Atmospheric Wind Retrievals from Satellite Soundings over the Middle- and High-Latitude Oceans

Abstract

A technique that uses satellite-based surface wind and temperature soundings for deriving three-dimensional atmospheric wind fields is developed for climate studies over the middle- and high-latitude oceans. In this technique, the thermal wind derived from the satellite soundings is added to the surface wind to obtain a first-guess, nonmass-conserved atmospheric wind profile. Then a Lagrange multiplier in a variational formalism is used to force the first-guess wind to conserve mass. Two mass conservation schemes are proposed. One is to use the meridional mass transport conservation equation as a constraint to derive the meridional wind first, and then the vertically integrated mass conservation equation is used to infer the zonal wind. The zonal and meridional winds are obtained separately in this approach. The second scheme is to use the vertically integrated mass conservation equation as a constraint to retrieve the zonal and meridional winds simultaneously from the first-guess field. Temperature soundings from the Television and Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder Path A dataset and a Special Sensor Microwave Imager (SSM/I) satellite-based surface wind field are used to derive the wind fields. The two mass conservation schemes yield two different wind fields. They are compared with the European Centre for Medium-Range Weather Forecasts (ECMWF) and National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses and radiosonde observations over the Southern Ocean. The general circulation structure of both wind fields is similar to the reanalysis winds. However, the annual-mean bias of the first method is small in both the zonal and meridional winds compared to radiosonde observations, while the zonal wind bias of the second method is as large as −4 m s−1. The main reason for the difference is that the second method requires that the Lagrange multiplier be zero on the latitudinal boundaries. This forces the retrieved zonal wind to approach the first-guess zonal wind. In contrast, the first method does not require latitudinal boundary conditions, allowing a larger correction to the first-guess zonal wind.