External vs. Self-Organization of Nocturnal MCSs from PECAN

The Plains Elevated Convection At Night (PECAN) field program was formulated in part to address the mechanisms by which mesoscale convective systems (MCSs) are maintained in environments with stable nocturnal boundary layers. One common assertion is that nocturnal MCSs typically are elevated and lack surface cold pools. A related assertion is that nocturnal MCSs therefore owe their maintenance and evolution primarily to external processes such as lifting along pre-existing fronts and interactions with the nocturnal low-level jet. This study addresses four intense, long-lived, overnight MCSs from PECAN (from 6/24/2015, 6/25/2015, 7/5/2015, and 7/12/2015). The WRF model is used in “case study mode” (nested down to dx=333 m) to simulate MCS development in a realistic, fully-heterogeneous, temporally-evolving environment. The CM1 model is used in “idealized mode” (with dx=250 m) to simulate a comparison MCS in a horizontally homogeneous and temporally steady environment drawn from each case.

Each of the four simulated MCSs is found to produce a substantial surface cold pool and to ingest considerable near-surface air into its updrafts in both the WRF and CM1 realizations. The simulated structures are rather realistic in both models (compared to radar observations), with a surprisingly strong correspondence in MCS evolution between the fully-varying WRF and the invariant CM1 environments. This implies that nocturnal MCSs may often “self-organize” via their own internally generated outflows and mesoscale wind fields (rather than being slaved to external fronts or low-level jets). The presentation will describe the dynamics of nocturnal self-organization, both in isolation and within the context of a fully varying synoptic and mesoscale environment.