Characteristics and Synoptic Environment of Drylines Occurring over the Higher Terrain of Southeastern Wyoming

Abstract

Commonly observed over the broadly sloped terrain of the southern Great Plains (SGP), drylines are frequent loci of warm season deep convection and have been the focus of numerous observational, theoretical, and climatological studies over last half century. In this study, a 3-yr (2010-12) analysis of the characteristics and synoptic environment of drylines occurring elsewhere, over the high terrain in southeastern Wyoming just east of the Rocky Mountains, is presented. Observed on ;11% of the days between May and August of the years examined, southeastern Wyoming drylines were often associated with large moisture gradients [;5-10 g kg 21 (100 km) 21 ], large horizontal virtual potential temperature differences (;2-5 K), and convergent zonal wind flow at the surface. The synoptic conditions leading to their formation and their relationship to thunderstorm activity are also explored in an effort to aid local forecasters in anticipating the development and convective impact of drylines across the region. Similarities exist between these drylines and those found over the SGP, especially with regard to their strength and close relationship to deep convection. However, the frequency at which they occur, some characteristics of their diurnal motion, and the synoptic conditions driving their formation differ noticeably.

Dryline characteristics in North America’s historical and future climates

“…However, this study shows the highest frequency percentage in SUS. This characteristic is consistent with the systematic overprediction by the multi-parametric algorithm, as mentioned before in this study and in Clark et al, 2013. In NUS, the dryline frequency is 33% (between May and August), which is also higher than previous findings in Wyoming of 11% of the days (Bergmaier and Geerts 2015) for three years (2010)(2011)(2012). This could be partly due to the use of a coarse-resolution gridded reanalysis data at 32-km resolution in Bergmaier and Geerts (2015).…”

“…This characteristic is consistent with the systematic overprediction by the multi-parametric algorithm, as mentioned before in this study and in Clark et al, 2013. In NUS, the dryline frequency is 33% (between May and August), which is also higher than previous findings in Wyoming of 11% of the days (Bergmaier and Geerts 2015) for three years (2010)(2011)(2012). This could be partly due to the use of a coarse-resolution gridded reanalysis data at 32-km resolution in Bergmaier and Geerts (2015). In NUS, the dryline average magnitude is 0.15 g kg -1 km -1 , which is similar to previous studies in Wyoming with values between 0.05 to 0.1 g kg -1 km -1 (Bergmaier and Geerts, 2015).…”

“…The dryline is characterized by its magnitude, frequency, location, diurnal and seasonal cycle, and its major axis distance, using the original model output (without smoothing). The mean monthly magnitude of the dryline is calculated as the average of all the events in each month between April and September over the 13-year period at 00 UTC for consistency with previous studies (Hoch and Markowski 2005;Coffer et al 2013;Bergmaier and Geerts 2015).…”

“…The dryline is an elongated feature commonly defined by a strong moisture gradient rather than a thermal gradient. For example, Hoch and Markowski (2005) defined the dryline using a water vapor mixing ratio gradient greater or equal than 0.03 g kg -1 km -1 in the southern U.S. Their definition has been used to identify drylines in different regions (Coffer et al 2013;Bergmaier and Geerts 2015;Duell and Van Den Broeke 2016;Bechis et al 2020) allowing for a comparison between studies and across regions. The humidity gradient produces an imbalance in the atmosphere, which is then compensated by a solenoidal circulation (increased solenoidal term in the vorticity equation) that interacts with the convergence flow at low levels close to the strongest humidity gradient .…”

“…Drylines have been observed on 32% of days between April and June over the Great Plains (30 years of analysis, Hoch and Markowski 2005), whereas in southwest Wyoming around 11% of days between April and June show a dryline with an average gradient of 5.3 g kg -1 100 km -1 (using three years of Reanalysis data; Bergmaier and Geerts 2015). Drylines with different widths have been observed and simulated ranging from less than 1-km (e.g.…”