Probability Distribution Functions of Liquid Water Path Associated With Marine Low Cloud

Probability distribution functions (PDFs) of liquid water path of marine low cloud over the eastern Pacific are obtained using geostationary satellite visible data, the main purpose being to improve low cloud parameterization for large scale models. The PDFs are calculated over an area of about 200km × 200km, which corresponds to a typical climate model grid size. A high time-interval (30 minutes) and high spatial resolution (1km) of the observation covering large areas provided us with fascinating details of the PDFs from diurnal to seasonal time scales. A shortcoming of the data is that it is available only during the daytime. The PDFs along the GPCI (GCSS Pacific Cross-Section Intercomparison)-line, an east-west line at 20S, and a north-south line at 90W off Peru, during the year 1999-2001 are analyzed. These areas are characterized by solid stratocumulus, breakup stratocumulus and shallow convections. The ECMWF reanalysis is used to identify the large scale meteorological parameters responsible for changes in the homogeneity, skewness, and kurtosis of the PDF. We found that the parameters which estimate the stability between 850hPa and surface and the parameters which relate to cloud top entrainment instability made a large contribution to the character of the PDF. Moreover, a mathematical form of the PDF was estimated from the observation and compared with the PDFs conventionally used in PDF-based cloud parameterizations. This study allowed us to provide a more general form of PDF which varies with large-scale meteorological parameters. We also estimated the impact of the use of the observed PDF on the cloud-water to precipitation conversion rate, as well as a rough estimate of the error generated by the plane parallel homogeneous assumption of cloud water in radiation schemes.