14th Conference on Satellite Meteorology and Oceanography


Variation of Oceanic Rain Rate Parameters from SSM/I: Mode of Brightness Temperature Histogram

Roongroj Chokngamwong, George Mason University, Fairfax, VA; and L. S. Chiu

We examined variations of oceanic rain rate parameters derived from microwave brightness temperature (Tb) collected by the Special Sensor Microwave Imager (SSM/I) on board the Defense Meteorological Satellite Program (DMSP) satellites using the technique developed by Wilheit, Chang and Chiu (1991).  Monthly oceanic rain rates are estimated from the monthly histogram by fitting it to a mixed lognormal rain rate distribution via a rain rate Tb relation derived from radiative transfer calculations based on a model.  To minimize the effect of water vapor on the rain signal, a combination channel of twice the vertical polarization at 19 GHz minus that at 22 GHz is used.  The mode of the Tb histogram, T0, is interpreted as the background Tb in the absence of rain. This paper examines the spatial and temporal variations of T0 using 17 years (1998-2004) of data. Analyses of global averages T0 show distinct changes in the characteristics of the time series with respect to different DMSP satellites.  The time series of global average T0 also shows a slight linear decreasing trend (~0.8K over 17 years) at the 95% level of significance. An Empirical Orthogonal Function (EOF) analysis is performed on the non-seasonal T0. The first EOF pattern has the same sign everywhere and the associated time series is similar to that of the global average. The second EOF pattern shows a pattern similar to the ENSO response in precipitation, and the associated time series is correlated to an Southern Oscillation index at -0.66.  Empirical Mode Decomposition (EMD) analyses of the zonal averages show decreasing trends of about 2K and 1K between 40N-50N and between 30S-40S, respectively.  These results are interpreted in terms of the trends of oceanic precipitation. 

extended abstract  Extended Abstract (760K)

Poster Session 2, Climatology and Long-Term Satellite Studies
Tuesday, 31 January 2006, 9:45 AM-9:45 AM, Exhibit Hall A2

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