A Nonhydrostatic Mesoscale Simulation of the 10–11 June 1994 Coastally Trapped Wind Reversal

William T. Thompson Marine Meteorology Division, Naval Research Laboratory, Monterey, California

Search for other papers by William T. Thompson in
Current site
Google Scholar
PubMed
Close
,
Tracy Haack Marine Meteorology Division, Naval Research Laboratory, Monterey, California

Search for other papers by Tracy Haack in
Current site
Google Scholar
PubMed
Close
,
James D. Doyle Marine Meteorology Division, Naval Research Laboratory, Monterey, California

Search for other papers by James D. Doyle in
Current site
Google Scholar
PubMed
Close
, and
Stephen D. Burk Marine Meteorology Division, Naval Research Laboratory, Monterey, California

Search for other papers by Stephen D. Burk in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

During the summer months, the California coast is under the influence of persistent northwesterly flow. Several times each summer, this regime is disrupted by coastally trapped wind reversals (CTWR) in which the northwesterly flow is replaced by southerlies in a narrow zone along the coast. Controversy exists as to the physical mechanisms responsible for initiation and maintenance of CTWRs. While it is clear that coastal terrain is important in creating the trapped response, the precise role played by terrain is unclear. In the present study, these issues are investigated using a nonhydrostatic mesoscale model to simulate the 10–11 June 1994 CTWR event. The results show that the model successfully reproduces many of the observed features of this event, including anomalous vertical structure involving the relatively shallow boundary layer with a warm, nearly neutral layer above; the northward propagation of southerly flow in advance of a tongue of coastal stratus/fog; and a substantial reduction in propagation speed due to the sea breeze. Of the several mechanisms that have been proposed in the literature to characterize these events, these results are most consistent with a topographically trapped gravity current. Further investigation, required to verify this hypothesis, is ongoing.

Two sensitivity studies are used to examine the role of terrain in producing and maintaining the CTWR. In the first sensitivity study, the coastline from Pt. Conception to Pt. Reyes is replaced with a straight line and a uniform 840-m-high ridge is placed adjacent to the coast. This simplification permits better isolation of the terrain influence on the mesoscale pressure field and the forcing of the CTWR by the pressure distribution. The results show that adiabatic warming associated with flow over the coastal terrain is required to produce the alongshore pressure gradient, which forces ageostrophic southerly flow, and that, in the absence of bays and gaps in this terrain, southerly flow extends to the location of the minimum pressure. In a second sensitivity study, the height of the ridge along the coast is set to zero. In this simulation there is no mesoscale organization of the southerly flow. Moreover, the structure of the marine boundary layer near the coast is altered by removal of downslope flow and the gravity current characteristics seen in the control and first sensitivity study are absent.

Corresponding author address: Dr. William T. Thompson, Naval Research Laboratory, Marine Meteorology Division, Monterey, CA 93943-5502.

Email: thompson@nrlmry.navy.mil

Abstract

During the summer months, the California coast is under the influence of persistent northwesterly flow. Several times each summer, this regime is disrupted by coastally trapped wind reversals (CTWR) in which the northwesterly flow is replaced by southerlies in a narrow zone along the coast. Controversy exists as to the physical mechanisms responsible for initiation and maintenance of CTWRs. While it is clear that coastal terrain is important in creating the trapped response, the precise role played by terrain is unclear. In the present study, these issues are investigated using a nonhydrostatic mesoscale model to simulate the 10–11 June 1994 CTWR event. The results show that the model successfully reproduces many of the observed features of this event, including anomalous vertical structure involving the relatively shallow boundary layer with a warm, nearly neutral layer above; the northward propagation of southerly flow in advance of a tongue of coastal stratus/fog; and a substantial reduction in propagation speed due to the sea breeze. Of the several mechanisms that have been proposed in the literature to characterize these events, these results are most consistent with a topographically trapped gravity current. Further investigation, required to verify this hypothesis, is ongoing.

Two sensitivity studies are used to examine the role of terrain in producing and maintaining the CTWR. In the first sensitivity study, the coastline from Pt. Conception to Pt. Reyes is replaced with a straight line and a uniform 840-m-high ridge is placed adjacent to the coast. This simplification permits better isolation of the terrain influence on the mesoscale pressure field and the forcing of the CTWR by the pressure distribution. The results show that adiabatic warming associated with flow over the coastal terrain is required to produce the alongshore pressure gradient, which forces ageostrophic southerly flow, and that, in the absence of bays and gaps in this terrain, southerly flow extends to the location of the minimum pressure. In a second sensitivity study, the height of the ridge along the coast is set to zero. In this simulation there is no mesoscale organization of the southerly flow. Moreover, the structure of the marine boundary layer near the coast is altered by removal of downslope flow and the gravity current characteristics seen in the control and first sensitivity study are absent.

Corresponding author address: Dr. William T. Thompson, Naval Research Laboratory, Marine Meteorology Division, Monterey, CA 93943-5502.

