风云三号C星微波湿温探测仪的定标和验证

风云三号C星(FY-3C)已经于2013年9月23日发射升空,其上装载的微波湿温探测仪(MWHTS)已于9月30日开机正常工作.MWHTS具有对大气温度和湿度垂直分布进行同步探测的能力.MWHTS为跨轨扫描式微波辐射计,在89~191GHz毫米波段内设置了十五个探测通道,其中包括118.75GHz氧气吸收线附近的8个大气温度探测通道,183.31GHz水汽吸收线附近的5个大气湿度探测通道,以及89GHz和150GHz两个窗区通道.设置在118.75GHz的一组毫米波探测通道是国际上业务卫星首次使用的大气探测通道,这组通道和183.31GHz通道对大气进行联合探测,将获得更加精细的大气温湿度垂直分布数据,为数值预报和气候研究提供丰富信息.为保证MWHTS观测资料的定量应用,对仪器性能和定标精度进行了在轨测试.利用MWHTS在轨正常工作后的三个月数据,对仪器在轨定标的基础数据:冷空和黑体计数值,黑体和仪器温度进行监测分析和质量检验,经过质量检验的在轨定标基础数据,结合发射前真空试验得到的非线性订正项在轨定标生成MWHTS观测亮温数据.评估MWHTS在轨辐射定标结果的精度和偏差特性使用了三种方法:1通过场地定标试验获取大气温湿廓线和地面温度等大气参数信息,结合微波逐线正演辐射传输模式MonoRTM(Monochromatic Radiative Transfer Model)模拟MWHTS的上行微波辐射亮温,与MWHTS实际观测结果进行对比分析;2两个通道特性一致的同类星载被动微波载荷同时观测同一目标,观测亮温的差异主要取决于两个载荷的定标系统偏差.选取美国SNPP上搭载的微波探测仪器ATMS作为MWHTS的参考载荷,基于SNO(simultaneous nadir overpass)技术,对两个仪器的观测亮温进行交叉比对,观测亮温时空匹配及均匀性检验的条件为:观测时间差异小于20min,观测像元中心距离小于3km,观测角度在星下点附近差异小于5°,观测像元周围3×3像元内的亮温标准差小于1K;3基于美国国家环境预测中心的全球数据同化系统GDAS(Global Data Assimilation System)数据,利用快速辐射传输模式CRTM(Community Radiative Transfer Model)对MWHTS各通道亮温进行正演模拟,模拟结果(O)和仪器实际观测的亮温(B)之间的差异记为"O-B",对偏差值"O-B"进行统计特征分析.仪器中心频率的变化、正演模式模拟精度和模式输入廓线自身的误差都会对"O-B"产生影响.但是对于首次使用的探测频点而言(如118.75GHz通道),由于国际上没有同类载荷可以进行交叉比对,借助于正演辐射传输模式计算得到"O-B"偏差的分析结果可以在一定程度上反映仪器整体定标情况.外场地定标试验结果显示除通道14外,其他14个通道的亮温差都在1.3K以内;与同类载荷ATMS的在轨观测进行直接交叉比对表明通道14与ATMS的亮温偏差最大,但中心频点一致的5个水汽探测通道的标准差都小于1K;将MWHTS观测结果和正演辐射传输模式模拟结果即"O-B"进行偏差分析显示,靠近118.75GHz吸收线中心的通道2—6"O-B"标准差小于0.5K,其他通道"O-B"标准差和ATMS相应通道的结果相当;MWHTS观测和模拟偏差随角度变化的研究表明通道1,7~13和15观测结果对角度有一定依赖性.

Calibration and validation of microwave humidity and temperature sounder onboard FY-3C satellite

The Fengyun (FY)-3C satellites was successfully launched on 23 December 2013 and carries the Microwave humidity and temperature sounder on board which started its measurements since December 30. MWHTS observe the vertical distribution of atmospheric temperature and moisture. MWHTS is a cross-track scanning instrument which observing in 15 channels at frequencies ranging from 89 to 191 GHz. Eight temperature sounding channels have center frequency at 118.75 GHz oxygen absorption line, five humidity sounding channels have center frequency at 183.31 GHz water vapor absorption line and two window channels centered at 89 and 150 GHz.118 GHz channel is first used to detect atmosphere on current operational satellite.118GHz and 183 GHz channels can obtain fine vertical distribution structure of atmosphere humidity and temperatures. These data will be used in data assimilation and climate research. Before MWHTS observationsquantitative application, the on-orbit test was carried out.The MWHTS post-launch instrument performance was conducted with on-orbit data during a period of three months. The main parameters monitored include the radiometric counts from the cold space and warm target, the warm target temperature and instrument temperature. These calibration data through quality control were converted to brightness temperature use non-linear correction and correction of antenna spill-over effects. There are three methods to validate MWHTS measurements: 1.Use the atmospheric humidity and temperature profiles and surface temperatures which were observed by site calibration test in the radiance transfer model Mono RTM(Monochromatic Radiative Transfer Model) to simulating the MWHTS radiation. The difference between simulations and measurements are assessed. 2.When the same kind sensors observe the same target at the same time, the observed brightness temperature difference should be the small and constant bias. The ATMS on a SNPP satellite was used to be reference sensor. The MWHTS calibration results are cross compared with ATMS by the SNO. The time gap for matched data is less than 20 minutes and the ground distance is less than 3 km. The scan angle difference is less than 5 degree around nadir. Spatial subsets are extracted for 3×3 MWHTS pixels for homogeneity checking, the brightness temperature standard deviation is less than the threshold of 1.0 K are qualified. 3.The CRTM(Community Radiative Transfer Model)was used to simulate the brightness temperature at MWHTS channels. The input data for CRTM come from the National Centers for Environmental Prediction (NCEP) Global Data Assimilation System (GDAS). The difference between observations and simulations which to be referred as O-B were analyzed. The statistic characteristics of O-B were also affected by the change of channel frequency, accuracy of radiative transfer model simulation and input profiles. The inter-satellite validation is useless for the first used channels (such as 118.75 GHz channels). The O-B characteristics were partly demonstrating the calibration accuracy of sensor.The results of post launch site calibration show that brightness temperature bias for every channel except 14 is less than 1.3 K. The brightness temperature observed by MWHTS and ATMS were compared to demonstrate the mean bias for channel 14 is the biggest than other channels and the standard deviations for five humidity sounding channels is less than 1 K. Furthermore, the differences between MWHTS observations and the forward radiativetransfer model simulations, referred to as "O-B", suggest that the standard deviations of "O-B" difference for channel 2 to 6 which is near the center of 118.75 GHz oxygen absorption line is less than 0.5 K and that for other channels is similar to that for corresponding ATMS channels. The scan-dependent biases for MWHTS indicate the temperature dependence of scan biases is noticed of channel 1, 7 to 13 and 15.