Graduate Exam Abstract

Direk Khajonrat

Ph.D. Final

June 23, 2008, 2 pm


Simulation of Space-based Radar Observations

Abstract: The Tropical Rainfall Measurement Mission (TRMM) satellite, carrying a space-based precipitation radar (PR), will end its age in the near future. The Global Precipitation Measurement (GPM) satellite will be the next generation of observation precipitation from space. The GPM will carry a dual-frequency precipitation radar (DPR) operating at 13.6 GHz (Ku-band) and 35.6 GHz (Ka-band), as opposed to a single-frequency 13.8 GHz (Ku-band) PR. The ultimate goal of deploying a dual-frequency radar is that a greater degree of accuracy of precipitation measurements can be achived, by using the two independent measurements from the two channels. The DPR on the GPM will be the first dual-frequency space-based precipitation radar, which operates at high frequencies. A high sensitivity of the Ka-band radar increase a dynamic range of precipitation measurement. However, the Ka-band radar suffers from a degradation of the signal caused by attenuation from precipitation. Since spaceborne precipitation observations have never been done in Ka-band before, an extensive research work on dual-frequency radar including electromagnetic wave propagation characteristics from space and retrieval algorithm are essentially required in system development and instrument performance evaluations. Because the DPR is the first of its kind, a simulation-based study is an approach that can have a direct impact on the GPM system designs, algorithm developments, and system evaluation. This research focuses on developing a methodology to simulate the precipitation characteristics that would be observed from space by DPR using space-based radar observations and earth-based radar measurements. The underlying microphysics of precipitation structures are important in developing a simulation model. Therefore, a realistic microphysical model of precipitation is desired for realistic simulation results. In this research, a microphysical model of precipitation is developed based on airborne radar measurements. Furthermore, in order to perform a cross-frequency simulation, a relationship between specific attenuation and radar reflectivity and variability of radar reflectivity with frequency are developed based on a theoretical computation of scattering characteristics of electromagnetic waves on precipitation particles. The simulation of precipitation observations in Ku- and Ka-band are performed using both TRMM-PR observations and ground-based radar measurements. The simulation was performed using both ground-based radar with and without colorimetric capability. Dual-polarization radar measurements offers an information on phase-height transition of precipitation particles along the vertical profile, which is an important factor in the simulation precess. The simulation of a wide variety of precipitation regimes reveals the characteristics the precipitation observed in Ku- and Ka-band, and allow a testing of different retrieval algorithms---either single-frequency (TRMM-like algorithm) or dual-frequency techniques. A significant degradation of signal in Ka-band channel in intense precipitation such as an intense convective storm and tropical storms, directly affect the retrieval algorithms that can be used.

Adviser: Dr. Chandrasekar V. Chandra
Co-Adviser: NA
Non-ECE Member: Paul W. Mielke
Member 3: Anura P. Jayasumana
Addional Members: NA


Program of Study: