267ÐñáêôéêÜ 5ïõ ÐáíåëëÞíéïõ Óõìðüóéïõ Ùêåáíïãñáößáò & Áëéåßáò, 1997 - ÔÏÌÏÓ É

SEA SURFACE TEMPERATURE IN ATSR SATELLITE IMAGE DATA FOR

THE NORTH AEGEAN SEA (EASTERN MEDITERRANEAN SEA).

D. Grillaki-Steiert, I. Robinson, M. Collins

Southampton Oceanography Center, University of Southampton Waterfront Campus, Southampton SO17 1BJ

ÐÅÑÉËÇØÇ

Ä. ÃñõëëÜêç-Steiert, I. Robinson, M. Collins. ÄïñõöïñéêÝò åðéöáíåéáêÞò èåñìïêñáóßáò èáëÜóóçò ãéá ôï Âüñåéï Áéãáßï.

Ôï ATSR (Along Track Scanning Radiometer) ìç÷Üíçìá óôïí ERS1 äïñõöüñï Ý÷åé áíáãíùñéóôåß üôé ìðïñåß íá ìåôñÞóåé ìåáêñßâåéá ôçí ðáãêüóìéá èåñìïêñáóßá èáëÜóóçò. Ç äéðëÞ ôïõ ïðôéêÞ ãùíßá êáé ç äõíáôüôçôá ôïõ íá êáôáãñÜöåé ðïëëÝò óõ÷íüôçôåòåöïäéÜæïõí ôï ATSR ìå ôçí éêáíüôçôá íá áðïôõðþíåé ôçí èåñìïêñáóßá èáëÜóóçò åðéöáíåßáò ìå áêñßâåéá 0.5 âáèìïýò ÊÝëâéí.ÄïñõöïñéêÝò åéêüíåò èåñìïêñáóßáò ëáìðñüôçôáò áðü ðïëëáðëÜ êáíÜëéá ãéá ôï Âüñåéï Áéãáßï ãéá ôï ÷åéìþíá êáé êáëïêáßñéôïõ 1993 Ý÷ïõí áíáëõèåß óå èåñìïêñáóßåò åðéöáíåßáò èáëÜóóçò. Åßíáé ç ðñþôç öïñÜ ðïõ áðïôåëÝóìáôá áðü ATSR äïñõöïñéêÝòåéêüíåò Ý÷ïõí ðáñïõóéáóôåß ãéá ôï Âüñåéï Áéãáßï.Ïé äïñõöïñéêÝò åéêüíåò Ý÷ïõí äåßîåé üôé ôï êáëïêáßñé, øõ÷ñü íåñü ðñïåñ÷üìåíï áíáôïëéêÜ áðü ôç Ìáýñç ÈÜëáóóá, åßíáé ðÜíùáðü ôï ñÞãìá ôïõ Âïñåßïõ Áéãáßïõ êáé ó÷çìáôßæåé äýï âñá÷ßïíåò. Ï Ýíáò âñá÷ßïíáò åéóÝñ÷åôáé áíôéêõêëùíéêÜ óôïí êüëðï ôçòÊáâÜëáò êáé ï Üëëïò åðßóçò áíôéêõêëùíéêÜ óôá áíïé÷ôÜ ôïõ êüëðïõ ôïõ Èåñìáúêïý. Ôï ÷åéìþíá ç øõ÷ñüôåñç èáëÜóóéá ìÜæáöáßíåôáé íá ðñïÝñ÷åôáé ðÜëé áðü áíáôïëéêÜ. Åðßóçò ôï åðéöáíåéáêü íåñü õöáëïêñçðßäáò ãéá âüñåéï Áéãáßï åßíáé øõ÷ñüôåñïáðü ôï íåñü áíïé÷ôÞò èÜëáóóáò. Ôá áðïôåëÝóìáôá áðü ôéò äïñõöïñéêÝò åéêüíåò åßíáé óå óõìöùíßá ìå ùêåáíïãñáöéêÝò ìåôñÞóåéòãéá ôï Âüñåéï Áéãáßï.

INTRODUCTION

The ERS-1 satellite is on orbit around the earth from the 17th of July 1991, at 785 Km from the earth’s surface.The satellite carries three sensors on board, the active microwave instrument, the radar altimeter and along track

scanning radiometer.The along track scanning radiometer captures the thermal radiation as reflected from the earth. The thermal infra-

red radiation, after corrections to account for radiation emitted by the atmosphere (Mc Clain 1981, Barton 1983, Schluesal1987, Mathur 1991), is directly related to the skin temperature of the sea. What distinguishes the ATSR from previousinfrared sensors such as the AVHRR flown on the NOAA satellites is its conical scanning mechanism and ability forimproved atmospheric correction using its two look capability.

The ATSR has been developed specifically to provide an accurate measure of sea surface temperature from space.One of the design features is the provision of six infrared measurements in highly transmissive spectral regions (3.7, 77and 12.5ìm), to assist in the determination of the atmospheric absorption which constrains the accurate determinationof the sea surface temperature. An ATSR image covers an area of 500 Km with a sampling distance every 1 Km and anaccuracy of 0.5 K absolute.

