Validation of MODIS aerosol retrievals and evaluation of potential cloud contamination in East Asia

MODIS aerosol retrievals onboard Terra/Aqua and ground truth data obtained from AERONET(Aerosol Robtic Network) solar direct radiance measurements are collocated to evaluate the quality of the former in East Asia. AERONET stations in East Asia are separated into two groups according to their locations and the preliminary validation results for each station. The validation results showed that the accuracy of MODIS aerosol retrievals in East Asia is a little worse than that obtained in other regions such as Eastern U.S., Western Europe, Brazil and so on. The primary reason is due to the improper aerosol model used in MODIS aerosol retrieval algorithm, so it is of significance to characterize aerosol properties properly according to long term ground-based remote sensing or other relevant in situ observations in order to improve MODIS retrievals in East Asia. Cloud contamination is proved to be one of large errors, which is demonstrated by the significant relation between MODIS aerosol retrievals versus cloud fraction, as well as notable improvement of linear relation between satellite and ground aerosol data after potential cloud contamination screened. Hence, it is suggested that more stringent clear sky condition be set in use of MODIS aerosol data. It should be pointed out that the improvement might be offset by other error sources in some cases because of complex relation between different errors. Large seasonal variation of surface reflection and uncertainties associated with it result in large intercepts and random error in MODIS aerosol retrievals in northern inland of East Asia. It remains to be a big problem to retrieve aerosols accurately in inland characterized by relatively larger surface reflection than the requirement in MODIS aerosol retrieval algorithm.     

Analysis of satellite imagery using a simple algorithm supports evidence that Trichodesmium supplies a significant new nitrogen load to the GBR lagoon

This work supports previous studies in the Great Barrier Reef lagoon that show the new nitrogen (N) load introduced by Trichodesmium is similar to or greater than that from riverine discharges. However, the current management programs aimed at improving the chronic eutrophic state of the GBR ignore the N load from Trichodesmium. These programs also ignore the evidence that Trichodesmium blooms could promote the bioavailability of heavy metals and be a source of toxins in the ciguatera food chain. Further work is urgently required to better quantify the potential impacts of Trichodesmium and develop management plans to reduce those impacts. A simple algorithm that uses MODIS imagery is developed for not only monitoring the spatial extent of Trichodesmium blooms but also for quantifying the concentration of those blooms. The algorithm is based on the readily available MODIS L2 data. A management plan that includes the harvesting of Trichodesmium is outlined.     

基于EOS/MODIS影像的青海省一次典型沙尘事件遥感监测研究

基于2012年11月至12月上午轨道星Terra共7景MODIS数据，对影响青海省的一次典型沙尘事件进行研究。结果表明，沙尘天气始于2012年11月18日，在12月1日及2日达到高峰，持续到12月18日趋于减弱；沙尘天气影响区域范围较广，主要影响省域西北、东北地区，面积占省域面积的11.26%至43.36%；沙尘天气扩散趋势与路径整体上由西北向东北地区，东部人口密集区沙尘天气出现时间相对西北地区、青南高原地区出现滞后。     

A focus group study of factors that promote and constrain the use of satellite-derived fire products by resource managers in southern Africa

Semi-structured focus group interviews were employed to examine factors that affect the likelihood that resource managers in southern Africa will use information on vegetation fires provided by two satellite-derived products: an active fire product and a burned area product. The two products are updated regularly and aim to deliver the state-of-the-art in the global monitoring of fires from satellite remote-sensing. Both products are derived from data transmitted by the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors carried onboard NASA's Aqua and Terra satellites. The active fire product can be accessed for free via the internet and on media by users working anywhere in the world; the burned area product will be accessible in a similar manner in 2006. The MODIS fire products provide systematic, near-global coverage and are freely available; as such, they give resource managers new opportunities to obtain or supplement information they need to manage vegetation fires effectively. However, the availability of these products does not mean that resource managers will use them, and many other factors are involved. To understand factors that affect whether southern African resource managers will use the two products, two focus groups were held with members of the Southern African Fire Network (SAFNet) in Malawi, Africa, August 2004. Analysis of the group discussions reveals a number of factors that influence whether they will use the products. The qualitative, in depth nature of the group discussions revealed 12 main factors that influence product use; not least the low international internet bandwidths for African countries outside of South Africa. Analysis of the group discussions also suggests how the uptake of MODIS fire products by resource managers in southern Africa might be enhanced by affecting specific changes to how MODIS products are packaged and delivered.     

