Influence of 3D thermal radiation on cloud development

This thesis aims to answer the question if 3D effects of thermal radiative transfer need to be considered in cloud resolving simulations and if an influence of 3D thermal heating and cooling rates exists in contrast to common 1D approximations. To study this question with the help of a cloud resolving model, an accurate, yet fast parameterization of 3D radiative transfer is needed. First, an accurate 3D Monte Carlo model was developed which was used as benchmark for developing the fast `Neighboring Column approximation' (NCA), which was then coupled to the UCLA-LES to study the effects of 3D thermal heating and cooling rates in comparison to common 1D radiative transfer approximations. First, differences between common 1D radiative transfer approximations and a correct 3D radiative transfer model were analyzed. For this, efficient Monte Carlo variance reduction methods have been developed and implemented in MYSTIC, a Monte Carlo radiative transfer model. The dependence of 1D and 3D heating and cooling rates on cloud geometry has been investigated by analyzing idealized clouds such as cubes or half spheres. Further more, 1D and 3D heating and cooling rates in realistic cloud fields were simulated and compared. It could be shown that cooling rates reach maximum values of several 100 K/d at cloud tops if the model resolution was between 50 m to 200 m. Additional cloud side cooling of several 10 to 100 K/d was found in 3D heating and cooling rate simulations. At the cloud bottom, modest warming of a few 10 K/d occurs. Heating and cooling rates depend on the vertical location of the cloud in the atmosphere, the liquid water content of the cloud, the shape of the cloud and the geometry of the cloud field (for example the distance between clouds). Based on the results of a detailed analysis of exact simulations of 3D thermal heating and cooling rates, a fast, but still accurate 3D parameterization for thermal heating and cooling rates has been developed. This parameterization, the `Neighboring Column Approximation' (NCA), is based on a 1D radiative transfer solution and uses the next neighboring columns of a column to estimate the 3D heating or cooling rate. The method can be used in parallelized models. With the NCA, it is possible to simulate 3D cloud side cooling and warming. It was shown that the NCA is a factor of 1.5 to 2 more expensive in terms of computational time when used in a cloud resolving model, compared to a 1D radiative transfer approximation. The NCA was implemented in UCLA-LES, a cloud resolving, large-eddy simulation model. With the UCLA-LES and the NCA it was possible for the first time to study the effects of 3D interactive thermal radiation on cloud development. Simulations without radiation, with 1D thermal radiation and 3D thermal (NCA) radiation have been performed and differences have been analyzed. First, single, isolated clouds were investigated. Depending on the cloud shape, 3D thermal radiation changes cloud development in comparison to 1D thermal radiation. Overall it could be shown that a thermal radiation effect on cloud development exists in general. Whether there is a differences between 1D and 3D thermal radiation on cloud development seems to depend on the specific situation. One of the main features of thermal radiation affecting a single cloud is a change in the cloud circulation. Stronger updrafts in the cloud core and stronger downdrafts at the cloud sides were found, causing an enhanced cloud development at first, but a faster decay of the cloud in the end. Second, large scale simulations of a shallow cumulus cloud field in a 25 x 25 km^2 domain with 100~m horizontal resolution were analyzed. To the authors knowledge, this is the first time that a cloud field of this size and resolution was simulated including 3D interactive thermal radiation. It was shown that on average, updrafts, downdrafts and liquid water increases if thermal radiation is accounted for. While most variables (for example liquid water mixing ratio or cloud cover) did not show significant systematic difference between no-radiation simulation and the simulations with 1D and 3D thermal radiation, the cloud size (or horizontal extent) was larger in the simulations with interactive 3D thermal radiation. Convective organization set in after a few hours already. This is a clear indication that 3D thermal radiation could trigger convective organization.