William M. F. Grey

Aerosol optical depth and land surface reflectance from multiangle AATSR measurements: global validation and intersensor comparisons

This paper presents the results and satellite intercomparisons for the retrieval of aerosol optical depth (AOD) and land surface bidirectional reflectance using the Multiangle Advanced Along-Track Scanning Radiometer (AATSR). The algorithm developed is based on inversion of a physical model of light scattering that requires no a priori knowledge of the land surface. The model is evaluated for a number of sites around the world to test its operation over a range of aerosol types and land covers including dark and bright surfaces. Validation is performed using Aerosol Robotic Network ground-based sun-photometer measurements and by intercomparison with independent estimates of AOD derived from spaceborne instruments including Multiangle Imaging Spectroradiometer (MISR), Moderate Resolution Imaging Spectroradiometer (MODIS), and Total Ozone Mapping Spectrometer (TOMS) aerosol products. Results show good agreement (Pearsons correlation coefficient r2=0.70 for all sites combined) between the AATSR-derived estimates of AOD and the sun-photometer measurements. There is also a high correlation (r2=0.84) between the AATSR- and MISR-derived AOD estimates, but the correlations of the AATSR-derived AOD with MODIS-derived AOD and TOMS aerosol index are lower. In addition, the ability of the sensor to discriminate between different aerosol types is evaluated. Moreover, the estimates of the aerosol properties are used for atmospheric correction of the top-of-atmosphere reflectance. The AATSR surface reflectances are compared with the MODIS bidirectional reflectance distribution function/Albedo and MISR surface products and are shown to correspond with root-mean-square errors of 0.03 and 0.06 or better, respectively. The retrieval method is applied on an image basis resulting in an image of surface reflectance and a separate map of AOD. A map of AOD at 550 nm covering the Sahel and southern Sahara region is presented to demonstrate operation at regional and potentially global scales

Mapping urban change in the UK using satellite radar interferometry

A multitemporal sequence of ERS interferometric coherence data acquired between 1993 and 1999 are utilised for automatically mapping urban change within South Wales, UK. Validation of the change map derived from the coherence data is performed using independent, multidate, digital survey data of the city of Cardiff, UK. All major building developments that have occurred within the study area are located. There is evidence to suggest that this approach is generalisable for a wide range of coherence data and to other regions with similar landscapes.

New Vegetation Albedo Parameters and Global Fields of Soil Background Albedo Derived from MODIS for Use in a Climate Model

Abstract New values are derived for snow-free albedo of five plant functional types (PFTs) and the soil/litter substrate from data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors on board Terra and Aqua. The derived albedo values are used to provide and test an improved specification of surface albedo for the land surface scheme known as the Joint U.K. Land Environment Simulator (JULES) that forms part of the Hadley Centre Global Environmental Model (HadGEM) climate model. The International Geosphere–Biosphere Programme (IGBP) global land cover map is used in combination with the MODIS albedo to estimate the albedo of each cover type in the IGBP classification scheme, from which the albedo values of the JULES PFTs are computed. The albedo of the soil/litter substrate, referred to as the soil background albedo, is derived from partially vegetated regions using a method that separates the vegetation contribution to the albedo signal from that of the soil/litter substrate. The g...

Computationally efficient method for retrieving aerosol optical depth from ATSR-2 and AATSR data

We present a robust and computationally efficient method for retrieving aerosol optical depth (AOD) from top-of-atmosphere ATSR-2 (Along-Track Scanning Radiometer) and AATSR (Advanced ATSR) reflectance data that is formulated to allow retrieval of the AOD from the 11 year archive of (A)ATSR data on the global scale. The approach uses a physical model of light scattering that requires no a priori information on the land surface. Computational efficiency is achieved by using precalculated lookup tables (LUTs) for the numerical inversion of a radiative-transfer model of the atmosphere. Estimates of AOD retrieved by the LUT approach are tested on AATSR data for a range of global land surfaces and are shown to be highly correlated with sunphotometer measurements of the AOD at 550 nm. (Pearsons correlation coefficient r(2) is 0.71.).

Mapping Urban Extent Using Satellite Radar Interferometry

Phase coherence between pairs of ERS SAR images is investigated as a method for mapping urban extent in South Wales, United Kingdom. Separability indices show that image pairs with time delays of greater than 2 months and baseline separations of less than 300 m can discriminate effectively between urban and non-urban land. Classification kappa coefficients greater than 90 percent are achieved, and there is evidence to suggest that a single coherence threshold is applicable for mapping urban extent in any similar landscape.

