The measurements made by the Mong-track scanning radiometer are now converted routinely into sea surface temperature (SST). The detMls of the atmospheric model which had been used for deriving the SST a•gorithms are given, together with tables of the coefficients in the a•gorithms for the different SST products. The accuracy of the retrieva• under norma• conditions and the effect of errors in the model on the retrieved SST are briefly discussed. IntroductionThe growing demand for more and more accurate global measurement of sea surface temperature (SST) has led to the development of the along-track scanning radiometer (ATSR), launched on the European Space Agency's (ESA) first European Remote Sensing Satellite (ERS 1) in July 1991. Before the instrument could be built or even before a proposal could be submitted to ESA for acceptance it was necessary to have a more or less complete design of the instrument and a prediction of its anticipated performance. The ATSR is fully described by, for example, Edwards et al. [1990]. One of its most important novel features is the measurement of the upwelling radiation from every picture element, not only in two or more different wavelength channels, but also at tw0different zenith angles. The larger the difference between the angles, and hence the difference in atmospheric pathlength from the satellite to the surface, the larger the difference in atmospheric effect, hence the easier the correction for the atmospheric absorption and emission. For obvious reasons, the optimum angle for one of the measurements is an angle as close to the vertical as the width of the swath permits. The second view must not be too oblique, however, and one of the reasons for this is that in this case the signal would have its dominant contribution from the middle atmosphere, and hence the compensation for the lower-tropospheric effects dominating the atmospheric signal in the nadir view would be poor. Although, in principle, by flying a prototype instrument with a range of viewing angles, the effect on the SST accuracy could be determined, the cost and time of build-x Now at ing the instrument, the range of atmospheric conditions over which the measurement campaigns would have had to be performed, and the very vigorous quality checking of the data would have made this method impractical. We have chosen the method of simulating the brightness temperatures that would be measured under different conditions through the writing of a computer code for a radiative transfer model. The brightness temperatures generated were then used to determine the optimum instrument parameters, to predict the performance of the instrument, and, finally, for calculating SST retrieval coefficients to be used with the ATSR measurements.Before the model could be used in this way, however, it was imperative to check its accuracy. Owing to the unavailability of accurate coincident atmospheric and infrared brightness temperature data measured from space, only an indirect validation was possible by using the infrared brightnes...
Abstract. The along-track scanning radiometer (ATSR), launched in July 1991 on ERS-1, is an infrared radiometer designed to permit retrieval of skin sea surface temperature (SST) to the accuracy required for many climate research purposes. Using the Prelaunch retrieval scheme, this accuracy (0.3 K) was achieved only when observations at 3.7 gm were available, i.e., SSTs derived from nighttime scenes before the failure of this channel in May 1992. Retrievals using only channels at 11 and 12 gm suffered significant biases. First, cold biases of up to 1.5 K arose from the radiative effects of the unanticipated presence of a significant loading of stratospheric aerosol following the eruption of Mount Pinatubo in June 1991. Second, cold biases of up to 0.4 K were associated with regions of high water vapor loading. We solve the first problem by choosing retrieval coefficients to be orthogonal to the modeled changes in brightness temperatures caused by variations in stratospheric aerosol optical depth. We attribute the second problem to deficiencies in radiative transfer modeling of water vapor continuum absorption and show that use of an updated parameterization reduces bias from wet atmospheres. Applying the new retrieval coefficients to ATSR data, we find good consistency between SSTs retrieved with and without the 3.7 gm channels, the global mean and standard deviation of differences between retrievals being of the order of 0.05 K and 0.25 K, respectively. We therefore anticipate that reprocessing ATSR data using our new retrieval scheme will result in a substantially improved record of ATSR SST, in that the following should be reduced to insignificant levels: (1) the artefactual trend (previously -•0.25 K yr -l in tropical regions) corresponding to the decaying load of post-Pinatubo aerosol, (2) the discontinuity in SST retrievals (previously up to 0.7 K) associated with the failure of the 3.7 gm channel, and (3) cold biases (previously -•0.4 K) in wet tropical regions. Thus this work represents a significant advance in terms of the quality of ATSR SSTs for climate research. The techniques are also applicable to both the ATSR-2, flying on ERS-2, and the advanced ATSR (planned for launch on the Envisat platform in 2000). However, we note that even with the improved physical modeling on which the new retrieval coefficients are based, we do not yet meet the stringent requirement of 0.1 K decade -1 stability in retrievals for climate change detection purposes. IntroductionThe measurement of sea surface temperature (SST) is essential to understand and monitor global climate. Radiative, sensible, and latent heat fluxes are largely governed by SST, and these fluxes, in turn, drive the general atmospheric circulation and determine In the ATSR scheme, bias in the SST retrieved from clear-sky brightness temperatures can arise from use of an unrepresentative sample of atmospheric and surface states, or from inadequate modeling of the corresponding brightness temperatures. We find that biases in the original SST retrievals o...
