Mountain Cumulus initation along the front range

The onset of cumulus clouds over the Colorado Rockies is modelled using a high-resolution (1km) simplified mesoscale model. The model has been designed to predict the daytime evolution of the convective boundary layer (CBL) under the combined action of a large-scale flow and thermally induced up-valley and upslope flows. Cumulus cloud formation is modelled by releasing buoyant parcels from the surface layer, simulating their ascent using entraining parcel equations, and checking whether they are able to reach their lifting condensation level and level of free convection. The purpose of the simulations is twofold.

First, the interplay of various forcing mechanisms in mountain cumulus formation is studied. For example, it is shown how the timing of cumulus initiation upwind from a ridge is controlled by the growth of the convective boundary layer. Moisture is transported upwards by CBL turbulence and then dynamically lifted by flow over the ridge. On the downstream side, the convergence of the thermally induced upslope flow with the large-scale flow is shown to be essential. This confirms the results of earlier studies of the 'lee-side convergence' mechanism of mountain cumulus formation.

Second, the performance of the model as a real-time forecasting tool is evaluated. Initial and boundary conditions are taken from MAPS (Mesoscale Analysis and Prediction System) analyses and forecasts. Predicted cumulus cloud patterns and onset times are compared to GOES data and visual observations.