Variational Inversion of the SSM/I Observations during the ASTEX Campaign

Abstract

Given the prospect of direct assimilation of satellite microwave radiances within numerical weather prediction (NWP) schemes, this study proposes to test a nonlinear inversion for retrieving geophysical parameters from Special Sensor Microwave/Imager (SSM/I) measurements using NWP forecasts as a priori information. The inversion method relies upon an accurate analysis of the radiative transfer and simultaneously retrieves the surface wind speed, the integrated water vapor, and the cloud liquid water. The method is tested in the framework of the well-documented Atlantic Stratocumulus Transition Experiment campaign. The validation approach involves radiosondes from islands, ships, and drifting buoys, cloud liquid water measurements from instruments onboard the U.K. Meteorological Office C-130 aircraft, and the International Satellite Cloud Climatology Project cloud liquid water estimates. The retrieved geophysical parameters are in satisfactory agreement with the available in situ measurements, and the inversion process is shown to be reliable even in the detection of low liquid water contents. The SSM/I inversion scheme is also a strategy for the validation of radiative transfer models, the discrepancy between the simulated and the observed radiances at the end of the inversion process being a powerful tool for testing the radiative transfer in use.