Radiative budget and cloud radiative effect over the Atlantic from ship-based observations

摘要

The aim of this study is to determine cloud-type resolved cloud radiativebudgets and cloud radiative effects from surface measurements of broadbandradiative fluxes over the Atlantic Ocean. Furthermore, based on simultaneousobservations of the state of the cloudy atmosphere, a radiative closure studyhas been performed by means of the ECHAM5 single column model in order toidentify the model's ability to realistically reproduce the effects of cloudson the climate system.An extensive database of radiative and atmospheric measurements has beenestablished along five meridional cruises of the German research icebreakerPolarstern. Besides pyranometer and pyrgeometer for downward broadband solarand thermal radiative fluxes, a sky imager and a microwave radiometer havebeen utilized to determine cloud fraction and cloud type on the one hand andtemperature and humidity profiles as well as liquid water path for warmnon-precipitating clouds on the other hand.Averaged over all cruise tracks, we obtain a total net (solar + thermal)radiative flux of 144 W m?2 that is dominated by the solar component.In general, the solar contribution is large for cirrus clouds and small forstratus clouds. No significant meridional dependencies were found for thesurface radiation budgets and cloud effects. The strongest surface longwavecloud effects were shown in the presence of low level clouds. Clouds with ahigh optical density induce strong negative solar radiative effects underhigh solar altitudes. The mean surface net cloud radiative effect is?33 W m?2.For the purpose of quickly estimating the mean surface longwave, shortwaveand net cloud effects in moderate, subtropical and tropical climate regimes, anew parameterisation was created, considering the total cloud amount and thesolar zenith angle.The ECHAM5 single column model provides a surface net cloud effect that ismore cooling by 17 W m?2 compared to the radiation observations. Thisoverestimation in solar cooling is mostly caused by the shortwave impact ofconvective clouds. The latter show a large overestimation in solar cooling ofup to 114 W m?2. Mean cloud radiative effects of cirrus and stratusclouds were simulated close to the observations.