A critical examination of the residual cloud contamination and diurnal sampling effects on MODIS estimates of aerosol over ocean

Observations of the aerosol optical thickness (AOT) by the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard Terra and Aqua satellites are being used extensively for applications to climate and air quality studies. Data quality is essential for these studies. Here we investigate the effects of unresolved clouds on the MODIS measurements of the AOT. The main cloud effect is from residual cirrus that increases the AOT by 0.015/spl plusmn/0.003 at 0.55 /spl mu/m. In addition, lower level clouds can add contamination. We examine the effect of lower clouds using the difference between simultaneously measured MODIS and AERONET AOT. The difference is positively correlated with the cloud fraction. However, interpretation of this difference is sensitive to the definition of cloud contamination versus aerosol growth. If we consider this consistent difference between MODIS and AERONET to be entirely due to cloud contamination we get a total cloud contamination of 0.025/spl plusmn/0.005, though a more likely estimate is closer to 0.020 after accounting for aerosol growth. This reduces the difference between MODIS-observed global aerosol optical thickness over the oceans and model simulations by half, from 0.04 to 0.02. However it is insignificant for studies of aerosol cloud interaction. We also examined how representative are the MODIS data of the diurnal average aerosol. Comparison to monthly averaged sunphotometer data confirms that either the Terra or Aqua estimate of global AOT is a valid representation of the daily average. Though in the vicinity of aerosol sources such as fires, we do not expect this to be true.     

Effects of neglecting polarization on the MODIS aerosol retrieval over land

Reflectance measurements in the visible and infrared wavelengths, from the Moderate Resolution Imaging Spectroradiometer (MODIS), are used to derive aerosol optical thicknesses (AOTs) and aerosol properties over ocean and land surfaces, separately. Both algorithms employ radiative transfer (RT) code to create lookup tables, simulating the top-of-atmosphere (TOA) reflectance measured by the satellite. Whereas the algorithm over ocean uses a vector RT code that includes the effects of atmospheric polarization, the algorithm over land assumes scalar RT, thus neglecting polarization effects. In the red (0.66 /spl mu/m) and infrared (2.12 /spl mu/m) MODIS channels, scattering by molecules (Rayleigh scattering) is minimal. In these bands, the use of a scalar RT code is of sufficient accuracy to model TOA reflectance. However, in the blue (0.47 /spl mu/m), the presence of larger Rayleigh scattering (optical thickness approaching 0.2) results in nonnegligible polarization. The absolute difference between vector- and scalar-calculated TOA reflectance, even in the presence of depolarizing aerosols, is large enough to lead to substantial errors in retrieved AOT. Using RT code that allows for both vector and scalar calculations, we examine the reflectance differences at the TOA, assuming discrete loadings of continental-type aerosol. We find that the differences in blue channel TOA reflectance (vector-scalar) may be greater than 0.01 such that errors in derived AOT may be greater than 0.1. Errors may be positive or negative, depending on the specific geometry, and tend to cancel out when averages over a large enough sample of satellite geometry. Thus, the neglect of polarization introduces little error into global and long-term averages, yet can produce very large errors on smaller scales and individual retrievals. As a result of this study, a future version of aerosol retrieval from MODIS over land will include polarization within the atmosphere.     

NASDARainfall algorithms for AMSR-E

Three related algorithms have been developed to retrieve rainfall from the Advanced Microwave Scanning Radiometer-Earth Observing System observations. First, land and ocean backgrounds must be treated separately. The highly reflective ocean background is ideal for observing atmospheric constituents including rainfall. The high and variable emissivity of the land background limits the usefulness of the signal from the liquid hydrometeors under most conditions. For an algorithm with global applicability, it is more reasonable to use the signal generated by scattering of microwaves from frozen hydrometeors. Unfortunately, this scattering is related to rainfall only indirectly. This dichotomy is reinforced by the differing availability of ground truth in the two environments. Thus, two distinct approaches must be used in generating the algorithms for the two cases. The oceanic algorithm is based on physical models, and the land algorithm is generated empirically. Functionally, the two algorithms are merged into a common structure for retrievals on a pixel-by-pixel (Level-2) basis, but the distinct philosophies remain. The third element is the monthly total product (Level-3) which could be generated by simply summing the Level-2 products. However, subtle biases, unimportant on a Level-2 basis, could introduce serious errors into the totals. Therefore, we have chosen to generate a separate product over the oceans consisting of monthly averaged rain rates over 5/spl deg/ /spl times/5/spl deg/ boxes. The Level-3 ocean algorithm assumes that the rainfall intensity is distributed log-normally and also absorbs small instrumental calibration errors and some modeling errors.     

