Improved representation of marine stratocumuli in GCMs

Although marine stratocumulus clouds are very important from a climate point of view they are not well represented in most numerical weather prediction and general circulation models (GCMs) because of their low vertical extension (few hundred meters). This proposal suggests using a (local) increase in vertical resolution in a GCM and including 2nd order effects affecting cloud thickness for more accurate and realistic future climate predictions. Low level, marine stratocumulus clouds have a strong cooling effect as the underlying sea has a low albedo. A correlation between the cloud coverage by low level clouds and the net forcing (cooling) by clouds is observed. But representing stratocumulus clouds is challenging in GCMs. Models forced by increasing CO 2 predict different increases in global average temperature. The large temperature range in these predictions corresponds to different responses in low cloud amount. In this proposal the applicant suggests to define a new Gaussian grid for the ECHAM6 GCM with two additional levels. One level follows the inversion on top of the stratocumulus cloud, the other follows the cloud base with time. This approach will reduce discretization errors which may occur due to large gradients at the inversion and improve the accuracy of the complex parameterizations of the physical processes involved in the stratocumulus life cycle. In the single column version of the preceding model version ECHAM5 this approach was successfully implemented by Siegenthaler-Le Drian (2010) leading to promising preliminary results. Once the thickness of stratocumulus clouds is well represented in the model, 2nd order effects influencing the evolution of stratocumulus clouds such as entrainment will be investigated. Recent large eddy simulation studies of nocturnal stratocumulus clouds show that the cloud thickness is determined by a competition between moistening from decreased surface precipitation and drying from increased entrainment of overlying air, both caused by an increase in aerosol load. The applicant D. Neubauer has also shown in his PhD thesis the importance of accurate cloud thickness for simulations of the effect of increased anthropogenic aerosol on stratiform clouds. In the proposed project, entrainment will be explicitly implemented to improve the response of the modelled marine stratocumulus clouds especially for increases in aerosol loading and diurnal effects. An improvement of the predictions of radiative forcing due to the cloud lifetime effect and a subsequent reduction of the large uncertainties in climate predictions due to the interaction of anthropogenic aerosol particles with clouds can be expected.

Within this project the representation of marine stratocumulus clouds in a global climate model was assessed and improvements for stratocumulus clouds were tested. Clouds are very important from a climate point of view. In particular low level, marine stratocumulus clouds have a strong cooling effect as the underlying sea is dark and the bright clouds effectively scatter the incoming radiation from the sun back to space. It is therefore important to understand how these clouds have changed and will change due to human activities, either due to emission of greenhouse gases and the subsequent warming of the ocean surface or due to emissions of particles (or emissions from which particles will form) so called aerosol particles that provide surfaces for the condensation of cloud droplets. In this project the latter, so called anthropogenic aerosol effect was investigated. It was found that the anthropogenic aerosol effect depends on how clouds are represented in a climate model. Small uncertainties in the representation of the clouds can have a large impact on the assessment of the human influence on clouds and thereby climate. This underscores the importance of simulating stratocumulus clouds as realistic as possible. Representing stratocumulus clouds in global climate models is challenging though because of their low vertical extension (few hundred meters) and climate models have to use a coarse vertical resolution otherwise the simulations would take too long. For a more realistic representation of clouds in a climate model a new parameterization was applied in this project that allows diagnosing the actual cloud top of stratocumulus clouds even at coarse vertical resolutions. First results show a more physical descent of a cloud in an atmospheric column.