Atmospheric back radiation in the tropical pacific: Intercomparison of in-situ measurements, simulations and satellite retrievals

The back radiation has been measured with an Eppley pyrgeometer on board the R/V Vickers in the tropical Pacific Ocean during the field campaigns COARE (Coupled Ocean Atmosphere Response Experiment) and CEPEX (Central Equatorial Pacific Experiment) in February and March 1993, respectively. As part of these compaigns radiosondes have been launched from the Vickers several times per day and cloud cover was observed frequently. The radiosonde and cloud observations are used together with a radiative transfer model to calculate the back radiation for a subsequent intercomparison with the pyrgeometer measurements. Another means of comparison is derived from space-borne SSM/I (Special Sensor Microwave/Imager) measurements. The mean difference between pyrgeometer measurements and simulated downwelling irradiance at the sea surface is less than 2 W/m2, at a mean of 425 W/m2 in the warm pool, with a standard deviation of 8 W/m2. The comparison of satellite measurements with pyrgeometer readings shows a mean difference of-3 W/m2 and a standard deviation of 14 W/m2. The mean difference between satellite-derived back radiation and simulated one is 3 W/m2 with a standard deviation of 14 W/m2. Comparisons with results obtained from bulk formulae applied to surface meteorological observations show a good performance of the bulk parameterisations in the cloud-free case but a general overestimation of the back radiation in cloudy situations.