2019 年 39 巻 3 号 p. 197-206
We describe the retrieval algorithms applied to study ice microphysics using the cloud profiling radar onboard the CloudSat satellite and the lidar onboard the CALIPSO satellite. Backscattering properties of ice particles for radar lie in the Rayleigh or Mie scattering regime, and those for lidar lie in the geometrical optics regime due to the difference in their wavelengths. Based on their differences, the radar and lidar observations offer unique opportunities to determine ice microphysics. In this study, we applied an extended version of the ice microphysics retrieval algorithm to the CloudSat and CALIPSO data. Input parameters for the algorithm are the radar reflectivity factor from CloudSat and the attenuated backscattering coefficient and depolarization ratio from CALIPSO. Since the laser tilt angle of the CALIPSO lidar changed from 0.3° to 3° off the nadir direction in November 2007, different treatments for ice clouds were required before and after the change. We prepared two different look-up-tables (LUTs) of a quasi-horizontally oriented ice plate (Q2D-plate) for CALIPSO for the two observation periods (i.e., June 2006-November 2007, November 2007-current) and implemented them in the retrieval algorithm for ice clouds. A mixture of randomly oriented bullet ice particles (3D-ice) and a Q2D-plate was considered in the algorithms. These modifications allow the long-term analysis of ice microphysics in a consistent manner. A sensitivity study indicated that the importance of depolarization ratio information in the retrieval of ice microphysics; i.e., when a depolarization ratio is not used and the LUT of a 3D-ice model is applied for all cloud regions, the effective radius is significantly overestimated. The Japan Aerospace Exploration Agency and European Space Agency joint mission ’EarthCARE’ is scheduled to be launched in 2021. EarthCARE will carry a 94-GHz cloud profiling radar with Doppler function (CPR), high spectral resolution atmospheric lidar (ATLID), a multi-spectral imager (MSI), and a broad-band radiometer (BBR). The information of the lidar ratio from the ATLID and the Doppler velocity from the CPR can further improve the retrieval accuracy of ice microphysics.