Email: thompson@nrlmry.navy.mil

Save
  • Blackadar, A. K., 1962: The vertical distribution of wind and turbulent exchange in a neutral atmosphere. J. Geophys Res.,67, 3095–3103.

  • Bond, N. A., C. F. Mass, and J. E. Overland, 1996: Coastally trapped wind reversals along the United States west coast during the warm season. Part I: Climatology and temporal evolution. Mon. Wea. Rev.,124, 430–445.

  • Businger, J. A., J. C. Wyngaard, Y. Izumi, and E. F. Bradley, 1971: Flux profile relationships in the atmospheric surface layer. J. Atmos. Sci.,28, 181–189.

  • Deardorff, J. W., 1978: Efficient prediction of ground surface temperature and moisture, with inclusion of a layer of vegetation. J. Geophys Res.,83, 1889–1903.

  • 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.

  • Harshvardhan, R. Davies, D. A. Randall, and T. G. Corsetti, 1987: A fast radiation parameterization for atmospheric models. J. Geophys. Res.,92, 1009–1016.

  • Hodur, R. M., 1997: The U. S. Navy’s coupled ocean/atmosphere model (COAMPS). Mon. Wea. Rev.,125, 1414–1430.

  • Hogan, T. F., and T. E. Rosmond, 1991: The description of the Navy Operational Global Atmospheric Prediction System’s spectral forecast model. Mon. Wea. Rev.,119, 1786–1815.

  • Klemp, J., and R. Wilhelmson, 1978: The simulation of three-dimensional convective storm dynamics. J. Atmos. Sci.,35, 1070–1096.

  • ——, R. Rotunno, and W. C. Skamrock, 1994: Propagation of atmospheric gravity currents along a coastal barrier. Preprints, Sixth Conf. on Mesoscale Processes, Portland, OR, Amer. Meteor. Soc., 497–500.

  • Louis, J.-F., 1979: A parametric model of vertical eddy fluxes in the atmosphere. Bound.-Layer Meteor.,17, 187–202.

  • Mass, C. F., 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 ———, 1989: Origin of the Catalina Eddy. Mon. Wea. Rev.,117, 2602–2436.

  • ——— and N. A. Bond, 1996: Coastally trapped wind reversals along the United States West Coast during the warm season. Part II: Synoptic evolution. Mon. Wea. Rev.,124, 446–461.

  • ———, ———, and D. J. Brees, 1986: The onshore surge of marine air into the Pacific Northwest: A coastal region of complex terrain. Mon. Wea. Rev.114, 2602–2627.

  • Mellor, G. L., and T. Yamada, 1982: Development of a turbulence closure for geophysical fluid problems. Rev. Geophys. Space Phys.,20, 851–875.

  • Oosterling, P. S., 1995: Coastally trapped disturbances along the U. S. west coast: Synoptic and mesoscale analysis of 9–12 June 1994. M. S. thesis, Naval Postgraduate School, Monterey, CA, 73 pp. [Available from Department of Meteorology, Naval Postgraduate School, Monterey, CA 93043-5000.].

  • Persson, P. O. G., P. J. Neiman, and F. M. Ralph, 1995: Topographically generated potential voricity anonolies: A proposed mechanism for initiating coastally trapped disturbances. Preprints, Seventh Conf. on Mountain Meteorology, Breckenridge, CO, Amer. Meteor. Soc., 216–222.

  • ——, ——, and ——, 1996: The role of a topographically generated potential vorticity anomaly in initiating a coastal wind reversal. Preprints, Conf. on Coastal Oceanic and Atmospheric Prediction, Atlanta, GA, Amer. Meteor. Soc., 120–124.

  • Ralph, F. M., P. J. Neiman, P. O. G. Persson, W. D. Neff, J. Miletta, L. Armi, and J. M. Bane, 1995: Observations of an orographically trapped disturbance along the California Coast on 10–11 June 1994. Preprints, Seventh Conf. on Mountain Meteorology, Breckenridge, CO, Amer. Meteor. Soc., 204–211.

  • ——, ——, ——, W. Nuss, C. Dorman, and W. D. Neff, 1996: The evolution of a coastally trapped atmospheric disturbance along the California Coast on 10–11 June 1994. Preprints, Conf. on Coastal Oceanic and Atmospheric Prediction, Atlanta, GA, Amer. Meteor. Soc., 129–135.

  • 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.

  • 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.

  • Rutlidge, S. A., and P. V. Hobbs, 1983: The mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones. VIII: A model for the “seeder-feeder” process in warm-frontal rainbands. J. Atmos. Sci.,40, 1185–1206.

  • Smagorinsky, J., 1963: General circulation experiments with the primitive equations. I. The basic experiment. Mon. Wea. Rev.,91, 99–164.

  • Therry, G., and T. LaCarr’re, 1983: Improving the eddy kinetic energy model for the planetary boundary layer description. Bound.-Layer Meteor.,25, 63–88.

  • Thompson, W. T., and S. D. Burk, 1991: An investigation of an Arctic front with a vertically nested mesoscale model. Mon. Wea. Rev.,119, 233–261.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 177 79 1
PDF Downloads 68 31 0