Evolution of Pre- and Postconvective Environmental Profiles from Mesoscale Convective Systems during PECAN

Abstract

During the Plains Elevated Convection at Night (PECAN) field campaign, 15 mesoscale convective system (MCS) environments were sampled by an array of instruments including radiosondes launched by three mobile sounding teams. Additional soundings were collected by fixed and mobile PECAN integrated sounding array (PISA) groups for a number of cases. Cluster analysis of observed vertical profiles established three primary preconvective categories: 1) those with an elevated maximum in equivalent potential temperature below a layer of potential instability; 2) those that maintain a daytime-like planetary boundary layer (PBL) and nearly potentially neutral low levels, sometimes even well after sunset despite the existence of a southerly low-level wind maximum; and 3) those that are potentially neutral at low levels, but have very weak or no southerly low-level winds. Profiles of equivalent potential temperature in elevated instability cases tend to evolve rapidly in time, while cases in the potentially neutral categories do not. Analysis of composite Rapid Refresh (RAP) environments indicate greater moisture content and moisture advection in an elevated layer in the elevated instability cases than in their potentially neutral counterparts. Postconvective soundings demonstrate significantly more variability, but cold pools were observed in nearly every PECAN MCS case. Following convection, perturbations range between −1.9 and −9.1 K over depths between 150 m and 4.35 km, but stronger, deeper stable layers lead to structures where the largest cold pool temperature perturbation is observed above the surface.

The Colorado State University Convective CLoud Outflows and UpDrafts Experiment (C3LOUD-Ex)

“…Cold pool processes have been successfully investigated through the use of instrumented towers (Charba 1974;Goff 1976), the Oklahoma (Engerer et al 2008) and mobile mesonets (Markowski et al 2002), radiosondes and radar (Wakimoto 1982;Bryan and Parker 2010;Hitchcock et al 2019;Borque et al 2020), Doppler lidars (Soderholm et al 2016), and drones (Houston et al 2012;Riganti and Houston 2017). While these studies have provided useful information on cold pool processes and variability, either the number of cold pools observed has been limited, the information was not obtained above the surface, the spatial resolution of observations was too coarse, or the observations were not obtained parallel to the gust front, to allow for the three-dimensional evaluation of cold pool processes simulated using high-resolution models.…”

“…The cold pools observed therefore have a wide range of temperature perturbations, depths, and vertical structures (Figurea). Maximum temperature perturbation magnitudes (relative to the pre-cold pool soundings) range from -8 K to close to 0 K, and cold pool depths range from 200 m to 2,300 m, where cold pool depth is defined simply as the lowest altitude at which the temperature perturbation  0 K, similar toHitchcock et al (2019). The cold pool observed on 17 May 2017 is now analyzed in detail.…”

An Overview of Low‐Level Jet Winds and Corresponding Mixed Layer Depths During PECAN

“…Jet features for these two events are less consistent throughout the night than NJs or LLJs driven by synoptic setups. A focused study of all PECAN MCSs is presented in Hitchcock et al ().…”

“…Jet features for these two events are less consistent throughout the night than NJs or LLJs driven by synoptic setups. A focused study of all PECAN MCSs is presented in Hitchcock et al (2019). Table 3 summarizes LLJ and NJ nightly jet classification into one of the four strength categories, with those not meeting the minimum classification criteria grouped together as nonjet nights.…”

“…PECAN data have also been used to investigate LLJ influence on nocturnal CI, both related to surface fronts (Trier et al, ) and more pristine environments (Gebauer et al, ; Shapiro et al, ). Hitchcock et al () performed an in‐depth study of all PECAN mesoscale convective system (MCS) environments, highlighting the LLJ's contribution to MCS development. Groundbreaking findings on the importance of spatial and temporal heterogeneities in LLJ evolution have been explored in Gebauer et al () and Smith et al (), who each examined three strong LLJ events and found that buoyancy gradients across the slope of the Great Plains can produce local maxima in the wind speed profile that descend over time at a fixed site.…”