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Assessing the Influence of Complex Terrain on Severe Convective Environments in Northeastern Alabama

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  • 1 a The Pennsylvania State University, University Park, Pennsylvania
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Abstract

Storms crossing complex terrain can potentially encounter rapidly changing convective environments. However, our understanding of terrain-induced variability in convective storm environments remains limited. HRRR data are used to create climatologies of popular convective storm forecasting parameters for different wind regimes. Self-organizing maps (SOMs) are used to generate six different low-level wind regimes, characterized by different wind directions, for which popular instability and vertical wind shear parameters are averaged. The climatologies show that both instability and vertical wind shear are highly variable in regions of complex terrain, and that the spatial distributions of perturbations relative to the terrain are dependent on the low-level wind direction. Idealized simulations are used to investigate the origins of some of the perturbations seen in the SOM climatologies. The idealized simulations replicate many of the features in the SOM climatologies, which facilitates analysis of their dynamical origins. Terrain influences are greatest when winds are approximately perpendicular to the terrain. In such cases, a standing wave can develop in the lee, leading to an increase in low-level wind speed and a reduction in vertical wind shear with the valley lee of the plateau. Additionally, CAPE tends to be decreased and LCL heights are increased in the lee of the terrain where relative humidity within the boundary layer is locally decreased.

Significance Statement

This work investigates how the environments associated with severe storms vary around a plateau in northeastern Alabama. We use operational high-resolution model output and idealized simulations to determine which parameters used to forecast severe storms are most affected by terrain, where the parameters tend to be increased or decreased relative to the terrain, and why such increases and decreases occur. These parameters that characterize the likelihood of severe thunderstorms and tornado formation are all affected by the terrain, primarily when the wind has a strong perpendicular component relative to the long axis of the terrain. It is not clear that these changes would tend to make storms consistently more or less strong near the terrain.

Katona’s current affiliation: NOAA/OAR/National Severe Storms Laboratory, Norman, Oklahoma.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Branden Katona, katona@psu.edu

Abstract

Storms crossing complex terrain can potentially encounter rapidly changing convective environments. However, our understanding of terrain-induced variability in convective storm environments remains limited. HRRR data are used to create climatologies of popular convective storm forecasting parameters for different wind regimes. Self-organizing maps (SOMs) are used to generate six different low-level wind regimes, characterized by different wind directions, for which popular instability and vertical wind shear parameters are averaged. The climatologies show that both instability and vertical wind shear are highly variable in regions of complex terrain, and that the spatial distributions of perturbations relative to the terrain are dependent on the low-level wind direction. Idealized simulations are used to investigate the origins of some of the perturbations seen in the SOM climatologies. The idealized simulations replicate many of the features in the SOM climatologies, which facilitates analysis of their dynamical origins. Terrain influences are greatest when winds are approximately perpendicular to the terrain. In such cases, a standing wave can develop in the lee, leading to an increase in low-level wind speed and a reduction in vertical wind shear with the valley lee of the plateau. Additionally, CAPE tends to be decreased and LCL heights are increased in the lee of the terrain where relative humidity within the boundary layer is locally decreased.

Significance Statement

This work investigates how the environments associated with severe storms vary around a plateau in northeastern Alabama. We use operational high-resolution model output and idealized simulations to determine which parameters used to forecast severe storms are most affected by terrain, where the parameters tend to be increased or decreased relative to the terrain, and why such increases and decreases occur. These parameters that characterize the likelihood of severe thunderstorms and tornado formation are all affected by the terrain, primarily when the wind has a strong perpendicular component relative to the long axis of the terrain. It is not clear that these changes would tend to make storms consistently more or less strong near the terrain.

Katona’s current affiliation: NOAA/OAR/National Severe Storms Laboratory, Norman, Oklahoma.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Branden Katona, katona@psu.edu
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