Editorial Type:
Article
Article Type:
Research Article

An Observational and Numerical Study of an Orographically Trapped Wind Reversal along the West Coast of the United States

Clifford F. Mass Department of Atmospheric Sciences, University of Washington, Seattle, Washington

Search for other papers by Clifford F. Mass in
Current site
Google Scholar
PubMed Close
and
W. James Steenburgh NOAA/Cooperative Institute for Regional Prediction, and Department of Meteorology, University of Utah, Salt Lake City, Utah

Search for other papers by W. James Steenburgh in
Current site
Google Scholar
PubMed Close
Print Publication:
01 Jul 2000
DOI:
https://doi.org/10.1175/1520-0493(2000)128<2363:AOANSO>2.0.CO;2
Page(s):
2363–2397
Restricted access

Abstract

Observational analyses and a high-resolution simulation using The Pennsylvania State University–NCAR Mesoscale Model Version 5 (MM5) were used to describe the coastally trapped wind reversal of 19–21 July 1994. Major findings include the following.

  • The event was initiated and controlled by changes in the synoptic-scale flow, and particularly by the development of offshore flow over the coastal terrain of Oregon. Although synoptic control was dominant, blocking of the inversion-capped marine layer and mesoscale coastal pressure ridging were important components of the event.

  • The synoptic evolution associated with the trapped reversal was characterized by a shift from climatological near-zonal 500-mb flow to high-amplitude upper-level ridging over the eastern Pacific. As this upper ridge moved northeastward, the 850-mb flow over Oregon became northeasterly and then southeasterly, and 850-mb heights fell over western Oregon.

  • The combination of falling heights aloft and increasing low-level offshore flow, with associated downslope subsidence warming and offshore advection of warm continental air, resulted in sea level pressure falling along the Oregon coast and the northward extension of the California thermal trough into northern Oregon. The extension of the thermal trough caused a reversal of the alongshore pressure gradient along the Oregon coast, so that pressure increased to the south, as well as the attenuation or reversal of the normal cross-shore pressure gradient over the coastal waters.

  • As the coastal trough intensified, there was an increase in nearly geostrophic, onshore-directed coastal flow, with appreciable blocking and deflection of the inversion-capped marine layer by the coastal terrain. With an increase in the northward-directed pressure gradient force due to the troughing to the north, and a lesser contribution from damming of the marine air on the coastal terrain, the blocked low-level winds developed a coastally trapped southerly component over a considerable expanse of the southern and central Oregon coast.

  • Before wind reversal, the northerly flow over the coast was nearly geostrophic. After the coastal winds turned southerly, near-geostrophic balance in the cross-shore direction was established, with the offshore-directed pressure gradient force associated with the coastal pressure ridge balanced by the eastward-directed Coriolis force associated with the southerly flow. In contrast, the momentum balance in the alongshore direction was highly ageostrophic after wind reversal, with near-antitriptic balance close to shore between the northward-directed pressure gradient force and friction, while farther offshore the pressure gradient and Lagrangian accelerations were roughly in balance.

  • The offshore scale of the blocking within the marine layer was less than in the stable layer immediately above, a result consistent with previous theoretical and modeling studies. This difference in offshore blocking scale resulted in the offshore wind reversal occurring in the stable layer aloft before it occurred at the surface.

Corresponding author address: Dr. Clifford Mass, Dept. of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195.

Abstract

Observational analyses and a high-resolution simulation using The Pennsylvania State University–NCAR Mesoscale Model Version 5 (MM5) were used to describe the coastally trapped wind reversal of 19–21 July 1994. Major findings include the following.

  • The event was initiated and controlled by changes in the synoptic-scale flow, and particularly by the development of offshore flow over the coastal terrain of Oregon. Although synoptic control was dominant, blocking of the inversion-capped marine layer and mesoscale coastal pressure ridging were important components of the event.

  • The synoptic evolution associated with the trapped reversal was characterized by a shift from climatological near-zonal 500-mb flow to high-amplitude upper-level ridging over the eastern Pacific. As this upper ridge moved northeastward, the 850-mb flow over Oregon became northeasterly and then southeasterly, and 850-mb heights fell over western Oregon.

  • The combination of falling heights aloft and increasing low-level offshore flow, with associated downslope subsidence warming and offshore advection of warm continental air, resulted in sea level pressure falling along the Oregon coast and the northward extension of the California thermal trough into northern Oregon. The extension of the thermal trough caused a reversal of the alongshore pressure gradient along the Oregon coast, so that pressure increased to the south, as well as the attenuation or reversal of the normal cross-shore pressure gradient over the coastal waters.

