E-Journal of Severe Storms Meteorology

E-Journal of Severe Storms Meteorology, Vol 8, No 4 (2013)

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Synoptic-Scale Precursors, Characteristics and Typing of Nocturnal Mesoscale Convective Complexes in the Great Plains

Shawn M Milrad, Cailee M Kelly

Abstract


Mesoscale convective complexes (MCCs) occur frequently during the warm season in the central U.S. and can produce flooding rains, hail and tornadoes.  Previous work has found that the synoptic-scale environment can greatly affect, and be affected by, the development and maintenance of MCCs.  Ninety-two MCC cases from 2006-2011 are manually identified using infrared satellite imagery and partitioned into three types (upstream trough, zonal and ridge) using a unique manual synoptic typing based on 500-hPa height patterns.  Upstream trough cases feature an amplified longwave 500-hPa trough upstream of the MCC genesis region (GR), while the 500-hPa flow is relatively flat in zonal cases, and a strong 500-hPa ridge is present over the Rockies in ridge cases. Individual case and storm-relative composite analyses of a subset of 28 cases show that of the three types, upstream trough cases feature both the strongest quasigeostrophic forcing for ascent and lower-tropospheric frontogenesis, the latter of which enhances ascent and is associated with a strong southerly low-level jet (LLJ).  Zonal and ridge cases feature smaller magnitudes (in descending order) of all ascent-forcing parameters.  Ridge cases, in particular, are characterized by weak Q-vector convergence, but easterly upslope flow likely acts as a compensating ascent mechanism.  A thermodynamic analysis shows that high-θe air is advected into the GR in all three MCC types, and serves as fuel for development and maintenance.  However, while the southerly LLJ advects high-θe air from the Gulf of Mexico in the upstream trough and zonal cases, such air is already pooled in the High Plains in the ridge cases and advected into the GR by easterly flow.  In accordance with the synoptic-dynamic analysis, upstream trough cases have the longest duration and largest impact on the synoptic-scale environment, while ridge cases are the shortest-lived.  The various underlying precipitation structures of each group are also explored; zonal cases, for example, appear to preferentially be associated with bow echoes.  


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