A. M. Zavody

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.

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.

Observations of sea-surface temperature for climate research.

The measurement of global sea-surface temperature (s.s.t.) from space, with high absolute accuracy, is one of the important requirements of the World Climate Research Programme (W.C.R.P.). This paper considers the definition of measurement aims based on considerations of specific types of scientific problem, and gives as examples discussion of two particular problems, first the possible influence of Pacific s.s.t. on the lower stratosphere, and second the role of s.s.t. in the cloud-climate feedback process. Following this, a brief review is presented on current status in satellite measurements of s.s.t. with both infrared and microwave techniques, and the paper concludes with a description of a future s.s.t.-measuring instrument, the Along-Track Scanning Radiometer (ATSR).

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.

Satellite measurements of sea-surface temperature for climate research.

Factors determining the accuracy with which the sea surface temperature can be measured from space using infrared radiometers are discussed. These include the clear atmosphere radiative effects, contamination of the signal by clouds in the field of view, and sun glitter at the 3.7 μm ‘atmospheric window’. The effects of near surface vertical temperature gradients caused by surface heat loss (the skin effect) and by solar heating (the diurnal thermocline) are also discussed. A review of present measurement capabilities is made and a brief description is given of the Along Track Scanning Radiometer (ATSR), which will fly on the European satellite ERS-1 to provide accurate measurements of sea surface temperature using a multi-angle, multichannel technique to overcome the effects of the clear atmopshere.

Sun Glint Contamination in ATSR-2 Data: Comparison of Observations and Values Calculated from the Measured 1.6-μm Reflectivities

Abstract The effect of radiation from the sun reflected at the sea surface on the 11- and 12-μm brightness temperatures measured by the Along-Track Scanning Radiometer ATSR-2 has been investigated. An attempt was made to determine its magnitude by comparing nadir-view and forward-view brightness temperature differences in glint and nonglint regions, and the results are compared with theoretical predictions computed by using the 1.6-μm reflectivity data. Atmospheric absorption and the polarization sensitivity of the instrument have been fully taken into account. The results show that temperature increases of a few tens of millikelvins are possible even over the open ocean, and that they correlate well with the measured 1.6-μm reflectivity values. A scheme for correcting the sun glint contamination is proposed.

A Novel Method for Calibrating the ATSR-2 1.6-μm Channel Using Simultaneous Measurements Made in the 3.7-μm Channel in Sun Glint

Abstract Reflection of the sun’s radiation from the sea surface can give rise to abnormally high brightness temperatures in the 3.7-μm channel of the Along-Track Scanning Radiometer (ATSR-2). Using a radiative transfer model, this effect is used to calibrate the instrument’s 1.6-μm channel by first characterizing the atmosphere with the aid of 11- and 12-μm measurements and then calculating the surface’s effective reflectivity for radiation in the 3.7-μm channel. Scaling of this reflectivity by a factor deduced from the refractive indices of water at 1.6 and 3.7 μm allows, in principle, the calculation of the 1.6-μm channel reflectivity. This can be used directly to give a calibration factor for the 1.6-μm ATSR-2 channel. The polarization sensitivity of the ATSR-2 instrument requires a small correction to be made, and this can be derived by using measurements made in sun-glint areas in both the nadir and the forward views.

Derivation of OH Concentrations from LIMS Measurements

Stratospheric concentrations of OH have been derived from LIMS measurements of minor constituents. Two methods have been used. Assuming that HNO3 and NO2 are in photochemical steady state, LIMS measurements of these species, with knowledge of appropriate rate constants and a calculation of the HNO3 photolysis rate, allow nearly global fields of OH to be derived. The derived profiles show satisfactory agreement with model calculations and the limited number of in situ observations. As a check on our method, OH has also been derived by calculations of its sources and sinks using the LIMS measurements of H2O. The two methods agree extremely well in low latitudes. At higher latitudes the agreement is less satisfactory. This is discussed in terms of the diurnal behaviour of the species and the time constant of the HNO3/NO2 equilibrium.