Comparison of Three Years of Terra and Aqua MODIS Aerosol Optical Thickness Over the Global Oceans

Far from land-based sources, long-term statistics of aerosol optical thickness (AOT) over the oceans is expected to have a negligible diurnal signal. Therefore, comparing large-scale regional and global mean AOT derived independently from Terra and Aqua Moderate Resolution Imaging Spectroradiometer data will provide a measure of the precision of the overall product. Three years of overlapping records of quality and pixel-weighted level-3 statistics were analyzed. These data show that the instruments' measurement of the long-term global annual mean agree to within 1% or 0.0014-in optical thickness with an rms difference of 0.004, which is calculated from global monthly means. Two-thirds of the regions have long-term annual regional means that agree to within 2.5%. The highest regional difference is 5%, which is found over the southern circumpolar ocean. Other aerosol parameters such as particle size will show larger differences. Datasets that are not quality and pixel weighted may also show larger differences     

江西千烟洲气溶胶光学厚度的反演与分析

2005年10-11月份，中国科学院遥感应用研究所联合北京师范大学遥感中心等几家单位在江西千烟洲地区进行了为期30天的遥感实验，在实验期间，采用法国CIMEL公司研制的太阳分光光度计CE318进行了连续测量，得到了大量的大气光学数据。利用这些数据，反演了气溶胶光学厚度和相应的Angstrom参数，分析了光学厚度和Angstrom参数的日变化和逐日变化，并讨论了这些变化的原因。本文还讨论了在反演气溶胶光学厚度过程中可能的误差源。     

Study on Regional Variations of Aerosol Loading Using Long Term Satellite Data Over Indian Region

Satellite-based measurements of aerosols are one of the most effective ways to understand the role of aerosols in climate in terms of spatial and temporal variability. In the present study, we attempted to analyse spatial and temporal variations of satellite derived aerosol optical depth (AOD) over Indian region using moderate resolution imaging spectrometer over a period of 2001–2011. Due to its vast spatial extent, Indian region and adjacent oceanic regions are divided into different zones for analysis. The land mass is sub divided into five different zones such as Indo Gangetic Plain (IGP), Indian mainland, North Eastern India (NE), South India-1 (SI-1), South India-2 (SI-2). Oceanic areas are divided into Arabian Sea and Bay of Bengal. Arabian Sea is further divided as three zones viz. Northern AS (NAS), Central AS (CAS) and Eastern AS (EAS) zones. Bay of Bengal is divided as North BoB (NBoB), West BoB (WBoB), Central BoB (CBoB), and East BoB (EBoB). The study revealed that among all the land regions, IGP showed the highest peak AOD value (0.52 ± 0.17) while SI-2 showed the lower values of AOD in all the months compared to all India average. The maximum AOD is observed during premonsoon season for all regions. During the winter, average AOD levels were substantially lower than the summer averages. Peak of aerosol loading (0.35 ± 0.159) is observed in March over NE region, whereas in all other regions, peak is observed during May. Frequency distribution of long term AOD (<0.2, 0.3–0.5, >0.5) shows a shift of frequency distribution of AOD from <0.3 to 0.3–0.5 during the study period in all regions except IGP. In IGP shift of frequency of AOD values occurs from 0.3–0.5 to >0.5. Oceanic areas also shows seasonal variation of AOD. Over Arabian Sea, high AOD values with greater variations were observed in summer monsoon season while in Bay of Bengal it is observed during winter monsoon. This is due to the high wind speed prevailing in Arabian Sea during monsoon season which results in production of more sea salt aerosol. Highest AOD values are observed over NAS during monsoon season and over NBOB during winter season. Lowest AOD values with its lower variations observed in both the central region of Arabian Sea and Bay of Bengal.     