Optical thickness of tropical cirrus clouds derived from the MODIS 0.66and 1.375-/spl mu/m channels

In this paper, we introduce a method to retrieve the optical thickness of tropical cirrus clouds using the isolated visible cirrus reflectance (without atmospheric and surface effects). The isolated cirrus reflectance is inferred from level 1b calibrated 0.66- and 1.375-/spl mu/m Moderate Resolution Imaging Spectroradiometer (MODIS) data. We created an optical properties database and optical thickness lookup library using previously calculated single-scattering data in conjunction with the discrete ordinates radiative transfer (DISORT) code. An algorithm was constructed based on this lookup library to infer the optical thickness of tropical cirrus clouds for each pixel in a MODIS image. We demonstrate the applicability of this algorithm using several independent MODIS images from the Terra satellite. The present method is complimentary to the MODIS operational cloud retrieval algorithm for the case of cirrus clouds.     

Aerosol-cloud interaction-Misclassification of MODIS clouds in heavy aerosol

The accuracy of the spaceborne Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask was evaluated for possible contamination by areas of heavy aerosol that may be misclassified as clouds. Analysis for several aerosol types shows that the cloud mask and products can be safely used in the presence of aerosol up to optical thickness of 0.6. Here we define as cloudy all MODIS 1-km (at nadir) pixels that were used to derive the cloud effective radius and optical thickness of water and ice clouds. The findings make it possible to study aerosol-cloud interaction from the MODIS aerosol and cloud products.     

Differences Between Collection 4 and 5 MODIS Ice Cloud Optical/Microphysical Products and Their Impact on Radiative Forcing Simulations

This paper reports on the comparison of two latest versions (collections 4 and 5) of ice cloud products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) measurements. The differences between the bulk optical properties of ice clouds used in collections 4 and 5 and the relevant impact on simulating the correlation of the bidirectional reflection functions at two MODIS bands centered at 0.65 (or 0.86) and 2.13 mum are investigated. The level-3 MODIS ice cloud properties (specifically, ice cloud fraction, optical thickness, and effective particle size in this paper) from the collection 4 and 5 datasets are compared for a tropical belt of 30deg S-30deg N. Furthermore, the impact of the differences between the MODIS collection 4 and 5 ice cloud products on the simulation of the radiative forcing of these clouds is investigated. Over the tropics, the averaged ice cloud fraction from collection 5 is 1.1% more than the collection 4 counterpart, the averaged optical thickness from collection 5 is 1.2 larger than the collection 4 counterpart, and the averaged effective particle radius from collection 5 is 1.8 mum smaller than the collection 4 counterpart. Moreover, the magnitude of the differences between collection 5 and 4 ice cloud properties also depends on the surface characteristics, i.e., over land or over ocean. The differences of these two datasets (collections 4 and 5) of cloud properties can have a significant impact on the simulation of the radiative forcing of ice clouds. In terms of total (longwave plus shortwave) cloud radiative forcing, the differences between the collection 5 and 4 results are distributed primarily between -60 and 20 W ldr m^-2 but peak at 0 W ldr m^-2.     

中国地区云量和云光学厚度的分布与变化趋势

利用最新的1983~2009年国际卫星云气候计划（ISCCP） D2月平均资料集,得到了中国地区总云量、低云量、中云量、高云量与云光学厚度的分布与变化趋势,结果表明：中国总云量和中云量呈现南多北少的分布,青藏高原地区高云量较大而低云量很小;总云量和中云量在东部呈增加趋势,西部呈减小趋势,低云量和高云量在大部分地区呈减小趋势。从不同季节来看,春季和秋季北方总云量增加,西部总云量减小;夏季大部分地区总云量增加,冬季大部分地区总云量减少。云的光学厚度呈现南多北少的分布,且在大部分地区呈增加趋势。同时利用中分辨率成像光谱仪（MODIS）资料分析了2001~2013年间中国不同污染地区总云量与云光学厚度的分布及变化趋势的异同,结果表明：中国不同污染地区云量与云光学厚度呈现不同的分布和变化规律。     

