High Resolution Numerical Simulations of Midwestern Quasi-Linear Mesoscale Convective Systems

ABSTRACT

The Pennsylvania State University/National Centers for Atmospheric Research Mesoscale Model Version 5 (MM5) is employed to investigate the structure, evolution, and circulation of mid-western quasi-linear meso-scale convective systems (MCSs) during the cool and/or transitional seasons. The model employs three domains with 9-, 3-, 1- and 0.3-km grid spacing, respectively. A “standard'' physical package was selected after running sensitivity tests. Two cases were chosen for this study. The first case is a classic transitional-season event in which the model predicted very well. The kinematic and dynamic structure of the simulated system, however, provided very detailed information as to how the model maintained a linear MCS some time after model initialization. The low-level simulated reflectivity fields show a strong similarity to the observations. Cross-sectional views show the detailed evolution occurring at different locations of the bowing segment of the squall line. Plan views from low- and mid-levels show the vertical structure of the dynamic interaction between the mass and momentum fields, which is further verified with selected cross sections. The low-level winds are also shown to create a reflectivity pattern similar to a line-echo-wave-pattern. The second case involved simulation of a warm-season convective event. This simulation was not as accurate as the first as evidenced largely by the decreasing accuracy of the surface fields over time. This suggests a limit on the usefulness of the model's forecasts. The MM5 model is able to develop some linear, simulated reflectivity patterns, but they are not as organized as observations for this event. However, it is encouraging to note that the model attempts to develop some degree of organization given the relative success of the first case.