Research on the method of constructing sea surface blended wind product considering vorticity and divergence variation

Abstract

The sea surface vector winds drive and modulate the heat and momentum transfer through the oceanatmosphere interface, influence the thermal and dynamical properties of ocean water body, and consequently determine the water cycle, ocean circulation and climate change on global scale. The spatial variation of sea surface winds is an important driving force for ocean dynamics. The vorticity of sea surface winds drives the ocean circulation through Sverdrup transport on basin scale, the divergence of surface winds efficiently affects the Ekman pumping, and thus the physical and biological processes over the global oceans.With the increasing sampling frequencies of satellite-borne scatterometer, it is vitally important to construct a long-time series of sea surface blended wind product for climate studies and numerical modeling studies as the forcing field. Previous studies have shown that wind fields with different spatial and temporal resolution have an apparent influence on numerical model results. Using the scatterometer winds and operational numerical model winds, this thesis aims to find an optimal wind blending method to merge winds from different sources, while effectively to retain the vorticity and divergence information in the blended wind products. The main findings of the thesis are listed as follows:Based on the 2-dimensional varational method (2DVAR) for wind blending, regularized terms for wind vorticity and divergence were introduced in the methodology to overcome the over-fitting problems associated with the 2DVAR method, and to retain the fine structure of scatterometer observed wind vorticity and divergence.Using the insitu wind observations in South China Sea, the QuikSCAT scatterometer winds and the global operational numerical prediction model winds of National Oceanic and Atmospheric Administratiton (NOAA) of the United States were evaluated. The analysis indicates that the accuracy and spatial/temporal variation of the sea surface winds are both reasonably good.Sensitivity studies of 2DVAR method with and without regularization of wind vorticity and divergence were performed for a typhoon case in 2008. The sensitivity studies show that 2DVAR with regularization effectively overcomes the over-fitting problems with smoother blended wind field. The spurious structure of false vorticity and divergence was effectively removed.The blended wind products with and without regularization terms in 2DVAR method were then evaluated using wind observations from an automatic meteorological observatorythan that produced from simple 2DVAR method, in terms of small bias and small root mean square errors (RMSE).As a final remark, the method described in this study can in South China Sea. The blended wind with regularization is more accurate in comparison with the insitu observation significantly reduce the noise level of scatterometer winds and keep the necessary information of wind vorticity and divergence. It is expected that the 2DVAR wind blending method with regularization terms can be implemented in the operational blended sea surfaced wind products in the future.