Towards Improvement of Hoaps: New Estimates of Evaporation and Precipitation from Trmm

This note describes new developments of retrievals of basic state variables at the air-sea interface as well as rainfall, both essential for the computation of turbulent heat fluxes and the water budget at the sea surface. The Visible Infrared Scanner (VIRS) and the Tropical Rainfall Measuring Mission (TRMM) microwave imager (TMI) are used to retrieve sea surface temperature, near-surface air specific humidity, and wind speed. All three variables are used within a bulk parameterization scheme to compute latent heat fluxes employing some additional empirical assumptions. Results of comparisons to buoy measurements led to several adjustments of the retrieval schemes. The application of the retrievals show that the sampling is sufficient to compute monthly averages. A comparison to HOAPS data showed substantially differences between the data sets primarily in the derived wind speed. For precipitation a new computationally efficient algorithm making use of TMI and the Precipitation Radar (PR) observations has been developed. The technique consists of a standard TMI algorithm that is compared to PR estimates of near-surface rainfall at TMI s spatial resolution and a reference altitude. The difference between TMI and PR estimates appear to be rather systematic which may be translated into a globally constant calibration thus easy to transfer to other passive microwave radiometers on other satellites.