中国东部海域气溶胶光学厚度时空变化研究

结合船基海上现场实测资料与卫星遥感MODIS光学厚度资料,分析了中国东部海域气溶胶光学厚度(AOD)的季节变化和地理分布特征。中国东部海域平均气溶胶光学厚度存在以中纬度为中心的纬向分布;同时受到陆源物质的影响,近海气溶胶光学厚度大于远海,且随着离海岸距离变大有线性递减的趋势。受沙尘、季风气候的影响,各海域气溶胶光学厚度存在明显季节变化和分布特征。渤海、黄海及东海有类似的变化特征,春季都受到沙尘气溶胶的影响,气溶胶平均光学厚度值为全年最高0.12,并且对东海的影响最明显;夏季最小,秋、冬季逐渐变大。     

环境与减灾小卫星高光谱成像仪陆地气溶胶光学厚度反演

环境与减灾小卫星高光谱成像仪是搭载在中国将于2007年发射的环境与减灾小卫星A星（HJ-1A）上的传感器之一，在0．45—0．95μm波段范围内设置了约135个通道，空间分辨率为100m，回访周期约为96h。本文根据传感器的参数设置，使用HYPERION数据仿真模拟了HJ-1A高光谱数据，探讨了它在陆地气溶胶光学厚度探测中的应用。文中使用浓密植被算法反演了模拟数据的气溶胶光学厚度。结果表明，HJ-1A高光谱成像仪能够很好地应用于陆地气溶胶光学厚度的反演。     

利用云下阴影实现陆地上空气溶胶和地表反射率的同时反演—理论方法和模拟

卫星探测信号包含大气中分子和粒子的散射贡献以及地表反射的贡献,在陆地上空二者的贡献相当,并且陆地地表反射率在时间和空间上极度不均一,因此,很难区分二者的各自贡献从而定量提取大气气溶胶和地表反射率,陆地上空气溶胶的反演也一直是一个极具挑战性的课题.而高分辨率卫星资料如TM5的可见光通道能够很好地区分云和云下阴影,如果云是不透光的,在阴影上空,卫星信号仅包含大气散射贡献和地表漫反射贡献,而在邻近的非阴影区上空,卫星探测信号还包含地表直接反射的贡献,根据这个原理,利用辐射传输模式分析了阴影区和非阴影区上空卫星探测的辐射量差别与地表反射率和大气气溶胶的关系,提出一种利用云下阴影来同时提取阴影上空大气气溶胶和地表反射率的单波长反演方案,并对气溶胶单次散射反照率,散射相函数,测量精度以及地表反射率的不均一性进行了敏感性分析.     

Intra-Seasonal Oscillations Associated with Indian Ocean Warm Pool and Summer Monsoon Rainfall and Their Inter-Annual Variability

Sea surface temperature (SST) and wind speed (WS) derived from Multichannel Scanning Microwave Radiometer (MSMR), onboard IRS-P4 (Oceansat-1) satellite were used to generate spatially averaged (80° to 100° E & 0° to 10° S) daily data during June to September from 1999 to 2001 along with collocated outgoing long wave radiation (OLR) data from NCEP/NCAR reanalysis product. Daily rainfall data over the peninsular India during the above period was taken from the weekly weather report published by the Indian Meteorological Department. The data were then subjected to power spectral analysis. Dominant 30 to 60 days oscillations were observed over both ocean and land during all the 3 years investigated. It is quite interesting to note that the intra-seasonal oscillation with 60 days periodicity dominates in both ocean and land during 1999 and then transforms to 30 and 40 days periodicities in 2000 and 2001 respectively with a phase lag of about 25 to 30 days. It was also observed that the time-latitude section of zonally averaged OLR and TMI derived cloud liquid water data clearly depict the propagation of convection and cloud from the equator to the north at the rate of 0.75° to 1° latitude per day which corroborates well with the rate of propagation derived from the phase lag obtained between the processes occurring at the equatorial Indian Ocean and rainfall recorded at the designated land segments. The results are in good agreement with the observations made by earlier investigators.     