基于MODIS资料的陕西水云特征研究

陕西位于西北内陆地区,是中国气候的敏感区,为准确认识其上空的水云特征,利用MODIS的MYD06二级云产品数据,对陕西水云的概率分布、云顶高度、粒子有效半径、光学厚度进行了统计分析。结果表明:(1) 水云概率分布显示出单峰结构,峰值出现在11月, 5月出现概率最低。水云在秋季出现概率最高,春季出现概率最低。 (2) 7月和9月水云云顶高度的概率分布会产生显著的变化,7月和8月的分布形态与其余各月有显著差别。水云云顶高度平均最大值出现在春季的4~5月,最小值出现在冬季的12~2月。(3)水云的粒子有效半径在10月~次年5月, 分布形态相似,6~9月分布形态与之明显不同。水云粒子尺度平均最大值出现在夏季的6~8月, 最小值出现在秋季的11月。(4) 光学厚度在0~5之间的水云在10月~次年5月,出现概率最高, 峰值出现在12~2月; 6~9月光学厚度在5~10之间的水云出现概率最高, 峰值出现在7~8月。水云光学厚度最大值出现在秋季的9~11月,最小值出现在夏季的7~8月。     

Retrievals of profiles of fine and coarse aerosols using lidar and radiometric space measurements

The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) spaceborne lidar, expected to be launched in 2004, will collect profiles of the lidar attenuated backscattering coefficients of aerosol and clouds at 0.53 and 1.06 /spl mu/m. The measurements are sensitive to the vertical distribution of aerosols. However, the information is insufficient to be mapped into unique aerosol physical properties and vertical distribution. Spectral radiances measured by the Moderate Resolution Imaging Spectrometer (MODIS) on the Aqua spacecraft, acquired simultaneously with the CALIPSO observations, can constrain the solutions. The combination of the MODIS and CALIPSO data can be used to derive extinction profiles of the fine and coarse modes of the aerosol size distribution for aerosol optical thickness of 0.1 and larger. Here we describe a new inversion method developed to invert simultaneously MODIS and CALIPSO data over glint-free ocean. The method is applied to aircraft lidar and MODIS data collected over a dust storm off the coast of West Africa during the Saharan Dust Experiment (SHADE). The backscattering-to-extinction ratio (BER) (BER=/spl omega//sub o/P(180)/4/spl pi/) can be retrieved from the synergism between measurements avoiding a priori hypotheses required for inverting lidar measurements alone. For dust, the resultant value of BER =0.016 sr/sup -1/ is over 50% smaller than what is expected using Mie theory, but in good agreement with recent results obtained from Raman lidar observations of dust episodes. The inversion is robust in the presence of 10% and 20% noise in the lidar signal at 0.53 and 1.06 /spl mu/m, respectively. Calibration errors of the lidar of 5% to 10% can cause an error in optical thickness of 20% to 40%, respectively, in the tested cases.     

A comparison of cloud droplet radii measured from space

Cloud droplet effective radius (CDR) can be estimated from the spectral signature of cloud reflectance. The technique has been applied to measurements of the Advanced Very High Resolution Radiometer instrument and more recently to the Moderate Resolution Imaging Spectroradiometer (MODIS). Another technique relies on the directional signature of the polarized reflectance and has been applied to observations from Polarization and Directionality of the Earth's Reflectances (POLDER) onboard Advanced Earth Observation Satellite (ADEOS). Although the latter technique requires very specific conditions, we argue that, when applicable, it is very accurate. A large fraction of successful POLDER estimates are derived from measurements over stratocumulus cloud fields. During portions of 2003, POLDER and MODIS acquired near coincident observations. The data can then be used for an evaluation of the two CDR products. The two datasets are highly correlated over the oceans albeit with a MODIS high bias of about 2 /spl mu/m. The correlation breaks down when POLDER retrieves small droplets (less than 7 /spl mu/m), which occurs over most land surfaces as well as polluted oceanic areas. We discuss the possible causes for biases and errors. Although differences in the two CDR estimates are expected because of the differences in the spatial scale and vertical weighting function, we did not find a fully satisfactory explanation for the bias and lack of correlation over land surfaces. It seems, however, that the spatial variability as seen by MODIS is larger than that deduced from POLDER measurements, in particular over land surfaces.     

