What Causes Weak Orographic Rain Shadows? Insights from Case Studies in the Cascades and Idealized Simulations

Nicholas Siler Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Dale Durran Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Abstract

Recent studies have shown that weak rain shadows in the Cascade Mountains are associated with passing warm fronts, but the specific mechanisms responsible for this connection have eluded consensus. One theory holds that weak rain shadows are the result of enhanced precipitation over eastern slopes caused by easterly upslope flow; the other suggests that condensation is produced primarily over the western slopes, with enhanced east-slope precipitation occurring in dynamical regimes that minimize descent and evaporation east of the crest. Here these mechanisms are investigated through numerical simulations involving both real and idealized topography. Consistent with the second theory, storms with weak rain shadows are found to exhibit much weaker mountain waves in the lee of the Cascades than storms with strong rain shadows, with correspondingly weaker leeside evaporation. The muted wave activity during weak-rain-shadow storms is found to be caused by cold, zonally stagnant air at low levels in the lee, which precedes the warm front, and remains in place as the progression of the front is impeded by the mountains. As the front brings warmer air aloft, the static stability of the zonally stagnant layer increases, making it more resistant to erosion by the overlying flow. This in turn allows the weak rain shadow to persist long after the front has passed. If the midlatitude storm tracks shift poleward in a warmer climate, the results suggest there could be an increase in the strength of the rain shadow in mountainous regions astride the current storm tracks.

Current affiliation: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California.

Corresponding author address: Nicholas Siler, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, #0206, La Jolla, CA 92093-0206. E-mail: nsiler@ucsd.edu

Abstract

Recent studies have shown that weak rain shadows in the Cascade Mountains are associated with passing warm fronts, but the specific mechanisms responsible for this connection have eluded consensus. One theory holds that weak rain shadows are the result of enhanced precipitation over eastern slopes caused by easterly upslope flow; the other suggests that condensation is produced primarily over the western slopes, with enhanced east-slope precipitation occurring in dynamical regimes that minimize descent and evaporation east of the crest. Here these mechanisms are investigated through numerical simulations involving both real and idealized topography. Consistent with the second theory, storms with weak rain shadows are found to exhibit much weaker mountain waves in the lee of the Cascades than storms with strong rain shadows, with correspondingly weaker leeside evaporation. The muted wave activity during weak-rain-shadow storms is found to be caused by cold, zonally stagnant air at low levels in the lee, which precedes the warm front, and remains in place as the progression of the front is impeded by the mountains. As the front brings warmer air aloft, the static stability of the zonally stagnant layer increases, making it more resistant to erosion by the overlying flow. This in turn allows the weak rain shadow to persist long after the front has passed. If the midlatitude storm tracks shift poleward in a warmer climate, the results suggest there could be an increase in the strength of the rain shadow in mountainous regions astride the current storm tracks.

Current affiliation: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California.

Corresponding author address: Nicholas Siler, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, #0206, La Jolla, CA 92093-0206. E-mail: nsiler@ucsd.edu
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  • Durran, D. R., and J. B. Klemp, 1983: A compressible model for the simulation of moist mountain waves. Mon. Wea. Rev., 111, 23412361, doi:10.1175/1520-0493(1983)111<2341:ACMFTS>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Gaberšek, S., and D. R. Durran, 2006: Gap flows through idealized topography. Part II: Effects of rotation and surface friction. J. Atmos. Sci., 63, 27202739, doi:10.1175/JAS3786.1.

    • Search Google Scholar
    • Export Citation
  • Hong, S.-Y., 2010: A new stable boundary-layer mixing scheme and its impact on the simulated East Asian summer monsoon. Quart. J. Roy Meteor. Soc., 136, 14811496, doi:10.1002/qj.665.

    • Search Google Scholar
    • Export Citation
  • Lee, T. J., R. A. Pielke, R. C. Kessler, and J. Weaver, 1989: Influence of cold pools downstream of mountain barriers on downslope winds and flushing. Mon. Wea. Rev., 117, 20412058, doi:10.1175/1520-0493(1989)117<2041:IOCPDO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Leung, L. R., Y. Qian, X. Bian, W. M. Washington, J. Han, and J. O. Roads, 2004: Mid-century ensemble regional climate change scenarios for the western United States. Climatic Change, 62, 75113, doi:10.1023/B:CLIM.0000013692.50640.55.

