Mesoscale convective systems: Structure, development and storm-environment interactions
This study describes Mesoscale Convective Systems (MCSs), clusterings of thunderstorms associated with significant weather events. Analyses focused on several objectives: a documentation of the conditions in which MCSs form; an elucÃdation of the life cycle and internal structure of a mid-latitude Mesoscale Convective Complex (MCC); and a systematic comparison of the precipitation structures and environments of many MCSs.
In meeting the objectives the evolution of an episode of f ive MCSs over West Texas was investigated using as a principal source data collected by Texas Tech University as a participant in the Texas High Plains Cooperative Program (HIPLEX). Results of the study show that the first MCS formed near a cold front where lifting along an elevated dew-point front released instability. Surface outflows from the MCSs advanced the surface baroclinic zone associated with the cold front and helped initiate subsequent convection. Convection in the mature MCSs was uncoupled from the surface layer and occurred just ahead of mid-level short-wave perturbations.
The third MCS in the sequence formed from isolated echoes over the mountains which merged and grew into an MCC with a low-level precipitation pattern whose organization resembled that of tropical MCSs, extended about 500 km across and had a lifetime of about 24 hours. Behind a squall line and a transition zone near the leading edge occurred an extensive region of precipitation that was organized during the system's mature stage as a set of curved rainbands. A composite wind analysis shows a center of cyclonic inflow at 500 mb near the common center of curvature of the rainbands. Upward motion in the middle- and upper-level cloud and a mesoscale unsaturated downdraft below are diagnosed.
MCSs consisting of a squall line followed by a wide region of lighter rain develop in an environment where the relative flow at 300 mb Is moist and directed from front to rear. Rainbands embedded in the trailing stratiform area are a common element of the wide systems. Narrow squall-line MCSs tend to occur in an environment having at 300 mb a rear-to-front flow, or a front-to-rear flow of dry air.