Aerosol properties over bright-reflecting source regions

Retrieving aerosol properties from satellite remote sensing over a bright surface is a challenging problem in the research of atmospheric and land applications. In this paper we propose a new approach to retrieve aerosol properties over surfaces such as arid, semiarid, and urban areas, where the surface reflectance is usually very bright in the red part of visible spectrum and in the near infrared, but is much darker in the blue spectral region (i.e., wavelength <500 nm). In order to infer atmospheric properties from these data, a global surface reflectance database of 0.1/spl deg/ latitude by 0.1/spl deg/ longitude resolution was constructed over bright surfaces for visible wavelengths using the minimum reflectivity technique (e.g., finding the clearest scene during each season for a given location). The aerosol optical thickness and aerosol type are then determined simultaneously in the algorithm using lookup tables to match the satellite observed spectral radiances. Examples of aerosol optical thickness derived using this algorithm over the Sahara Desert and Arabian Peninsula reveal various dust sources, which are important contributors to airborne dust transported over long distances. Comparisons of the satellite inferred aerosol optical thickness and the values from ground-based Aerosol Robotic Network (AERONET) sun/sky radiometer measurements indicate good agreement (i.e., within 30%) over the sites in Nigeria and Saudi Arabia. This new algorithm, when applied to Moderate Resolution Imaging Spectroradiometer (MODIS), Sea-viewing Wide Field of view Sensor (SeaWiFS), and Global Imager (GLI) satellite data, will provide high spatial resolution (/spl sim/1 km) global information of aerosol optical thickness over bright surfaces on a daily basis.     

基于CALIOP和MODIS数据的气溶胶时空分布特征对比分析

利用两个AERONET站点（Hangzhou_ZFU、SACOL）的Level 2 气溶胶光学厚度（aerosol optical depth, AOD）数据对比验证CALIOP Level 2 AOD数据。结果表明：Hangzhou_ZFU、SACOL站的相关系数为0.87、0.85,回归方程的斜率为0.76、0.92,这表明CALIOP AOD与AERONET AOD显著相关,在这两个站点及附近区域具有适应性。基于2008~2015年无云条件下的CALIOP Level 3月气溶胶产品和同期的MODIS Terra/Aqua Level 3月气溶胶产品,对比分析中国东南和西北区域气溶胶光学厚度（aerosol optical depth, AOD）时空分布特征。分析表明：中国东南区域AOD季节与空间变化明显,AOD高值主要分布在长三角、珠三角等地,且夏季最高、春季次之,秋冬季相当。MODIS AOD月均值保持在在0.25~0.8之间,且与CALIOP 夜间AOD值接近,但与CALIOP白天AOD值差异较大,最大相差值可达0.45。中国西北区域两种卫星获取的AOD值空间分布非常相似,其高值区都位于塔里木盆地、准格尔盆地和柴达木盆地;AOD值春季最高,夏季减少、冬季次之、秋季最低;MODIS AOD值波动显著且普遍高于CALIOP AOD值。     

The determination of the atmospheric optical thickness over Western Europe using SeaWiFS imagery

The first results obtained from the aerosol-cloud retrieval algorithm (developed at the University of Bremen) are presented. The algorithm enables the observation of the regional characteristics of aerosol and cloud optical thickness both over land and ocean surfaces. The aerosol and cloud optical thickness over Western Europe is derived from the high-resolution SeaWiFS data for October 11, 2001 (11:30 UTC). The most probable value of the aerosol optical thickness was found to be equal approximately 0.25. The frequency distributions of the aerosol and cloud optical thickness are skewed and have long tails for larger optical thickness. It was found that retrieved values of the aerosol optical thickness at wavelengths 0.412 and 0.440 /spl mu/m are close to those measured by five ground-based instruments placed at different locations. The problems related to the retrieval of the atmospheric optical thickness from space are discussed.     

