Stratus over rolling terrain: Large‐eddy simulation reference and sensitivity to grid spacing and numerics

The formation of low stratus cloud over idealized hills is investigated using numerical model simulations. main driver for the radiative cooling due to outgoing longwave radiation. Despite a purely horizontal flow, advection terms in prognostic equations heat and moisture produce vertical mixing across upper edge, leading loss water content. This behavior depicted via budget analysis. More precisely, this spurious caused by diffusive error scheme regions where sloping surfaces terrain-following coordinate intersect top. study shows that intensity (spurious) diffusion depends strongly on resolution, order schemes, choice scalar scheme. A large-eddy simulation with 4- m $$ \mathrm{m} resolution serves as reference. For resolutions few hundred meters simulations carried out setup used weather prediction, strong reduction simulated liquid-water path observed. In keep at coarser small, least fifth-order should be used. present case, weighted essentially nonoscillatory turns increase along sharp edge compared an upwind Furthermore, has impact orography. With modified definition sigma coordinate, it possible classical does not allow any formation.