Multiscale Upstream and In Situ Precursors to the Elevated Mixed Layer and High-Impact Weather over the Midwest United States

Abstract

Two severe MCSs over the upper Midwest United States resulted in .100 mm of rain in a ;24-h period and .200 severe weather reports, respectively, during 30 June-2 July 2011. This period also featured 100 (104) daily maximum high (low) temperature records across the same region. These high-impact weather events occurred in the presence of an elevated mixed layer (EML) that influenced the development of the severe MCSs and the numerous record high temperatures. The antecedent large-scale flow evolution was influenced by early season Tropical Cyclone Meari over the western North Pacific. The recurvature and subsequent interaction of Meari with the extratropical large-scale flow occurred in conjunction with Rossby wave train amplification over the North Pacific and dispersion across North America during 22 June-2 July 2011. The Rossby wave train dispersion contributed to trough (ridge) development over western (central) North America and the development of an EML and the two MCSs over the upper Midwest United States. A composite analysis of 99 warm-season days with an EML at Minneapolis, Minnesota, suggests that Rossby wave train amplification and dispersion across the North Pacific may frequently occur in the 7 days leading up to EMLs across the upper Midwest. The composite analysis also demonstrates an increased frequency of severe weather and elevated temperatures relative to climatology on days with an EML. These results suggest that EMLs over the upper Midwest may often be preceded by Rossby wave train amplification over the North Pacific and be followed by a period of severe weather and elevated temperatures.

Elevated Mixed Layers and Associated Severe Thunderstorm Environments in South and North America

“…The base is considered the first pressure level, from bottom to top, to have g greater than or equal to 7.5 K km 21 . This g threshold is lower than that used in other research [BE10 and Cordeira et al (2017) used 8.0 K km 21 ] because the CFSR has less steep g in the EML than observed soundings (Table 1). An analysis of different thresholds was conducted, and a g of 7.5 K km 21 in the CFSR soundings was in good agreement with EMLs in observed soundings (Fig.…”

“…2). A threshold of 150 hPa for EML thickness was used by Cordeira et al (2017) and is less restrictive than the other thresholds tested (between 150 and 250 hPa). The mean EML thicknesses, shown in section 3a, are generally greater than 200 hPa, so 150 hPa is a suitable EML thickness threshold.…”

“…These layers occur mainly to the east (downstream) of great mountain ranges, a result of horizontal advection of surface-heated air over higher terrain and/or ageostrophic circulations in the lee of the mountains. The Rockies in North America (NA; e.g., Carlson and Ludlam 1968;Banacos and Ekster 2010, hereafter BE10) and the Tibetan Plateau in East Asia (e.g., Das et al 2014) are known to favor the formation of EMLs that can have a great impact on the atmospheric conditions over the lower terrain downstream due to their association with hazardous weather events (Cordeira et al 2017). Observations and case studies by Rasmussen and Houze (2016) show EMLs occur east of the Andes in South America (SA).…”

“…Severe thunderstorms occur in thermodynamically unstable environments where the vertical wind shear is high and there is sufficient lifting capable to trigger convection and release the thermodynamic instability (Thompson et al 2003;Brooks et al 2003). In this context, EMLs are known to be associated with severe thunderstorms (e.g., Carlson and Ludlam 1968;LW91;Lanicci and Warner 1991c;BE10;Cordeira et al 2017). The concomitant occurrence of moderate convective inhibition (CIN) caused by the temperature inversion in the base of the EML, high convective available potential energy (CAPE) associated with steep g in the EML over a warm/moist PBL, and sufficient vertical wind shear are recognized to favor severe thunderstorms in NA.…”

“…The concomitant occurrence of moderate convective inhibition (CIN) caused by the temperature inversion in the base of the EML, high convective available potential energy (CAPE) associated with steep g in the EML over a warm/moist PBL, and sufficient vertical wind shear are recognized to favor severe thunderstorms in NA. Despite being an important feature, the EML is not a sufficient ingredient for severe thunderstorm formation (Cordeira et al 2017) and can prevent convective initiation in some cases (Carlson and Ludlam 1968). In SA, the EMLs' influences on severe thunderstorm environments are likely similar to NA.…”