumptions on particle mass and area; if VarCloud mimics the MODIS assumptions then better agreement is found in effective radius and optical depth is overestimated. However, MODIS predicts the mean vertically integrated ice water content to be around a factor-of-3 lower than VarCloud for the same retrievals, because the MODIS algorithm assumes that its retrieved effective radius (which is mostly representative of cloud top) is constant throughout the depth of the cloud. These comparisons highlight the need to refine microphysical assumptions in all retrieval algorithms, and also for future studies to compare not only the mean values but also the full probability density function.

Domaines: Météorologie

Thorwald H. M. Stein, Julien Delanoë, Robin J. Hogan. A comparison among four different retrieval methods for ice-cloud properties using data from CloudSat, CALIPSO, and MODIS. Journal of Applied Meteorology and Climatology, 2011, 50 (9), pp.1952-1969. ⟨10.1175/2011JAMC2646.1⟩. ⟨hal-00590875⟩