# ESRL/PSD Seminar Series

PSD Flash Seminars: Using Ground-Based Observations to Validate Assumptions used in Satellite Rainfall Retrieval Algorithms

Christopher Williams

NOAA/ESRL PSD Weather and Climate Physics Branch

## Abstracts

Rainfall retrieval algorithms often assume the raindrop size distribution (DSD) can be described by a gamma function using three mathematical DSD parameters. But, if only two independent measurements are available, as with the dual-frequency precipitation radar (DPR) on the Global Precipitation Mission (GPM) core satellite, then retrieval algorithms are under-constrained and need to make assumptions about the DSD parameters. Satellite rainfall algorithms reduce the number of free parameters by either assuming one DSD parameter is a constant (e.g., the DSD has an inverse exponential distribution), or, by assuming a mathematical relationship between two DSD parameters. Controversies exist over whether relationships between DSD parameters result from physical processes or mathematical artifacts due to high correlations between gamma DSD parameters.

This study uses ground-based observations to validate if either assumption occurs in nature. Without assuming a mathematical shape of the DSD, this study analyzes raindrop mass spectra to describe the DSD shape with a mean raindrop diameter and a normalized mass spectrum standard deviation. When mapped into the gamma DSD parameter space, ground-based rainfall observations show that keeping one parameter constant is not observed in nature. And, a mathematical relationship between two DSD parameters does provide an improved rainfall retrieval estimate. And since the two mass spectrum physical attributes are statistically independent, they are well suited for probabilistic rainfall retrieval algorithms as proposed for the GPM satellite program.

Wednesday, Apr 9

2:00pm

Seminar Coordinator: barbara.s.herrli@noaa.gov

SECURITY: If you are coming from outside the NOAA campus, you must stop at the Visitor Center to obtain a vistor badge. Please allow 10 extra minutes for this procedure. If you are a foreign national coming from outside the NOAA campus, please email the seminar coordinator at least 48 hours prior to the seminar to provide information required for security purposes.

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