Accession Number:

ADA522191

Title:

Momentum Fluxes of Gravity Waves Generated by Variable Froude Number Flow Over Three-Dimensional Obstacles

Descriptive Note:

Journal article preprint

Corporate Author:

NAVAL RESEARCH LAB WASHINGTON DC SPACE SCIENCE DIV

Report Date:

2010-01-01

Pagination or Media Count:

58.0

Abstract:

Fully nonlinear mesoscale model simulations are used to investigate the momentum fluxes of gravity waves that emerge at a far-field height of 6 km from steady unsheared flow over both an axisymmetric and elliptical obstacle for nondimensional mountain heights hsub m 1Fr in the range 0.1-5, where Fr is the surface Froude number. Fourier- and Hilbert-transform diagnostics of model output yield local estimates of phase-averaged momentum flux, while area integrals of momentum flux quantify the amount of surface pressure drag that translates into far-field gravity waves, referred to here as the wave drag component. Estimates of surface and wave drag are compared to parameterization predictions and theory. Surface dynamics transition from linear to high-drag wave-breaking states at critical inverse Froude numbers 1Frsub c predicted to within 10 by the transform relations of Smith 1989b. Wave drag peaks at 1Frsub c hm 2, where for the elliptical obstacle both surface and wave drag vacillate due to cyclical buildup and breakdown of waves. For the axisymmetric obstacle, this occurs only at hm 1.2. At hm 2-3 vacillation abates and normalized pressure drag assumes a common normalized form for both obstacles that varies approximately as hmexp -1.3. Wave drag in this range asymptotes to a constant absolute value that, despite its theoretical shortcomings, is predicted to within 10-40 by an analytical relation based on linear clipped-obstacle drag for a Sheppard-based prediction of dividing streamline height. Constant wave drag at hm approx. 2-5 arises despite large variations with hm in the three-dimensional morphology of the local wave momentum fluxes. Specific implications of these results to the parameterization of subgrid-scale orographic drag in global climate and weather models are discussed.

Subject Categories:

  • Atmospheric Physics
  • Meteorology
  • Numerical Mathematics
  • Test Facilities, Equipment and Methods
  • Fluid Mechanics

Distribution Statement:

APPROVED FOR PUBLIC RELEASE