Revisiting satellite radiative flux computations at the top of the atmosphere

Most satellite observations of radiative fluxes at the top of the atmosphere (TOA) are at narrow spectral intervals and at particular viewing angles. Critical elements in the formulation of TOA shortwave (SW) radiative fluxes are (1) the transformation from narrowband to broadband values (n/b) and (2) the application of angular distribution models (ADMs) to correct for anisotropy. In this article, the n/b transformations are based on theoretical simulations with a radiative transfer model Moderate Resolution Atmospheric Transmission (MODTRAN) 3.7 using land classification types based on the International Geosphere-Biosphere Programme (IGBP) scheme and a range of realistic atmospheric conditions. The newly developed ADMs are a combination of MODTRAN-3.7 simulations and the Clouds and the Earth's Radiant Energy System (CERES)-observed ADMs. To evaluate the impact of the proposed corrections, they are implemented with observations from the Spinning Enhanced Visible Infrared Imager (SEVIRI) on the Meteorological Satellite (METEOSAT) 8 to derive TOA fluxes and compared to similar quantities from CERES. It is shown that the estimated TOA radiative fluxes have –3% bias and 7% root mean square error (RMSE) when compared with CERES observations at a monthly timescale.     

An efficient scheme to calculate radiative transfer in mesoscale models

Mesoscale meteorological models require the parameterization of radiative transfer. In this paper a radiation scheme is presented which is found to be reasonably accurate for inclusion in a three-dimensional mesoscale model. The scheme provides atmospheric radiative heating/cooling and radiative fluxes at ground level for both clear and polluted or cloudy atmospheres. The parameterization of longwave radiation is based on the emissivity method and takes into account the spectral overlap in the 15 μm band. Shortwave radiative transfer is described by means of a new approach allowing the consideration of multiple reflections; this is found to be of particular importance in the case of a cloudy atmosphere. Approximative methods are applied to parameterize both extinction due to pollution and cloud transmission. By a proper numerical treatment the conservation of radiative fluxes is ensured and computational efficiency is achieved. Selected simulation results are used to illustrate the features of the proposed scheme and to point out its advantages compared to previous methods.     

青藏高原积雪对气候反馈的初步研究

积雪具有很高的反照率,能反射回绝大部分的太阳短波辐射;同时,积雪是热的不良导体,其热阻隔性会抑制地表的长波辐射。因此,积雪的积累和消融会强烈地改变大气层顶的辐射平衡,进而对气候产生反馈。采用ERA-Interim再分析资料和MODIS去云积雪产品,通过改进的偏辐射扰动思想,对青藏高原地区2001~2010年积雪影响下大气层顶的辐射能量收支状况进行模拟,计算对应的积雪辐射强迫,并在此基础上估算积雪反馈。结果表明:研究区99.5%以上地区的大气层顶辐射平衡为负,即积雪对气候存在正的辐射强迫,年平均辐射强迫为3.97 W·m-2。时空分布特征表明,积雪辐射强迫的年际差异不大,但空间差异很大,其空间分布与积雪覆盖率有很强的正相关关系,在绝大多数情况下,短波反照率辐射强迫对积雪辐射强迫起着决定性作用,且青藏高原的积雪反馈强度约为9.35 W·m-2·℃-1。     

Estimating surface radiation fluxes in the Arctic from TOVS brightness temperatures

A new method for estimating downwelling shortwave and longwave radiation fluxes in the Arctic from TOVS brightness temperatures has been developed. The method employs a neural network to bypass computationally intensive inverse and forward radiative transfer calculations. Results from two drifting ice camps (CEAREX, LeadEx) and from one coastal station show that downwelling fluxes can be estimated with r.m.s. errors of 20Wm-2 for longwave radiation and 35Wm-2for shortwave radiation. Mean errors are less than 4 Wm-2 and are well within the bounds required for many climate process studies.     

