Drop Size Distribution Model Estimation by Comparing CloudSat/CPR and TRMM/PR

In this study, an analysis method to estimate the climatological drop size distribution (DSD) parameter using spaceborne radars (CloudSat's w-band radar and TRMM's Ku-band radar) is proposed and global map of a DSD parameters (e.g. median diameter, D0) is demonstrated. The basic idea of the analysis method is to compare the histogram of radar reflectivity factor (Z) at near-surface range bin in the over-lapping Z range (weak to moderate rain echo). Because of the Mie effect, the Z value of rainfall is different between w- and Ku-band and it reflects the difference in the histograms of w- and Ku-band. The analysis is to find the most appropriate DSD model by matching the histograms each other. In this study, D0 (median diameter of a DSD) is used as a DSD parameter. The histograms are created in 10 degree grid in latitude and longitude for each season, local time (ascending/descending node of CloudSat) and the Earth's surface type. The final product is a global map of D0. Data used in this study are near-surface Z data in L2B GEOPROF from CloudSat and in 2A25 from TRMM/PR from December 2006 to November 2007 (12 months). Attenuation corrected Z data are available for TRMM/PR 2A25 whereas only uncorrected Z data are available for CloudSat 2B GEOPROF. For this reason, a simple attenuation correction method, surface reference technique, is introduced for the data from L2B GEOPROF. In this study, data in 20 to 40 dBZ (in Ku-band) range are used for the comparison. A general characteristic of global maps of D0 through the year and local time is apparent land-ocean contrast; larger D0 appears over land and smaller D0 appears over ocean except for relatively small D0 over southeastern Asia to China. Also, relatively larger D0 appears in tropical area and mid latitude summer. Diurnal change of D0 can be seen by comparing the day/night time D0; D0 is larger in the night time over ocean while day time D0 is larger over land. On the seasonal change, smaller amplitude appears over Tropical Ocean, and larger amplitude appears mid-latitude.