ATSR DATA

ERS-1 ATSR data for the 10th of June and the 18th of December 1993 have been provided by the Rutherford AppletonLaboratory and the Department of Oceanography and Remote Sensing University of Southampton.

The calibrated and collocated data are in the form of equivalent blackbody brightness temperature of the 11 and 12.5ìm thermal channels. Sea surface temperature has been computed by using a chosen combination of ATSR nadir andforward channels (Mathur 1992).

SST = -2.2646 - 2.0498 * CH4_N+3.0589 * CH3_N- 2.1865 * (CH4_N - CG4_O)+2.8129 * (CH3_N - CH3_O)

Where CH3 and CH4 stand for the equivalent black body temperature for the channels 3 and 4 at 11ìm and 12 ìmrespectively and N and O stand for the nadir and off-nadir observations. The retrieved sea surface temperature valuesare transformed into image and contours for display, (Grillaki - Steiert, Brelstaff 1994).

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RESULTS

Fig. 1 is a summer ATSR image, of the North Aegean sea. The Thermaikos Gulf is on the left of the ChalkidikiPeninsula and the Gulf of Kavala and Alexandroupolis are on the right. Surface temperatures are vary from 19.5°C to24°C.

The warmer surface water is depicted by the orange-yellow -green color in the image and the colder water of thedeeper Aegean sea water is represented by the blue color. The violet blue colors describe even colder Aegean sea water.

The following surface summer circulation pattern is revealed. The water originating from the Black Sea (violet blue)appears to be lying on top of the North Aegean through. One of its branches is seen to enter the deep Sporades basin ofthe Thermaikos Gulf in an anti-clockwise direction. For the Thermaikos Gulf in particular the above summer surfacecirculation pattern is also supported by conventional shelf of the Samothraki plateau and then the Gulf of Kavala againin an anti-clockwise direction. For the Thermaikos Gulf in particular the above summer surface circulation pattern isalso supported by conventional oceanographic data analysis (Durrieu de Madron 1992).

The winter image, fig. 2, shows a colder Black sea origin water represented by the blue, violet blue colors travelingalong the North Aegean sea water which is featured by the green-yellow colors in the image. The continental shelf wateras shown in the ATSR image is the coldest water all along most of the North Aegean sea. This is also supported byconventional oceanographic data (Theocharis et al 1993). Temperature values for the winter image vary from 11.5°C to16°C.

For the Thermaikos Gulf in particular, the inner bays of the Gulf have a much colder fluvial origin water blue color.The warmer Sporades basin water, green-yellow color, has entered into the continental shelf of the Gulf in the winter.

The above circulation pattern which is the reverse of the summer one is also supported by conventional oceano-graphic data (Balopoulos, 1987).

These satellite images are the first ATSR data presented for the North Aegean sea to describe seasonal variation ofsynoptic mesoscale surface circulation. The results of this initial analysis of satellite data are very interesting and we planto use monthly time series of satellite data to study the interannual variability and evolution of mesoscale surface pat-terns as well the climatic changes and trends of the North Aegean sea.

REFERENCES

Balopoulos, E., Chronis, G., Lykousis, V., Papageorgiou, E. (1987). Hydrodynamical and sedimentological processes in the northAegean sea: Thermaikos plateau: In Colloque International d’ Oceanologie. Ecosystemes des Marges continentales. C.I.E.S.M.,Perpignan, 24Barton, I. J. (1983). Dual channel satellite measurements of sea surface temperature, Q.J.R. Meteorol. Soc. 109, 365-378.Durrieu de Madron, X., Nyffeler, F., Balopoulos, E., Chronis, G. (1992). Circulation and distribution of suspended matter in theSporades basin (northwestern Aegean sea), Journal of Marine Systems, 3, 237-248.Grillaki - Steiert, D., Brelstaff, G. (1984). ATSR Satellite data: Data Base and Demo using the Interactive Data Language System,National Remote Sensing Center PV Report, p24.Mc Clain, E.P. (1981). Multiple atmospheric window techniques for satellite derived sea surface temperatures. Oceanography formSpace, Plenum, 73 - 85.Mathur, A.K., Agarwal, V.C. (1991). A quantitative study on the effect of water vapour on estimation of sea surface temperatureusing satellite infrared observations. Oceanography of the Indian ocean, Oxford - IBH, p. 350.Robinson, I.S. (1985). Satellite Oceanography: an introduction for oceanographers and remote sensing scientists. Publishers EllisHorwood, p. 455.Robinson, I.S & Donlon, C. (1993). Filaments and finestructure of sea surface temperature in ATSR image data. 2d ERS-1 Sympos.,Vol II, 1087 - 1092.Schluessal, P.k Shin, H., Emery, N. (1987). Computation of satellite derived sea surface temperature with in situ skin measurements,Journal of Geophysical Research, vol 92, No C2, 2859 - 2874.Theocharis, A., Georgopoulos, D. (1993). Dense water formation over the Samothraki and Limnos Plateau in the North Aegean Sea(Eastern Mediterranean Sea), Continental Shelf Research, vol 13, No 8/9, 919 - 939.