The along-track scanning radiometer (ATSR) was launched in July 1991 on the European Space Agency's first remote sensing satellite, ERS 1. An initial analysis of ATSR data demonstrates that the sea surface temperature (SST) can be measured from space with very high accuracy. Comparison of simultaneous measurements of SST made from ATSR and from a ship-borne radiometer show that they agree to within 0.3øC. To assess data consistency, a complementary analysis of SST data from ATSR was also carded out. The ATSR global SST field was compared on a daily basis with daily SST analysis of the United Kingdom Meteorological Office (UKMO). The ATSR global field is consistently within 1.0øC of the UKMO analysis. Also, to demonstrate the benefits of along-track scanning SST determination, the ATSR SST data were compared with high-quality bulk temperature observations from drifting buoys. The likely causes of the differences between ATSR and the bulk temperature data are briefly discussed. These results provide early confidence in the quantitative benefit of ATSR's two-angle view of the Earth and its high radiometric performance and show a significant advance on the data obtained from other spaceborne sensors. It should be noted that these measurements were made at a time when the atmosphere was severely contaminated with volcanic aerosol particles, which degrade infrared measurements of the Earth's surface made from space. 22,575 22,576 MUTLOW ET AL.: SEA SURFACE TEMPERATURE MEASUREMENTS FROM ERS 1 teorological Office daily bulk SST analysis, and (3) comparison of the ATSR spatially averaged SST products with SST measurements from the drifting buoy network. Brief Description of the ATSR InstrumentThe ATSR instrument and its unique features are described in detail elsewhere [Delderfield et al., 1985; Edwards et al., 1990]. In short, ATSR is a four-channel infrared imaging radiometer with spatially coregistered spectral channels centered at 1.6, 3.7, 10.8, and 12.0 prn. It has been designed for exceptional sensitivity and stability of calibra-MEASUREMENTS FROM ERS 1 22,577 the cloud-free scene brightness temperature observed by ATSR. The coefficient a i is evaluated by linear regression of T s with T i calculated from a representative set of atmospheres using modeled radiation transfer of the atmosphere. The atmospheric data set was obtained from meteorological data provided by the United Kingdom Meteorological Office (UKMO). The comprehensive radiative transfer calculations are performed using a line-by-line model of molecular absorption. Spectral line parameters were taken from the Air Force Geophysical Laboratory line compilation [Rothman et al., 1987], which has been augmented by experimental measurements of the chlorofluorocarbons CFC-11 and CFC-12.
Reliable climate forecasting using numerical models of the ocean‐atmosphere system requires accurate data sets of sea surface temperature (SST) and surface wind stress. Global sets of these data will be supplied by the instruments to fly on the ERS 1 satellite in 1990. One of these instruments, the Along‐Track Scanning Radiometer (ATSR), has been specifically designed to provide SST in cloud‐free areas with an accuracy of 0.3 K. The expected capabilities of the ATSR can be assessed using transmission models of infrared radiative transfer through the atmosphere. The performances of several different models are compared by estimating the infrared brightness temperatures measured by the NOAA 9 AVHRR for three standard atmospheres. Of these, a computationally quick spectral band model is used to derive typical AVHRR and ATSR SST algorithms in the form of linear equations. These algorithms show that a low‐noise 3.7‐μm channel is required to give the best satellite‐derived SST and that the design accuracy of the ATSR is likely to be achievable. The inclusion of extra water vapor information in the analysis did not improve the accuracy of multiwavelength SST algorithms, but some improvement was noted with the multiangle technique. Further modeling is required with atmospheric data that include both aerosol variations and abnormal vertical profiles of water vapor and temperature.
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