从ASTER和Landsat／TM卫星数据反演日本千叶地区地表反射率和气溶胶光学厚度分布

在卫星遥感大气研究中，已有的精确反演海洋表面反射率和其上空气溶胶光学厚度分布的算法，由于陆地地表像元反射率的不均一性，使得这些方法在应用于陆地表面反射率的反演中具有一定的局限性。而利用三步校正法可以去除地表邻近像元的影响，从而消除这种局限性。文中介绍了三步校正的算法，并对日本千叶地区ASTER卫星数据进行了大气和地表邻近像元的影响校正，精确地反演了该地区的地表反射率，通过地表反射率和气溶胶光学厚度之间的关系计算得到了气溶胶光学厚度的分布。同时证明了在洁净天反演的地表反射率可以应用于反演同一季节中非洁净天的气溶胶光学厚度分布，这样不但减小了气溶胶模式选择的影响，而且实现了在缺少太阳辐射计数据情况下获得气溶胶光学厚度分布的目的。     

Machine Learning and Bias Correction of MODIS Aerosol Optical Depth

Machine-learning approaches (neural networks and support vector machines) are used to explore the reasons for a persistent bias between aerosol optical depth (AOD) retrieved from the MODerate resolution Imaging Spectroradiometer (MODIS) and the accurate ground-based Aerosol Robotic Network. While this bias falls within the expected uncertainty of the MODIS algorithms, there is room for algorithm improvement. The results of the machine-learning approaches suggest a link between the MODIS AOD biases and surface type. MODIS-derived AOD may be showing dependence on the surface type either because of the link between surface type and surface reflectance or because of the covariance between aerosol properties and surface type.      

Description of a New Method for Retrieving the Aerosol Optical Thickness from Satellite Remotely Sensed Imagery Using the Maximum Contrast Value and Darkest Pixel Approach

Satellite sensors have provided new datasets for monitoring regional and urban air quality. Satellite sensors provide comprehensive geospatial information on air quality with both qualitative remotely sensed imagery and quantitative data, such as aerosol optical depth which is the basic unknown parameter for any atmospheric correction method in the pre‐processing of satellite imagery. This article presents a new method for retrieving aerosol optical thickness directly from satellite remotely sensed imagery for short wavelength bands in which atmospheric scattering is the dominant contribution to the at‐satellite recorded signal. The method is based on the determination of the aerosol optical thickness through the application of the contrast tool (maximum contrast value), the radiative transfer calculations and the ‘tracking’ of the suitable darkest pixel in the scene. The proposed method that needs no a‐priori information has been applied to LANDSAT‐5 TM, LANDSAT‐7 ETM+, SPOT‐5 and IKONOS data of two different geographical areas: West London and Cyprus. The retrieved aerosol optical thickness values show high correlations with in‐situ visibility data acquired during the satellite overpass. Indeed, for the West London area a logarithmic regression was fitted for relating the determined aerosol optical thickness with the in‐situ visibility values. A high correlation coefficient (r²= 0.82; p= 0.2) was found. Plots obtained from Tanre et al. (1979, 1990) and Forster (1984 ) were reproduced and estimates for these areas were generated with the proposed method so as to compare the results. The author's results show good agreement with Forster's aerosol optical thickness vs. visibility results and a small deviation from Tanre's model estimates.     

Seasonal Variation of MODIS Vegetation Indexes and Their Statistical Relationship With Climate Over the Subtropic Evergreen Forest in Zhejiang, China

Moderate Resolution Imaging Spectroradiometer (MODIS) 16-day 1-km vegetation index products, daily temperature, photosynthetically active radiation (PAR), and precipitation from 2001 to 2004 were utilized to analyze the temporal variations of the MODIS normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI), as well as their correlations with climate over the evergreen forested sites in Zhejiang-a humid subtropical region in the southeast of China. The results showed that both NDVI and EVI could discern the seasonal variation of the evergreen forests. Attributed to the sufficient precipitation in the study area, the growth of vegetation is mainly controlled by energy; as a result, NDVI, and especially EVI, is more correlated with temperature and PAR than precipitation. Compared with NDVI, EVI is more sensitive to climate condition and is a better indicator to study vegetation variations in the study region     

Satellite Observations on Cyclone-Induced Upper Ocean Cooling and Modulation of Surface Winds—A Study on Tropical Ocean Region

