Potential Limitations of Light Rain Retrieval from Spaceborne Cloud Radar Observations

Light rain (rain rates less than 1 mm/hr) that typically occurs over larger areas and lasts longer than heavy rain, may account for a large fraction of the total precipitation globally and have a greater impact on the global energy balance. However, it is difficult to detect light rain using the current TRMM precipitation radar or the upcoming GPM dual-wavelength radar since the reflectivities are near or below the respective minimum sensitivity of the radars. A cloud radar operating at 95 GHz, can usually detect this light rain because of its higher sensitivity. This has been shown by observations from the recently launched Cloud Profiling Radar on CloudSat [Stephens et al. 2001, L'Ecuyer and Stephens 2002].

In principle, single-frequency rain retrieval algorithms such as the surface reference technique (SRT) [Iguchi and Meneghini, 1994] used by the TRMM Precipitation Radar (PR) can be applied to the CloudSat data to retrieve light rain. However, the accuracy is limited by the uncertainty in the raindrop size distribution (RSD), surface return, and water vapor distribution that vary both inside and outside of the rain region. Dual-frequency radar can resolve the uncertainty in RSD and may be used as a tool to address some of the difficulties encountered by a single - wavelength cloud radar. In this research, we compare the profiles of rainfall rate retrieved from single- and dual- wavelength algorithms using the observations from 10 and 94 GHz radars onboard NASA's ER-2 aircraft. Since the 94 GHz radar is operating at same frequency as CloudSat, the results from this research can provide valuable information for the CloudSat light rain retrieval algorithms.