This article investigates errors in forecasts of the environment near an elevated mesoscale convective system (MCS) in Iowa on 24-25 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. The eastern flank of this MCS produced an outflow boundary (OFB) and moved southeastward along this OFB as a squall line. The western flank of the MCS remained quasi stationary approximately 100 km north of the system's OFB and produced localized flooding. A total of 16 radiosondes were launched near the MCS's eastern flank and 4 were launched near the MCS's western flank.Convective available potential energy (CAPE) increased and convective inhibition (CIN) decreased substantially in observations during the 4 h prior to the arrival of the squall line. In contrast, the model analyses and forecasts substantially underpredicted CAPE and overpredicted CIN owing to their underrepresentation of moisture. Numerical simulations that placed the MCS at varying distances too far to the northeast were analyzed. MCS displacement error was strongly correlated with models' underrepresentation of low-level moisture and their associated overrepresentation of the vertical distance between a parcel's initial height and its level of free convection (Dz LFC , which is correlated with CIN). The overpredicted Dz LFC in models resulted in air parcels requiring unrealistically far northeastward travel in a region of gradual meso-a-scale lift before these parcels initiated convection. These results suggest that erroneous MCS predictions by NWP models may sometimes result from poorly analyzed low-level moisture fields.

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.

This study examines the impact of assimilating preconvective radiosonde observations obtained by mobile sounding systems on short-term forecasts of convection. Ensemble data assimilation is performed on a mesoscale (15 km) grid and the resulting analyses are downscaled to produce forecasts on a convection-permitting grid (3 km). The ensembles of forecasts are evaluated through their depiction of radar reflectivity compared to observed radar reflectivity. Examination of fractions skill scores over eight cases shows that, for four of the cases, assimilation of radiosonde observations nearby to subsequent convection has a positive impact on the initiation and early evolution during the first 3–4 h of the forecasts, even for the smallest resolvable scales of the 3-km grid. For the four cases in which positive impacts near the smallest resolvable scales of the grid are not seen, analysis of the changes to the preconvective environment suggests that suboptimal locations of the soundings compared to the location of convective initiation are to blame. The aggregate positive impacts on forecasts of convection is more clearly seen when spatial scales larger than individual thunderstorms are examined.

Capsule SummaryExploring convective updrafts and cold pools using novel observational strategies, including a “Flying Curtain” of drones, radiosondes, and surface stations, to characterize cold pool heterogeneities, and targeting updrafts using radiosondes.