C. T. Mutlow

A radiative transfer model for sea surface temperature retrieval for the along‐track scanning radiometer

The measurements made by the along-track scanning radiometer are now converted routinely into sea surface temperature (SST). The details of the atmospheric model which had been used for deriving the SST algorithms are given, together with tables of the coefficients in the algorithms for the different SST products. The accuracy of the retrieval under normal conditions and the effect of errors in the model on the retrieved SST are briefly discussed.

Sea surface temperature measurements by the along-track scanning radiometer on the ERS 1 satellite: Early results

The along-track scanning radiometer (ATSR) was launched in July 1991 on the European Space Agencys 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 0.3°C. To assess data consistency, a complementary analysis of SST data from ATSR was also carried 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 1.0°C 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 ATSRs 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 Earths surface made from space.

Cloud Clearing over the Ocean in the Processing of Data from the Along-Track Scanning Radiometer (ATSR)

Abstract Infrared radiometric measurements of surface parameters are prone to error if clouds are present in the observation path. The along-track scanning radiometer (ATSR) with its novel dual-view feature is able to correct for absorption effects in the clear atmosphere more precisely than previous instruments; hence, it is especially important in this case for the retrieved surface temperatures not to be cloud contaminated. The algorithms used for identifying cloud in the routine processing of the ATSR data are described. These tests rely heavily on the previous experience gained by using data from the Advanced Very High Resolution Radiometer (AVHRR). Modifications to the original AVHRR tests, and the new tests developed, are given in detail.

Calibration of the AATSR instrument

Abstract The Advanced Along Track Scanning Radiometer (AATSR) on ESAs ENVISAT mission is the latest in a series of instruments designed to measure global sea-surface-temperatures to an accuracy of 0.3K and to monitor global vegetation coverage and cloud properties. The accuracy of the SST and other scientific products depends on the accuracy of the top-of-atmosphere radiometric measurements. To ensure that AATSR meets its scientific goals, the instrument is equipped with precision on-board calibration targets. To verify the accuracy of the on-board calibration, a thorough pre-launch calibration exercise was performed, covering field-of-view measurements, visible and infrared radiometric calibrations. The radiometric responses of the visible/near infrared channels were measured, and the in-flight VISible Calibration unit (VISCAL) was calibrated. The calibrations of the thermal infrared channels were verified over a range of target temperatures between 210K to 315K and corrections derived for detector non-linearity. Tests were also performed to verify freedom from any significant scan dependent variations or effects due to changes in the thermal environment.

The ATSR data processing scheme developed for the EODC

Abstract Precise global measurements of sea surface temperature (SST) are of great importance for climate research and our ability to model the ocean/ atmosphere. The ATSR instrument is an Announcement of Opportunity experimental package on the ERS-1 satellite, and it is designed to measure global SST with the accuracy levels (better than 0·5 K) that are required by modern climate models. The ATSR instruments ability to meet its demanding performance objectives depends critically upon a number of novel design features. The way in which these features enable ATSR to achieve its measurement objectives are outlined, and the main tasks of the data processing scheme developed for the U.K. Earth Observation Data Centre are described, including in particular, the ways in which the telemetry data are decoded, the brightness temperature images are geolocated, and the scientific products are derived.

Actual and potential information in dual‐view radiometric observations of sea surface temperature from ATSR

The along-track scanning radiometer (ATSR) on ERS-1 has delivered a continuous global record of radiometric (skin) sea surface temperature (SST) since August 1991. We present a comprehensive analysis of the large-scale and low-frequency characteristics of the data set using direct comparison with other global SST analyses to develop a quantitative understanding of the various factors contributing to the accuracy of and sources of bias in the first 4 years of the ATSR SST record. Such a global analysis is a necessary complement to direct validation against in situ observations, since large-scale sources of bias may be indistinguishable from instrument noise or sampling uncertainty in individual validation campaigns. No large-scale features attributable to atmospheric contamination through aerosols or water vapor are discernible in the difference between the three-channel, dual-view ATSR SSTs and the National Oceanic and Atmospheric Administration (NOAA) blended analysis of advanced very high resolution radiometer (AVHRR) and in situ SST observations. Features in the difference field can be traced to known deficiencies in the data used in the NOAA operational analysis or tentatively related to skin-bulk temperature differences in certain regions. Atmospheric contamination is, however, evident in the ATSR two-channel SST retrieval (used in daytime and after the failure of the 3.7 μm channel) compared to the NOAA operational analysis. The Pinatubo aerosol plume is the dominant feature of the difference field in the first year of ATSR operation. In all cases, however, the amplitude of the atmospheric signature is significantly lower in dual-view than in corresponding single-view ATSR SSTs, indicating that the potential remains for unbiased two-channel SST retrieval even in the presence of aerosol.

