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

Abstract

This study presents a climatological and composite analysis of elevated mixed layers (EMLs) in South and North America derived from the NCEP Climate Forecast System Reanalysis. The EMLs are identified based on objective criteria applied to the reanalysis data. Composite analyses of synoptic-scale conditions and severe weather parameters associated with spring EML cases are presented. EMLs are more frequent immediately to the east of the Andes and the Rockies. The North American EMLs form by surface heating over the higher terrain of the Rockies, with peak frequency occurring in spring and summer. EMLs in South America are generated by differential temperature advection due to ageostrophic circulations east of the Andes, as indicated by the temperature lapse rate tendency equation, which relates to the higher frequency of EMLs during the cold season in South America. EMLs over North America are about 100 hPa lower than over South America due to the lower height of the Rockies in comparison to the Andes. The synoptic conditions associated with EMLs in South and North America are characterized by an upper-level trough upstream and low-level moisture flux convergence due to poleward-directed flow, favoring synoptic-scale ascent poleward of the EML location, where the convective inhibition is relatively low. When EMLs occur, higher surface-based convective available potential energy and low-level storm-relative helicity, in association with lower lifting condensation level heights observed in North America, indicate that surface-based supercell storms and tornadoes are more likely to occur on this continent in comparison with South America, corroborating observations.

Variations of Thunderstorm Charge Structures in West Texas on 4 June 2012

“…We propose that entrainment could have likewise contributed in previous studies. Dry air aloft originating from higher elevation to the west is a common feature in the western Great Plains (Ribeiro & Bosart, 2018), where the largest percentage of +CG-dominated storms are found (e.g., Carey et al, 2003;Medici et al, 2017;Reap & MacGorman, 1989). Prior work has focused on surface-based parcels, except for Carey et al (2003) who explicitly noted the dry air aloft when discussing synoptic-scale predictors of +CG-dominated storms.…”

A Demonstration of Modern Geostationary and Polar-Orbiting Satellite Products for the Identification and Tracking of Elevated Mixed Layers

“…For an understanding of why the GOES 7.34-µm band can be utilized in monitoring an EML and why it is a better choice than other water vapor bands, it is important to understand where in the vertical an EML typically exists. Base heights of the EML in the spring east of the Rockies vary from around 700 to 750 hPa (Ribeiro and Bosart 2018), while the EML top is typically around 500 hPa (Lanicci and Warner 1991a). These studies suggest that the majority of EML events would be in a layer around 750 to 500 hPa, with some variability at either end depending on season and location (Ribeiro and Bosart 2018).…”

“…For example, they most frequently form over the Mexican Plateau and Desert Southwest in the early-to-mid spring, but then shift northward in the late spring to summer. The EML is important to the severe thunderstorm environment; severe storms often develop along the edges of the EML (Carlson et al 1983;Farrell and Carlson 1989;Lanicci and Warner 1991c;Ribeiro and Bosart 2018). Tracking an EML plume as it advects eastward is a critical component to forecasting the potential of severe thunderstorm events.…”

A climatology of quasi‐linear convective systems and associated synoptic‐scale environments in southern Brazil

“…The severe weather parameters composited are most‐unstable convective available potential energy (MUCAPE), surface‐based CAPE (SBCAPE), downdraft CAPE (DCAPE), convective inhibition of the most unstable parcel (CIN) and bulk wind difference. These parameters are commonly used in severe weather forecast and research (e.g., Rasmussen and Blanchard, ; Evans and Doswell III, ; Thompson et al ., ; Ribeiro and Bosart, ). All the types of CAPE and the CIN were calculated by vertically interpolating the CFSR data into vertical levels spaced 500 m from each other and then applying the parcel method (Nascimento, ).…”