The inter-comparison of AATSR dual-view aerosol optical thickness retrievals with results from various algorithms and instruments

The Advanced Along-Track Scanning Radiometer (AATSR) dual-view (ATSR-DV) aerosol retrieval algorithm is evaluated for a single scene over Germany (49–53˚ N, 7–12˚ E) on 13 October 2005 by comparison of the aerosol optical thickness (AOT) at 550 nm with products from Multiangle Imaging SpectroRadiometer (MISR), Moderate Resolution Imaging Spectroradiometer (MODIS) and Medium Resolution Imaging Spectrometer (MERIS), in addition to the Atmospheric Aerosol Retrieval using Dual-View Angle Reflectance Channels (AARDVARC) algorithm developed at Swansea University. The AOT was retrieved from the AATSR using the ATSR-DV algorithm, for the pixel size of 1 km × 1 km (at nadir). Then these values were meshed to be consistent with the sampling of the products from the other satellite instruments. The ATSR-DV results compare favourably with the products from orbiting optical instruments dedicated to aerosol retrieval, such as MODIS and MISR, which leads to the conclusion that AATSR is well suited for aerosol retrieval over land when the dual view is used with the ATSR-DV algorithm.

Urban building height variance from multibaseline ERS coherence

Multibaseline European Remote Sensing (ERS) interferometric synthetic aperture radar coherence images from Cardiff, U.K. are investigated with respect to urban form. A model of spatial coherence, taking into account the vertical distribution of scatterers, is inverted to allow urban building height variance to be retrieved. Sixty-nine coherence maps are employed, generated from 20 ERS images. No a priori information is required in the analysis. However, realistic vertical scatterer distributions are retrieved, and Cardiffs central business district is automatically identified. This analysis demonstrates the utility of the urban decorrelation models employed.

Improvements in Aerosol Optical Depth Estimation Using Multiangle CHRIS/PROBA Images

A method has been developed to estimate aerosol optical depth (AOD) over land surfaces using high spatial resolution, hyperspectral, and multiangle Compact High Resolution Imaging Spectrometer (CHRIS)/Project for On Board Autonomy (PROBA) images. The CHRIS instrument is mounted aboard the PROBA satellite and provides up to 62 bands. The PROBA satellite allows pointing to obtain imagery from five different view angles within a short time interval. The method uses inversion of a coupled surface/atmosphere radiative transfer model and includes a general physical model of angular surface reflectance. An iterative process is used to determine the optimum value providing the best fit of the corrected reflectance values for a number of view angles and wavelengths with those provided by the physical model. This method has previously been demonstrated on data from the Advanced Along-Track Scanning Radiometer and is extended here to the spectral and angular sampling of CHRIS/PROBA. The values obtained from these observations are validated using ground-based sun-photometer measurements. Results from 22 image sets show an rms error of 0.11 in AOD at 550 nm, which is reduced to 0.06 after an automatic screening procedure.

Aerosol optical depth from dual-view (A)ATSR satellite observations

Atmospheric aerosol particles play a critical role in the Earth’s radiation budget, yet the global radiative forcing by aerosols is widely recognized as a major uncertainty in our understanding of the climate [IPCC, 2007]. The radiative characteristics of aerosol particles are determined by their shape, size, total amount and chemical composition [Kaufman et al., 1997a]. Overall though, aerosols have a cooling effect at the Earth’s surface by reducing the amount of solar radiation arriving at the surface below the layer of aerosols in the atmosphere. This cooling effect by aerosols is achieved by increasing the planetary albedo at the top-of-the-atmosphere (TOA) through directly scattering some of incoming sunlight back into space. However, some of the radiation can also be absorbed in the atmosphere by aerosols and reemitted. The volcanic eruption of Mount Pinatubo in 1991 provides an excellent natural experiment to demonstrate the surface cooling effect of aerosols. In the following two years after the eruption the average global surface temperature was reduced by about half a degree Celsius, principally owing to the scattering of sunlight by volcanically enhanced stratospheric sulfate aerosol [Hansen et al., 1992]. In addition, to the direct influence that aerosols have on the climate system, aerosols have an indirect effect on the radiative forcing through their interaction with cloud droplets and influence on cloud albedo.

Global atmospheric aerosol optical depth retrievals over land and ocean from AATSR

Aerosol radiative forcing is a major unknown in climate modelling. Owing to the large spatial and temporal variability exhibited by atmospheric aerosol concentrations remote sensing is the only feasible way to obtain global measurements. The ATSR-2 (1995–2002) and AATSR (2002-) radiometer instruments together provide one of the longest available, well calibrated datasets of satellite radiance measurements. The algorithm presented here enables the retrieval of aerosol optical depths (AODs) from these data over a wide variety of surface types including ocean, vegetated land surfaces, and desert. This paper demonstrates the potential for retrievals based on ATSR-2 and AATSR data to reveal spatial and temporal signals in AOD from 1995 onwards. AODs based on AATSR data are validated against surface-based measurements and 6-year time series of regional monthly composites are presented.