The radiation climate of the dead sea

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Abstract

Three years of continuous measurements on the NW coast of the Dead Sea, together with a series of shipborne observations made above its water surface, were used to characterize and analyze the radiation climate of the sea.

Incident global radiation on the coast, averaged 6.64 GJm−2 per year, less than values measured both at sea and at nearby pyranometer stations despite the greater cloud cover of the latter. The relatively low insolation was attributed to the considerable back scatter from the sky shown by the high values of diffuse sky radiation recorded even during cloudless conditions. Short-wave radiation reflected from the sea was calculated, on the basis of a quadratic relationship between reflectivity and solar altitude, to total O50GJ m−2 annually. Above a critical solar altitude of 45° reflectivity averaged 6 per cent. Monthly values of reflectivity from the Dead Sea agreed with those calculated for a standard sea surface.

Atmospheric long wave radiation measured on the coast totalled ll.44 GJ m−2 annually. Its seasonal and diurnal variations closely followed those in air temperature. Small variations in the apparent sky emissivity, which averaged 0.80, could be accounted for by reference to accompanying changes in the water content and transmissivity of the atmosphere.

Midsummer and midwinter in situ determinations of the emissivity of the Dead Sea yielded near-black body values which, with the high surface temperatures of the sea, resulted in a terrestrial radiation flux totalling 14.69 GJ m−2 annually.

Direct pyrradiometer measurements over the Dead Sea were used to establish the accuracy of long term estimates of the radiation balance of the sea and to develop a method of calculating it from routine, shore-based measurements of global radiation.

The radiation balance of the Dead Sea, normalized to the incident radiation, was 0.45,22 per cent less than the average for eight other non-saline water bodies in comparable mid-latitude climates. The relatively low radiation balance of the Dead Sea was attributed to the enhanced long-wave radiation loss caused by the elevated surface temperature of its salt-saturated waters.

Thus the salinity of the Dead Sea reduces its evaporation loss by two mechanisms; the direct effect of the reduced vapour pressure of the water surface and the indirect effect of the reduced radiation balance.

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