PM2.5 over North China based on MODIS AOD and effect of meteorological elements during 2003‒2015

• The Taihang Mountains was the boundary between high and low pollution areas. • There were one high value center for PM_2.5 pollution and two low value centers. • In 2004, 2009 and after 2013, PM_2.5 concentration was relatively low. Over the past 40 years, PM_2.5 pollution in North China has become increasingly serious and progressively exposes the densely populated areas to pollutants. However, due to limited ground data, it is challenging to estimate accurate PM_2.5 exposure levels, further making it unfavorable for the prediction and prevention of PM_2.5 pollutions. This paper therefore uses the mixed effect model to estimate daily PM_2.5 concentrations of North China between 2003 and 2015 with ground observation data and MODIS AOD satellite data. The tempo-spatial characteristics of PM_2.5 and the influence of meteorological elements on PM_2.5 is discussed with EOF and canonical correlation analysis respectively. Results show that overall R² is 0.36 and the root mean squared predicted error was 30.1 μg/m³ for the model prediction. Our time series analysis showed that, the Taihang Mountains acted as a boundary between the high and low pollution areas in North China; while the northern part of Henan Province, the southern part of Hebei Province and the western part of Shandong Province were the most polluted areas. Although, in 2004, 2009 and dates after 2013, PM_2.5 concentrations were relatively low. Meteorological/topography conditions, that include high surface humidity of area in the range of 34°‒40°N and 119°‒124°E, relatively low boundary layer heights, and southerly and easterly winds from the east and north area were common factors attributed to haze in the most polluted area. Overall, the spatial distribution of increasingly concentrated PM_2.5 pollution in North China are consistent with the local emission level, unfavorable meteorological conditions and topographic changes.     

丹江口库区陆地植被物候空间格局及其与海拔的响应关系

海拔高度变化对区域温度、降水都起着至关重要的作用,从而会对植被物候特征产生影响。以丹江口库区为研究区,分析库区植被物候随海拔变化特征,该工作的开展对进一步认识库区植物物候空间特征,进而监测库区土地覆盖变化具有重要实践意义。研究采用Savitzky-Golay滤波算法重建库区2001~2012年MODIS 16天最大合成EVI时序影像数据,对重建后的时间序列影像采用动态阈值法提取库区陆地植被关键物候特征信息,并对库区陆地植被物候特征随海拔梯度变化特征进行分析。研究结果表明,丹江口库区陆地植被生长季为4月上旬至10月上旬,南部山区林地生长季最长,而库区中部、东部耕地生长季较短。植被物候特征随海拔梯度变化呈现两个较为明显的区域,低海拔区域植被生长季开始时间(Start of Season,SOS)随海拔升高而提前,生长季结束时间(End of Season,EOS)随海拔升高而推迟,进而导致生长季长度(Length of Season,LOS)随海拔升高而延长。而在海拔较高山区,林地植被物候呈现完全相反变化趋势。受丹江口水库和人类活动的影响,丹江口库区植被分布随海拔变化呈现两个较为明显的区域。     

Sensitivity of modelled gross primary productivity to topographic effects on surface radiation: A case study in the Cotter River Catchment, Australia

