Characterization of Cloud Liquid Water Content Distributions from CloudSat

The development of realistic cloud parameterizations requires accurate characterizations of sub-grid distributions of thermodynamic variables. To this end, cloud liquid water content (CLWC) distributions are characterized with respect to cloud phase, cloud type, precipitation occurrence, and geo-location using CloudSat radar measurements. The probability density function (PDF) of CLWC is estimated using Maximum Likelihood Estimation. The best-estimated PDF of CLWC is found to closely follow either a gamma or a lognormal distribution depending on temperature (cloud phase), cloud type, the occurrence of precipitation, and geo-location. In the lower to mid-troposphere (altitudes of 1-6 km) in the tropical and subtropical latitudes, where non-precipitating and pure-water phase clouds are dominant, the PDFs of CLWC are best described by lognormal distributions. In contrast, at altitudes above 6 km and in regions poleward of the midlatitudes, the CLWC more closely resembles a gamma distribution due to a high frequency of occurrence of supercooled liquid clouds, which contain low CLWC values. The data sub-sampling with respect to cloud phase and precipitation significantly affects the distribution characteristics of CLWC in some regions. When the contributions of supercooled water and precipitation are removed, the CLWC PDFs transition from gamma to lognormal distributions in two areas: (1) the high altitude and middle-to-polar latitude regions where the contribution of supercooled cloud is significant; (2) near the surface where precipitation is frequently detected. In particular, the boundary layer with the lognormal distributions exhibit that the peaks of the mean values of CLWC are encountered in the stratocumulus regions off the subtropical west coast of continents. When precipitating events are eliminated, the peak mean-values of CLWC in the boundary layer are found in the midlatitudes and the variability of CLWC is strongly diminished in the stratocumulus regions.