Sensitivity of coastal squall line evolution to numerical sea breeze initialization method and coastal mountain characteristics

Fan Wu Department of Meteorology and Atmospheric Science, The Pennsylvania State University, University Park, Pennsylvania, USA

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Kelly Lombardo Department of Meteorology and Atmospheric Science, The Pennsylvania State University, University Park, Pennsylvania, USA

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Abstract

This study employs 3D idealized numerical experiments to investigate the physical processes associated with coastal convection initiation (CI) as an offshore-moving squall line traverses a mountainous coastal region. A squall line can propagate discretely as convection initiates over the lee slope downstream of the primary storm as the cold pool collides with a sea breeze. Intensity of the initiating convection, thus the downstream squall line, is sensitive to the sea breeze numerical initialization method, since it influences sea breeze and cold pool characteristics, instability and vertical wind shear in the sea breeze environment, and ultimately the vertical acceleration of air parcels during CI. Here, sea breezes are generated through four commonly used numerical methods: a cold-block marine atmospheric boundary layer (MABL), prescribed surface sensible heat flux function, prescribed surface sensible plus latent heat flux functions, and radiation plus surface-layer parameterization schemes. For MABL-initialized sea breezes, shallow weak sea breeze flow in a relatively low instability environment results in weak CI. For the remainder, deeper stronger sea breeze flow in an environment of enhanced instability supports more robust CI. In a subset of experiments, however, the vertical trajectory of air parcels is suppressed leading to weaker convection. Downward acceleration forms due to the horizontal rotation of the sea breeze flow. Accurate simulations of coastal convective storms rely on an accurate representation of sea breezes. For idealized experiments such as the present simulations, a combination of initialization methods likely produces a more realistic representation of sea breeze and the associated physical processes.

© 2024 American Meteorological Society. This is an Author Accepted Manuscript distributed under the terms of the default AMS reuse license. For information regarding reuse and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Fan Wu (fvw5116@psu.edu)

Abstract

This study employs 3D idealized numerical experiments to investigate the physical processes associated with coastal convection initiation (CI) as an offshore-moving squall line traverses a mountainous coastal region. A squall line can propagate discretely as convection initiates over the lee slope downstream of the primary storm as the cold pool collides with a sea breeze. Intensity of the initiating convection, thus the downstream squall line, is sensitive to the sea breeze numerical initialization method, since it influences sea breeze and cold pool characteristics, instability and vertical wind shear in the sea breeze environment, and ultimately the vertical acceleration of air parcels during CI. Here, sea breezes are generated through four commonly used numerical methods: a cold-block marine atmospheric boundary layer (MABL), prescribed surface sensible heat flux function, prescribed surface sensible plus latent heat flux functions, and radiation plus surface-layer parameterization schemes. For MABL-initialized sea breezes, shallow weak sea breeze flow in a relatively low instability environment results in weak CI. For the remainder, deeper stronger sea breeze flow in an environment of enhanced instability supports more robust CI. In a subset of experiments, however, the vertical trajectory of air parcels is suppressed leading to weaker convection. Downward acceleration forms due to the horizontal rotation of the sea breeze flow. Accurate simulations of coastal convective storms rely on an accurate representation of sea breezes. For idealized experiments such as the present simulations, a combination of initialization methods likely produces a more realistic representation of sea breeze and the associated physical processes.

© 2024 American Meteorological Society. This is an Author Accepted Manuscript distributed under the terms of the default AMS reuse license. For information regarding reuse and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Fan Wu (fvw5116@psu.edu)
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