Precipitation Retrievals for GMI with Consideration of Multi-viewing Directions Based on 3-D and 1-D Radiative Transfer Models

Abstract:

Vertically and horizontally inhomogeneous distributions of hydrometeors are often observed in precipitating clouds. The three dimensional (3D) characteristics can then cause errors in the passive microwave rainfall measurements with the current off-nadir viewing sensors’ specifications. This result is due to the fact that the same surface rainfall could be associated with different amounts of hydrometeors depending on the sensors’ viewing paths. In this study, we confirmed that the plane-parallel radiative treatment to the atmosphere leaves a notable deficiency in the microwave radiometric signatures, especially at the higher frequency channels for different viewing directions when largely inhomogeneous precipitating clouds are accompanied by significant ice particles. The impact of the 3-D effect on the passive microwave rainfall estimations is then examined by synthetic retrievals employing a Bayesian methodology. The results showed that the uncertainty in the rainfall estimations due to the 3D effect depended on the viewing directions considered in the a-priori information. It was also found that taking more viewing angles or the azimuth angles in the a-priori information into consideration tended to moderate the retrieval difference that resulted from the different viewing directions. In addition, the retrieval uncertainty related to the 3D effect appeared to be more significant for heavy rainfall cases with large amounts of ice particles, as expected. We additionally performed the retrieval experiments with the databases constructed with the one dimensional slant path (1D-SP) calculation. In general, the 3D model-based experiments slightly outperformed the 1D-SP model-based experiments. However, the 1D-SP model may be considered as an alternative model for the full 3D radiative transfer model with limited computing resources and a required level of the retrieval accuracy.