Two-dimensional simulation of orographic effects on mesoscale boundary-layer convection

In this paper, orographic effects on mesoscale boundary-layer convection are studied through a series of idealized numerical experiments. It is found that hills tend to weaken convective activity at their summits under higher background winds, whilst strong updraughts can be observed at the summits, or slightly downwind of the peaks, under light-wind conditions; these can be associated with so-called convective cores. When the background winds are strong, the effects of the hill length on the results are only significant when the height of the hill reaches 500 m. The combined effect of a sensible-heating maximum on the hill summit and baroclinic tendencies due to the elevated heating is a tendency to produce a convective core under reasonably light-wind conditions. Under higher-wind conditions, the strong ascents and descents on both sides of the hill, as well as the lee-wave dynamics, seem to pose a more important impact on the convective features than the thermal forcing of the hill. A Richardson-number balance has been proposed to explain these two kinds of response. Finally, under stronger-wind conditions, the convective available potential energy (CAPE) is small at the summit and reaches its maximum value in the lee of the hill while, under light-wind conditions, the air parcels at the summit have more buoyancy and the CAPE downwind of the top of the hill is slightly larger than that elsewhere.