利用MODIS数据对北极夏季卷云特性的研究

利用2011~2014年MODIS云产品数据对北极地区夏季卷云的出现概率、云顶温度、云顶高度、光学厚度、有效粒径大小进行统计分析,并讨论了北极地区夏季卷云有效粒径大小和卷云高度的关系。结果表明,北极地区上空夏季卷云出现概率最高,水云较少。卷云云顶温度主要分布在230~272 K(即-43~-5℃),其云顶高度主要在2~8 km,4.5~6 km出现概率最大。卷云的光学厚度主要在小于10范围内。卷云的有效粒径在5~40 m之间,10~20 m出现概率最大。卷云的有效粒径和高度的关系与中纬度地区相反,北极地区卷云高度越高,卷云有效粒径越大。北极地区卷云随着纬度增大,卷云出现概率增加,卷云云顶温度降低,卷云高度增加,卷云有效粒径增大,卷云光学厚度增大。     

利用FY-1C资料反演水云的光学厚度和粒子有效半径

利用FY-IC极轨气象卫星扫描辐射计的通道1(0．58—0．68μm)、通道3(3．55—3．95μm)和通道6(1．58—1．64μm)所提供的探测数据进行了水云光学厚度和粒子有效半径的反演试验．在非水汽吸收波段(如通道1)，云的反射函数主要是云光学厚度的函数，而在水汽吸收波段(如通道6)，云的反射函数主要是云粒子大小的函数．因此当云光学厚度较厚时，可利用通道1和通道6反射率，或通道1反射率和通道3亮度温度同时计算出云的光学厚度和有效粒子半径。     

Thickness dependence of Weibull slopes of HfO/sub 2/ gate dielectrics

Breakdown voltage distribution, Weibull slopes, and area scaling factors have been investigated for HfO/sub 2/ gate dielectrics in order to gain a better understanding of the breakdown mechanism. Weibull slope of thick HfO/sub 2/ (e.g., /spl beta//spl ap/4 for EOT=2.5 nm) is smaller than that of SiO/sub 2/ with similar physical thickness, whereas /spl beta/ of the thinner HfO/sub 2/ (e.g., /spl beta//spl ap/2 for EOT=1.4 nm) is similar to that of SiO/sub 2/. The implication of the thickness dependence of /spl beta/ is discussed.     

Bendability of single-crystal Si MOSFETs investigated on flexible substrate

This letter reports on a device layer transfer (based on thermal bonding and grinding backside Si) process and device characteristics of Si MOSFETs on a flexible substrate, focusing mainly on the mechanical bendability of the device and resistance to fatigue. The results demonstrated a well-optimized bonding process, as indicated by the nearly indiscernible performance difference (e.g., subthreshold slope, V/sub th/, and I/sub dsat/) before and after the bonding of Si with the flexible substrate. The device characteristics indicate excellent bendability of Si MOSFETs on flexible substrate (e.g., for radius tested down to /spl plusmn/72 mm) and good immunity to fatigue (e.g., negligible performance drift tested up to /spl sim/10/sup 3/ bending cycles with a radius of /spl plusmn/126 mm). Results suggest the feasibility of this approach in achieving high-performance MOSFETs for applications in performance-sensitive and flexible electronics.     

Retrieval, validation, and application of the 1-km aerosol optical depth from MODIS measurements over Hong Kong

The Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol retrieval algorithm was developed to derive aerosol properties at a global scale, suitable for climate studies. Under favorable conditions (clear sky and over dark surfaces), the standard 10/spl times/10 km MODIS aerosol products are also useful on regional scales to monitor aerosol distributions and transports. However, the 10-km resolution is insufficient to depict aerosol variation on local or urban scales, due to inherent aerosol variability as well as complex surface terrain. In this study, we have modified the MODIS algorithm to retrieve aerosol optical depth (AOD) at 1-km resolution over Hong Kong, a city of just over 1000 km/sup 2/ with very complex surface features. Accompanied by the increased spatial resolution are new aerosol models derived with single-scattering albedo (SSA) around 0.91-0.94 to accommodate higher aerosol absorption encountered in Hong Kong than that was presumed for MODIS standard products (SSA/spl sim/0.97) over the region. The derived AOD data are compared to handheld Microtops II sunphotometer observations at the Hong Kong University of Science and Technology and other locations across Hong Kong. Retrieval errors within 15% to 20% of sunphotometer measurements are found. Moreover, when compared with the standard 10-km AOD products, the 1-km AOD data are much better correlated with PM/sub 10/ measurements across Hong Kong, suggesting that the new 1-km AOD data can be used to better characterize the particulate matter distribution for cities like Hong Kong than the MODIS standard products.     

