Downwelling Longwave Irradiance at the Ocean Surface: An Assessment of In Situ Measurements and Parameterizations

Abstract

Two sets of ocean surface longwave irradiance measurements collected during the FASINEX and MILDEX experiments are analyzed for quality and variability studies. Using concomitant radiosonde data, the clear-sky contribution to the downward flux at the surface is computed and, subsequently, the effect of clouds from the surface measurements is deduced. The longwave irradiance computations are performed using a broadband model, a simpler parameterization, and an empirical formula. The three schemes are chosen because they represent different, possible approaches for computing surface longwave irradiance. They are intercompared, separating clear and cloud components, and verified against in situ measurements. During both experiments, which took place in midlatitudes during different seasons, variations in the downward longwave flux associated with clear-sky variations (air temperature, humidity changes) and cloud effects are found to be of the same order of magnitude (≈70 W m?2). Applied to radiosonde profiles, the two more physical schemes provide consistent results with a 4 W m?2 the standard deviation of the differences is only 4 W m?2. These schemes, however, exhibit a 4 W m?2 relative bias, which is comparable to the desired accuracy for monthly-mean longwave flux estimates. Intercomparisons with the empirical formula yield larger standard deviations, demonstrating the formula's inability to reproduce adequately the clear-sky flux variability. For all three schemes, the scatter around the measured values is rather large (20?25 W m?2); surprisingly, the empirical formula gives the best results. This is explained by the large uncertainty in the cloud parameters used as input to the schemes; when no reliable estimates of these critical variables can be made, it may be more accurate to take a simple parameterization for the cloud effect on the longwave flux.