敦煌地区大气气溶胶对卫星传感器辐射校正的影响

１９９４年６月，利用光谱辐射计在中国西北敦煌西戈壁地区进行了一次大气衰减测量试验。利用测量结果和有关资料，给出了该地区气溶胶光学厚度及其粒子大小谱分布。并将实验结果与我国东部典型地区的气溶胶特性测量结果作了比较，结果表明：敦煌西戈壁地区气溶胶类似于我国东部地区乡村气溶胶模型，并显示出该地区大气中气溶胶大粒子（ｒ≥１．０μｍ）含量占有一定比例。为对即将开展的遥感卫星传感器辐射校正工作作准备，进一步利用模拟计算分析的方法在可见（以０．５５μｍ代表）、中红外（以１０．６μｍ代表）波段，探讨了气溶胶粒子折射指数虚部、粒径大小对气溶胶消光和吸收作用的影响，得到了气溶胶吸收在一般情况下与折射指数虚部呈线性关系及在可见波段对气溶胶消光有影响作用的折射指数应部的变化范围很小的结果。在所给粒径范围（０．０１μｍ≤ｒ≤１０．０μｍ）内，可见波段气溶胶吸收对粒子半径（ｒ）的依赖性较大；气溶胶散射在０．０１μｍ≤ｒ≤０．１μｍ范围内对ｒ的依赖性几乎为线性关系，随ｒ的进一步增大，气溶胶散射和吸收趋于常数。     

Comparison and evaluation of dust detection algorithms using MODIS Aqua/Terra Level 1B data and MODIS/OMI dust products in the Middle East

Dust storms have a major impact on air quality, economic loss, and human health over large regions of the Middle East. Because of the broad extent of dust storms and also political–security issues in this region, satellite data are an important source of dust detection and mapping. The aim of this study was to compare and evaluate the performance of five main dust detection algorithms, including Ackerman, Miller, normalized difference dust index (NDDI), Roskovensky and Liou, and thermal-infrared dust index (TDI), using MODIS Level 1B and also MODIS Deep Blue AOD and OMI AI products in two dust events originating from Iraq and Saudi Arabia. Overall, results showed that the performance of the algorithms varied from event to event and it was not possible to use the published dust/no-dust thresholds for the algorithms tested in the study area. The MODIS AOD and OMI AI products were very effective for initial dust detection and the AOD and AI images correlated highly with the dust images at provincial scale (p-value <0.001), but the application of these products was limited at local scale due to their poor spatial resolution. Results also indicated that algorithms based on MODIS thermal infrared (TIR) bands or a combination of TIR and reflectance bands were better indicators of dust than reflectance-based ones. Among the TIR- based algorithms, TDI performed the best over water surfaces and dust sources, and accounted for approximately 93% and 90% of variations in the AOD and OMI AI data.     

Comparison of SeaWiFS,MODIS‐Terra and MODIS‐Aqua in the Southern Ocean

The surface chlorophyll‐a concentrations measured by SeaWiFS, MODIS‐Terra and MODIS‐Aqua are compared in the Southern Ocean in summer 2003. The radiometers generally agree within their estimated accuracy. Residual discrepancies could be reduced by regional calibrations of the bio‐optical algorithms.     

On the calibration and use of in situ ocean colour measurements for monitoring algal blooms

Simultaneous in situ measurements of chlorophyll concentration and water colour are reported at three diverse sites: a sea loch, the west Scottish shelf and the north Atlantic. A good (R² =0.97) log-log relation exists between the ratio, gamma, of the irradiance reflection coefficients at 490 and 570 nm and the chlorophyll concentration, C, over a range of chlorophyll concentrations from 0.01 to nearly 50 mg m ^-3     

Validation of collection of 6 MODIS/Terra and MODIS/Aqua gross primary production in an alpine meadow of the Northern Tibetan Plateau

Moderate Resolution Imaging Spectroradiometer (MODIS) continuously monitors gross primary production (GPP), which is an extremely important component of carbon cycling, at the global scale. Uncertainties about MODIS GPP limit our ability to accurately quantify GPP at the regional scales. The Collection 6 MODIS/Terra and MODIS/Aqua GPP products (i.e. MOD17A2H and MYD17A2H) were compared with the estimated GPP (GPP_EC) by eddy covariance measurements in an alpine meadow in the Northern Tibetan Plateau during three consecutive growing seasons of 2005–2007. The Collection 6 MODIS/Terra and MODIS/Aqua fractional photosynthetically active radiation (FPAR) products (i.e. MOD15A2H and MYD15A2H) were also validated. The MOD17A2H and MYD17A2H products tended to overestimate GPP_EC by 2.17% and 7.35% in 2005–2007, respectively, although these differences were not significant. The MOD15A2H and MYD15A2H products also tended to overestimate ground-based FPAR (FPAR_G) by 20.31% and 24.73% in 2005–2007, respectively. The overestimation of FPAR resulted in about 17.51–23.97% overestimation of GPP_EC. The default maximum light-use efficiency (ε_max) of 0.86 g C MJ^?1 only underestimated the ground-based ε_max (0.88 g C MJ^?1) by 2.27%, which in turn resulted in about 2.13–2.72% underestimation of GPP_EC. The meteorology data errors only caused about 0.48–1.06% underestimation of GPP_EC. Therefore, although MODIS Collection 6 GPP had a very high accuracy, the input parameters had relative greater errors in the alpine meadow of the Northern Tibetan Plateau. The differences between MODIS GPP and GPP_EC mainly resulted from FPAR, followed by ε_max and meteorological data.     

