| Enhanced infrared satellite imagery has been used to document the existence and frequent occurrence, over middle latitudes of the United States, of large, convectively driven weather systems. These systems, named Mesoscale Convective Complexes (MCCs), have not been heretofore recognized or documented in the scientific literature. It is shown that these systems frequently produce a variety of severe convective weather phenomena (such as tornadoes, hailstorms and flash floods) that significantly impact human activities. A preliminary climatology of MCCs, based on satellite data from two warm seasons, indicates that these systems frequently affect United States agricultural regions and it is hypothesized that MCCs produce a highly significant portion of the growing season precipitation over these areas.; Objective analyses of composited meteorological conditions attending ten MCC weather systems have revealed a number of distinctive characteristics and interactions with their larger scale environment. These analyses are used to develop a physically realistic model of the life-cycle of the typical midlatitude Mesoscale Convective Complex. The systems develop within a relatively weak and stagnant large-scale setting and are usually closely linked to the eastward progression of a weak middle tropospheric short-wave trough. Initial thunderstorms develop within a region of mesoscale convergence and lifting that is primarily forced by low-level warm advection. The MCC system rapidly grows and takes on a mesoscale organization while it moves slowly eastward ahead of the short-wave trough. Diabatic heating eventually produces a system that is warm core in the middle troposphere and cold core in lower and upper levels. The mature MCC, although it occurs within a considerably different large-scale setting, exhibits many similarities to tropical convective systems. Strong inflow within the lower half of the toposphere forces an intense mesoscale updraft that maintains a region of moist ascent and widespread precipitation. Thickness increases within this meso-updraft produce anomalously high heights in the upper-troposphere above the MCC and an intense outflow jetstreak develops along the northern periphery of the system in the region where the height gradient has increased. As the system decays atmospheric response to residual temperature perturbations results in intensification of the pre-existent short-wave trough within the upper half of the troposphere. Decay occurs when the system moves east of the conditionally unstable airmass and region of warm advection over the Plains. Although the demise of the intense, highly organized convective system is abrupt, residual cloudiness and light shower activity may persist for many hours and affect large regions of the eastern United States. It is found that synoptic data (i.e., data taken at 12 h intervals) are not sufficient to quantitatively describe the energetics or the small-to-medium scale processes and interactions attending these systems. |