Control of tropical instability waves in the Pacific

Westward-propagating waves with periods of 20–30 days and wavelengths of ∼ 1,100km are a prominent feature of sea-surface temperatures (SSTs) in the equatorial Pacific and Atlantic Oceans. They have been attributed to instabilities due to current shear. We compare SST observations from the spaceborne Along Track Scanning Radiometer (ATSR) and TOGA-TAO moored buoys with SSTs from a model of the tropical Pacific forced with observed daily windstress data. The phases of the strongest “Tropical Instability Waves” (TIWs) in the model are in closer correspondence with those observed than we would expect if these waves simply developed from infinitesimal disturbances (in which case their phases would be arbitrary). If we filter out the intraseasonal component of the windstress, all phase-correspondence is lost. We conclude that the phases of these waves are not arbitrary, but partially determined by the intraseasonal winds. The subsurface evolution of the model suggests a possible control mechanism is through interaction with remotely-forced subsurface Kelvin and Rossby waves. This is supported by an experiment which shows how zonal wind bursts in the west Pacific can modify the TIW field, but other mechanisms, such as local feedbacks, are also possible.

Improved sea surface temperature measurements from space

We investigate ATSRs skin SST precision by comparing all the available high resolution (1km) ATSR data coincident with global drifting buoy records for the months of February and March 1992. Comparisons are made at two spatial levels: using spatially averaged ATSR data, and at the full (1km) resolution. SSTs in each case are derived using both the standard algorithm and one using the 3.7µm and 11µm nadir channels. The mean bulk-skin temperature bias is 0.45K (night) and 0.13K (day). The precision is greater: (i) at night than during the day, (ii) when 12µm noise contamination in the standard SST product is removed, (iii) when using a ½° subset for which a corresponding 1km match-up exists. Our highest precision subset has an r.m.s. scatter of just 0.25K and must approach the geophysical limit set by the inherent variability of the skin effect.

The Mutsu Bay experiment : validation of ATSR-1 and ATSR-2 sea surface temperature

A comparison is presented between sea surface temperature measurements from the Along Track Scanning Radiometer (ATSR) space instruments and coincident in situ measurements made using the high-accuracy Scanning Infrared Sea Surface Temperature Radiometer (SISTeR). The in situ measurements were obtained as part of the Mutsu Bay Experiment (MUBEX) conducted in northern Japan during July and August 1996. Differences between the satellite and in situ measurements are lower than 0.2 deg K for ATSR-2 and 0.5 deg K for ATSR-1. This indicates that both ATSR-1 (after 5 years of continuous operation) and ATSR-2 are operating well within their accuracy design specification.

On the Earth's surface energy exchange determination from ERS satellite ATSR data. Part I : Long-wave radiation

This is the first in a series of papers which addresses the determination of the Earths surface energy exchange using data from the Along-Track Scanning Radiometer (ATSR). This paper focuses on long-wave radiation from sea and land surfaces and a technique is proposed for the derivation of land surface temperature (LST) and land surface emissivity retrieval using ATSR data in a new simultaneous split-window method. Two points regarding net long-wave radiation are also considered. Firstly, over land and sea, differences in several previously published are discussed. Secondly, over sea, the effect on the net longwave radiation of using sea surface skin temperatures, which can be derived accurately from satellite thermal band data, as input to the empirical formulae is compared to the use of bulk water temperature taken from in situ measurements. Finally, a new formula is developed for the calculation of net long-wave radiation at the surface. The equivalent sky temperature, T, is used and the results agree...