Microwave radiative transfer in the mixed-phase regions of tropical rainfall

Abstract

Current physically-based Radiative Transfer Model (RTM) algorithms for estimating oceanic rain use a very simplified hydrometeor profile that ignores the mixed-phase regions (Wilheit et al., 1977). However, to estimate hydrometeor profiles more reasonably in the tropical precipitation regions, understanding of the brightness temperature (Tb) variations in the mixed-phase regions is essential. Further, establishing physical assumptions for microwave radiative transfer in the mixed-phase regions is necessary for quantifying and minimizing the uncertainties in the rainfall retrieval. Consequently, the objective of this study was to quantify uncertainties and to achieve a solid basis for improvement of the current rainfall retrieval, which is based on an RTM. To accomplish this, we examined data taken by the Convair-580 aircraft during the KWAJEX (Kwajalein Experiment). In order to calculate radiative transfer, the AMMR (Airborne Multi-channel Microwave Radiometer) data were combined with radiosonde data and aircraft microphysics data. Analyses were performed for the stratiform and convective rainfall regions respectively. In stratiform precipitation with a bright band, the main concern was to examine the needed physical assumptions for describing the abrupt change of Tb just below the freezing level. In the case of convective precipitation, the focus was to investigate the effective additional rain layer thickness corresponding to the super-cooled water layer above the freezing level. From the results, it was required to assume the bright-band (around 1/2 km below the FL) as having 2 times the absorption as the rain below to explain the Tb variation due to the phase change of hydrometeors. On the other hand, in the case of convective rainfall, it was suggested that approximately from 1/4km to 3km of super-cooled layer thickness should be considered to describe the additional hydrometeor layer due to the strong updrafts in the convective regions.