Evaluation of cloud liquid water product of Cloudsat using surface echo data

The strength of surface return echo (sigma-zero) from air/space borne radar reflects accumulated attenuation trough its ray path. The cloud profiling radar (CPR) onboard Cloudsat is aimed to measure cloud profiles over globe. It also measures sigma-zero as well as cloud echo. One of the major products of the Cloudsat is vertical properties of clouds such as cloud liquid water content (LWC), cloud liquid path (LWP), cloud ice water content (IWC) and cloud ice path (IWP). The principle to estimate those parameters is to utilize the radar reflectivity factor (Z). Sigma zero information, on the other hands, contains the total path attenuation (path integrated attenuation: PIA) that mainly comes from attenuation in liquid water. This information is not used for the LWC (or LWP) estimation for instantaneous data simply because the relatively larger variability of surface echo than cloud echo (in other words, poor signal to noise ratio: S/N) especially over land. However, sigma zero information can be utilized for statistical approach. This study tries to evaluate the Cloudsat products mainly LWP by using sigma zero data. The method is to accumulate the sigma zero data for each LWP categories of the Cloudsat product and the mean value is compared with the PIA. Since the attenuation data is less sensitive to the drop size distribution of cloud than radar reflectivity factor, it can give more accurate estimation if there is no S/N problem.