Correction of Marine Air Temperature Observations for Solar Radiation Effects

Abstract

The effect of incoming solar radiation on merchant ships' observations of air temperature was assessed as part of the Voluntary Observing Ships' Special Observing Project for the North Atlantic (VSOP-NA), The ships' reports were compared with interpolated output from a numerical weather model. Differences between the ship values and the model values for air temperature (?Ta) were found, in the mean, to be independent of instrument type, ship size, and, except for very badly exposed sensors, exposure. The differences were related to the relative wind speed over the ship (V) and the incoming shortwave radiation (R). The formula derived for the radiative heating error δT was δT = 2.7 ? 10?3 R ? 3.2 ? 10?5 RV, where δt has units of degrees Celsius, R is in watts per square meter, and V is in knots. After correcting the ?Ta values, an approximately constant bias remained with the ship reports on average 0.4°C lower than the model air temperatures. This offset probably represents a mean bias in the model estimates; however, a residual bias in the ship observations is also a possibility. There was also evidence that heat generated by the ship caused a temperature overestimate of about 0.4°C at zero relative wind, decreasing to a negligible level at a relative wind speed of 20 kt. For the North Atlantic dataset used, the correction reduced daytime marine air temperature reports by 0.63°C on average. Applying the correction to the VSOP-NA air temperature data was found to significantly change estimates of sensible and latent heat fluxes.