Books received

Satellite-derived broad-band albedo offers a useful tool for monitoring surface conditions. Given the limited wavelength window of most satellite radiometers, satellite albedo studies need to define narrow-band to a broad-band transformations. Signals from the AVHRR channels on board the NOAA-11 satellite, the Meteosat visible channel and a rectangular spectral band from 0-3 to 2.5 μm were simulated for a set of 20 representative land surfaces. The radiative transfer code described in Tanre et al. (1990) was used to obtain top of the atmosphere radiances. The derived signals were then correlated to predict broadband albedo and the Meteosal response via the two AVHRR signals. The results indicated that the use of the two AVHRR bands makes the narrow-band to broad-band conversion independent of the surface type. Finally, the regression was applied to two concurrent AVHRR and Meteosat images so as to compare a broad-band AVHRR-derived top of the atmosphere (TOA) albedo for the Meteosat-band with a corresponding TOA albedo from Meteosat data. The results of the comparison illustrated the anisotropic character of surface reflection.     

A multi-sensor approach for assessing the impacts of ultraviolet-absorbing aerosols on top of atmosphere radiative fluxes

One year (June 2006–May 2007) of the Clouds and the Earth's Radiant Energy System (CERES) top of atmosphere (TOA) shortwave (SW) and longwave (LW) fluxes are used with Ozone Monitoring Instrument-Aerosol Index (OMI-AI) data to assess the direct radiative effects of ultraviolet (UV)-absorbing aerosols. We show that TOA SW aerosol radiative effects from UV-absorbing aerosols alone are almost double that compared to when considering all aerosols. These aerosols account for nearly all of the seasonal variation in direct SW radiative effect (SWRE) over the oceans and a significant portion of that over land.

Overall, TOA clear-sky SWREs over ocean and land surfaces were estimated to be –4.3 and –4.9 W m^–2, respectively, with both values being in line with previous estimates. When only regions where UV-absorbing aerosols are considered, the SWRE over the ocean becomes more negative between March and August, when the concentration of dust aerosols in the North Atlantic is greatest. Larger concentrations of UV-absorbing aerosols generally exist over land surfaces, increasing the OMI-AI to 0.54 compared to 0.20 over ocean, increasing SW cooling estimates over land. The SWRE also varies as a function of land-surface type and surface albedo. The lowest concentration of absorbing aerosols and therefore the least SW cooling occurs over forests, with the OMI-AI?=?0.36 and the non-bias-adjusted SWRE?=?–1.45 W m^–2. Over other surfaces, the concentrations of absorbing aerosols are greater (AI > 0.5), thereby increasing the SWRE to between –2 and –3 W m^–2. However, as surface brightness increases, the correlation between the AI and the SWRE decreases, and it becomes more difficult to attribute the SWRE estimates to absorbing aerosol concentrations. In particular, the SWRE estimates over bare-soil regions were found to be negative, but completely uncorrelated with the OMI-AI. We concluded that the uncertainty of the SWRE estimates increases as a function of increasing albedo.

The TOA radiative effects of absorbing aerosols to longwave radiation (LWREs) are also examined over North Africa and the North Atlantic, where substantial concentrations of elevated dust aerosols exist. LW warming was found to offset SW cooling over the ocean 20–30% during the spring and summer months. Even greater LW warming was found over land in North Africa and South Asia, with a mean value of +3.8 W m^–2 between March and August, representing an increase of over 100% compared to the other seasons. Our analysis indicates that the OMI provides complementary information to the Moderate Resolution Imaging Spectroradiometer (MODIS) for studying aerosol radiative effects.     

An algorithm for the retrieval of 30-m snow-free albedo from Landsat surface reflectance and MODIS BRDF

We present a new methodology to generate 30-m resolution land surface albedo using Landsat surface reflectance and anisotropy information from concurrent MODIS 500-m observations. Albedo information at fine spatial resolution is particularly useful for quantifying climate impacts associated with land use change and ecosystem disturbance. The derived white-sky and black-sky spectral albedos may be used to estimate actual spectral albedos by taking into account the proportion of direct and diffuse solar radiation arriving at the ground. A further spectral-to-broadband conversion based on extensive radiative transfer simulations is applied to produce the broadband albedos at visible, near infrared, and shortwave regimes. The accuracy of this approach has been evaluated using 270 Landsat scenes covering six field stations supported by the SURFace RADiation Budget Network (SURFRAD) and Atmospheric Radiation Measurement Southern Great Plains (ARM/SGP) network. Comparison with field measurements shows that Landsat 30-m snow-free shortwave albedos from all seasons generally achieve an absolute accuracy of ±0.02-0.05 for these validation sites during available clear days in 2003-2005, with a root mean square error less than 0.03 and a bias less than 0.02. This level of accuracy has been regarded as sufficient for driving global and regional climate models. The Landsat-based retrievals have also been compared to the operational 16-day MODIS albedo produced every 8-days from MODIS on Terra and Aqua (MCD43A). The Landsat albedo provides more detailed landscape texture, and achieves better agreement (correlation and dynamic range) with in-situ data at the validation stations, particularly when the stations include a heterogeneous mix of surface covers.     