风云三号微波湿度计航空校飞辐射定标原理及数据分析

风云三号微波湿度计是我国首次在轨定量应用于大气探测的星载被动微波辐射计,为验证载荷辐射性能,发射前于2007年9月至11月开展了航空校飞试验,历时2个月共转场青海湖、敦煌和云南普洱3个试验场,地面同步实施了辐射校正测量。在阐述星载微波大气探测载荷航空校飞辐射定标原理基础上,开展航空校飞数据分析,并与同步辐射校正测量结果进行了比对验证,结果表明风云三号微波湿度计初样产品探测结果符合设计指标要求。风云三号微波湿度计航空校飞试验为其资料在轨业务定量应用奠定了基础,其中航空校飞辐射定标模型支撑了后续风云三号卫星微波湿度计在轨业务定标。     

风云三号微波湿度计航空校飞辐射定标原理及数据分析

风云三号微波湿度计是我国首次在轨定量应用于大气探测的星载被动微波辐射计，为验证载荷辐射性能，发射前于2007年9月至11月开展了航空校飞试验，历时2个月共转场青海湖、敦煌和云南普洱3个试验场，地面同步实施了辐射校正测量。在阐述星载微波大气探测载荷航空校飞辐射定标原理基础上，开展航空校飞数据分析，并与同步辐射校正测量结果进行了比对验证，结果表明风云三号微波湿度计初样产品探测结果符合设计指标要求。风云三号微波湿度计航空校飞试验为其资料在轨业务定量应用奠定了基础，其中航空校飞辐射定标模型支撑了后续风云三号卫星微波湿度计在轨业务定标。     

Accuracy assessment of the MODIS 16-day albedo product for snow: comparisons with Greenland in situ measurements

The accuracy of the Moderate Resolution Imaging Spectroradiometer (MODIS) 16-day albedo product (MOD43) is assessed using ground-based albedo observations from automatic weather stations (AWS) over spatially homogeneous snow and semihomogeneous ice-covered surfaces on the Greenland ice sheet. Data from 16 AWS locations, spanning the years 2000-2003, were used for this assessment. In situ reflected shortwave data were corrected for a systematic positive spectral sensitivity bias of between 0.01 and 0.09 on a site-by-site basis using precise optical black radiometer data. Results indicate that the MOD43 albedo product retrieves snow albedo with an average root mean square error (RMSE) of ±0.07 as compared to the station measurements, which have ±0.035 RMSE uncertainty. If we eliminate all satellite retrievals that rely on the backup algorithm and consider only the highest quality results from the primary bidirectional reflectance distribution function (BRDF) algorithm, the MODIS albedo RMSE is ±0.04, slightly larger than the in situ measurement uncertainty. There is general agreement between MODIS and in situ observations for albedo <0.7, while near the upper limit, a −0.05 MODIS albedo bias is evident from the scatter of the 16-site composite.     

高分辨率SAR辐射定标中两种角反射器响应测量方法的应用对比研究

利用角反射器进行SAR图像辐射定标是一种简单、实用、精度较高的绝对定标方法。其关键技术包括角反射器响应测量、天线方向图拟合和定标常数确定。针对机载分米级高分SAR影像定标问题,在同一场景中布设了30和10cm两组三面角反射器,依据雷达中心入射角估计值、均方根误差和变异系数等指标,分析了已有的两种角反射器响应测量方法,即峰值法和积分法的适用性,并提出了一种新的天线方向图拟合方法。最后给出了两种方法对高分辨率SAR辐射定标的适用性结论。     

A new area-specific bio-optical algorithm for the Bay of Biscay and assessment of its potential for SeaWiFS and MODIS/Aqua data merging

Based on a feed-forward and error-back-propagated neural network (NN), a new bio-optical algorithm is developed for the Bay of Biscay. It is designed as a set of NNs individually dedicated to the retrieval of the phytoplankton chlorophyll (chl), and total suspended matter (tsm) from Sea-viewing Wide Field-of-View Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua data. The retrieved versus in situ measured concentrations of chl and tsm correlation coefficients for chl proved to be ～0.8 (SeaWiFS) and 0.72 (MODIS), and for tsm 0.71 (SeaWiFS) and 0.74 (MODIS). The developed NN-based bio-optical algorithms are employed to assess the compatibility of SeaWiFS and MODIS data on chl and tsm in the coastal zone of the Bay of Biscay (case 2 waters). The value of the ratio between the concentration of chl and tsm derived from the same-day SeaWiFS and MODIS data (the overflight time difference, Δt is ≤2.5 hours) has in most cases values of approximately 1, however, in specific cases it varies appreciably. These results indicate that, unlike the reportedly very successful cases of merging of SeaWiFS and MODIS data on chl in open ocean waters (case 1 waters), the merging of chl (and tsm) data from these sensors collected over case 2 waters needs to be supervised at a level of a few pixels. At the same time, when averaged over the entire coastal zone of the Bay of Biscay, the retrieved monthly mean chl and tsm concentrations from SeaWiFS and MODIS practically coincide throughout the years (2002–2004) of contemporaneous operation of these two satellite sensors. Thus, even in the case of such dynamic and optically complex case 2 waters that are inherent in the Bay of Biscay, the potentials for ocean colour data merging are very good. The merging efficiency is assessed and illustrated via documenting the spatio-temporal dynamics of bottom sediment re-suspension in the bay occurring in winter – the season of heaviest cloudiness over the bay.