• Etling, D., 1989: On atmospheric vortex streets in the wake of large islands. Meteor. Atmos. Phys.,41, 157–164.

    • Crossref
    • Export Citation
  • ——, and S. Raasch, 1987: Numerical simulation of vortex roll development during a cold air outbreak. Dyn. Atmos. Oceans,10, 277–290.

  • Gal-Chen, T., and R. Somerville, 1975: On the use of coordinate a transformation for the solution of the Navier–Stokes equations. J. Comput. Phys.,17, 209–228.

    • Crossref
    • Export Citation
  • Gallus, W. A., Jr., and M. Rancic, 1996: A nonhydrostatic version of the NMC’s regional eta model. Quart. J. Roy. Meteor. Soc.,122, 495–513.

    • Crossref
    • Export Citation
  • ——, and J. B. Klemp, 2000: On the behavior of flow over step orography. Mon. Wea. Rev.,128, 1153–1164.

    • Crossref
    • Export Citation
  • Gerrity, J. P., T. L. Black, and R. E. Treadon, 1994: On the numerical solution of the Mellor–Yamada level 2.5 turbulent kinetic energy equation in the Eta Model. Mon. Wea. Rev.,122 1640–1646.

    • Crossref
    • Export Citation
  • Janjic, Z. I., 1990: The step-mountain coordinate: Physical package. Mon. Wea. Rev.,118, 1429–1443.

    • Crossref
    • Export Citation
  • ——, 1994: The step-mountain eta coordinate model: Further developments of the convection, viscous sublayer, and turbulence closure schemes. Mon. Wea. Rev.,122, 928–945.

  • Kanamitsu, M., 1989: Description of the NMC Global Data Assimilation and Forecast System. Wea. Forecasting,4, 335–342.

    • Crossref
    • Export Citation
  • Lilly, D. K., and E. J. Zipser, 1972: The front range windstorm of 11 January 1972—A meteorological narrative. Weatherwise,25, 56–63.

    • Crossref
    • Export Citation
  • Lobocki, L., 1993: A procedure for the derivation of surface-layer bulk relationships from simplified second-order closure models. J. Appl. Meteor.,32, 126–138.

    • Crossref
    • Export Citation
  • Mellor, G. L., and T. Yamada, 1974: A hierarchy of turbulence closure models for planetary boundary layers. J. Atmos. Sci.,31, 1791–1806.

    • Crossref
    • Export Citation
  • ——, and ——, 1982: Development of a turbulence closure model for geophysical fluid problems. Rev. Geophys. Space Phys.,20, 851–875.

    • Crossref
    • Export Citation
  • Mesinger, F., 1984: A blocking technique for representation of mountains in atmospheric models. Riv. Meteor. Aeronaut.,44, 195–202.

  • ——, and Z. I. Janjic, 1985: Problems and numerical methods of incorporation of mountains in atmospheric models. Large-Scale Computations in Fluid Mechanics, Part 2, B. E. Engquist, S. Osher, R. C. J. Somerville, Eds., Lectures in Applied Mathematics, Vol. 22, American Mathematical Society, 81–120.

  • ——, and T. L. Block, 1992: On the impact on forecast accuracy of the step-mountain (eta) vs. sigma coordinate. Meteor. Atmos. Phys.,50, 47–60.

    • Crossref
    • Export Citation
  • ——, Z. I. Janjic, S. Nickovic, D. Gavrilov, and D. G. Deaven, 1988:The step mountain coordinate: Model description and performance for cases of Alpine cyclogenesis and for a case of an Appalachian redevelopment. Mon. Wea. Rev.,116, 1493–1518.

    • Crossref
    • Export Citation
  • Nuss, W. A., and D. W. Titley, 1994: Use of multiquadric interpolation for meteorological objective analysis. Mon. Wea. Rev.,122, 1611–1631.

    • Crossref
    • Export Citation
  • Phillips, N. A., 1957: A coordinate system having some special advantages for numerical forecasting. J. Meteor.,14, 184–185.