Gross primary productivity (GPP) is a critical response variable for many environmental problems, including terrestrial carbon accounting and the calculation of catchment water balances. Various approaches for modelling GPP have been developed and applied at continental and landscapes scales, but little attention has been given to the sensitivity of GPP to the spatial scale of its driving variables. A key driving variable is surface radiation (R_s) which is influenced by both meso-scale factors (latitude, time of year, cloudiness) and the topographic variables of slope, aspect and horizon shading. We compared the sensitivity of modelled GPP to two different sources of surface radiation (R_s): (1) the ANUCLIM method which only captures the meso-scaled factors; and (2) the SRAD method which incorporates the topographic effects GPP was calculated using the radiation use efficiency (RUE) model (Roderick et al., 2001) to discern general patterns of vegetation productivity at a sub-catchment (i.e. sub-water shed) scale. The radiation use efficiency approach uses the normalized difference vegetation index (NDVI) derived from satellite data (MODIS TERRA), along with estimates of solar radiation at the top of the atmosphere (R_o) and canopy (R_s). In this approach, R_o and R_s capture the influence of diffuse irradiance in canopy photosynthesis and vegetation productivity respectively. This research showed that R_s calculated using the SRAD program provides important discrimination of GPP regimes at a sub-catchment scale, as the result of minimum and maximum daily radiation varying between shaded and exposed surfaces. However, mean daily radiation at a whole-of-catchment scale did not differ between the two sources as the differences in the minimum and maximum daily values tend to cancel each other out. Applications of GPP models therefore need to consider whether topographic factors are important and select the appropriate source of R_s values. GPP models should also reflect understanding of radiation use efficiency. However, further research is required especially with respect to the influence of water stress on plant response.     

基于MODIS数据的云南省地表温度反演研究

基于MODIS数据反演云南省地表温度,采用分裂窗算法需要参数较少、易实现。研究表明:云南省地表温度由北向南递增,东部高、西部低,基本符合云南省实际情况。利用气象观测站实测数据进行验证,平均误差为3.16℃,两组数据基本接近,证明基于MODIS数据和分裂窗算法反演云南省地表温度是可行的。     

二类水体短波红外波段大气校正方法的改进

海洋水色遥感技术在海洋生态环境研究中越来越受到重视.大气校正是海洋水色遥感中的一个关键问题,短波红外波段(SWIR)方法是目前在二类水体大气校正中应用较多的一种方法.现有的SWIR方法假设在对数坐标下,大气校正因子在外推波段与基准波段间按线性关系变化.当波段跨度不大时,这一线性近似与辐射传输模型模拟结果相符,而当波段跨度较大时,则存在一定差异.针对这一问题,本文采用分段外推的方法,修正了大气校正因子,对SWIR方法进行了改进.同时,选取2010年10月4日和2012年4月4日两景渤海海域的MODIS遥感图像,先引入浑浊度来分析渤海海域水体清洁程度,进而在区分出的二类水体区域分析改进方法的效果.结果表明,改进后方法的结果比原方法整体偏小,在渤海区域的差异在1%~15%之间.在卫星遥感数据的反演中有必要关注这一问题.     

2003~2014年东北三省气溶胶光学厚度变化分析

利用2003~2014年MODIS-Aqua气溶胶光学厚度（AOD）产品、DMSP卫星夜间灯光时间资料和基本气象资料,分析我国东北三省（辽宁、吉林、黑龙江）大气气溶胶光学厚度年际变化及季节变化的空间分布特征.结果表明,东北三省多年平均AOD空间分布存在由大连、沈阳、长春和哈尔滨等城市构成的一个高值带,呈东北-西南走向,多年平均AOD值为0.4~0.8;东北三省植被覆盖率较高的东部和北部是AOD的低值区,多年平均AOD小于0.3;东北三省AOD季节变化为AOD春季到夏季升高,秋季下降,冬季再次升高.东北三省AOD年际变化特征为大部分低值地区呈减小趋势,但以沈阳、长春和哈尔滨为轴线的东北-西南走向的高值区域呈增大趋势,反映了近10多年出现的空气质量两极分化趋势.此外研究了东北三省年均AOD在强、弱西北太平洋夏季风年时的空间分布差异,受地面风场影响,AOD在强季风年时较弱季风年偏低.