Microwave Brightness Temperatures from Tilted Convective Systems

Abstract

Aircraft and ground-based radar data from the Tropical Ocean and Global Atmosphere Coupled Ocean?Atmosphere Response Experiment show that convective systems are not always vertical. Instead, many are tilted from vertical. Satellite passive microwave radiometers observe the atmosphere at an oblique angle. For example, the Special Sensor Microwave Imager on Defense Meteorological Satellite Program satellites and the Tropical Rainfall Measurement Mission (TRMM) Microwave Imager (TMI) on the TRMM satellite view at an incident angle of about 50°. Thus, the brightness temperature measured from one direction of tilt may be different than that viewed from the opposite direction because of the different optical path. This paper presents an investigation of passive microwave brightness temperatures upwelling from tilted convective systems. To account for the effect of tilt, a 3D backward Monte Carlo radiative transfer model has been applied to a simple tilted cloud model and a dynamically evolving cloud model to derive the brightness temperature. The radiative transfer results indicate that brightness temperature varies when the viewing angle changes because of the different optical path. The tilt increases the displacements between the high 19-GHz brightness temperature (Tb19) due to liquid emission from the lower level of cloud and the low 85-GHz brightness temperature (Tb85) due to ice scattering from the upper level of cloud. As the resolution degrades, the difference of brightness temperature due to the change of viewing angle decrease dramatically. The displacement between Tb19 and Tb85, however, remains prominent. The successful launch and operation of the TRMM satellite provide an opportunity to examine tilted convective systems using collocated radar and radiometer data. TMI observations of tilted systems indicate that displacement between Tb19 and Tb85 can be as far as 100 km. Such displacement not only poses a problem to rainfall retrieval algorithms that use only scattering information but also causes large uncertainty in rainfall retrieval from multichannel retrieval algorithms. This study suggests that combined radar and radiometer data are needed to reduce the effect of tilt and to improve surface rainfall retrieval.