Numerical Study of Nocturnal Low-Level Jets over Gently Sloping Terrain

Numerical Study of Nocturnal Low-Level Jets over Gently Sloping Terrain AbstractNocturnal low-level jets (LLJs) over gently sloping terrain typical of the U.S. Great Plains are investigated by means of direct numerical simulation. Such LLJs develop in a tilted atmospheric boundary layer as a result of inertia–gravity oscillations initiated by a change of the surface thermal forcing during the evening transition. External parameters are the free-atmospheric geostrophic wind, ambient atmospheric stratification, surface buoyancy forcing, and slope angle. The governing momentum and buoyancy balance equations are written in slope-following coordinates, and solved numerically in the Boussinesq approximation. The surface forcing is prescribed in a form of surface buoyancy or buoyancy flux, both of which are slope-uniform but change in time. LLJs over slopes are contrasted with LLJs over flat terrain.Slope-induced effects essentially modify the entire structure of nocturnal LLJs. The shape of the LLJ wind profile over a slope is characterized by a sharper and larger-magnitude maximum. The presence of the slope causes the along-slope advection of environmental potential temperature during the night. This advection can reignite static instability in the LLJ flow developing after the evening transition. The resulting turbulence leads to a complete or partial remix of the boundary layer flow and drastically changes the appearance of the LLJ in terms of its shape and vertical position. A pronounced nighttime jet can also develop from the daytime convective boundary layer in the absence of any free-atmospheric geostrophic forcing. The daytime flow preconditioning, an important precursor of the nocturnal LLJ development, plays an especially important role in LLJs over a slope. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the Atmospheric Sciences American Meteorological Society

Numerical Study of Nocturnal Low-Level Jets over Gently Sloping Terrain

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Publisher
American Meteorological Society
Copyright
Copyright © American Meteorological Society
ISSN
1520-0469
D.O.I.
10.1175/JAS-D-17-0013.1
Publisher site
See Article on Publisher Site

Abstract

AbstractNocturnal low-level jets (LLJs) over gently sloping terrain typical of the U.S. Great Plains are investigated by means of direct numerical simulation. Such LLJs develop in a tilted atmospheric boundary layer as a result of inertia–gravity oscillations initiated by a change of the surface thermal forcing during the evening transition. External parameters are the free-atmospheric geostrophic wind, ambient atmospheric stratification, surface buoyancy forcing, and slope angle. The governing momentum and buoyancy balance equations are written in slope-following coordinates, and solved numerically in the Boussinesq approximation. The surface forcing is prescribed in a form of surface buoyancy or buoyancy flux, both of which are slope-uniform but change in time. LLJs over slopes are contrasted with LLJs over flat terrain.Slope-induced effects essentially modify the entire structure of nocturnal LLJs. The shape of the LLJ wind profile over a slope is characterized by a sharper and larger-magnitude maximum. The presence of the slope causes the along-slope advection of environmental potential temperature during the night. This advection can reignite static instability in the LLJ flow developing after the evening transition. The resulting turbulence leads to a complete or partial remix of the boundary layer flow and drastically changes the appearance of the LLJ in terms of its shape and vertical position. A pronounced nighttime jet can also develop from the daytime convective boundary layer in the absence of any free-atmospheric geostrophic forcing. The daytime flow preconditioning, an important precursor of the nocturnal LLJ development, plays an especially important role in LLJs over a slope.

Journal

Journal of the Atmospheric SciencesAmerican Meteorological Society

Published: Sep 18, 2017

References

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