Composite RUC analyses of weakly-forced mesoscale convective systems

This study examines weakly-forced mesoscale convective systems (MCSs) using Rapid Update Cycle (RUC) analyses on a 20-km resolution grid, which allows an examination of mesoscale features common to MCSs. Thirty MCSs that were not associated with well-defined surface cold fronts were chosen from the months of June, July and August from 2005 to the present using 30-minute national base reflectivity images (archived at http://locust.mmm.ucar.edu/case-selection) as guidance. Each case was divided into six stages of the MCS lifecycle from its first storms to the final dissipation of the convective echoes associated with the MCS. RUC analysis fields were interpolated to a 81 x 81 domain with 20 km grid spacing that was centered on the area of strongest convection and aligned with the MCS direction of motion. Various composites of fields derived from the RUC data were produced to identify important mesoscale features in the pre-MCS environment, to examine how features of the MCSs themselves are represented in the RUC analyses, and to explore any environmental signals associated with MCS evolution.

Initial inspection of the individual cases and the composites show that the 20-km RUC analyses can identify mesoscale features common to MCSs, which were reported in earlier studies. For example, MCS-scale features, such as strengthening of cold pools and strong divergence aloft, are captured in the RUC composites. An interesting initial finding is the depiction of short-wave features in upper levels as the first storms develop, which are not always evident in the height field. The analyses will be inspected further to determine the potential vorticity structure in upper levels associated with the MCS and explore whether or not they can be used as an indicator for the ensuing convective development.