ABSTRACT

Long-lived mesoscale convective systems (MCSs) generate a significant fraction of the warm season rainfall in the central United States and frequently produce severe straight-line winds and tornadoes. The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX), conducted over the central United States in the late spring and early summer of 2003, investigated the processes leading to the formation of bow echoes, severe windstorms, tornadoes, and mesoscale vortices that are often part of MCS circulations. The overarching goal of this research is to provide a quantitative understanding of the temporal and spatial scales, source air, and dynamic and thermodynamic forcing for downdrafts generated within MCSs and to relate the downdraft circulations to severe surface winds in the case of the MCS and mesoscale gravity waves in the case where squall lines occur above a deep stable layer. The research involves analysis of airborne dual and quad Doppler radar, microphysical data collected in the trailing stratiform region of MCSs using laser optical array probes, dropsonde data taken throughout MCS systems, and WSR-88D Doppler radar data, as well as numerical modeling studies using the Weather Research and Forecasting model.

Specific objectives include: 1) characterize the microphysical structure of the trailing stratiform region of warm season MCSs using BAMEX analyses to further understanding of the roles of microphysical processes in MCS evolution; 2) determine the degree to which severe surface winds are a manifestation of downburst circulations generated by intense evaporative cooling near the trailing edge of convection, or a manifestation of high momentum air within the rear inflow jet descending slantwise to the earth's surface; 3) use numerical modeling studies to understand the dynamical vs. microphysical contributions to the formation, structure and evolution of mesoscale and convective downdrafts and elevated and descending rear inflow jets; and 4) complete ongoing idealized numerical modeling studies of mesoscale gravity wave generation to test the hypothesis that the processes that create mesoscale gravity waves in elevated squall line environments are dynamically similar to processes creating wake lows in MCSs.

The intellectual merit of this research derives from the new understanding it will provide concerning severe weather phenomena including bow echoes, downbursts, and mesoscale gravity waves. The research is designed to develop a new link and clear understanding of the interaction between cloud microphysical processes and storm dynamics in these phenomena. The novel approaches and unique measurements from BAMEX will provide basic new scientific understanding of mesoscale convective systems. The research likely will lead to new discoveries, since the processes to be investigated have been poorly observed in the past and the specialized observations taken in MCSs in BAMEX are truly unique.

The broader impacts of this research are substantial. Observational analyses and modeling of the generation of strong surface winds will contribute to improvements in operational weather forecasting and nowcasting techniques, warning lead times, and understanding of severe weather systems. The understanding of the fundamental physics associated with MCSs that will result from this research may lead to improved prediction of the high-impact weather associated with MCSs, bow echoes and severe windstorms, contributing substantially to the goals of the U.S. Weather Research Program. At least four graduate students at the University of Illinois will have a significant role in the research to be performed. Furthermore, BAMEX data will be incorporated into courses including Mesoscale Meteorology, Radar Meteorology, and Precipitation Physics, survey courses such as Severe and Unusual Weather, and the 2nd edition of a general education textbook Severe and Hazardous Weather. Findings will be communicated to the research and operational communities through meetings and symposia.

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