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

Abstract

Abstract. The aim of this study is to determine cloud-type resolved cloud radiative budgets and cloud radiative effects from surface measurements of broadband radiative fluxes over the Atlantic Ocean. Furthermore, based on simultaneous observations of the state of the cloudy atmosphere, a radiative closure study has been performed by means of the ECHAM5 single column model in order to identify the model's ability to realistically reproduce the effects of clouds on the climate system.An extensive database of radiative and atmospheric measurements has been established along five meridional cruises of the German research icebreaker Polarstern. Besides pyranometer and pyrgeometer for downward broadband solar and thermal radiative fluxes, a sky imager and a microwave radiometer have been utilized to determine cloud fraction and cloud type on the one hand and temperature and humidity profiles as well as liquid water path for warm nonprecipitating 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 for stratus clouds. No significant meridional dependencies were found for the surface radiation budgets and cloud effects. The strongest surface longwave cloud effects were shown in the presence of low level clouds. Clouds with a high optical density induce strong negative solar radiative effects under high solar altitudes. The mean surface net cloud radiative effect is −33 W m −2 .For the purpose of quickly estimating the mean surface longwave, shortwave and net cloud effects in moderate, subtropical and tropical climate regimes, a new parameterisation was created, considering the total cloud amount and the solar zenith angle.The ECHAM5 single column model provides a surface net cloud effect that is more cooling by 17 W m −2 compared to the radiation observations. This overestimation in solar cooling is mostly caused by the shortwave impact of convective clouds. The latter show a large overestimation in solar cooling of up to 114 W m −2 . Mean cloud radiative effects of cirrus and stratus clouds were simulated close to the observations.

Global atmospheric downward longwave radiation at the surface from ground‐based observations, satellite retrievals, and reanalyses

“…It is lower by ~30% over middle latitudes and higher by more than 30% over tropical regions, in particular over the Intertropical Convergence Zone (ITCZ). Its negative bias over middle latitudes is mostly for low clouds and appears to explain much of the negative bias of MERRA L d since low clouds generally have an effect of 70 W m −2 on L d over middle latitude oceans [ Ghate et al ., ; Kalisch and Macke , ], especially the low level stratus clouds over cool ocean surfaces [ Harshvardhan , ]. Clouds can also increase L d by ~50 W m −2 in polar regions [ Cho et al ., ; Shupe and Intrieri , ] as well as ~ 50 W m −2 over middle latitude land [ Dong et al ., ; Wang et al ., ].…”

Observation and Parameterization of Broadband Sea Surface Albedo

“…The atmospheric transmittance is a dimensionless number between 0 and 1. This value mainly depends on the cloud properties, with a high value in clear skies and a low value under cloudy conditions (Kalisch & Macke, ). Figure a shows the atmospheric transmittance during the observations, which varied between 0.06 and 0.85.…”

“…On the other hand, it is difficult to measure the upwelling solar radiation at the sea surface (Katsaros et al, 1985). In most cases, only the incident solar radiation has been measured in conventional marine observations (Kalisch & Macke, 2012;MacWhorter & Weller, 1991). Thus, an accurate SSA observation is necessary to complete the heat budget of the upper ocean.…”

Comparison of Cloudy‐Sky Downward Longwave Radiation Algorithms Using Synthetic Data, Ground‐Based Data, and Satellite Data

“…For cloudless skies, surface DLR is largely determined by vertical profiles of atmospheric temperature and humidity, especially the several hundred‐meter‐thick atmospheric layer near the Earth's surface. When clouds are present, they attenuate the solar radiation before it reaches the surface, but enhance DLR by absorbing surface outgoing longwave radiation and reemitting longwave radiation (Kalisch & Macke, ).…”