An Active Forward Model for Simulating Satellite Radar Observations

Abstract:

Modern data assimilation frameworks require sophisticated physical and radiative models to guide assimilation and interpretation of satellite-based observations. To date, satellite-based infrared and passive microwave radiances, in various scenarios, are being assimilated operationally at multiple centers around the world (e.g., ECMWF, NOAA/NCEP), however precipitating/cloudy radiances assimilation is still under active development for most observation streams. Additionally, with the advent of space-based precipitation radars (e.g., TRMM, GPM, CloudSAT) and active microwave scatterometers (e.g., QuickSCAT, ASCAT), interest in directly assimilating observed radar reflectivities from satellite-based observations is rapidly increasing.

The present study describes a fully self-contained forward model (forward operator) specifically designed for use in data assimilation applications of active radar observations. The model is a 1D, fully polarized, plane-parallel, adding doubling, model based on the RT4 model (K.F. Evans, 1995). The hydrometeor description is updated to include non-spherical scattering and extinction properties derived from a large database developed by K.-S. Kuo at NASA GSFC. Updated surface properties, over both land and ocean have been rigorously tested. This model also includes an explicit melting-layer model for simulating the melting of non-spherical particles, providing a continuous physical basis throughout the vertical column as snow transitions to rainfall.

Here we present the model description, key elements, and sensitivity to the various inputs and implicit / explicit assumptions within. Examples of forward modeled reflectivities and attenuations, consistent with the GPM DPR instrument will be shown for idealized cases. Plans for implementation within the data assimilation framework at NOAA/NCEP will also be discussed.