    • Crossref
    • Export Citation
  • Rogers, E., and Coauthors, 1998: Changes to the NCEP operational“early” Eta Analysis/Forecast System. NWS Tech. Procedures Bull. 447, National Oceanic and Atmospheric Administration/National Weather Service, 34 pp. [Available from National Weather Service, Office of Meteorology, 1325 East–West Highway, Silver Spring, MD 20910.].

  • Scorer, R. S., 1955: Theory of airflow over mountains. IV. Separation of flow from the surface. Quart. J. Roy. Meteor. Soc.,81, 340–350.

    • Crossref
    • Export Citation
  • Smith, R. B., A. C. Gleason, P. A. Gluhosky, and V. Grubisic, 1997:The wake of St. Vincent. J. Atmos. Sci.,54, 606–622.

    • Crossref
    • Export Citation
  • Smolarkiewicz, P. K., and R. Rotunno, 1989: Low Froude number flow past three-dimensional obstacles. Part I: Baroclinically generated lee vortices. J. Atmos. Sci.,46, 1154–1164.

  • Staudenmaier, M. J., and J. Mittelstadt, 1997: Results of the western region evaluation of the Eta-10 model. Western Region Tech. Attachment No. 97-18, 12 pp. [Available from National Weather Service Western Region-SSD, 125 S. State St., Rm 1311, Salt Lake City, UT 84147; or online at http://nimbo.wrh.noaa.gov/wrhq/97TAs/TA9718/TA97-18.html.].

  • Tsuchiya, K., 1969: The clouds with the shape of Karman vortex street in the wake of Cheju Island, Korea. J. Meteor. Soc. Japan, II,47, 457–464.

    • Crossref
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 22 22 1
PDF Downloads 9 9 1

The Impact of Step Orography on Flow in the Eta Model: Two Contrasting Examples

View More View Less
  • 1 Department of Geological and Atmospheric Science, Iowa State University, Ames, Iowa
© Get Permissions Rent on DeepDyve
Restricted access

Abstract

Simulations were performed using the Eta Model with its eta vertical coordinate and stepwise treatment of terrain, and with a substitution of the terrain-following sigma vertical coordinate to investigate the impact of step orography on flow near high mountains. Two different cases were simulated: (i) a downslope windstorm along the Front Range of the Rocky Mountains, and (ii) stably stratified flow blocked by high mountains in Taiwan. Flow separation on the lee side of the mountains, previously shown to occur in idealized two-dimensional Eta simulations, was also apparent in these real data cases, even for the downslope wind event. The step orography resulted in a substantial underestimate of wind speeds to the lee of the Rockies during the windstorm. Near the surface, both the eta and sigma simulations of the Taiwan blocking event were comparable. For both events, the use of step orography resulted in much weaker mountain waves than occurred when the sigma vertical coordinate was used. Localized vertical velocity perturbations associated directly with the step corners were minor for these cases.

Corresponding author address: Dr. William A. Gallus Jr., Dept. of Geological and Atmospheric Science, Iowa State University, 3025 Agronomy Hall, Ames, IA 50111.

Email: wgallus@iastate.edu

Abstract

Simulations were performed using the Eta Model with its eta vertical coordinate and stepwise treatment of terrain, and with a substitution of the terrain-following sigma vertical coordinate to investigate the impact of step orography on flow near high mountains. Two different cases were simulated: (i) a downslope windstorm along the Front Range of the Rocky Mountains, and (ii) stably stratified flow blocked by high mountains in Taiwan. Flow separation on the lee side of the mountains, previously shown to occur in idealized two-dimensional Eta simulations, was also apparent in these real data cases, even for the downslope wind event. The step orography resulted in a substantial underestimate of wind speeds to the lee of the Rockies during the windstorm. Near the surface, both the eta and sigma simulations of the Taiwan blocking event were comparable. For both events, the use of step orography resulted in much weaker mountain waves than occurred when the sigma vertical coordinate was used. Localized vertical velocity perturbations associated directly with the step corners were minor for these cases.

Corresponding author address: Dr. William A. Gallus Jr., Dept. of Geological and Atmospheric Science, Iowa State University, 3025 Agronomy Hall, Ames, IA 50111.

Email: wgallus@iastate.edu

Save