Global analysis of ice microphysics from CloudSat and CALIPSO: calibration of 1064nm channel, color ratio and ice microphysics

We presents a method that can be applied to CloudSat and CALIPSO data to retrieve ice microphysics. The key feature of the algorithm is that it can treat specular reflection of lidar signals often observed by CALIPSO where large backscattering coefficient and small depolarization ratio were observed due to the existence of horizontally oriented plate. We implemented new look up tables for horizontally oriented plate for the algorithm and specular reflection model in the radar-lidar algorithm could drastically improve the cloud portions where ice microphysics can be obtained by the radar-lidar algorithm. The new radar-lidar algorithm required depolarization ratio measured by the CALIPSO in addition to radar reflectivity factor and backscattering coefficient at 532nm. Global analyses of ice microphysics for the CloudSat-CALIPSO overlap-regions are performed. The effective radius decreases as altitude increases. Effective radius in the specular reflection ranges from 100 µm to 300 µm. IWC ranges from 10-4 to several tenth of g/m3. The large mixing ratio of oriented particles occurred between -20oC to -5oC. The land-ocean differences of ice microphysics are examined for one year. IWC tends to be larger over land than over ocean due possibly to large vertical air motion over land. Next, we analyzed the color ratio, defined as backscattering coefficient at 1064nm to 532nm, of ice particles observed by CALIPSO lidar. Since the problem of calibration of 1064nm channel has been known (Hunt et al., 2009), we developed a method to calibrate 1064nm channel of CALIPSO lidar by using water clouds. The correction factor was derived for the backscattering coefficient at 1064nm in the CALIPSO Lidar Level 1B (version 2.01) products. The factor turned out to increase from south to north despite day or night. This could indicate that the thermal deformation of the instrument disturbs the 1064nm signal and this effect has to be corrected within one granule. Then the color ratio of ice clouds is estimated. The global mean value of the color ratio of ice clouds was about 0.8 and the color ratio is generally less than unity, which is consistent to the theoretical estimated values for non-spherical ice particles. The relation between color ratio and ice microphysics is also examined for CloudSat-CALIPSO overlapping clouds.