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K. D. Hage

Abstract

A network of seven thermographs has been operated continuously within the city of Edmonton, Alberta, since February 1968 by Geoscience Research Associates, Ltd., under contract to the Department of Health of the Government of Alberta. Data from these stations, together with hourly observations from two rural airports and one urban airport in the Edmonton area, are inadequate for mapping the temperature field, but provide an unusual opportunity for the study of some climatological characteristics of urban temperatures over relatively flat terrain undisturbed by lake or sea influences. Annual variations in maximum heat island intensity based on monthly mean data are ill-defined because of large variations in month-to-month frequencies of favorable nights. Stratified monthly samples consisting only of nights with intense heat islands show weak annual maxima in January and June. A well-defined diurnal cycle in heat island intensity is found with a maximum 3–4 hr after sunset in all seasons. The time of maximum heat island intensity precedes the time of maximum vertical temperature gradient over the city in an seasons. In the presence of strong vertical wind shear, inversion breakdowns occur and these are found to be patchy and of small horizontal extent.

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K. D. Hage

Abstract

In a series of nine experiments, embracing a range of wind speeds and stability conditions, fluorescent-dyed glass microspheres of nominal diameter 100µ were emitted continuously from a point source at 15 m over gently rolling prairie. The particles were collected on flat-plate adhesive sampling surfaces at ground level along arcs between the source and a distance of 800 m. The observed crosswind-integrated deposits are compared with the predictions of Rounds' solution to the K theory of eddy diffusion and with Godson’s modification of Rounds' solution. The results indicate that Rounds' solution tends to overemphasize the effects of vertical diffusion on large particles. Godson's modified solution improves the predictions in stable cases but results in excessive dispersion of the particles in the direction of the wind under lapse conditions. Some evidence is provided to suggest that, in certain cases, measurements of vertical temperature gradient and wind-speed profiles between ground level and emission height are not sufficient to provide unique or reliable indicators of the intensity of vertical turbulence for the purpose of predicting the dispersion of particles.

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K. D. Hage

About twice in an average summer season, it is observed that sea-level cyclones of extraordinary intensity develop in the lee area of the northern Rocky Mountains. These cyclones differ from the customary frontal waves in that the onset of the development is sudden, the development period is short and the system remains quasi-stationary until full maturity is reached. In common with the frontal waves and most minor disturbances in the same region is the feature that the geneses are preceded by a forward march of an upper cold low or trough from the northeastern Pacific Ocean.

An analysis of individual cases shows that the sea-level development begins at the time when an area of positive vorticity advection aloft spreads over the eastern slopes of the mountain range. The movement of the upper cold low is found to be strongly influenced by the behavior of the next cold upper low or trough upstream.

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K. D. Hage

Abstract

Urban and rural airport surface weather observations in a 13-year period of rapid city growth are used to document city effects on absolute and relative humidity in a dry climate at fairly high latitudes. The city is found to be dry at all hours (relative humidity) and dry by day but moist at night (absolute humidity) in all but winter months. Some but not all of the major features of the humidity differences conform to those found by Ackerman for Chicago. In winter, relative and absolute humidities are high in the city at all hours because of vertical mixing and combustion sources. Maximum differences in absolute humidity at night occur in March and August. The former is attributed primarily to urban snowmelt on occasions when rural temperatures are below freezing. The August peak occurs near sunrise and is attributed mainly to rural dewfall. The times of maximum cooling and maximum absolute humidity in the city on clear hights in summer are strongly dependent on wind speed. For this reason it is argued that interaction of advection processes and vertical flux divergence (radiative plus turbulent) seems to be essential for realistic simulation of urban cooling rates at night. Moisture differences appear not to play a crucial role in heat island development.

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R. M. Holmes
and
K. D. Hage

Abstract

Three chinook situations which occurred in southern Alberta during the winter of 1967–68 were studied using an instrumented aircraft. On the days of observation, local areas experienced warm air penetration toward the surface. On 29 October 1967, severe turbulence and significant warming neat Rolling Hills, Alberta, marked the occurrence. On 18 January 1968, warm air intrusion was found from three large isolated areas of melting snow near Brooks. On 3 February 1968, a shallow layer of cold air covered the southern prairies, with a marked temperature inversion at 50 m above the surface. Local wavelike intrusions of the warm air occurred near Calgary on this day, which was one day previous to the general invasion of warm air from the south.

The available data, while somewhat incomplete, were subjected to analysis according to a modified Scorer equation to test for atmospheric and/or topographic inducement of the wave motions observed. Neither method of analysis was completely successful. More detailed and accurate observations of atmospheric motions by aircraft are required.

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Raymond K. W. Wong
,
Keith D. Hage
, and
Leslie D. Phillips

Abstract

A two-dimensional numerical model is used to simulate nocturnal drainage flow in a small urban valley with light prevailing winds and conditions of supercritical Richardson numbers (Ri). The model uses a hydrostatic and Boussinesq system of equations written in terrain-following coordinates. Radiative transfer is represented by Brunt's method of radiative diffusivity. Eddy diffusivities are specified in the subgrid parameterization for conditions where Ri is supercritical. Tests show the dependence of drainage wind on slope angle, cooling rate, surface drag and prevailing wind speed, and also the insensitivity of wind and temperature to the eddy diffusivities under supercritical Ri conditions. The drainage wind cells are asymmetric, with a shallow surface layer of drainage flow and a thicker upper region of slower return flow. The predicted wind profiles show low-level maxima and the predicted temperature profiles are exponential in shape, in good agreement with observations obtained in Edmonton, Alberta in the summer of 1978. The model is also able to predict the quasi-stationary slope flow observed in the field.

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K. D. Hage
,
J. Gray
, and
J. C. Linton

Abstract

Regional analyses of oxygen and deuterium isotope abundances in precipitation from selected stations of the International Atomic Energy Agency-World Meteorological Organization global netwok are presented as background for a more detailed study of the origin and history of atmospheric water in western Canada east of the Rocky Mountains. Departures from Dansgaard's regression between oxygen-18 concentrations and surface temperatures are attributed mainly to differences in intial water vapor isotope concentrations at inland stations in western Canada. Values of δO18 in rain at Edmonton are best correlated with 800 mb temperatures. However, snow data showed little variation in correlation with height up to 800 mb, and larger unexplained variance than rain, despite the fact that evaporation and isotope exchange effects are small for snow. Using simultaneous upper air temperature and wind data the δO18 variations in snow are attributed both to large condensation temperature variations that can occur in winter storms, and to the condensation of water vapor of different origin and history. Isotope concentrations characteristic of precipitation derived from re-evaporated water are associated with strong westerly flow below cloud level. On the other hand isotope concentrations characteristic of coastal station precipitation are associated with easterly low-level flow or light winds. The magnitude of the differences in concentrations in the two situations is so large that isotope measurements should be useful in the study of the structure of such storms. Previous studies have attributed the small slopes of δO18-δD regression lines at Fort Smith, Bethel, and Whitehorse to the effects of rapid evaporation of precipitation below cloud base. An alternative approach using a fixed slope of 8 and varying intercepts suggests that the observed isotope concentrations can be accounted for by variations from near-equilibrium evaporation of North Pacific Ocean water in summer to rapid evaporation of the ocean water in winter. This explanation appears to be more consistent with the climate characteristics of these stations.

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