a Numerical Investigation of AN Orogenic Mesoscale Convective System

The interaction of topographically induced thermally and mechanically driven diurnal flow regimes in the lee of the Rockies is shown to lead to the growth of a mesoscale convective system (MCS). The findings are based on a series of two-dimensional and three-dimensional non-hydrostatic numerical simulations of an intensively-studied convective event that occurred during the 1977 SPACE (South Park Area Cumulus Experiment)/HIPLEX (High Plains Experiment). The results suggest a multi-scale interaction leading to the formation of a quasi-geostrophically balanced warm core deep circulation system on the scale of the Rossby Radius. The genesis is initiated by the formation of deep cumuli in association with mountain wave and slope flows. The simultaneous maintenance of a thermal boundary layer capping inversion over the High Plains to the east, keeps the convection localized and therefore leads to a systematic balanced atmospheric response over a period of 12 hours. The convective core itself is organized on the meso-(beta) scale and is transient in nature. Because favorable conditions for deep convection are localized by the plains inversion, convection does not move with the meso-(beta) pulses emitted and instead reforms continually as the vertical motion oscillates. The meso-(alpha) scale system moves eastward with a speed near the mean tropospheric wind. As nightfall occurs, it is shown that the meso -(alpha) system is transformed from one with a single dominant meso-(beta) scale core, to one with several more diffuse meso-(beta) scale regions of vertical motion. It is concluded that the transformation is because of destabilization of the upper troposphere by longwave radiative cooling which leads to horizontal ducting of the transient meso-(beta) response.