Combined Radar-Radiometer Rainfall Retrieval for TRMM Using Structure Function-Based Optimization
Abstract
The Tropical Rainfall Measuring Mission (TRMM) satellite, a joint US-Japanese mission to explore tropical rainfall and its effects on the earth's energy budget, general circulation, and climate, represents the first dual deployment of a precipitation radar and passive microwave radiometer on an earth-viewing satellite. While both radiometers and radars have been widely used independently to retrieve rainfall, previous attempts to combine the two types of measurements have mostly maintained independent treatment of the data, as these instruments are designed to measure precipitation based on different physical principles. A method has been developed which objectively combines the measurements within the context of a unified radiative transfer equation (RTE) model, by treating the radar as a radiation source within the RTE model's source function. The retrieval method, referred to as 'tall vector' inversion, uses a physical inversion scheme which initially searches a cloud-radiation database of profiles, produced by a 3-dimensional non-hydrostatic cloud model simulation of a tropical storm, to find first guess hydrometeor profiles whose brightness temperatures (TBs) and reflectivities (Zs) best match the radiometer and radar measurements as determined by a combined normalized error function. The first guess profiles, expressed in terms of structure functions, are modified within a steepest descent optimization scheme, using the structure function coefficients as the optimization control variables, until the error function cannot be reduced. The solution hydrometeor profiles are then subjected to a set of gravity fallout equations to calculate the surface rainfall rates. Precipitation type is also diagnosed from an algorithm using TB threshold tests in conjunction with radar-derived bright band information.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 22, 1997
- Accession Number
- ADA325404
Entities
People
- Michael R. Farrar
Organizations
- Air Force Institute of Technology