Cloud cover comparisons of the MODIS daytime cloud mask with surface instruments at the north slope of Alaska ARM site

This paper compares daytime cloud fraction derived from the Moderate Resolution Imaging Spectrometer (MODIS), an imager on the National Aeronautics and Space Administration's Earth Observing System Aqua and Terra platforms, to observations from a suite of surface-based instrumentation located at the Department of Energy's atmospheric radiation measurement (ARM) program North Slope of Alaska (NSA) Clouds and Radiation Testbed site. In this systematic comparison of satellite-to-surface measurements, 3650 cases are analyzed from February through September 2001. The surface instruments used in these comparisons include the Vaisala Ceilometer (VCEIL), the Micropulse Lidar (MPL), the Active Remote Sensing of Clouds (ARSCL) composite laser-derived data product, the Whole-Sky Imager (WSI), and the Normal Incidence Pyrheliometer (NIP). In terms of the active sensors, VCEIL cloud cover results compare to within /spl plusmn/20% of MODIS results 77% of the time. As expected, VCEIL is found to be insensitive to optically thin high-level clouds. MPL results are consistent with MODIS in 83% of the cases; however, the MPL preliminary.cbh variable reports spurious clouds in clear-sky conditions. The ARSCL composite laser-derived data product agrees with MODIS in 81% of the cases, improving upon high cloud detection of the VCEIL, while eliminating the spurious clear-sky cloud detections in the MPL preliminary.cbh variable. For the passive WSI, cloud cover agrees with the MODIS cloud fraction in 74% of the cases, with the difference primarily caused by the insensitivity of the WSI to thin clouds. Detailed analysis of individual cases shows that the MODIS cloud mask generally detects more thin cirrus than the surface-based instruments, but it sometimes fails to detect low-level cumulus and fog over the ARM NSA site.     

Emissivity maps to retrieve land-surface temperature from MSG/SEVIRI

Retrieval of land-surface temperature (LST) using data from the METEOSAT Second Generation-1 (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI) requires adequate estimates of land-surface emissivity (LSE). In this context, LSE maps for SEVIRI channels IR3.9, IR8.7, IR10.8, and IR12.0 were developed based on the vegetation cover method. A broadband LSE map (3-14 /spl mu/m) was also developed for estimating longwave surface fluxes that may prove to be useful in both energy balance and climate modeling studies. LSE is estimated from conventional static land-cover classifications, LSE spectral data for each land cover, and fractional vegetation cover (FVC) information. Both International Geosphere-Biosphere Program (IGBP) Data and Information System (DIS) and Moderate Resolution Imaging Spectrometer (MODIS) MOD12Q1 land-cover products were used to build the LSE maps. Data on LSE were obtained from the Johns Hopkins University and Jet Propulsion Laboratory spectral libraries included in the Advanced Spaceborne Thermal Emission and Reflection Radiometer spectral library, as well as from the MODIS University of California-Santa Barbara spectral library. FVC data for each pixel were derived based on the normalized differential vegetation index. Depending on land cover, the LSE errors for channels IR3.9 and IR8.7 spatially vary from /spl plusmn/0.6% to /spl plusmn/24% and /spl plusmn/0.1% to /spl plusmn/33%, respectively, whereas the broadband spectrum errors lie between /spl plusmn/0.3% and /spl plusmn/7%. In the case of channels IR10.8 and IR12.0, 73% of the land surfaces within the MSG disk present relative errors less than /spl plusmn/1.5%, and almost all (26%) of the remaining areas have relative errors of /spl plusmn/2.0%. Developed LSE maps provide a first estimate of the ranges of LSE in SEVIRI channels for each surface type, and obtained results may be used to assess the sensitivity of algorithms where an a priori knowledge of LSE is required.