Large Eddy Simulation of Roll Convection during a strong Cold Air Outbreak

Roll convection is a common phenomena in convective boundary layers (CBL) with background wind. So far there is no large eddy simulation which could represent clear signals of roll convection under strong heating and weak CBL wind shear (like observed in cold air outbreaks), that is with large -z_i/L,in which z_i denotes the the boundary layer height and L the Monin-Obukhov stability length. In this contribution we present a large eddy simulation (LES) which successfully represents roll convection during a strong cold air outbreak (air-sea temperature difference of 26K). The model domain covered an area of 64km X 400km with a resolution of 50m horizontally and 25m vertically (1.4*10⁹ grid points), large enough to capture the evolution of the large scale organized structures and able to explicitly resolve the small scale unorganized turbulence. In contrast to most other LES studies of cold air outbreaks, which used periodic boundary conditions, this simulation was carried out with an inflow and an outflow boundary so that thermal wind effects (resulting from the inclination of z_i and warming of CBL in flow direction) could develop. The cloud streets in the simulation, associated with the updraft in the roll convection, look very similar to those in satellite pictures of cold air outbreaks. Furthermore we present results from some additional LES studies with smaller domain, but varying -z_i/L. Herefrom we conclude that there are two types of roll convection both caused by thermal instability, but with different characteristics: One type only exists for small -z_i/L and oriented with the roll axis in direction of the CBL wind shear vector and an other type existing also for large -z_i/L but oriented in direction of the CBL wind. Moreover the cloud structure between this two types differ in the horizontal aspect ratios. In the former case there are single cloud streets which extend over long distances up to 100 kilometers, whereas in the latter case single cloud streets only extend at most up to some tenth of kilometers. For this study we used the LES model PALM (parallelized LES model). The following figure shows the simulated cloud field for a part of the model domain of one of the above mentioned studies.