Diurnal variability matters to those who wish
- to make optimum use of SST data captured at different times of day
- to maximize the utilization of high-resolution satellite data, particularly in coastal and frontal areas
- to make accurate estimates of ocean-atmosphere fluxes that are modified by any diurnal warming cycle
- to estimate the skin SST at a particular time of day
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A satellite measurement of a warm-layer situation gives an SST that does not accurately represent the heat content of the upper ocean. This can adversely affect ocean forecasting models that assimilate the SSTs as representing the temperature of their top model layer (typically of order 10 m thick). Some assimilation and analysis systems therefore discard SST data that are at risk of significant diurnal warming. Others may use all data and simply tolerate the increased residuals and SST bias introduced by diurnal variability.
Data denial is a problem most particularly in coastal and frontal areas, where for high-resolution SSTs we depend on infra-red observations. Why? Cloud screening is usually more efficient for the daytime imagery where reflectance channels are available. Daytime IR SSTs are thus less prone to cloud-related biases and should be the most reliable SSTs at high spatial resolution; it is disappointing to discard them because of potential diurnal warming. If DV can be adequately estimated, these data can be used and their information content of these data for quantifying the state of the upper ocean can be exploited.
The instantaneous radiative and turbulent heat fluxes between ocean and atmosphere are fundamentally functions of skin SST. An analysis of the evolution of the skin temperature through the day is therefore both supportable by satellite observations and useful in improving ocean-atmosphere flux estimates. The air-sea flux of gas is likewise affected by SST if the gas's solubility depends on temperature. This is the case for carbon dioxide, for example.
An unbiased estimate of the skin SST is also relevant as a prior in atmospheric sounding (assimilation of radiances).
(Last Updated: 03-08-2007)