HY-2卫星微波散射计海面风场地球物理模式函数研究

Development of a geophysical model function for HY-2 satellite microwave scatterometer wind retrievals

地球物理模式函数GMF (Geophysical Model Function)在星载微波散射计海面风场反演中具有关键作用。本文以C波段MetOp/ASCAT散射计风场数据为参考数据,通过改进现有Ku波段NSCAT-4 GMF中后向散射系数对海面风速和风向的依赖关系,以及增加海表温度(SST)因子项,构建了适用于HY-2卫星微波散射计海面风场反演的NSCAT-5.HY-2 GMF。研究结果表明,相比NSCAT-4 GMF,采用NSCAT-5.HY-2 GMF反演得到的HY-2散射计海面风速与ASCAT散射计风速的一致性提高了约11%,特别是风速反演误差不再依赖于SST变化、不再呈现显著经向分布特点。本论文研究结果对进一步提升国内外其他Ku波段星载微波散射计数据的海面风场遥感应用具有重要参考价值。

The Geophysical Model Function(GMF) is indispensable in space-born scatterometer wind retrieval data processing because it is the main factor that dominates the error characteristics of retrieved winds.In this study,a new Ku-band GMF(named NSCAT-5.HY-2),which includes Sea Surface Temperature(SST) dependence,is developed for improved HY-2 satellite scatterometer(HSCAT) wind retrieval.Based on the existing NSCAT-4 GMF,the dependence of σ0 on wind speed and direction will be refined,and that of σ0 on SST is then added.The sea surface winds from the C-band advanced scatterometer(ASCAT) onboard the MetOp-B and C satellites are of high quality and have no dependence on SST.Accordingly,the ASCAT winds are used as reference for developing the NSCAT-5.HY-2 GMF.HY-2C and HY-2D are in non-sun-synchronous orbits with a 66.0° inclination,and their equator crossing times shift each orbit.The HY-2C or HY-2D scatterometer represents a unique opportunity to acquire closely collocated HSCAT and ASCAT scatterometer winds at different times of the day.In this study,the wind vector cells between HSCAT and ASCAT are matched by limiting the spatial distance within km and time differences within 45 min.The close collocations of HSCAT and ASCAT winds allow accurate attribution of the different correlated residual geophysical effects of wind speed bias and distribution,SST,sea state,rain,wind variability,and geographical sampling biases.Here,we explore the opportunity of close collocations of HSCAT-C and ASCAT and develop an SST-dependent Ku-band GMF for HY-2 scatterometers.The wind speed corrections as a function of wind speed can be calculated by using the cumulative distribution function matching technique,which aligns the Probability Density Function(PDF) of HSCAT wind speeds with the referenced PDF.During the wind direction modulations,five terms of the Fourier series are used,and the harmonic coefficients are derived utilizing ASCAT wind direction as reference.The variations of σ0 as a function of SST are given as a polynomial expansion for each given wind speed,and the polynomial coefficients are obtained from the observed and simulated radar cross-sections using closely collocated ASCAT winds of either vertical or horizontal polarizations.The validation of the new HSCAT wind products retrieved using NSCAT-5.HY-2 GMF shows clear improvements over those obtained with NSCAT-4 GMF.The consistency between HSCAT and ASCAT winds is much improved by at least 10%,and wind speed differences no longer depend on SST.