Comparison of CERES surface radiation fluxes with surface observations over Loess Plateau

Surface energy budget is an important factor in weather and climate processes. To estimate the errors in satellite-retrieved surface radiation budget over the interior of China, instantaneous-footprint surface radiation fluxes from the Terra/Aqua FLASHFlux SSF product are compared with the measurements taken at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) from July 2008 to March 2010. Validation is performed separately for different conditions: clear-sky and cloudy-sky, daytime and nighttime for four seasons. Differences between the FLASHFlux CERES shortwave radiation flux and surface measurements have larger standard deviations in cloudy-sky conditions than in clear-sky conditions, indicating that cloud contamination increases uncertainty in the retrieval algorithm. Upward shortwave radiation flux (USW) is overestimated in cloudy conditions suggesting that the cloud parameters and surface scene type in the retrieval process are not optimal for northwestern China. The CERES downward longwave radiation fluxes (DLW) accurately follow the variation of surface measurements during daytime, but are slightly underestimated during nighttime due to the coarse sounding profile and undetected low clouds at nighttime. The CERES upwelling longwave radiation fluxes (ULW) are strongly underestimated during daytime but are slightly underestimated during nighttime regardless of cloud coverage. This large bias could be caused by an underestimate of surface skin temperature and/or surface emissivity, or spatial inhomogeneity around the site. Generally, except for diurnal ULW, other components of the surface radiative fluxes obtained from CERES SSF datasets are close to meeting the accuracy requirements for climate research.     

Top-of-atmosphere flux retrievals from CERES using artificial neural networks

The Clouds and the Earth's Radiant Energy System (CERES) instruments on the Terra spacecraft provide accurate shortwave (SW), longwave (LW) and window (WN) region top-of-atmosphere (TOA) radiance measurements from which TOA radiative flux values are obtained by applying Angular Distribution Models (ADMs). These models are developed empirically as functions of the surface and cloud properties provided by coincident high-resolution imager measurements over CERES field-of-view. However, approximately 5.6% of the CERES/Terra footprints lack sufficient imager information for a reliable scene identification. To avoid any systematic biases in regional mean radiative fluxes, it is important to provide TOA fluxes for these footprints. For this purpose, we apply a feedforward error-backpropagation Artificial Neural Network (ANN) technique to reproduce CERES/Terra ADMs relying only on CERES measurements. All-sky ANN-based angular distribution models are developed for 10 surface types separately for shortwave, longwave and window TOA flux retrievals. To optimize the ANN performance, we use a partially connected first hidden neuron layer and compact training sets with reduced data noise. We demonstrate the performance of the ANN-based ADMs by comparing TOA fluxes inferred from ANN and CERES anisotropic factors. The global annual average bias in ANN-derived fluxes relative to CERES is less than 0.5% for all ANN scene types. The maximum bias occurs over sea ice and permanent snow surfaces. For all surface types, instantaneous ANN-derived TOA fluxes are self-consistent in viewing zenith angle to within 9% for shortwave, 3.5% and 3% longwave daytime and nighttime, respectively.     

The vertical structure of aerosols and clouds derived from satellites equipped with high-resolution polarization sensors

The passive retrieval technique for investigation of the vertical structure of clouds and aerosols from satellites is discussed. This technique uses a combination of cross-nadir polarimetry and high-resolution infrared (IR) spectroscopy. The real potential of this technique is demonstrated here with the help of a set of numerical experiments, e.g. for the detection of cirrus clouds (Ci). In this regard, new vector versions for the longwave and shortwave in the FLBLM (Fast Line-by-Line Model) radiative transfer model are presented. These new versions are based on the Line-by-Line (LbL) and ‘local estimation’ Monte Carlo (MC) methods and can calculate the Stokes parameters of the outgoing radiation in vertically inhomogeneous atmospheres with any spectral resolution.     

