A Hierarchical Dissection of Multiscale Forcing on the Springtime Mesoscale Convective Systems in the United States

Abstract

Abstract 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 multiscale 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 by low-level kinematic and moisture convergence driven by low-level jets (LLJs). Nearly 50% 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−1 and CIN of ∼150 J kg−1. 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 at 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. Significance Statement The purpose of this study is to provide a systematic classification of multiscale forcing factors triggering mesoscale convective system development over the United States. These storms are very active in spring and often lead to intense rainfall and other weather hazards such as lightning, hail, and tornadoes. They play a key role in the U.S. hydrological cycle and have been occurring more frequently over the past several decades. Our study reveals the detailed characteristics of atmospheric forcing leading to these storms. Such information lays theoretical grounds for designing prediction schemes of warm season severe weather and provides guidance for model development to improve climate models’ simulation and long-term projection of these storms.