The GPM Validation Network and Evaluation of Satellite-Based Retrievals of the Rain Drop Size Distribution

Patrick N. Gatlin,David B. Wolff,Jason L. Pippitt,Ali Tokay,Todd A. Berendes,Walter A. Petersen

doi:10.3390/atmos11091010

Abstract

A unique capability of the Global Precipitation Measurement (GPM) mission is its ability to better estimate the raindrop size distribution (DSD) on a global scale. To validate the GPM DSD retrievals, a network of more than 100 ground-based polarimetric radars from across the globe are utilized within the broader context of the GPM Validation Network (VN) processing architecture. The GPM VN ensures quality controlled dual-polarimetric radar moments for use in providing reference estimates of the DSD. The VN DSD estimates are carefully geometrically matched with the GPM core satellite measurements for evaluation of the GPM algorithms. We use the GPM VN to compare the DSD retrievals from the GPM’s Dual-frequency Precipitation Radar (DPR) and combined DPR–GPM Microwave Imager (GMI) Level-2 algorithms. Results suggested that the Version 06A GPM core satellite algorithms provide estimates of the mass-weighted mean diameter (Dm) that are biased 0.2 mm too large when considered across all precipitation types. In convective precipitation, the algorithms tend to overestimate Dm by 0.5–0.6 mm, leading the DPR algorithm to underestimate the normalized DSD intercept parameter (Nw) by a factor of two, and introduce a significant bias to the DPR retrievals of rainfall rate for DSDs with large Dm. The GPM Combined algorithm performs better than the DPR algorithm in convection but provides a severely limited range of Nw estimates, highlighting the need to broaden its a priori database in convective precipitation.

Highlights

Understanding the distribution of precipitation is vital to gaining new insights into the water and energy cycle of the Earth system and how this life-essential resource evolves in a changing climate.accurate precipitation estimates are required on a global scale—something only possible withEarth observing satellites
Figure shows the retrievals of stratiform and convective rainfall, which is based on the precipitation type classification and for Note stratiform convective based on the focuses precipitation type of 2ADPR
We have identified a few issues with the drop size distribution (DSD) retrievals from the Global Precipitation Measurement (GPM) V06A algorithms, We have identified a few issues with the DSD

Summary

Introduction

Understanding the distribution of precipitation is vital to gaining new insights into the water and energy cycle of the Earth system and how this life-essential resource evolves in a changing climate.accurate precipitation estimates are required on a global scale—something only possible withEarth observing satellites. Measurement (TRMM) mission [3,4], the GPM extends the precipitation program of record to higher latitudes and facilitates new satellite-based measurements of raindrop size, which improves the retrieved rainfall estimate. One of the science requirements for the GPM core satellite is to provide an estimate of raindrop sizes accurate to within 0.5 mm. This retrieval is largely facilitated by DPR’s ability to obtain radar measurements at two different frequencies—the Ku-(13.6 GHz) and Ka-band (35 GHz), whose differential scattering in rain is directly related to the size of the raindrops [5,6]

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Results

Discussion

Conclusion