In this letter, we evaluate the relationships between the sea-surface temperatures (SSTs) and meteorological parameters over the Bay of Bengal region, India, using microwave satellite remote sensing data. Most of the cyclones in this region occur during the premonsoon period in April–June and are associated with SSTs greater than 26 $^{circ}hbox{C}$. We particularly analyzed the data from two recent cyclonic events: Mala that occurred in April 24, 2006 and Tropical Cyclone 01B (TC 01B) that occurred in May 11, 2003. We used two different remote sensing data sets, sea surface temperature (SST) from the Tropical Rainfall Measurement Mission and the NASA QuikSCAT ocean surface wind vectors to characterize the ocean–atmosphere interactions in cold SST regions formed in the trail of the aforementioned two cyclone events. The results from the satellite data analysis suggested the systematic weakening of wind speed over the cold patch, along the trail of the cyclone. A cooling of around 4$^{circ}$–5 $^{circ}$ was observed to be associated with the passage of cyclone Mala. Wind speed gradually increased from 2 to 9 m/s from the center to the boundary of the cold patch and showed good correlation with SST $(r = 0.97)$. These observations have been validated with another cyclone data (TC 01B) over the Bay of Bengal region that occurred during May 2003. Our results were consistent with the Wallace hypothesis that SST modulates the surface winds via stability.      

水平能见度与气溶胶光学厚度转换模型的适用性分析

以实测的水平能见度和时间同步MODIS的气溶胶光学厚度(aerosol optical depth,AOD)数据为基础,验证了多个经典的AOD与能见度转换模型在全球不同区域的适用性。结果表明,模型中含有气溶胶标高的Peterson模型可以得到较高精度。     

Ocean Mixed-Layer Depth and Current Variation Estimated From Imagery of Surfactant Streaks

The ability of high-resolution imaging systems to resolve small-scale structure on the ocean surface suggests the possibility of using characteristics of Langmuir circulation to map the surface mixed-layer depth and near-surface current. We illustrate this using synthetic aperture radar and infrared imagery that are collected across the edge of the Gulf Stream (GS), which reveals surfactant streaks, or ldquowindrows,rdquo that are induced by Langmuir circulation. Based on changes in the windrow spacing and orientation, the mixed layer is estimated to deepen from 7 to 12 m across the edge of the GS and the current to increase from about 1 to 2 m/s. These spatial changes compare reasonably well with independent data, suggesting that the approach is plausible. It may also be possible to extract additional environmental information from the windrows.     

利用TERRA和AQUA共同反演陆地上空的气溶胶光学厚度

利用TERRA和AQUA共同反演气溶胶光学厚度和地表反射率特征，对其原理及方法进行了详细的讨论。通过Terra和Aqua两颗卫星对同一地点的不同角度的观测，结合多个光学通道的信息，反演了北京地区光学厚度及地表反射率信息。反演的气溶胶光学厚度同地面观测的结果相比具有很好的一致性。同时，对地表反射率及气溶胶波长指数等也进行了讨论和对比，结果显示，对北京地区，MODIS1通道地表反射率和7通道地表反射率的比在0．66左右，3通道和7通道的比在0．28左右。相比于NASA暗背景全球反演算法中1、3通道和7通道的比为0．50和0．25的处理方法，反演得到的气溶胶光学厚度结果也较好。     

A comparison of altimeter and gravimetric geoids in the Tonga Trench and Indian Ocean areas

A gravimetric geoid computed using different techniques has been compared to a geoid derived from Geos-3 altimeter data in two 30°×30° areas: one in the Tonga Trench area and one in the Indian Ocean. The specific techniques used were the usual Stokes integration (using 1°×1° mean anomalies) with the Molodenskii truncation procedure; a modified Stokes integration with a modified truncation method; and computations using three sets of potential coefficients including one complete to degree 180. In the Tonga Trench area the standard deviation of the difference between the modified Stokes’ procedure and the altimeter geoid was ±1.1 m while in the Indian Ocean area the difference was ±0.6 m. Similar results were found from the 180×180 potential coefficient field. However, the differences in using the usual Stokes integration procedure were about a factor of two greater as was predicted from an error analysis. We conclude that there is good agreement at the ±1 m level between the two types of geoids. In addition, systematic differences are at the half-meter level. The modified Stokes procedure clearly is superior to the usual Stokes method although the 180×180 solution is of comparable accuracy with the computational effort six times less than the integration procedures.     

