An Analysis Scheme for Determination of True Surface Winds at Sea from Ship Synoptic Wind and Pressure Observations

Abstract

For oceanographie studies, a high resolution description of the wind field at the sea surface is required. In order to estimate the air-sea interaction fluxes correctly, it is important that the derived wind field has the characteristics of actual surface winds. Ship synoptic observations are at irregular positions, while applications require boundary conditions at a regular grid. The problem is solved by locally fitting a second-order pressure surface to both wind and pressure observations, with the aid of a boundary-layer formulation. The results show that fitting wind and pressure data together provides better spatial resolution than using wind or pressure data alone, because more information is available. Wind data provide at least the same accuracy and less bias than pressure data, tested against independent wind observations. Also a stability and wind-dependent boundary-layer formulation results in less bias than a constant one. For the ship synoptic data available via GTS for the North Atlantic Ocean, spatial resolution, as expressed by an equivalent filter half-width, is 220 km, considerably improved compared to typical data assimilation schemes used in numerical forecast models.