Microwave brightness temperatures from tilted convective systems

Aircraft and ground-based radar data from the Tropical Ocean and Global Atm osphere Coupled Ocean-Atmosphere Response Experiment show that convective s ystems are not always vertical. Instead, many are tilted from vertical. Sat ellite passive microwave radiometers observe the atmosphere at an oblique a ngle. For example, the Special Sensor Microwave Imager on Defense Meteorolo gical Satellite Program satellites and the Tropical Rainfall Measurement Mi ssion (TRMM) Microwave Imager (TMI) on the TRMM satellite view at an incide nt angle of about 50 degrees. Thus, the brightness temperature measured fro m one direction of till may be different than that viewed from the opposite direction because of the different optical path. This paper presents an in vestigation of passive microwave brightness temperatures upwelling from lil ted convective systems. To account for the effect of tilt, a 3D backward Monte Carlo radiative tran sfer model has been applied to a simple tilted cloud model and a dynamicall y evolving cloud model to derive the brightness temperature. The radiative transfer results indicate that brightness temperature varies when the viewi ng angle changes because of the different optical path. The tilt increases the displacements between the high 19-GHz brightness temperature (Tb-19) du e to liquid emission from the lower level of cloud and the low 85-GHz brigh tness temperature (Tb-85) due to ice scattering from the upper level of clo ud. As the resolution degrades, the difference of brightness temperature du e to the change of viewing angle decrease dramatically. The displacement be tween Tb-19 and Tb-85, however, remains prominent. The successful launch and operation of the TRMM satellite provide an opport unity to examine tilted convective systems using collocated radar and radio meter data. TMI observations of tilted systems indicate that displacement b etween Tb,, and Tb,, can be as far as 100 km. Such displacement not only po ses a problem to rainfall retrieval algorithms that use only scattering inf ormation but also causes large uncertainty in rainfall retrieval from multi channel retrieval algorithms. This study suggests that combined radar and r adiometer data are needed to reduce the effect of tilt and to improve surfa ce rainfall retrieval.