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



Details

  • Journal Title:
    Journal of the Atmospheric Sciences
  • Personal Author:
  • NOAA Program & Office:
    CIMMS (Cooperative Institute for Mesoscale Meteorological Studies)
  • Description:
    Nocturnal 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.
  • Source:
    Journal of the Atmospheric Sciences, 74(9), 2813-2834
  • DOI:
  • ISSN:
    0022-4928 ; 1520-0469
  • Format:
    pdf
  • Publisher:
    American Meteorological Society
  • Document Type:
    Journal Article
  • Rights Information:
    Other
  • Compliance:
    Library
  • Main Document Checksum:
    urn:sha-512:39afe59534d35290f821e4fbf9a6255afecce8f76e8e18cabda6b350ea5e38c152d1674508564d9346753b2fb230f63782e79061f83a64fdb0d30bf36926b88e
  • Download URL:
  • File Type:
    Filetype[PDF - 2.23 MB ]
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