Initiation Mechanisms of Nocturnal Convection without Nearby Surface Boundaries over the Central and Southern Great Plains during the Warm Season

Abstract

The number of case studies in the literature of nocturnal convection has increased during the past decade, especially those that utilize high-spatiotemporal-resolution datasets from field experiments such as the International H2O Project (IHOP_2002) and Plains Elevated Convection at Night (PECAN). However, there are few case studies of events for convection initiation without a nearby surface boundary. These events account for approximately 25% of all nocturnal convection initiation (CI) events. Unique characteristics of these events include a peak initiation time later at night, a preferred initiation location in northern Kansas and southern Nebraska, and a preferred north–south orientation to linear convective systems. In this study, four case studies of convection that is initiated without a nearby surface boundary are detailed to reveal a number of possible initiation mechanisms, including quasigeostrophic-aided ascent, elevated ascent associated with convergent layers (of unknown causes), the low-level jet, and gravity waves. The case studies chosen illustrate the wide variety of synoptic-scale conditions under which these events can occur.

Where, When, and Why Did It Rain during PECAN?

“…The border events are not plotted because they first appeared on the edge of the radar domain and therefore the actual time of CI was unknown. Nocturnal CI events (0100-1300 UTC) in the plains maximized around sunset (;0200 UTC) and then reached a secondary broad maximum 0400-0800 UTC, generally consistent with the 20-yr climatological study of nocturnal convection initiation (Reif and Bluestein 2018), as well as the study focusing on PECAN nocturnal CI (Stelten and Gallus 2017). Nocturnal CI events occurred nearly twice as frequently in the plains (144) than in the mountains (76).…”

“…They are more difficult to forecast than daytime CI partly because the most unstable layer is typically elevated and is not as readily observed (e.g., Weckwerth et al 2019). Some of the complex and interacting factors affecting nocturnal CI and MCS maintenance include a surface-based stable layer (e.g., Carbone et al 2002;Billings and Parker 2012), the LLJ (e.g., Trier et al 2006;Tuttle and Davis 2006;French and Parker 2010;Shapiro et al 2016;Trier et al 2017;Gebauer et al 2018;Reif and Bluestein 2018), frontal boundaries (e.g., Maddox et al 1979;Trier and Parsons 1993;Horgan et al 2007;Reif and Bluestein 2017), gravity waves (e.g., Wilson et al 2018;Reif and Bluestein 2018), and nocturnal bores (e.g., Koch et al 2008a,b;Marsham et al 2011;Coleman and Knupp 2011;. In contrast to relatively high skill in warm season quantitative precipitation forecasts (QPFs) under strong synoptic-scale forcing (e.g., Jankov and Gallus 2004;Squitieri and Gallus 2016), the forecasting skill is relatively low for weakly synoptically forced systems in the central United States (e.g., Fritsch and Carbone 2004).…”

Nocturnal Convection Initiation during PECAN 2015

“…While the impact of NCI in the U.S. Great Plains is high, the forecasting skill remains relatively low. A 20-yr radar climatology illustrated that NCI events can produce hail, high winds, f looding, and in some cases, tornadoes (Reif and Bluestein 2017, hereafter RB17;Reif and Bluestein 2018). Figure 1 illustrates that locally forced NCI episodes contributed up to 30%-60% of the PECAN nocturnal accumulated rainfall derived from radar (Weckwerth and Romatschke 2019).…”

Initiation of an Elevated Mesoscale Convective System With the Influence of Complex Terrain During Meiyu Season

“…The current study with a combination of real observations, model simulation, and data assimilation‐based analysis has provided another likely mechanism for elevated CI, namely, mountain‐plain thermal circulations corresponding to local‐scale terrain variation in a region with the existence of complex terrain and a large‐scale Meiyu front. According to previous studies, terrain‐related initiation mechanisms, such as gravity wave (Parsons et al., 2019; Reif & Bluestein, 2018) and vorticity anomaly (He et al., 2018; Li & Smith, 2010), tend to have a downstream moving characteristic and initiate an MCS far away from the terrain region. By contrast, the influence range of the proposed mechanism associated with mountain‐plain thermal circulations is local (Zardi & Whiteman, 2013), and the subsequent elevated CI like the case we studied occurs in the nearby vicinity of mountains and takes place around sunset.…”

“…The mechanism of CIs can be rather different under different environmental conditions in which they occur (Nesbitt & Zipser, 2003; Reif & Bluestein, 2018). In central‐east China, during the warm season, the meteorological condition is typically characterized by a quasi‐stationary frontal system, called the Meiyu front (Ninomiya & Shibagaki, 2007), which can be identified as a sharp gradient in equivalent potential temperature (theta‐e) (K. Ninomiya, 1984).…”