Radiative budget and cloud radiative effect over the Atlantic from ship-based observations 1Helmholtz Centre for Ocean Research Kiel (GEOMAR), Duesternbrooker Weg 20, 24105 Kiel, Germany
16 Oct 2012
2Leibniz-Institute for Tropospheric Research (IfT), Permoserstraße 15, 04318 Leipzig, Germany
Received: 18 January 2012 – Published in Atmos. Meas. Tech. Discuss.: 01 March 2012 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.
Revised: 23 August 2012 – Accepted: 05 September 2012 – Published: 16 October 2012
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
non-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 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.
Citation: Kalisch, J. and Macke, A.: Radiative budget and cloud radiative effect over the Atlantic from ship-based observations, Atmos. Meas. Tech., 5, 2391-2401, doi:10.5194/amt-5-2391-2012, 2012.