10:45 AM - 12:15 PM
[AAS03-P02] A Hierarchical Dissection of the Multi-Scale Forcing on the Springtime Mesoscale Convective Systems in the United States
Keywords:mesoscale convective system, multi-scale processes, Pacific Decadal Oscillation
Mesoscale Convective Systems (MCSs) play a key role in regulating variability in the U.S. water and energy cycle. Here a hierarchical dissection of the multi-scale forcing of springtime MCSs is carried out through a two-step classification process. Hierarchical clustering is first applied to spatiotemporally evolving upper-tropospheric height fields to reveal large-scale forcing patterns of MCSs. Five distinct forcing patterns (clusters) are identified with three being “remotely forced” and two associated with “local growth”. The upper-level troughs associated with these forcing patterns create broad envelopes downstream within which large-scale ascent and MCS genesis tend to occur. Further classification of MCSs based on MCS track locations reveals that local dynamic and thermodynamic forcing determines the precise locations of MCS genesis in the envelope created by large-scale forcing. Specifically, MCSs often occur near surface fronts in warm sectors of surface low pressure systems and are accompanied with low-level kinematic and moisture convergence driven by low level jets (LLJs). Nearly fifty percent of spring MCSs are associated with potential instability realized through frontal lifting, and the highest probability of MCS genesis is seen with an environmental CAPE of ~1400 J/kg and CIN of ~150 J/kg. The positive trend of the U.S. MCS genesis frequency observed in recent decades is found to be driven by the cluster of MCSs forced in large-scale by the Pacific storm track. Regression analysis further suggests that the growing phase of the Pacific Decadal Oscillation (PDO) modulates the associated MCS large-scale forcing and is ultimately responsible for the positive MCS trend.