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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 radiosond...