Forward and Backscattering Measurements of Rainfall over a path at the GPM Frequencies

This paper will describe the theoretical basis and conceptual design for a dual-frequency microwave measuring system capable of forward and backscatter measurements at the Global Precipitation Mission (GPM) frequencies of 13.6 GHz and 35.5 GHz. The forward and backscattering measurements will provide rain profile estimates over a 2.3 km path permitting a detailed study of the rainfall process. The measurements will be used in conjunction with a ground-based network of disdrometers and rain gauges located under the propagation path, creating a tightly controlled environment for the research of spaceborne and ground-based radar rainfall retrieval. The new concept is expected to improve rainfall estimation and validation significantly and help form the foundation of a new generation of rainfall validation instruments.

The forward and backscattering measuring system will enable small-scale rain studies essential in understanding the rainfall process, and make significant contributions to 1) the investigation of radar inversion algorithms for the estimation of precipitation parameters that include liquid water content, median mass diameter, number concentration, and rainfall rate, 2) the testing and validation of established radar retrieval algorithms such as those used by TRMM single-frequency radar and those proposed for GPM dual-frequency radar, and 3) the validation of GPM overflight rainfall estimates.

The system will be able to measure forward and backscatter signals using a combination of radar and a microwave Link techniques. The radar will use Frequency Modulated Continuous Wave (FMCW) to measure backscatter power. The microwave Link will use the radar as the transmitter and a coherent receiver phase-locked to the radar's local oscillator to measure the path-average attenuation and frequency differential phase-shift.