  • As the coastal trough intensified, there was an increase in nearly geostrophic, onshore-directed coastal flow, with appreciable blocking and deflection of the inversion-capped marine layer by the coastal terrain. With an increase in the northward-directed pressure gradient force due to the troughing to the north, and a lesser contribution from damming of the marine air on the coastal terrain, the blocked low-level winds developed a coastally trapped southerly component over a considerable expanse of the southern and central Oregon coast.

  • Before wind reversal, the northerly flow over the coast was nearly geostrophic. After the coastal winds turned southerly, near-geostrophic balance in the cross-shore direction was established, with the offshore-directed pressure gradient force associated with the coastal pressure ridge balanced by the eastward-directed Coriolis force associated with the southerly flow. In contrast, the momentum balance in the alongshore direction was highly ageostrophic after wind reversal, with near-antitriptic balance close to shore between the northward-directed pressure gradient force and friction, while farther offshore the pressure gradient and Lagrangian accelerations were roughly in balance.

  • The offshore scale of the blocking within the marine layer was less than in the stable layer immediately above, a result consistent with previous theoretical and modeling studies. This difference in offshore blocking scale resulted in the offshore wind reversal occurring in the stable layer aloft before it occurred at the surface.

Corresponding author address: Dr. Clifford Mass, Dept. of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195.

Save
Cover Monthly Weather Review
Monthly Weather Review

  • Bell, G., and L. F. Bosart, 1989: The large-scale atmospheric structure accompanying New England coastal frontogenesis and associated North American east coast cyclogenesis. Quart. J. Roy. Meteor. Soc.,115, 1133–1146.

  • Bond, N., C. F. Mass, and J. Overland, 1996: Coastally trapped southerly flow along the U.S. west coast. Part I: Climatology and temporal evolution. Mon. Wea. Rev.,124, 430–445.

  • Bresch, J. F., and S. Low-Nam, 1995: Implementation of new objective analysis scheme in RAWINS. Preprints, Fifth PSU/NCAR Mesoscale Model User’s Workshop, Boulder, CO, NCAR Mesoscale and Microscale Meteorology Division, 28–29.

  • Colle, B., and C. F. Mass, 1995: The structure and evolution of shallow cold surge east of the Rocky Mountains. Mon. Wea. Rev.,123, 2577–2610.

  • Dorman, C. E., 1985: Evidence of Kelvin waves in California’s marine layer and related eddy generation. Mon. Wea. Rev.,113, 827–839.

  • ——, 1987: Possible role of gravity currents in northern California’s coastal summer wind reversals. J. Geophys. Res.,92, 1497–1506.

  • Dudhia, J., 1989: Numerical study of convection observed during the Winter Monsoon Experiment using a mesoscale two-dimensional model. J. Atmos. Sci.,46, 3077–3107.

  • Grell, G. A., 1993: Prognostic evaluation of assumptions used by cumulus parameterizations. Mon. Wea. Rev.,121, 764–787.

  • ——, J. Dudhia, and D. R. Stauffer, 1994: A description of the fifth-generation Penn State/NCAR Mesoscale Model (MM5). NCAR Tech. Note NCAR/TN-398+STR, 138 pp. [Available from the National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307.].

  • Guan, S., P. L. Jackson, and C. J. C. Reason, 1998: Numerical modeling of a coastal trapped disturbance. Part I: Comparison with observations. Mon. Wea. Rev.,126, 972–990.

  • Heymsfield, A. J., and L. J. Donner, 1990: A scheme for parameterizing ice-cloud water content in general circulation models. J. Atmos. Sci.,47, 1865–1877.

  • Jackson, P. L., C. J. C. Reason, and S. Guan, 1999: Numerical modeling of a coastally trapped disturbance. Part II: Structure and dynamics. Mon. Wea. Rev.,127, 535–550.

  • Klemp, J. B., and D. R. Durran, 1983: An upper boundary condition permitting internal gravity wave radiation in numerical mesoscale models. Mon. Wea. Rev.,111, 430–444.

  • Leidner, S. M., 1995: Improving California coastal-zone numerical weather prediction by dynamic initialization of the marine boundary layer. M.S. thesis, Dept. of Meteorology, The Pennsylvania State University, 158 pp. [Available from Dept. of Meteorology, The Pennsylvania State University, University Park, PA 16802.].

