The relationship of the Great Plains low level jet to nocturnal MCS development

The role of the Low Level Jet (LLJ) in the initiation of Mesoscale Convective Systems (MCSs) over the Great Plains is investigated for the 2005 warm season. The data set for this study were compiled using cases from June, July and August of 2005, within a domain roughly defined by the area enclosed by 102° W to 90° W longitude and 33° N to 49° N latitude. The emphasis of this study is on the relationship between the LLJ and nocturnal MCS initiation, so only MCSs with initiation times between 0300 UTC and 1200 UTC are examined.

Infrared satellite imagery was used to identify candidate cases, using criteria developed for Mesoscale Convective Complexes (MCCs – Maddox 1980). From this initial dataset, national base reflectivity data were screened for each case. Cases were included in the final data sample if the storms evolved into an organized squall line or organized cluster of cells, with a common cirrus canopy near MCC criteria. The date, time of initiation, dissipation, and location of each of the 45 MCS events was documented. In addition, severe storm reports (tornado, hail, and wind) and flooding reports (both flash floods and floods) from each case were collected. Finally, the initiation locations were plotted and a climatology of MCS initiation was constructed.

Using this dataset, archived wind profiler data from the National Climatic Data Center were utilized to detect the LLJ wind maxima, geographic location of the maxima, height, and time of the maximum wind speed for each case. Plots of the MCS and wind profiler data for each case were created to determine the location of the MCS initiation relative to the LLJ maximum. Point-forecast soundings from the RUC model at the time of the initiation, located immediately downstream from each MCS, were collected and many kinematic and thermodynamic parameters were calculated for each case.

Results indicate that the Great Plains LLJ is an integral part of the occurrence of nocturnal warm season MCSs. While the LLJ is not the only phenomenon involved in the initiation of nocturnal MCSs, virtually every case from the summer of 2005 showed a southerly LLJ in place prior to MCS initiation. Of these LLJs, a majority (71.1%) of them exceeded 16 ms-1 and (41%) exceeded 20 m s-1, stressing the importance of strong LLJs in MCS initiation. The LLJs are most frequently oriented along the western Great Plains, roughly near 100° W longitude, and are most commonly anticyclonically-curved.

The left front exit region of the LLJ maxima tend to be preferred locations for MCS initiation. A majority of the MCSs in this study initiated in the western regions of the Great Plains and propagated generally southeastward or eastward across the Plains. Initial results indicate that there are no strong correlations between any of the examined thermodynamic or kinematic parameters and the severity of the MCS, which is based on the occurrence of severe weather and/or flooding. However, several moderate correlations exist. The best indicators of MCS severity appear to be downdraft CAPE (DCAPE), convective inhibition for the most unstable parcel (MUCIN), and the 3 – 6 km lapse rate.