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I. J. Barton and R. P. Cechet

Abstract

Satellite measurements of sea surface temperature (SST) are regularly available from data supplied by the AVHRR instruments on the NOAA meteorological satellites. In cloudless areas SST is derived from the infrared data using a differential absorption technique to correct for the effect of the atmosphere. For the AVHRR data a multichannel (multiwavelength) approach is used and global operational algorithms are in use. During 1990 a new instrument that has been specifically designed to measure SST will be launched on the European satellite, ERS-1. The Along Track Scanning Radiometer (ATSR) will provide six infrared measurements for each pixel on the earth's surface. Using the same differential absorption techniques, a multitude of algorithms for providing SST will then be possible. In this note a technique is described that will enable the comparison and optimization of SST algorithms and will also aid in the selection of the most appropriate algorithm for ATSR data analysis.

To demonstrate the technique mosaic images were constructed from small areas of cloud-free infrared images of the sea surface as seen by the NOAA-9 AVHRR. Each area was approximately 55 km by 55 km and, by arranging them in order of decreasing mean temperature and increasing mean zenith angle, it was possible to use an image analysis system to compare the relative performance of different algorithms for deriving surface temperature. The images were also used to compare some NOAA-7 SST algorithms.

A second set of mosaic images was constructed using NOAA-10 AVHRR data collected on the same night and for the same surface location. Images of SST derived with theoretical NOAA-10 algorithms were compared with those from an operational NOAA-9 algorithm. Then a simple optimization technique was used to obtain a new algorithm for deriving SST from channels 3 and 4 of the NOAA-10 instrument. This optimization scheme, using an ordered mosaic image that covers a wide range of conditions (location, local zenith angle, or some other parameter), should be applicable to the comparison and optimization of other satellite data products.

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I. J. Barton, A. J. Prata, and R. P. Cechet

Abstract

The Along Track Scanning Radiometer (ATSR) was launched on the ERS-1 satellite on 17 July 1991. During the following six months, a concentrated effort was made to validate the sea surface temperature (SST) derived from data supplied by this new generation radiometer. Ship and aircraft radiometers collected “ground truth” data that were coincident with ATSR measurements and thus allowed a comparison of the surface and space measurements. A large proportion of the early validation data was obtained during four research vessel cruises in Australian waters, and a detailed analysis of those results is presented here. Ancillary data were collected to support the shipborne radiometer measurements and to allow further analyses beyond the important validation task. These data included the standard surface meteorological data, bulk SST, and, in most cases, ship-launched radiosondes. Four different algorithms derived using a theoretical atmospheric transmission model were applied to the ATSR data to provide estimates of SST, and these estimates were compared to the surface-based measurements. All the algorithms gave reasonable agreement with each other as well as agreement with the surface data. The algorithm using all six infrared measurements gave the lowest standard deviation but showed a warm bias of 0.2 K when compared to the temperature of the skin layer of the ocean. The validation results show that the ATSR instrument can provide SST within the design accuracy of 0.3 K. The results of the validation presented here are in good agreement with those reported elsewhere using other datasets. Further improvements in SST accuracy, perhaps to 0.2 K, may be expected with a more rigorous analysis of the data.

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I. F. Grant, A. J. Prata, and R. P. Cechet

Abstract

The correction of a land surface albedo estimate made at one solar zenith angle (SZA) from a polar-orbiting satellite to a standard SZA or to a daily mean albedo requires knowledge of the dependence of the albedo on SZA. This paper uses ground-based measurements of the clear-sky albedo at a uniform grassland site at Uardry (34.39°S, 145.30°E) in southeastern Australia to investigate the accuracy to which the daily mean albedo can be inferred from the albedo at 1030 LST, given knowledge of the SZA dependence of albedo to various levels of detail. During nine months in which the daily mean albedo varied from 0.20 to 0.27, the albedo always had the expected minimum near noon but the strength of the albedo’s SZA dependence varied greatly. For a few months, albedos were up to 0.04 higher in the afternoon than in the morning, and variations on finer timescales of up to 0.02 also appeared in the diurnal albedo cycle for days or weeks. These features of the diurnal variation were all seen at two or three surface points separated by up to 750 m and so are expected to appear at the ∼1-km resolution of many satellite sensors. For the Uardry grassland site, the error in estimating the daily mean albedo from the 1030 LST, albedo can be up to 0.03, which is 15% of an albedo of 0.20, if the albedo is assumed to be constant through the day. The maximum error is reduced to about 0.02 if a simple model of the SZA dependence is used with even an approximate value for the parameter that controls the strength of the dependence, and to 0.01 or less if the strength of the dependence is appropriate to the state of the vegetation on the day. Afternoon–morning asymmetry in the albedo can contribute almost 0.01 to the error in inferring a daily albedo from a morning measurement.

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R. P. Cechet, J. Bennett, I. Helmond, P. A. Coppin, E. F. Bradley, I. J. Bapton, and J. S. Godfrey

Abstract

An accurate platinum RTD-based (resistive temperature device) system has been developed to measure the vertical temperature profile in the region of the atmosphere-ocean interface. TASITA, the towed air–sea interaction temperature analyzer, continuously measures the vertical temperature profile using 17 fixed temperature probes mounted on the instrument: 9 in the uppermost meter of the ocean, and 8 in the lowest 2 m of the atmosphere. The absolute accuracy is better than ±0.05°C, and the relative accuracy between RTDs is ±0.01°C. The instrument is designed to be towed beside a research vessel in undisturbed water outside the ship's wake. Towing speeds between 4 and 8 kt are possible. Instrument operational use is aimed specifically at low wind conditions when the sea surface is smooth to slight and mixing in the top few meters of the ocean is inhibited. Under these conditions a diurnal warm surface water layer is often present in which the surface temperature of the water is markedly different to that tens of centimeters below. Data collected in the western equatorial Pacific show variations in the temperature structure of the surface mixed layer caused by solar beating of the ocean surface and freshwater lenses resulting from heavy precipitation.

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