Droplet growth in warm water clouds observed by synergistic use of MODIS and CloudSat

Global distribution of the cloud optical depths (COD) and cloud droplet radii (CDR) have been retrieved from space-borne passive visible-to-infrared wide-swath imagers such as the MODIS aboard the Terra and Aqua (Platnick et al. 2003), and GLI aboard the Midori-II (T. Nakajima et al. 2009, TY. Nakajima et al 2009). In the retrievals of COT and CDR, a non-absorbing channel in visible (0.67 and/or 0.86 micrometers) and absorbing channel in the short wave infrared channel (1.6, 2.1, and/or 3.7 micrometers) are used (TY. Nakajima et al 2009). On the other hand, the Cloud Profiling Radar (CPR) aboard the CloudSat observes vertical structure of clouds in the form of profiles of radar reflectivities (dBZ) (Stephens et al. 2008). If the CDRs retrieved from a passive imager represent the transition of cloud droplet evolutions (e.g. condensation, coalescence etc.), the radar reflectivities classified by CDR will present significant structures of the vertical profile in every stage of such evolutions.

In this paper, we will show that the CDR retrieved from 2.1-micrometer channel (hereafter, R21) of the MODIS traces the cloud droplet evolution monotonically (TY. Nakajima et al 2010a, 2010b). In this case, CDR retrieved from 3.7-micrometer channel (R37) is not suitable for this objective because the R37 has a strong sensitivity to the cloud droplet size at the most upper layer where the particle size tends to be small due to evaporation process whereas the R21 has information about cloud droplet size at deeper layer of the cloud and also it has sensitivity to the drizzle particles. We will propose a new approach of constructing the frequency diagram of the radar reflectivity, Contoured Frequency by Optical Depth Diagram (CFODD), which represents typical distributions of the radar reflectivities as a function of COD (TY. Nakajima et al. 2010b, Suzuki et al. 2010). It is quite interesting that the reflectivity profiles in the form of CFODDs, when grouped according to the R21 values, clearly represent every stages of the cloud droplet evolution including cloud, drizzle, and rain modes and the transitions between them. It is also of some interest to find that the CFODDs over ocean and land appear to be different. Drizzle seems to occur in lower layer of the clouds over the ocean than over the land. This possible difference could be explained by the difference in updraft velocity induced by atmospheric stability difference between land and ocean.