MODIS卫星数据地表反照率反演的简化模式

以内蒙西部地区的MODIS遥感图像数据和地表野外同步观测的光谱数据为例,在野外数据量较少且有定标数据的条件下反演地表反照率。使用6S大气1辐射传输模型进行大气校正,并通过MODTRAN4.0模型获取各波段地表入射光通量和窄波段的地表反照率;在窄波段反照率与宽波段反照率之间存在线性关系的前提下,以各波段的入射光通量占总入射通量的比例作为反演参数,实现窄波段到宽波段的反演。反演结果证明此方法简便可行。     

The relationship of surface air temperature,heat balance at the surface,and radiative balance at the top of atmosphere over the Asian territory of Russia using reanalysis and remote-sensing data

In this study over the Asian territory of Russia (ATR) (45° N–80° N, 60° E–180° E) for the period of 1979–2010 the temporal variability of the surface air temperature field was investigated. There are several climatic factors which can influence temperature variability including radiative balance at the top of atmosphere (TOA), heat balance at the surface, total cloud cover, and large-scale atmospheric circulation in the Northern Hemisphere. The contribution of these factors to temperature variability is also investigated. It was found that during the past decade, over the ATR, the process of warming prevails mainly in the warm season, but in the cold season it is either not as marked or there is cooling instead. In the winter season there is a positive relationship between temperature anomalies and anomalies in cloudiness, effective radiation, and the Arctic Oscillation (AO) index. During the same period, a negative relationship between anomalies of temperature and anomalies of net radiation at the TOA, net shortwave radiation at the surface, and the Scandinavian (SCAND) index was observed. In the summer season, the relationship between temperature and cloudiness becomes negative and the relationship between temperature and atmospheric circulation indices decreases. For the period 2001–2010, radiative fluxes obtained from reanalysis data sets Japanese Reanalysis Data (JRA-25) and Modern Era-Retrospective Analysis for Research and Applications (MERRA) were compared to satellite remotely sensed data project Clouds and the Earth’s Radiant Energy System (CERES). It was found that there is a good agreement between estimates of the net radiation at the TOA calculated using reanalysis data and satellite data: the difference is about 1.5 W m^?2 and the correlation coefficient is more than 0.7. As for the comparison of radiative fluxes estimates at the surface for clear sky, there is less difference between MERRA and CERES. So, during the period 2001–2010 the relation between atmospheric circulation and surface air temperature variability increased in winter months. Obtained regression models allow us to describe from 27% to 82% of temperature variability in different months if we take into account both circulation and radiative factors.     

Mathematical aspects of the meteorological interpretation of satellite hyperspectral infrared measurements part I: statement of the inverse problem for estimation of the cloud absorption vertical profile

Satellite high spectral resolution infrared measurements provide information for cloud vertical characterization when optically semi-transparent clouds at high altitude cover cloud layers at lower altitudes. It is an important issue because such atmospheric conditions are common and clouds are characterized by large-scale vertical development. An approximation radiative transfer model for a cloudy atmosphere is introduced. Cloud particle absorption of infrared (IR) radiation depends on the spectral frequency. So, the effective cloud parameters such as amount (absorption) and height, derived from IR spectral measurements, will be spectral functions as well. The degree of uncertainty in the determination of effective cloud parameters cannot be eliminated by increasing the number of spectral measurements. A cloud model should have extra degrees of freedom to address the spectral and spatial (vertical) variability of cloud absorption. A semi-discrete multilevel cloud model is used to describe the perturbation of the outgoing IR thermal radiation caused by cloudiness in the field of view of a satellite instrument. The model delineates cloudiness in a number of layers at fixed heights. Each layer (level) is characterized by the effective cloud absorption. An inverse problem of cloud absorption vertical profile (CAVP) estimation is described. The estimate of an effective cloud absorption profile is considered as predictor for identification of cloud presence at specific atmospheric layers. The problem is numerically examined for real satellite IR spectral measurements and solution estimates are compared with lidar measurements. Results show that the resulting estimate of CAVPs provides a realistic characterization of cloud top and cloud vertical scale.     

