GPM dual-frequency ratio characteristics in the melting layer

ABSTRACT

The Global Precipitation Measurement (GPM) mission is centered on the deployment of a core observatory satellite with an active dual-frequency precipitation radar (DPR), operating at Ku (13.6 GHz) and Ka (35.6 GHz) band. The DPR is expected to improve our knowledge of precipitation processes on microphysics. DPR offers dual-frequency observations along the vertical profile which allow us to investigate the microphysical properties using the difference between the reflectivity at two frequency channels ( measured dual frequency ratio or DFRm ). It was shown in [1], that DFRm profile is rich in information and can be used to produce precipitation type classification and hydrometeor identification.

There are two main aspects that control the shape of DFRm profiles: one is the non-Rayleigh scattering effect; another is path integrated attenuation difference between two frequency channels. In the ice region, DFRm is mainly caused by non-Rayleigh scattering effect while in the rain, it is dominated by path integrated attenuation difference between Ka and Ku band. However, in the melting region, both non-Rayleigh scattering effects and attenuation difference play a role. DFRm profile within the melting region has the trend to increase with decrease in height till it reaches a peak value, in the early melting stage, then it starts to decrease with decrease in height in the late melting stage when it reaches a local minimum value.

The DFRm profiles have different features for different microphysical composition along the vertical profiles. Some of these features could be the DFRm maximum value; the DFRm local minimum value; the slope of the DFRm between the maximum and minimum points. This paper is focused on characterizing these features towards potential implication for microphysical retrievals.

In order to verify that different microphysics have different features on DFRm profile, we study these features mentioned above using both theoretical models and APR2 (airborne precipitation radar generation 2) radar data of NAMMA (NASA African Monsoon Multidisciplinary Analysis) and GRIP (Genesis and Rapid Intensification Processes) experiment. The theoretical model and airborne radar data are used to drive classification of dual frequency radar observation of precipitation, such as convective and stratiform profiles, so that the subsequent retrievals can benefit from such a scheme.