PNNL

Abstract

Past studies implicate dynamic anomalies operating on sub-synoptic scales as a possible initiation source of CONUS summertime (July-August) mesoscale convective systems (MCSs) during northwesterly flow regimes. To improve our understanding of summertime MCSs occurring over a variety of flow regimes, we track sub-synoptic scale midlevel (600 hPa) vorticity perturbations (“MPs”) as 2D objects comprising spatial scales of 500-2500 km over the central CONUS from May-August of 2004-2021. We perform statistical analysis of relationships between metrics of MP objects (e.g., duration, size, intensity, origin) and high-resolution MCS precipitation characteristics (e.g., duration, total rainfall, rain coverage area, motion) that occur while collocated with or in the absence of MPs to discern predictive capability of sub-synoptic dynamic features on storm precipitation potential. Although the majority of MPs collocated with MCS initiation occur in the months of July-August, a significant number (41%) occur between May-June. Northwesterly flow MPs comprise a relative minority of our events, suggesting that MPs can affect MCSs across a variety of warm season flow regimes. MPs affecting MCSs initiated primarily over the high plains near the central Rockies. Only approximately 20% of tracked MCS initiation events were collocated with MPs, but these storms produced statistically greater lifetime rainfall, coverage area, and more stratiform rain than non MP-induced storms. In general, larger and more vigorous MPs translated to more hydrologically impactful MCSs. The most directly attributable benefit to MCS initiation was from MP-enhanced background vertical motion and thermodynamic instability (increased CAPE and reduced CIN)