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D. G. Steyn
and
M. Segal

Abstract

An estimate is given of the relative importance of wind veering and turbulent diffusion in the mean horizontal spread of pollutant plumes in the atmosphere. Documented veering rates in sea breezes are used to illustrate the effect, and it is concluded that for typical sea breeze induced veering, the effect will be significant over much of the range of applicability of the Gaussian plume model.

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B. Ainslie
and
D. G. Steyn

Abstract

A cluster analysis of wind measurements from two meteorological stations in the Lower Fraser Valley, British Columbia, Canada, has been performed to identify mesoscale circulation regimes that are common to days on which ozone mixing ratios at one or more measuring stations in the region’s fixed monitoring network exceed the National Ambient Air Quality Objective of 82 ppb. The analysis, using 20 yr of data (1984–2003), identifies the following four regimes: two with morning winds at the coastal Vancouver International Airport (YVR) meteorological station from the northwest direction and two with southerly YVR morning winds. Not all exceedance days are associated with sea-breeze circulations, but days with southerly morning winds have a higher proportion of well-developed sea-breeze circulations. Composite synoptic patterns associated with each regime all show high pressure over the eastern Pacific Ocean with a thermal trough over Washington State and southwestern British Columbia. Composite ozone patterns, corresponding to each mesoscale circulation regime and taken at the hour of maximum ozone concentration, show similar general features, including strong ozone titration in and around the urban source regions and higher values downwind. This suggests that precursor buildup, prior to the exceedance day, plays an important role in the spatial ozone pattern on exceedance days. A simple multiple linear regression of the plume centroid with the number of days elapsed from the start of the analysis period suggests the centroid of the ozone plume has shifted eastward since 1990. There also appears to be a north–south shift in the ozone plume. It is impossible to tell if these shifts are due to changes in emission levels or to changes in spatial emission patterns, because both changes have occurred over the study period.

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A. Porson
,
D. G. Steyn
, and
G. Schayes

Abstract

A scaling analysis is conducted to explore the dependence of sea-breeze speed and inland occurrence in the presence of opposing winds on a set of dynamical parameters. The overall aim of the analysis is to develop an index for sea-breeze occurrence in the face of opposing winds, similar to the Biggs and Graves lake-breeze index. Most studies separate sea-breeze speed and sea-breeze inland occurrence or, at best, link the two in linear analyses. This work analyzes the output of a nonlinear numerical mesoscale model (in idealized simulations) using scaling methods commonly applied in observational studies. It is found that the scaled sea-breeze speed, in response to increasing magnitude of opposing wind, shows two distinct phases: a phase of increasing speed while the sea breeze progresses inland and a phase of sharply decreasing speed when the sea breeze is no longer detected inland. The analysis also allows the development of an index for sea-breeze inland occurrence. This index is an improvement over existing analyses through the use of nonlinear scaling and the use of surface heat flux as opposed to simpler land–sea temperature contrasts.

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D. G. Steyn
and
I. G. Mckendry

Abstract

This study presents an evaluation of the performance of the Colorado State University (CSU) three-dimensional numerical mesoscale model. The evaluation consists of quantitative and qualitative comparisons of the model output with observed data. The observations were undertaken in the lower Fraser Valley of British Columbia, Canada on 23 August 1985 utilizing up to 23 fixed stations for anemometry, three acoustic sounders, one tethersonde and one set of instruments for determining the surface energy budget terms.

The modeling covers a 24-hour period during which a wed-developed sea breeze was observed. While the results of the model evaluation apply to the single case presented, they have implications for a wide range of cases. The statistical methods of Willmott are applied to hourly averaged variables to assess the model's performance.

The evaluation shows that the model is capable of providing very realistic wind and temperature fields within the broad coastal valley which makes up the bulk of the domain. The modeled mixed layer depth, surface energy budget terms and wind profiles are in good agreement with observed data.

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C. J. C. Reason
and
D. G. Steyn

Abstract

No abstract available

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D. G. Steyn
and
T. J. Lyons

Abstract

No abstract available.

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C. J. C. Reason
and
D. G. Steyn

Abstract

The dynamics of coastally trapped ridges that propagate in the marine layers of western North America and southeastern Australia is examined. A nonlinear semigeostrophic theory shows that the coastal ridges develop initially as an alongshore intrusion of denser marine air that is driven by the synoptic-scale pressure gradient. Nonlinear Kelvin waves evolve with the intruding flow on a slower time scale governed by the dynamic parameters. If dispersive effects balance the nonlinearities, then these waves evolve into solitary form. Otherwise, the nonlinear waves steepen so that the leading edge of the ridge eventually propagates as a shock.The theory is applied to two ridging events in California and one in southeastern Australia. In each case, good agreement is found between theory and observations of the evolution times and propagation speeds of the coastal ridges. The theory also explains the observed behavior of the events at prominent convex bends and gaps in the coastal topography.

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Peter L. Jackson
and
D. G. Steyn

Abstract

A simple shallow-water model of gap wind in a channel that is based upon hydraulic theory is presented and compared with observations and output from a 3D mesoscale numerical model. The model is found to be successful in simulating gap winds. The speed and depth of gap wind flow is strongly controlled by topography. Horizontal or vertical channel contractions can act to force strong, shallow supercritical flow downwind and light, deep subcritical flow upwind. Force-balance analysis of the hydraulic model output confirms mesoscale model results and indicates that the prime force balance in gap wind is between external pressure gradient and friction for supercritical flow and between external pressure gradient and height pressure gradient for subcritical flow. This force balance changes near channel controls when the balance is between advection and height pressure gradient. Sensitivity analyses show positive sensitivity of gap wind speed to changes in discharge and external pressure gradient, negative sensitivity to changes in friction and boundary layer height at the channel exit, and mixed sensitivity of gap wind speed to changes in reduced gravity.

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Peter L. Jackson
and
D. G. Steyn

Abstract

Gap winds in Howe Sound, British Columbia, are described and placed in context by reviewing studies of similar phenomena in other locations. An observational program consisting of a surface mesonetwork and vertical soundings shows that gap winds vary considerably along and across the channel, as well as vertically. Wind strength generally increases down channel, and strongest winds are found below 1000-m depth. Results from application of a 3D mesoscale numerical model to a gap wind case compare reasonably well with observations. Model output reveals more details of horizontal and especially vertical flow structure than is possible from observations. Model vertical cross sections and Froude number output indicate similarity with hydraulic flow. This is further substantiated by a force-balance analysis of model output.

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D. G. Steyn
and
K. W. Ayotte

Abstract

A caution is offered with regard to the use of one-dimensional terrain spectra to indicate the grid resolution needed to resolve terrain forcing in mesoscale numerical modeling exercises. To illustrate this, two-dimensional terrain height spectra are presented for two contrasting terrains: a relatively direction free topography (a portion of southcentral British Columbia, Canada) and a highly ordered topography (a portion of the ridge and valley terrain in Pennsylvania). Isoamplitude plots of the two spectra show clearly the morphological differences between the two regions and indicate the degree of directionality of the ordered terrain.

An investigation of the wavenumber dependence of the terrain height spectra shows the spectral roll-off for the first case to be essentially independent of direction and to decay roughly as wavenumber to the -5/2 power over a wavenumber range of 0.04 to 8.33 km−1. By contrast, the spectral roll-off in the second case is strongly dependent on direction with an exponent that may be either greater than or less than the convergence limit (−2.0 for the amplitude spectrum) indicated by Young and Pielke.

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