Toward improved prediction of MCS dissipation

The dissipation of Mesoscale Convective Systems (MCSs) over the Central United States is investigated using observations and model data from roughly 50 cases, which occurred during May-August 1998 and 1999.

Rotunno and colleagues' 1988 model results suggested that squall line longevity is determined by the balance between the ambient low-level shear circulation and the opposing evaporation-induced surface cold pool circulation. If the ambient low-level shear circulation dominates, the updraft will tilt downshear, while if the cold pool circulation dominates, the updraft will tilt upshear. If the two circulations are balanced, the updraft is erect, and the squall line will have increased longevity. In this study, observational surface and profiler data were analyzed to determine the strengths of the low-level shear and cold pool circulations and test the circulation balance theory. Preliminary results indicate that the amount of circulation balance may play an important role in MCS dissipation, but the balance is not the only aspect influencing dissipation and its importance may be case-dependent. For example, in elevated MCSs (such as those occurring north of a warm or stationary front), the cold pool may be too weak to identify and measure. The position of each system relative to fronts has been noted in this study.

Other observational parameters computed include storm-relative inflow, low-level flow, lapse rates (surface-700 mb, 850-500 mb, 700-500 mb), and maximum 0-2 km theta-e. Analysis of these parameters has just begun, but preliminary results suggest that a substantial decrease in storm-relative inflow may be one pre-cursor of MCS dissipation, although some MCSs dissipated without a noticeable decrease in storm-relative inflow. The instability parameters (lapse rates and theta-e) generally were not factors in MCS dissipation except for a few cases in which the MCS moved into a much more stable environment. Most often, the MCS dissipated in an environment of similar instability to the environment in which it developed and matured. Changes in low-level flow, determined from profiler data, also seem to have limited ability to predict MCS dissipation. An analysis of low-level jet (LLJ) parameters is ongoing. The LLJ is indirectly included in the calculation of storm-relative inflow and low-level flow.

Because MCS dissipation is often difficult to predict, many parameters have been computed using Eta model data. Parameters computed include LLJ strength, low-level convergence and moisture convergence, frontogenesis, 850 mb theta-e and theta-e advection, 500 mb vorticity advection, upper-level divergence, and jet streak influences. These parameters and their correlation with MCS dissipation will also be discussed.