风云三号卫星微波湿度计的扫描角偏差订正

首先统计分析了FY-3A卫星MWHS(Micro Wave Humidity Sounder,微波湿度计)2010年1月整月和8月28日—9月6日Level-1b全球观测亮度温度T_O和背景场(NCEP GFS 6 h预报场)用辐射传输模式(美国通用辐射传输模式CRTM 2.0版本)模拟的亮度温度T_B随扫描角的分布特征,发现通道3和4的观测随仪器扫描角有抖动、不连续现象。同时沿着仪器扫描线在星下点两测存在观测不对称现象,而且权重函数峰值越接近地面的通道该不对称现象越明显。在统计观测增量T_O-T_B随扫描角和纬度变化的基础上,定量给出了不同纬度带内(每隔5个纬度)MWHS通道3、4和5的扫描角偏差订正系数,该系数可直接提供给各种资料同化系统同化FY-3A MWHS资料时使用。

Scan correction scheme of FY-3A microwave atmospheric humidity sounder radiance for data assimilation

Meteorological satellites have become an irreplaceable weather and ocean observing tool in China.Recently,much progress has been made in direct assimilation of satellite radiance measurements in numerical weather prediction(NWP).The Microwave Atmospheric Humidity Sounder(MWHS),carried onboard the Chinese new generation of polar-orbiting weather satellites,the Feng-Yun(FY-3) series,provides passive measurements of the radiation emitted from Earth''s surface and throughout the atmosphere.MWHS is similar to AMSU-B and the Microwave Humidity Sounder of NOAA,with a primary purpose of moisture sounding in cloudy regions,designed to obtain information on atmospheric humidity profiles,water vapor,rainfall,cloud liquid water,and so on.Several operational NWP centers currently rely on a variational analysis system to define the initial state for their NWP models.From the point of view of variational assimilation,errors from observations and models should have normal and unbiased distributions.However,statistical errors of radiance between observation and prediction from the NWP model are usually not random but systemic.MWHS is a cross-track scanning microwave radiometer.A feature of a cross-track sounder is that the measurements vary with scan angle because of the change in the optical path length between Earth and the satellite.This feature is called the limb effect.Cross-track scan bias correction is a key part of an NWP assimilation system.In order to use radiances from MWHS,biases between the observed radiances and those simulated from the model first guess must be corrected.The bias of global FY-3A MWHS observed brightness temperature(T_O) and simulated brightness temperature(T_B) by the Community Radiative Transfer Model(CRTM) based on 6 h forecast fields of the NCEP GFS from 28 August to 6 September 2010 are calculated in this study.The statistical features of observation increment(T_O minus T_B) in channels 3,4 and 5 of MWHS are analyzed.The scan angle bias of channels 3-5 varying along scan position reveal that the scan angle bias generally increases with the scan position far away from the nadir,but does not increase in a strictly progressive manner.Large positive scan biases are present at large scan angles at all latitudes for channels 3 and 4 on both sides of the scan lines.Relatively large negative scan biases for channel 5 are seen at most scan angles and are highly asymmetric on both sides of nadir.The closer to the surface the peak height of the channel weighting functions,the more serious the asymmetry.All of this further verifies the importance of scan angle bias correction.Analysis of radiance data shows a significant residual scan bias that depends strongly on latitude for these channels.The scheme applies a latitudinally dependent scan correction to take this into account.In this study,the correction coefficients are calculated within every 5° latitudinal band for each scan position.Moreover,nadir biases are subtracted.These biases will be subtracted from T_O minus-values in the formulation of data assimilation.Therefore,a constant scan bias for each channel needs be removed before MWHS radiance assimilation.The statistical distribution of brightness temperature bias(the departures of T_O minus T_B) after correction locates mostly in the vicinity of zero,with a more Gaussian distribution.The bias and standard deviations of T_O minus T_B differences between observations and model simulations are reduced after bias correction.The scan-angle and latitudinal-dependence of the observation increment for the MWHS channels 3 to 5 within 5° latitudinal bands are given quantitatively.This scan-bias correction coefficient can be used directly for MWHS data assimilation.This work can be extended to the FY-3 MWHS series.The methodology will be incorporated into the Global/Regional Assimilation and Prediction System,operational NWP data assimilation systems in China,or other NWP modeling systems.