Evolution of Environmental Conditions Preceding the Development of a Nocturnal Mesoscale Convective Complex

Abstract

A large nocturnal mesoscale convective complex (MCC) developed on 4 June 1985 during the Oklaboma-Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) field phase. It occurred near the climatological center of the nocturnal maximum in warm-season precipitation situated over the central United States. In this study special rawinsonde and surface mesonet data have been used to examine how the environmental conditions, which supported MCC development, evolved at night over this region. The MCC of interest was the fourth in a series of MCCS, three of which propagated east-northeastward, 100?300 km north of a quasi-stationary surface front. The region where the MCC experienced its most intensive growth was initially characterized by dry and hydrostatically stable conditions (associated with the passage of the previous MCC) above the shallow, wedge-shaped cold air mass. In less than 3 h, interaction between the diurnally varying low-level jet and the frontal boundary led to a local increase in convective available potential energy (CAPE) of over 2000 J kg?1 for air parcels averaged through a 50-mb-deep layer immediately above the Frontal surface. Our analysis shows that the region north of the quasi-stationary surface front became a favored zone for nocturnal MCC development when 1) particularly high CAPE arose due to the transport of moist, high-?E air northward above the frontal surface by the diurnally modulated low-level jet into a region of significantly colder midtropospheric conditions, and 2) adiabatic mesoscale ascent, which was particularly strong near the northern terminus of the low-level jet, resulted in significant cooling above the jet axis. The cooling acted together with the strong moisture advection to eliminate convective inhibition [negative CAPE below the level of free convection (LFC)], thus enabling air parcels over a mesoscale region to more easily attain their LFC. Strong and deep mesoscale ascent was absent south of the front. In this region the surface-based deep convection that was supported during the evening hours weakened overnight as the low-level jet veered to a southwesterly direction, resulting in less favorable vertical shear for the sustenance of convective updrafts, while diurnal cooling increased the convective inhibition and raised the height of the LFC.