The Role of Elevated Terrain and the Gulf of Mexico in the Production of Severe Local Storm Environments over North America

Abstract

The prevailing conceptual model for the production of severe local storm (SLS) environments over North America asserts that upstream elevated terrain and the Gulf of Mexico are both essential to their formation. This work tests this hypothesis using two prescribed-ocean climate model experiments with North American topography removed or the Gulf of Mexico converted to land and analyzes how SLS environments and associated synoptic-scale drivers (southerly Great Plains low-level jets, drylines, elevated mixed layers, and extratropical cyclones) change relative to a control historical run. Overall, SLS environments depend strongly on upstream elevated terrain but more weakly on the Gulf of Mexico. Removing elevated terrain substantially reduces SLS environments especially over the continental interior due to broad reductions in both thermodynamic instability and vertical wind shear, leaving a more zonally uniform residual distribution that is maximized near the Gulf coast and decays toward the continental interior. This response is associated with a strong reduction in synoptic-scale drivers and a cooler and drier mean-state atmosphere. Replacing the Gulf of Mexico with land modestly reduces SLS environments over the Great Plains (driven primarily thermodynamically) and increases them over the eastern United States (driven primarily kinematically), shifting the primary local maximum eastward into Illinois; it also eliminates the secondary, smaller local maximum over southern Texas. This response is associated with modest changes in synoptic-scale drivers and a warmer and drier lower troposphere. These experiments provide insight into the role of elevated terrain and the Gulf of Mexico in modifying the spatial distribution and seasonality of SLS environments.

Biases in CMIP6 Historical U.S. Severe Convective Storm Environments Driven by Biases in Mean‐State Near‐Surface Moist Static Energy

“…Following Li et al. (2021), we decompose the bias in CAPES06 of each CMIP6 model relative to ERA5 according to $$$\frac{{\Delta}CAPES06}{CAPES06}=\frac{{\Delta}CAPE}{CAPE}+\frac{{\Delta}S06}{S06}+\frac{{\Delta}CAPE}{CAPE}\cdot \frac{{\Delta}S06}{S06}+{\epsilon},$$$ where we drop the ERA5 subscript in the denominator to simplify the notation. A given term may be multiplied by 100 to express it as a percent bias as in Equation .…”

“…High values of the product of the two quantities is a widely used proxy for a favorable environment for SCS activity (hereafter “SCS environments”). These proxies calculated from environmental data have been successfully used to explain spatiotemporal variations in severe thunderstorm activity and their associated hazards using reanalysis data (Coffer et al., 2020; Taszarek et al., 2020, 2021) and in individual global climate model simulations (Chen et al., 2020; Hoogewind et al., 2017; Li et al., 2021; Marsh et al., 2007). Climate models have been used to examine future climate change impacts on SCS environments (Del Genio et al., 2007; Trapp et al., 2009).…”

Upstream surface roughness and terrain drive contrast in tornado potential between North and South America

“…Tornado Alley) (10,26), where over 1200 tornadoes and over 10 extreme tornado outbreaks occur annually (27,28), an order of magnitude greater than in any other region (29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39). This behavior is commonly ascribed to elevated terrain to the west (Rocky Mountains and Mexico's Sierra Madre mountains) and equatorward oceans to the south (Gulf of Mexico and Caribbean Sea) (40)(41)(42). The north-south oriented mountain range drives the strong climatological southerly low-level ow over the Great Plains (43)(44)(45).…”

“…This daily cycle is associated with diurnal oscillations of near-surface processes including boundary layer frictional stress (47), heating of the sloping terrain (48), and their combination (49)(50)(51). In addition, southerly winds associated with the warm sector of synoptically driven troughs are also found to enhance the southerly mean ow and LLJ over the Great Plains (40,42,115). This low-level southerly ow transports warm, moist near-surface air from upstream oceans into CNA (52,53,112,113) beneath a well-mixed eastward ow aloft (54).…”

“…Starting from the CESM2.1.1 aquaplanet set-up (89-91), we develop an idealized con guration including the minimal geographic ingredients expected to be necessary to produce a regional severe thunderstorm hotspot: a mid-latitude continent with a western elevated plateau and poleward of a large source of warm moist air (40)(41)(42). The smooth experiment (denoted as aquasmooth) simulates an aquaplanet with prescribed monthly-varying sea surface temperature (SST) and includes an idealized rectangular midlatitude continent with a 3-km north-south oriented narrow plateau along its western edge (Fig.…”