Analysis of vertically-pointing profiler and polarimetric scanning radar observations is proposed to quantify the vertical and spatial structure of raindrop size distributions (DSDs) in precipitating cloud systems and will address NASA's request for (i) improving current precipitation algorithms from satellite observations, (ii) developing, testing and validating future dual-frequency precipitation radar (DPR) algorithms, and (iii) developing statistical validation techniques that include improving ground-based comparison products.
The DSD is a fundamental precipitation property from which nearly all other precipitation products, such as rainfall rate, can be derived. Vertically-pointing profilers retrieve the DSD from near the surface to just below the melting layer in precipitating cloud systems that advect directly overhead. Scanning polarimetric radars use polarization diversity to estimate the DSD parameters throughout the precipitating cloud system. These unique instruments provide valuable information on the DSD that is not available in conventional scanning weather radars (e.g. WSR-88D) and not available from spaceborne radars.
The proposed research is divided into three themes. The first theme deals with the retrieval of raindrop size distributions from profilers and from scanning polarimetric X-band radars. The first task in this theme will retrieve and analyze DSDs from multiple years of profiler observations producing tens-of-thousands of DSD profiles in different rain regimes that will lead to physically based DSD parameterizations needed to reduce the errors in current and future precipitation satellite algorithms. The second DSD retrieval task will use a polarimetric X-band radar in combination with profilers to analyze the spatial and temporal variability of DSD parameters to develop and improve ground-based comparison products needed for ground validation facilities supporting the Global Precipitation Measurement (GPM) mission.
The second theme of the proposed research focuses on the attenuation at Ku- and Ka-bands through precipitation. The first task will use profiler observations to statistically determine the errors in the Tropical Rainfall Measuring Mission (TRMM) Ku-band Precipitation Radar (PR) attenuation correction algorithm. Profiler and TRMM PR reflectivity distributions above the brightband agree very well. In contrast, there is a 5 dBZ disagreement in the reflectivity distributions below the brightband suggesting errors in the attenuation correction algorithm through the brightband. The proposed analysis will isolate these errors in the current PR algorithms providing feedback from ground observations to improve TRMM PR precipitation algorithms. The second attenuation task will exploit the fact that the attenuation at Ka-band is linearly proportional to the rain rate in the attenuation path. The proposed analysis will convert zenith-looking ground-based attenuation-based algorithms estimating rain rate to the nadir geometry for use with the GPM Ka-band radar to provide independent rain rate estimates to supplement DPR algorithms.
The third theme continues collaborations with North Carolina State University, University of Alabama in Huntsville, and North Dakota University by analyzing profiler observations collected in support of their respective NASA Precipitation Measurement Mission (PMM) proposals. These collaborations will focus on retrieving ice particle size distributions above the freezing level complementing the DSD work proposed here.