积水沉陷区水体的表观光学特性参数测量与分析

通过实际测量试验区的积水沉陷区水深值,了解水体光学特性随水深不同而引起的衰减特征变化,建立水深与光谱特性响应间的关系表达式,确定获取水深信息的最佳波谱范围;揭示了叶绿素、悬浮物质、CDOM等水体组分与水体光谱特征变化的关系,探讨因水体各主要组分变化而引起的水体光谱吸收光谱、散射光谱的变化规律.     

Evaluation of Precipitation Features in High-Frequency SSM/I Measurements Over Indian Land and Oceanic Regions

Evaluation of the Special Sensor Microwave Imager (SSM/I) precipitation features is presented from various passes over Indian oceanic and land regions under the framework of radiative transfer simulations for both emission and scattering atmospheres. There is considerable uncertainty in the interpretation of the SSM/I high-frequency scattering features for development of rainfall algorithms. Specifically large areas of very low emissivity regimes showing false rain signatures due to the presence of low brightness temperatures (TBs) that are often present in the vicinity of colder sea surface areas in the 85-GHz TB from SSM/I. In this connection, the Polarization Corrected Temperature (PCT) defined by Spencer using the 85-GHz channels, vertical (V) and horizontal (H), has been studied to delineate these surface effects. These false scattering signatures, once corrected using PCT as a suitable parameter, significantly improve the quality of the SSM/I-derived precipitation areas. These results are also confirmed with the cloud optical depth data from the Moderate Resolution Imaging Spectroradiometer sensor onboard the Aqua satellite and the standard merged rain product (geostationary infrared and Tropical Rainfall Measuring Mission (TRMM) microwave data) 3B42 from TRMM. This letter is aimed for selecting PCT as one of the suitable predictor variables for the development of operational rainfall retrieval algorithm for the Indo-French Megha-Tropiques satellite's microwave radiometer at high frequencies.      

Estimation of Seasonal Sun-Induced Fluorescence Dynamics of Indian Tropical Deciduous Forests using SCOPE and Sentinel-2 MSI

This paper presents a new approach to estimate spatial Sun-Induced Fluorescence (SIF) using the empirical relationship between simulated Canopy Chlorophyll Concentration (CCC) and simulated SIF. PROSAIL model [PROpriétésSPECTrales (PROSPECT) and Scattering by Arbitrarily Inclined Leaves (SAIL) models] was used to simulate CCC. CCC maps were generated through an Automated Radiative Transfer Model Operator (ARTMO) using the PROSAIL model and Sentinel-2 Multi-Spectral Imager (MSI) imagery. The Soil Canopy Observation, Photochemistry, and Energy fluxes (SCOPE) model was used to simulate SIF emitted at 740 nm (SIF₇₄₀), at 760 nm (SIF₇₆₀), and top of canopy (SIF_TOC) (640-850 nm). The SCOPE model, configured with the specification of the Sentinel-2 sensor, simulates SIF within the spectrum range of 640-850 nm. A non-linear logarithmic relationship (R²>0.9, p < 0.05) was observed between simulated SIF and simulated CCC. Simulated CCC was linearly related to observed CCC with R² 0.88, 0.92 and 0.89 and RMSE = 0.04, 0.17 and 0.09 gm/m² at p < 0.05 for summer, post-monsoon and early winter respectively. Whereas, the simulated CCC did not capture the full range of CCC variability for the post-monsoon season. Simulated SIF (SIF₇₆₀) was well correlated with SIF from Orbiting Carbon Observatory-2 (OCO-2) satellite with R² 0.68, 0.73 and 0.73 (RMSE = <1 W/m²/sr/μm, p < 0.05) for the month of summer (April), pre-monsoon (May) and early winter season (November) respectively. Temporal SIF_TOC effectively captured the seasonal variability associated with the phenology of deciduous tree species. Among various Sentinel-2 MSI derived VIs, Red Edge NDVI (RENDVI) exhibited maximum sensitivity with SIF (highest monthly average R²> 0.6, p < 0.05). The spatial SIF would serve as an useful link between airborne /satellite derived SIF and in-situ fluorescence measurements to understand multiscale SIF variability of terrestrial vegetation.