Landsat8卫星影像大气校正及其植被指数与SEVI性能比较

遥感影像受大气的吸收散射以及地形起伏变化的影响,使得传感器接收到的辐射信号既包含了地物的信息,同时也包含了大气以及地形的信息。为了提高地表反射率的反演精度,需要去除遥感影像中大气和地形的影响。提出了一种基于查找表的Landsat8-OLI遥感影像的大气校正方法,该方法由6S辐射传输模型生成查找表,其中输入的参数包括大气水蒸汽含量、臭氧浓度和气溶胶光学厚度等MODIS大气参数产品。利用传统方法建立的大气参数查找表通常只考虑一部分因素,这对于以MODIS产品为输入参数的大气校正是不适用的。本文建立了一个包括大部分输入参数的高维大气校正查找表,对于Landsat-8 OLI传感器具有很高的通用性,通过进行光谱分析、与USGS地表反射率产品交叉验证等方式来验证模型的精度。验证结果表明该方法能有效地反演精确可靠的地表反射率。最后,采用目视解译、统计分析将校正结果与SEVI做对比分析,比较地形影响消减的效果。结果表明该模型与SEVI在地形消减的效果上作用相当。     

Landsat8卫星影像大气校正及其植被指数与SEVI性能比较

遥感影像受大气的吸收散射以及地形起伏变化的影响，使得传感器接收到的辐射信号既包含了地物的信息，同时也包含了大气以及地形的信息。为了提高地表反射率的反演精度，需要去除遥感影像中大气和地形的影响。提出了一种基于查找表的Landsat8-OLI遥感影像的大气校正方法，该方法由6S辐射传输模型生成查找表，其中输入的参数包括大气水蒸汽含量、臭氧浓度和气溶胶光学厚度等MODIS大气参数产品。利用传统方法建立的大气参数查找表通常只考虑一部分因素，这对于以MODIS产品为输入参数的大气校正是不适用的。本文建立了一个包括大部分输入参数的高维大气校正查找表，对于Landsat-8 OLI传感器具有很高的通用性，通过进行光谱分析、与USGS地表反射率产品交叉验证等方式来验证模型的精度。验证结果表明该方法能有效地反演精确可靠的地表反射率。最后，采用目视解译、统计分析将校正结果与SEVI做对比分析，比较地形影响消减的效果。结果表明该模型与SEVI在地形消减的效果上作用相当。     

Interannual variability of smoke and warm cloud relationships in the Amazon as inferred from MODIS retrievals

Aerosol and cloud data from the MODerate resolution Imaging Spectroradiometer (MODIS) onboard the Earth Observing System (EOS) Aqua are used to investigate interannual variability of smoke and warm cloud relationships during the dry-to-wet transition season (August-October) over the Amazon for two years and its association with meteorological conditions. In one year (2003), smoke aerosols are associated with an increase of cloud fraction and a decrease of cloud effective radius. These effects amplify the cooling at the surface and at the top of the atmosphere (TOA) caused by the aerosol extinction. However, in another year (2002) the cloud fraction decreases with increasing aerosol optical depth. Such a decrease of cloud fraction could offset the effect of increased reflection of solar radiation by the aerosols both at the surface and at TOA. The changes in radiative fluxes between these years would contribute to interannual changes of surface energy fluxes and radiative balance at the top of the atmosphere and influence variability of the wet season onset in the basin. In 2003, the atmosphere was more humid and less stable. These conditions may be relatively favorable for the activation of aerosol particles into cloud condensation nuclei and hence cloud droplets. In 2002, the clouds were less extensive and thinner in a relatively dry atmosphere and presumably dissipated more easily. This study suggests that the aerosol-cloud relation can be influenced by atmospheric structure and convective motions, in addition to changes in aerosols properties. An adequate characterization of aerosol-cloud relationship would require a longer time series of data that includes a variety of climate conditions. The caveat of this analysis is that differences in aerosol absorption and its vertical distribution may have contributed to the observed interannual change of smoke-cloud relationship but could not be determined due to lack of adequate measurements.     