    • Search Google Scholar
    • Export Citation
  • Mass, C., N. Johnson, M. Warner, and R. Vargas, 2015: Synoptic control of cross-barrier precipitation ratios for the Cascade Mountains. J. Hydrometeor., 16, 10141028, doi:10.1175/JHM-D-14-0149.1.

    • Search Google Scholar
    • Export Citation
  • Niu, G.-Y., and Coauthors, 2011: The community Noah land surface model with multiparameterization options (Noah-MP): 1. Model description and evaluation with local-scale measurements. J. Geophys. Res., 116, D12109, doi:10.1029/2010JD015139.

    • Search Google Scholar
    • Export Citation
  • Pavelsky, T. M., S. Sobolowski, S. B. Kapnick, and J. B. Barnes, 2012: Changes in orographic precipitation patterns caused by a shift from snow to rain. Geophys. Res. Lett., 39, L18706, doi:10.1029/2012GL052741.

    • Search Google Scholar
    • Export Citation
  • Shi, X., and D. R. Durran, 2014: The response of orographic precipitation over idealized midlatitude mountains due to global increases in CO2. J. Climate, 27, 39383956, doi:10.1175/JCLI-D-13-00460.1.

    • Search Google Scholar
    • Export Citation
  • Siler, N., and G. Roe, 2014: How will orographic precipitation respond to surface warming? An idealized thermodynamic perspective. Geophys. Res. Lett., 41, 26062613, doi:10.1002/2013GL059095.

    • Search Google Scholar
    • Export Citation
  • Siler, N., and D. Durran, 2015: Assessing the impact of the tropopause on mountain waves and orographic precipitation using linear theory and numerical simulations. J. Atmos. Sci., 72, 803820, doi:10.1175/JAS-D-14-0200.1.

    • Search Google Scholar
    • Export Citation
  • Siler, N., G. Roe, and D. Durran, 2013: On the dynamical causes of variability in the rain-shadow effect: A case study of the Washington Cascades. J. Hydrometeor., 14, 122139, doi:10.1175/JHM-D-12-045.1.

    • Search Google Scholar
    • Export Citation
  • Smith, R. B., 2006: Progress on the theory of orographic precipitation. Geol. Soc. Amer. Spec. Pap., 398, 116, doi:10.1130/2006.2398(01).

    • Search Google Scholar
    • Export Citation
  • Thompson, G., P. R. Field, R. M. Rasmussen, and W. D. Hall, 2008: Explicit forecasts of winter precipitation using an improved bulk microphysics scheme. Part II: Implementation of a new snow parameterization. Mon. Wea. Rev., 136, 50955115, doi:10.1175/2008MWR2387.1.

    • Search Google Scholar
    • Export Citation
  • Yin, J. H., 2005: A consistent poleward shift of the storm tracks in simulations of 21st century climate. Geophys. Res. Lett., 32, L18701, doi:10.1029/2005GL023684.

    • Search Google Scholar
    • Export Citation
  • Zängl, G., 2005: The impact of lee-side stratification on the spatial distribution of orographic precipitation. Quart. J. Roy. Meteor. Soc., 131, 10751091, doi:10.1256/qj.04.118.

    • Search Google Scholar
    • Export Citation
  • Zängl, G., and M. Hornsteiner, 2007: The exceptional Alpine south foehn event of 14–16 November 2002: A case study. Meteor. Atmos. Phys., 98, 217238, doi:10.1007/s00703-006-0257-9.

    • Search Google Scholar
    • Export Citation
  • Zhang, D., and R. A. Anthes, 1982: A high-resolution model of the planetary boundary layer—Sensitivity tests and comparisons with SESAME-79 data. J. Appl. Meteor., 21, 15941609, doi:10.1175/1520-0450(1982)021<1594:AHRMOT>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
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