The Impact of Varied Latent Cooling Parameters on the Propagation of a Simulated Nocturnal Mesoscale Convective System

One of the goals of the Plains Elevated Convection at Night (PECAN) field project was to understand the environmental factors responsible for a nocturnal Mesoscale Convective System (MCS) exhibiting either surface-based or elevated convection, and how that affects the mechanisms for the MCS’ propagation. Past studies from this campaign have suggested that many cases were likely surface-based to some extent, as evidenced by the observation of strong surface cold pools in nearly every case and modeling studies which supported the conclusion that at least several of those cases were capable of lifting surface-based air through their updrafts, despite the stable environments in which many of these systems developed.

This presentation will analyze the 20 June 2015 PECAN MCS case and document its progression into a stable environment during the nighttime transition. This case is unique for several reasons, one being that it was one of the strongest storms observed during PECAN and conformed most closely to the traditional leading line/trailing stratiform conceptual model. More importantly, however, as it propagated eastward across the Great Plains it intercepted the nocturnal low-level jet partway through its life cycle, presenting an opportunity to analyze the MCS’ behavior before and after this transition. Using a high resolution WRF simulation, the tendency of the MCS to exhibit surface-based or elevated convection was explored through a detailed series of parcel trajectory calculations, showing that once the system encountered the low-level jet and an elevated maximum in convective available potential energy (CAPE), updraft-bound parcels tended to more likely originate above the boundary layer than before. It was also found that the system did produce a strong cold pool that either remained strong or intensified over the duration of the simulation, even as the nocturnal transition set in, suggesting that the role of the cold pool in promoting surface-based convection may not be as straightforward as expected. To better understand this interaction, additional simulations were run in which latent cooling parameters, most importantly evaporation, were either increased or decreased in magnitude in order to test the sensitivity of the system to cold pool strength. Results indicated that the cold pool strength was successfully altered due to increased or decreased evaporation, and the average starting elevation of parcels entering the updrafts shifted correspondingly. It was also found that the effect that the varied latent cooling had on the behavior of the rear-inflow jet may have been partially responsible for the shift in starting parcel elevation.