THE INFLUENCE OF THE ROCKY-MOUNTAINS ON THE 13-14 APRIL 1986 SEVERE WEATHER OUTBREAK .1. MESOSCALE LEE CYCLOGENESIS AND ITS RELATIONSHIP TOSEVERE WEATHER AND DUST STORMS

In this first of a two paper series, a sequence of dynamical processes involving the evolution of a mesoscale lee cyclone and its subsequent interaction with a mesoscale tropopause fold downstream of the Rocky Mountains is investigated. These scale-interactive phenomena, which re sulted from the jet streak interaction with the topography, were exami ned in detail using the observational data obtained from the Program f or Regional Observing and Forecasting Services' mesonetwork and wind p rofilers, as well as conventional surface and rawinsonde data and Tota l Ozone Mapping Spectrometer satellite data over the Colorado region f or the severe weather event that occurred during 13-14 April 1986. Lar ge-scale analysis indicated that as a baroclinic low pressure system a pproached the Rockies with its attendant upper-level jet streak, a typ ical prestorm environment over western Kansas formed in the early morn ing hours of 13 April. Hourly mesonet data analysis revealed the forma tion and eastward progression of a mesoscale lee cyclone with a traili ng wind-shift line identified as an internal bore initiated by a cold front (i.e., a prefrontal bore) in Part II. Analysis of winds and dive rgence including diagnostically derived temperature and height fields from Colorado wind profilers indicated that as the jet streak momentum propagated into a less stable region in the midtroposphere created by low-level adiabatic warming and midlevel cooling on the leeside of th e Rockies, unbalanced flow conditions resulted at scales less than the Rossby radius of deformation. As a consequence of geostrophic adjustm ent processes, mesoscale tropopause folding and upper-level frontogene sis occurred over the profiler network, Unbalanced upper-level frontog enesis resulted from the tilting of the isentropes by along-stream age ostrophic indirect circulations comprised of horizontal vertical veloc ity gradients across the tropopause fold. As the mesoscale tropopause fold extruded downwards to midlevels in association with the descendin g secondary upper-level jet streak forced by the geostrophic adjustmen t process, lee cyclogenesis occurred due to the phasing of the upper-l evel front with the low-level lee cyclone. Synthesis of the mesonetwor k and profiler observations suggest that high momentum in the midtropo sphere associated with the descending branch of the jet stream just ah ead of the tropopause fold was entrained and mixed downward to the sur face by the growing boundary layer that developed ahead of the prefron tal bore but behind the dryline. This surge of southwesterly momentum at the surface, largely responsible for blowing dust, was mostly ageos trophic and contributed to an increase in surface vorticity and moistu re convergence as well as frontogenesis around the lee cyclone, A meso scale conceptual model is proposed in order to explain the dynamical S equence of events involving lee cyclogenesis, dust storms, and a tropo pause fold that led to the severe weather environment over the Great P lains. In the companion paper (Part II), observational evidence of an internal bore occurring ahead of a cold front and comparisons with sim ple numerical model results are presented in order to understand the i nitiation and propagation of the prefrontal bore and its influence in triggering a squall line farther downstream.