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Ian G. Mckendry

Abstract

The Kirchhofer synoptic classification procedure is applied to both mean sea level and 500-hPa NMC gridded pressure fields for the vicinity of southwestern British Columbia. Exceedances of the Canadian 1-h Ambient O3 Air Quality Objective of 82 ppb at Port Moody, Vancouver, are associated with the coincidence of a low-level thermal trough and an upper-level ridge of high pressure. Analysis of synoptic sequences also reveals the importance of persistence in the development of elevated O3 concentrations. The application of synoptic climatology to ground-level O3 in Vancouver highlights the need for consideration of more than one atmospheric level in map-typing schemes. An extension of the basic Kirchhofer approach to permit multilevel computer-assisted map typing is advocated.

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Ian G. McKendry and Cliff G. Revell

Abstract

Evidence is presented confirming the existence of a late-afternoon mesoscale eddy primarily caused by local diabatic heating. Simulations with the Colorado State University (CSU) mesoscale model show that the eddy forms in a zone of strong sea-breeze convergence under light southeasterly gradient flow. Although showing good agreement with simulations in respect to the timing and location of eddy development, observations from two days demonstrate that eddy formation may be complicated by mesoscale interactions that result in concurrent cloud development and precipitation in the vicinity of eddy genesis. The ability of the CSU model to capture the salient mesoscale features in the region, and the association of the cyclonic eddy and sea-breeze convergence in south Auckland with a preferred synoptic regime give cause for optimism in forecasting such phenomena.

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Joshua P. Hacker, Ian G. McKendry, and Roland B. Stull

Abstract

An intense Gobi Desert dust storm in April 1998 loaded the midtroposphere with dust that was transported across the Pacific to western North America. The Mesoscale Compressible Community (MC2) model was used to investigate mechanisms causing downward transport of the midtropospheric dust and to explain the high concentrations of particulate matter of less than 10-μm diameter measured in the coastal urban areas of Washington and southern British Columbia. The MC2 was initialized with a thin, horizontally homogeneous layer of passive tracer centered at 650 hPa for a simulation from 0000 UTC 26 April to 0000 UTC 30 April 1998. Model results were in qualitative agreement with observed spatial and temporal patterns of particulate matter, indicating that it captured the important meteorological processes responsible for the horizontal and vertical transport over the last few days of the dust event. A second simulation was performed without topography to isolate the effects of topography on downward transport.

Results show that the dust was advected well east of the North American coast in southwesterly midtropospheric flow, with negligible dust concentration reaching the surface initially. Vertically propagating mountain waves formed during this stage, and differences between downward and upward velocities in these waves could account for a rapid descent of dust to terrain height, where the dust was entrained into the turbulent planetary boundary layer. A deepening outflow (easterly) layer near the surface transported the tracer westward and created a zonal-shear layer that further controlled the tracer advection. Later, the shear layer lifted, leading to a downward hydraulic acceleration along the western slopes, as waves generated in the easterly flow amplified below the shear layer that was just above mountain-crest height. Examination of 10 yr of National Centers for Environmental Prediction–National Center for Atmospheric Research reanalyses suggests that such events are rare.

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John P. Gallagher, Ian G. McKendry, Anne Marie Macdonald, and W. Richard Leaitch

Abstract

A mountain air chemistry observatory has been operational on the summit of Whistler Mountain in British Columbia, Canada, since 2002. A 1-yr dataset of condensation nuclei (CN) concentration from this site has been analyzed along with corresponding meteorological data to assess the frequency and patterns of influence from the planetary boundary layer (PBL). Characterization of air masses sampled from the site as either PBL influenced or representative of the free troposphere (FT) is important to subsequent analysis of the chemistry data. Median CN concentrations and seasonal trends were found to be comparable to other midlatitude mountain sites. Monthly median number concentrations ranged from 120 cm−3 in January to 1601 cm−3 in July. Using well-defined diurnal cycles in CN concentration as an indicator of air arriving from nearby valleys, PBL influence was found to occur on a majority of days during spring and summer and less frequently in late autumn and winter. Days with PBL influence were usually associated with synoptic-scale weather patterns that were conducive to convective mixing processes. Although more common in the warm season, vertical mixing capable of transporting valley air to the mountaintop also occurred in February during a period of high pressure aloft. In contrast, an August case study indicated that the more stable character of marine air masses can at times keep the PBL below the summit on summer days. Considerable variability in the synoptic-scale weather conditions at Whistler means that careful analysis of available datasets must be made to discriminate FT from PBL periods at the observatory.

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John P. Gallagher, Ian G. McKendry, Paul W. Cottle, Anne Marie Macdonald, W. Richard Leaitch, and Kevin Strawbridge

Abstract

A ground-based lidar system that has been deployed in Whistler, British Columbia, Canada, since the spring of 2010 provides a means of evaluating vertical aerosol structure in a mountainous environment. This information is used to help to determine when an air chemistry observatory atop Whistler Mountain (2182 m MSL) is within the free troposphere or is influenced by the valley-based planetary boundary layer (PBL). Three case studies are presented in which 1-day time series images of backscatter data from the lidar are analyzed along with concurrent meteorological and air-chemistry datasets from the mountaintop site. The cases were selected to illustrate different scenarios of diurnal PBL evolution that are expected to be common during their respective seasons. The lidar images corroborate assumptions about PBL influence as derived from analysis of diurnal trends in water vapor, condensation nuclei, and ozone. Use of all of these datasets together bolsters efforts to determine which atmospheric layer the site best represents, which is important when evaluating the provenance of air samples.

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John P. Gallagher, Ian G. McKendry, Kevin Strawbridge, Anne Marie Macdonald, W. Richard Leaitch, and Paul W. Cottle
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