Convection Initiation Caused by Heterogeneous Low-Level Jets over the Great Plains

Abstract

Observations from three nights of the Plains Elevated Convection at Night (PECAN) field campaign were used in conjunction with Rapid Refresh model forecasts to find the cause of north–south lines of convection, which initiated away from obvious surface boundaries. Such pristine convection initiation (CI) is relatively common during the warm season over the Great Plains of the United States. The observations and model forecasts revealed that all three nights had horizontally heterogeneous and veering-with-height low-level jets (LLJs) of nonuniform depth. The veering and heterogeneity were associated with convergence at the top-eastern edge of the LLJ, where moisture advection was also occurring. As time progressed, this upper region became saturated and, due to its placement above the capping inversion, formed moist absolutely unstable layers, from which the convergence helped initiate elevated convection. The structure of the LLJs on the CI nights was likely influenced by nonuniform heating across the sloped terrain, which led to the uneven LLJ depth and contributed toward the wind veering with height through the creation of horizontal buoyancy gradients. These three CI events highlight the importance of assessing the full three-dimensional structure of the LLJ when forecasting nocturnal convection over the Great Plains.

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

“…As listed in Table 3, a greater percentage of nights at FP2 had LLJ-3s, agreeing with the findings of stronger LLJs at western sites by Smith et al (2019). The diagonal striations presented in Gebauer et al (2018) and Smith et al (2019) are not apparent in the mean LLJ wind profiles at any site because these striations are not found in all LLJs and they are weak features with variable timing. Visual inspection of the winds from each night revealed diagonal striations to be most common at FP6, present there in half of the LLJ-3s.…”

“…While in‐depth case studies of PECAN LLJs influencing CI have been published (Gebauer et al, ; Reif & Bluestein, ; Shapiro et al, ; Trier et al, ), we present here jet coincidence with PNCI for the entire PECAN campaign. This is not meant to be an exhaustive study, but rather a brief opportunistic presentation of a known important jet impact, made available by the preceding work of categorizing the jets.…”

“…Fedorovich et al () used numerical modeling and sounding profiles to discuss LLJ impacts on turbulence and advection of environmental potential temperature and how these feed back to affect LLJ structure, also showing that weak LLJs can form on the sloped terrain of the Great Plains in the absence of the geostrophic wind that is fundamental to IO theory. 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.…”

“…These wind features, called “diagonal striations,” are indicative of the LLJ strengthening earlier in the west, enhancing wind at higher altitudes to the east as the westerly component grows. This elevated advection and convergence of the westerly component can generate CI in north‐south line structures (Gebauer et al, ). While this heterogeneity is important to forecasting CI, the diagonal striations are not found in all LLJs (Smith et al, ).…”

“…The most studied LLJ is that of the Great Plains region of the United States due to its high frequency of occurrence and strong wind speeds spanning hundreds of kilometers along-track and cross-track. The frequency, strength, and location of the Great Plains LLJ allow it to play a significant role in regional weather; it is a source of wind field convergence and the southerly LLJs typically carry warm and moist air northward to fuel convective systems (Barandiaran et al, 2013;Gebauer et al, 2018;Helfand & Schubert, 1995;Higgins et al, 1997;Houze, 2004;Stelten & Gallus, 2017;Wu & Raman, 1998). LLJs are also important for air quality, capable of transporting atmospheric constituents hundreds of kilometers and in some cases are associated with high concentrations at the surface (Banta et al, 1998;Delgado et al, 2015;Klein et al, 2014;Mao & Talbot, 2004).…”