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Dynamical and Physical Processes Associated with Orographic Precipitation in a Conditionally Unstable Uniform Flow: Variation in Basic Wind Speed

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  • 1 Department of Energy and Environmental Systems, North Carolina Agricultural and Technical State University, Greensboro, North Carolina, and Environmental Science Division, Argonne National Laboratory, Argonne, Illinois
  • | 2 Department of Physics, and Department of Energy and Environmental Systems, North Carolina Agricultural and Technical State University, Greensboro, North Carolina
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Abstract

A series of systematic two- and three-dimensional (2D and 3D, respectively) idealized numerical experiments were conducted to investigate the combined effects of dynamical and physical processes on orographic precipitation with varying incoming basic-flow speed U in a conditionally unstable uniform flow. In addition to the three moist flow regimes found in Chu and Lin at lower wind speeds, a new flow regime, regime IV, is found for higher wind speeds (U > 36 m s−1) and is characterized by gravity waves and heavy precipitation and lack of upper-level wave breaking and turbulence over the lee slope. The transition from regime III to regime IV at 36 m s−1 is explained by the transition from upward-propagating gravity waves to evanescent flow, which can be predicted with a modified mountain wave theory. Although the basic features are captured well in low grid resolution (Δx = 1 km), high-resolution (Δx = 100 m) 2D and 3D simulations are required to resolve precipitation distribution and intensity at higher basic winds (U > 30 m s−1). Based on 3D simulations, gravity wave–induced severe downslope winds and turbulent mixing within hydraulic jump reduce orographic precipitation in regime III. A preliminary budget analysis indicated that, in regime IV, orographic precipitation further increases as a result of enhanced rain processes when the blocking effect of wave breaking vanishes.

© 2017 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 e-mail: Yuh-Lang Lin, ylin@ncat.edu

Abstract

A series of systematic two- and three-dimensional (2D and 3D, respectively) idealized numerical experiments were conducted to investigate the combined effects of dynamical and physical processes on orographic precipitation with varying incoming basic-flow speed U in a conditionally unstable uniform flow. In addition to the three moist flow regimes found in Chu and Lin at lower wind speeds, a new flow regime, regime IV, is found for higher wind speeds (U > 36 m s−1) and is characterized by gravity waves and heavy precipitation and lack of upper-level wave breaking and turbulence over the lee slope. The transition from regime III to regime IV at 36 m s−1 is explained by the transition from upward-propagating gravity waves to evanescent flow, which can be predicted with a modified mountain wave theory. Although the basic features are captured well in low grid resolution (Δx = 1 km), high-resolution (Δx = 100 m) 2D and 3D simulations are required to resolve precipitation distribution and intensity at higher basic winds (U > 30 m s−1). Based on 3D simulations, gravity wave–induced severe downslope winds and turbulent mixing within hydraulic jump reduce orographic precipitation in regime III. A preliminary budget analysis indicated that, in regime IV, orographic precipitation further increases as a result of enhanced rain processes when the blocking effect of wave breaking vanishes.

© 2017 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 e-mail: Yuh-Lang Lin, ylin@ncat.edu
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