Seasonal behaviour of NCEP-NCAR longwave cloud radiative forcing and its relationship with all-India summer monsoon rainfall

Using National Center for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) longwave cloud radiative forcing (LWCRF) reanalysis at the top of the atmosphere (TOA) for the period 1949–2006, the seasonal behaviour of the LWCRF and its relationship with the all-India summer monsoon rainfall (AISMR) during the winter (December–January–February, DJF), pre-monsoon (March–April–May, MAM) and summer monsoon (June– July–August–September, JJAS) seasons has been examined. The LWCRF over the Bay of Bengal region (15–20° N and 87.5–92.5° E) during the pre-monsoon season (MAM) is found to be significantly related to AISMR. The correlation coefficient (CC) between AISMR and LWCRF is 0.419, significant at the 1% level. The composite anomalies for excess minus deficient rainfall years of the LWCRF during MAM over the same region strongly support a significant relationship between LWCRF and AISMR. Thus, the LWCRF over the Bay of Bengal region in the pre-monsoon season appears to be a good indicator of the forthcoming monsoon rainfall.     

Quantitative study of net radiation from MODIS data in the lower boundary layer in Poyang Lake area of Jiangxi Province,China

This paper focuses on quantitative calculation of longwave radiation and shortwave radiation from MODIS data in the Poyang Lake area of Jiangxi Province, China. The sum of the net longwave radiation and the shortwave radiation is the net radiation. These parameters are critical for the study of energy exchange in the lower boundary layer on land surface. Two of the most important factors for the retrieval of longwave radiation are the land surface temperature and emissivity. In this paper, the land surface temperature and emissivity were calculated from MODIS data using the regional self‐iterative split‐window method. The most important factor in the determination of the shortwave radiation is Earth surface albedo. The spectral reflectance and surface albedo were derived from MODIS data using the Synergy of TERRA and AQUA MODIS data (SYNTAM) algorithm. The net shortwave and longwave radiation were calculated and compared with the in situ measurement data. Our results indicate that the methods for quantitative calculation of net longwave radiation, shortwave radiation and net radiation from MODIS data can have a good accuracy. The relative errors are between 2.1% to 9.72% for longwave radiation, 0.15% to 10.48% for shortwave radiation and 0.64% to 13.7% for net radiation. We can conclude that a good accuracy can be achieved for deriving longwave radiation, shortwave radiation and net radiation, which are helpful for heat exchange, environmental, hydrology and ecology research in land areas.     

Analysis of two-dimensional transient conduction–radiation problems in an anisotropically scattering participating enclosure using the lattice Boltzmann method and the control volume finite element method

This study deals with the performance evaluation of the lattice Boltzmann method (LBM) and the control volume finite element method (CVFEM) in terms of their abilities to provide accurate results in solving combined transient conduction and radiation mode problems in a two-dimensional rectangular enclosure containing an absorbing, emitting and anisotropically scattering medium. Coupling problems for mixed kind thermal boundary are worked out for reflective interfaces. Effects of various parameters are studied on the distributions of temperature, radiative and conductive heat fluxes. The results of the LBM in conjunction with the CVFEM have been found to compare very well with available results in the literature. So, the numerical approach is extended to deal with a practical combination of mixed boundary conditions in a transient multi-dimensional combined conductive radiative heat transfer problems in an emitting, absorbing, anisotropically scattering enclosure.     

Broadband solar radiances from visible band measurements: A method based on ScaRaB observations and model simulations

Using simultaneous and co-located broadband and narrowband observations of solar radiation from the Scanner for Radiation Budget (ScaRaB) instrument on Meteor-3 during 1994-1995, empirical relationships between ScaRaB visible band and broadband solar radiances are derived for different surface types, cloud amounts, solar zenith angles and satellite viewing angles. Relationships between ScaRaB visible channel and National Ocean and Atmosphere Administration (NOAA) Advance Very High Resolution Radiometer (AVHRR) visible channel radiances are derived from radiation transfer model simulations for different surface types and cloud cover. Combining the narrow-to-broadband (NTB) and narrowband-to-narrowband (NTN) relationships, broadband solar radiance can be derived from narrowband radiances measured by the AVHRR on operational meteorological satellites. The derived NTB conversion coefficients are evaluated against independent data. Radiances are converted to fluxes by the application of angular distribution models. Typical differences between fluxes derived from ScaRaB narrowband and broadband channels are of the order of 1 W m-2 with standard deviations of the order 15 W m-2.