Review Article. The sea surface thermal boundary layer and its relevance to the measurement of sea surface temperature by airborne and spaceborne radiometers†

Abstract

Abstract This review examines the importance of the thermal boundary layer of the ocean to the measurement of sea surface temperature (SST) by satcllitc-bornc infrared (IR) radiometers. Attention is focused on the difference between the temperature of the top 01mm which is observed by radiometers, and the temperature found at depths between a few centimetres and one metre, which is that recorded as bulk SST by ship measurement techniques. The question addressed is whether corrections for this effect are necessary in the light of the accuracy of IR radiometers, and the demands made by the application of SST data. A brief preview is made of the accuracy of the best presently available sensor for space observations of SST, the AVHRR, the associated atmospheric corrections and other sources of error. Current knowledge of the thermal boundary layer of the ocean is then presented in a thorough review of the scientific literature. The evidence of field observations suggests that the surface skin can be typically 0.1-0.5 degK cooler than the water a few centimetres deeper. Theoretical models of the effect are available and there is some understanding of the dependence of spatial and temporal variability of the skin effect on other environmental parameters, but there is a need for more systematic observation before prediction of the effect can confidently be achieved. The existing and potential applications of SST in oceanography and meteorology are summarized, with emphasis on the required accuracy, in certain areas 0 2 degK being desirable. Following a survey of the likely capabilities of the next generation of spaceborne IR radiometers (notably the ATSR) it is concluded that while the inaccuracies of present systems tend to be greater than the skin effect, if future systems meet their specification then the skin temperature deviation will become a significant factor in the calibration procedure. In the light of this, the review concludes with some remarks regarding the value of further studies of the thermal skin effect.