A physically based algorithm to remotely sense the rain rate profile and underlying surface wind speed from an airborne radar and radiometer

A new method is developed to retrieve the vertical profile of precipitation and the underlying surface wind speed from an airborne Ku/Ka-band radar and a horizontally polarized X-band radiometer. The dual-frequency radar backscatter measurements are used to determine two parameters of the gamma drop size distribution (DSD) at each range gate. This is accomplished using a non-linear iterative inversion technique with Mie theory. A Hitschfeld-Bordan approach for attenuation correction is used in light rain and the surface reference technique is employed for heavier rain. The parameterized DSD profile is used to determine the extinction and absorption profile from precipitation at 10.7 GHz, which are used, along with estimates of the absorption from clouds and gas, to determine the atmospheric contribution to the measured brightness temperatures (TB). The radiative transfer equation and a model for the ocean surface emissivity are then inverted to estimate the near surface wind speed.; It is found that retrievals in stratiform precipitation require a model to describe the thermodynamic and electromagnetic properties of melting hydrometeors. The melting layer can contribute up to 50% of the total atmospheric absorption, making it a key component for accurate retrievals in stratiform rain. Several melting layer models were evaluated based on their fit to the dual-frequency reflectivity measurements in the melting layer. A candidate model was selected and tuned to match the radar measurements. The difference between the modeled and measured peak reflectivity is -0.16 dB at Ku-band and 0.33 dB at Ka-band. This model was then incorporated in the retrieval algorithm and shown to produce reasonable wind speed retrievals in stratiform rain. A case study is presented of the wind speed and rain rate retrievals. The retrieved wind speeds are uncorrelated with the reflectivity and the average value is within 1 m/s of that retrieved in a clear area adjacent to the rain. The standard deviation of the retrieved wind speed is 2 m/s in stratiform rain and less than 4 m/s in moderate convective rain. Current active and passive wind speed retrievals have uncertainties of +/-5 m/s in light rain and +/-10 m/s in moderate rain.