Douw G. Steyn

Simulation of surface urban heat islands under ‘ideal’ conditions at night part 2: Diagnosis of causation

A simple energy balance model which simulates the thermal regime of urban and rural surfaces under calm, cloudless conditions at night is used to assess the relative importance of the commonly stated causes of urban heat islands. Results show that the effects of street canyon geometry on radiation and of thermal properties on heat storage release, are the primary and almost equal causes on most occasions. In very cold conditions, space heating of buildings can become a dominant cause but this depends on wall insulation. The effects of the urban ‘greenhouse’ and surface emissivity are relatively minor. The model confirms the importance of local control especially the relation between street geometry and the heat island and highlights the importance of rural thermal properties and their ability to produce seasonal variation in the heat island. A possible explanation for the small heat

Evaluation and comparison of statistical forecast models for daily maximum ozone concentrations

Abstract Three statistical models that estimate daily maximum ozone (O3) concentrations in the lower Fraser Valley of British Columbia (BC) are specified using measured concentrations from two monitoring stations during the time period 1978–1985. The three models are (1) a univariate deterministic/stochastic model, (2) a univariate autoregressive integrated moving average (ARIMA) model, and (3) a bivariate temperature and persistence based regression model. The three models as well as a persistence forecast are tested by comparison with O3 concentrations observed during 1986; it is concluded that the bivariate model is superior to both unvariate models and persistence. The ARIMA model has nearly the same predictive capability as persistance while the mixed deterministic/stochastic model performs the worst. This suggests that the traditional time series technique of decomposing a series into a trend, a cycle and a stochastic component may not be appropriate for O3 air quality forecasting.

The Detection of Mixed Layer Depth and Entrainment Zone Thickness from Lidar Backscatter Profiles

Abstract A new method is presented for the extraction of mixed layer depth and entrainment zone thickness from lidar, backscatter ratio profiles. The method is based on fitting a four parameter, idealized profile to observed profiles. Optimization of the fit yields values for mixed layer depth and entrainment zone thickness. Since the fitting procedure is based on the entire measured profile, it has a robustness not found in methods based on critical backscatter or backscatter gradient. The method is tested by application to four measured profiles and three synthetic profiles. The sets of profiles include some that are very demanding because of small mixed layer to upper layer backscatter ratio contrasts, or have plumes of high backscatter imbedded in mixed and upper layers. It is shown that the method is robust and simple to implement, even for a sequence of independent profiles.

Simulation of Surface Urban Heat Islands under ‘IDEAL’ Conditions at Night Part 1: Theory and Tests against Field Data

Observations show that the urban heat island in the atmospheric layer below roof level is most strongly developed during calm, cloudless conditions at night. This paper outlines two versions of a numerical model to describe the cooling of rural and street canyon surfaces under these conditions using surface thermal and radiative properties and the radiative geometry of the canyons. One version uses a full system of differential equations and the other the simpler force-restore approach. The two approaches are shown to be in general agreement and the output of the simpler model is shown to give a faithful representation of cooling of rural and urban surfaces, and therefore heat islands, when compared with field observations.

Nocturnal cleansing flows in a tributary valley

Abstract During photochemical air pollution episodes in the Lower Fraser Valley (LFV) near Vancouver, BC, daytime upvalley flows carried polluted air, with high ozone (0 3 ) concentrations, into tributary valleys to the north of the LFV. Nighttime flows out of the valleys had low 0 3 concentrations, according to surface measurements, and also had low aerosol concentrations, as measured by a scanning Doppler lidar. Analysis of lidar scan data showed that the flows were highly complex, that the relatively clean flow was confined to the lower levels (lowest ∼ 500 m) of the valley, and that regions of strongest outflow were also the regions of “cleanest” air. Measurements of NO 2 concentrations well above background levels in the outflow indicate that it was formerly polluted air from which 0 3 and aerosols had been removed. Possible removal mechanisms were found to be dry deposition in the katabatic (downslope) flows down the valley sidewalls, in agreement with a previous study in a Swiss valley, or fast chemical reactions with NO and N0 3 . Nearly horizontal lidar scans showed that the valley exit flows penetrated into the LFV, where they merged with the downvalley/land-breeze system along the Fraser River.

The calculation of view factors from fisheye‐lens photographs: Research note

(1980). The calculation of view factors from fisheye‐lens photographs: Research note. Atmosphere-Ocean: Vol. 18, No. 3, pp. 254-258.

Overview of tropospheric ozone in the Lower Fraser Valley, and the Pacific '93 field study

This paper provides an overview of ozone pollution in the Lower Fraser Valley, and summarizes a large, multi-agency field study of that problem. The paper provides a context and general background for the 12 papers in this special issue, and serves to provide a general environmental context for their more detailed reports. In addition, we summarize the major findings of all papers in this collection. While most of the findings are significant additions to our knowledge of ozone pollution in the region, the most notable findings relate to the great importance of biogenically emitted volatile organic compounds as ozone precursors in this region.

Diurnal variations of boundary-layer carbon dioxide in a coastal city—Observations and comparison with model results

Abstract Climate change expected from increasing atmospheric CO2 concentrations has been studied widely. Further, it is recognized that cities are a major source of anthropogenic CO2 However, few studies of CO2 concentrations in, or near, cities have been conducted. A LI-COR infrared gas analyzer was operated in a suburban region of Vancouver during June 1993. The observed summer-time concentrations show a late afternoon minimum, and overnight maximum around the upwind background concentration. The afternoon CO2 minimum is attributed to the strength of biospheric photosynthesis and strong mixing of local anthropogenic sources within a deep mixed layer. Poor nighttime mixing, lower mixed layer depths, and biospheric respiration account for the observed nighttime maximum, often more than 80 ppmv greater than the background concentration. A numerical multiple-box transport and mixing model was developed to simulate the observed diurnal pattern of CO2 concentration at the suburban site. CO2 emissions inventories for important mobile sources, stationary sources, and biospheric sources and sinks provide input to the model for upwind fetch areas. Other CO2 inputs include advection, entrainment from above the mixed layer and modelled mixed layer depth for the CO2 mixing volume. Close agreement between observations and model results show the importance of boundary layer structure on CO2 concentrations at a specific location. In terms of CO2 the role of the city is placed in its global context.

Turbulence Structure and Exchange Processes in an Alpine Valley: The Riviera Project.

During a special observing period (SOP) of the Mesoscale Alpine Programme (MAP), boundary layer processes in highly complex topography were investigated in the Riviera Valley in southern Switzerland. The main focus was on the turbulence structure and turbulent exchange processes near the valley surfaces and free troposphere. Due to the anticipated spatial inhomogeneity, a number of different turbulence probes were deployed on a cross section through the valley. Together with a suite of more conventional instrumentation, to observe mean meteorological structure in the valley, this effort yielded a highly valuable dataset. The latter is presently being exploited to yield insight into the turbulence structure in very complex terrain, and its relation to flow regimes and associated mean flow characteristics. Specific questions, such as a detailed investigation of turbulent exchange processes over complex topography and the validity of surface exchange parameterizations in numerical models for such surfaces, t...

Gap Winds in a Fjord. Part I: Observations and Numerical Simulation

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.