A Long-Lived Mesoscale Convective Complex. Part I: The Mountain–Generated Component

Abstract

Using data collected during Colorado State University's South Park Area Cumulus Experiment in 1977, a sequence of multi-scale convective events leading to the formation of a mesoscale convective complex is described. In the first phase, surface-based cool advection in the elevated mountain basin delayed the full transition of the morning boundary layer into a deep mixed layer until well after convective instability was reached over the adjacent ridges. The second phase was earmarked by the formation of convective precipitation echoes at ?hot spots? over the high mountain terrain. Two groups of cells then propagated. eastward across the mountain basin, forming a line of discrete cells which moved across the foothills toward the High Plains. The cells further intensified at the. foothills/High Plains interface and formed a still larger, north-south line of thunderstorms. In the third phase, this north?south line of thunderstorms evolved into an expanding meso-?-scale convective cluster as it continued its eastward propagation over eastern Colorado. The convective intensity of the line was apparently modulated by moisture availability over the plains, with the southern cells being most intense initially. As the northern end of the line encountered greater low-level moisture in western Kansas, the convection rapidly intensified to severe levels and produced in excess of 50 mm of precipitation over a large area. In Part II of this article it is shown that this meso-?-scale system participated in the formation of a meso-&α-scale convective complex.