Study on a Platform Effect in the In Situ Sea Surface Temperature Observations under Weak Wind and Clear Sky Conditions Using Numerical Models

Abstract

Estimation of the vertical sea water temperature difference between 0- and 1-m depths is necessary to validate the accuracy of satellite-derived sea surface temperatures (SSTs). The authors have examined diurnal temperature variations in the sea surface layer under weak wind and clear sky conditions by two kinds of numerical models. Under such conditions, a large temperature gradient develops in the vicinity of the sea surface and little turbulence is induced by wind stress mixing. The vertical temperature profiles between 0- and 2-m depths observed with a sea surface temperature profiler buoy (SSTPB) are well simulated by the models. SSTPB was specially designed not to disturb the water column in the vicinity of the surface. However, the 1-m-depth temperatures simulated by the models do not agree with that observed with a buoy under weak wind conditions. The hull of buoy is large and can be inferred to generate turbulence in the near-surface layer through the interaction with the swell, wind waves, and currents. This disagreement may be attributed to in-water turbulence induced by the measuring platforms. If the eddy diffusion coefficients are forcibly enlarged in 0?1-m depth, so that the heat near the sea surface can be transferred efficiently to 1-m depth, the simulated 1-m-depth temperature becomes closer to the observed temperature to a certain extent. It is inferred that, under these conditions, the turbulence induced by measuring platforms and instruments such as buoys, CTDs, and ships may be more intense than that induced by the wind stress. This effect, called ?platform effect,? will weaken a large vertical temperature gradient and reduce SST. In contrast, the SSTPB has a shape that does not induce this effect. The platform effect described here may be important for SST retrievals from satellite infrared measurements assisted by the in situ SSTs observed with buoys under weak wind and clear sky conditions.