  • Manning, K. W., and C. A. Davis, 1997: Verification and sensitivity experiments for the WISP94 MM5 forecasts. Wea. Forecasting,12, 719–735.

  • Mass, C., 1995: Comments on “The dynamics of coastally trapped mesoscale ridges in the lower atmosphere.” J. Atmos. Sci.,52, 2313–2318.

  • ——, and M. D. Albright, 1987: Coastal southerlies and alongshore surges of the west coast of North America: Evidence of mesoscale topographically trapped response to synoptic forcing. Mon. Wea. Rev.,115, 1707–1738.

  • ——, and ——, 1988: Reply. Mon. Wea. Rev.,116, 2407–2410.

  • ——, and N. A. Bond, 1996: Coastally trapped southerly flow along the U.S. west coast. Part II: Synoptic evolution. Mon. Wea. Rev.,124, 446–461.

  • ——, J. Steenburgh, and D. Schultz, 1991: Diurnal surface pressure variations over the continental U.S. and the influence of sea level reduction. Mon. Wea. Rev.,119, 2814–2830.

  • Nichols, S., 1984: The dynamics of stratocumulus: Aircraft observations and comparisons with a mixed layer model. Quart. J. Roy. Meteor. Soc.,110, 783–820.

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

  • Overland, J. E., and N. A. Bond, 1995: Observations and scale analysis of coastal wind jets. Mon. Wea. Rev.,123, 2934–2941.

  • Pierrehumbert, R. T., and B. Wyman, 1985: Upstream effects of mesoscale mountains. J. Atmos. Sci.,42, 977–1003.

  • Ralph, F. M., L. Armi, J. M. Bane, C. Dorman, W. D. Neff, P. J. Neiman, W. Nuss, and P. G. Persson, 1998: Observations and analysis of the 10–11 June 1994 coastally trapped disturbance. Mon. Wea. Rev.,126, 2435–2465.

  • ——, P. J. Neiman, J. M. Bane, P. G. Persson, M. L. Cancillo, J. M. Wilczak, and W. Nuss, 2000: Kelvin waves and internal bores in the marine boundary layer inversion and their relationship to coastally trapped wind reversals. Mon. Wea. Rev.,128, 283–300.

  • Reason, C. J. C., and D. G. Steyn, 1992: The dynamics of coastally trapped mesoscale ridges in the lower atmosphere. J. Atmos. Sci.,49, 1677–1692.

  • Rogers, D. P., and Coauthors, 1998: Highlights of Coastal Waves 1996. Bull. Amer. Meteor Soc.,79, 1307–1326.

  • Rogerson, A. M., and R. M. Samelson, 1995: Synoptic forcing of coastal-trapped disturbances in the marine atmospheric boundary layer. J. Atmos. Sci.,52, 2025–2040.

  • Skamarock, W. C., R. Rotunno, and J. B. Klemp, 1999: Models of coastally trapped disturbances. J. Atmos. Sci.,56, 3349–3365.

  • Slingo, A., and H. M. Schrecker, 1982: On the shortwave radiative properties of stratiform water clouds. Quart. J. Roy. Meteor. Soc.,108, 407–426.

  • Stauffer, D. R., and N. L. Seaman, 1990: Use of four-dimensional data assimilation in a limited-area mesoscale model. Part I: Experiments with synoptic scale data. Mon. Wea. Rev.,118, 1250–1277.

  • Thompson, W. T., S. D. Burk, and J. Rosenthal, 1997a: An investigation of the Catalina eddy. Mon. Wea. Rev.,125, 1135–1146.

  • ——, T. Haack, J. D. Doyle, and S. D. Burk, 1997b: A nonhydrostatic mesoscale simulation of the 10–11 June 1994 coastally trapped wind reversal. Mon. Wea. Rev.,125, 3211–3230.

  • Ueyoshi, B., and J. Roads, 1993: Simulation and prediction of the Catalina eddy. Mon. Wea. Rev.,121, 2975–3000.

  • Xu, Q., S. Gao, and B. H. Fiedler, 1996: A theoretical study of cold air damming with upstream cold air inflow. J. Atmos. Sci.,53, 312–326.

  • Zhang, D., and R. A. Anthes, 1982: A high resolution model of the planetary boundary layer: Sensitivity test and comparisons with SESAME-79 data. J. Appl. Meteor.,21, 1594–1609.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 526 402 66
PDF Downloads 81 24 2