An urban-induced meso-circulation

A two dimensional model has been set up to investigate the circulation induced by an urban heat island in the absence of synoptic winds. The boundary conditions need to be formulated carefully and due to difficulties arising here, we restrict our attention to cases of initially stable thermal stratification. Heat island circulations are allowed to develop from rest and prior to the appearance of the final symmetric double cell pattern, a transitional multi-cell pattern is observed in some cases. The influence on the steady state circulation of various parameters is studied, among which are eddy transfer coefficients, the heat island intensity, the initial temperature stratification and the heat island size. Some results are presented for a case in which differential surface cooling beneath an initially stable atmosphere produces a circulation and an unstable layer capped by an elevated inversion over the city. It is hoped that this case is vaguely representative of the night-time heat island with no geostrophic wind.

Section: The Intensity Of the Heat Island (O)mentioning

confidence: 89%

“…The base of the inversion was found on the average to be near 300 m and the temperature at this height was often lower over the city than over the country. Observations of surface winds by Pooler (1963) and Findlay and Hirt (1969) suggest the existence of a heat island circulation when the synoptic winds are light.…”

Section: Introductionmentioning

confidence: 99%

Numerical studies of heat island circulations

Delage

Taylor

1970

“…The effects of various topographic features on low-level winds have been widely examined, both in non-urban areas (e.g., Buettner and Thyer, 1966;Steele, 1976;Daniels and Schroeder, 1978) and within cities (e.g., Lyons, 1977;Colacino and Dell'Osso, 1978;Paterson and Hage, 1979). Several recent studies have also examined the influence of the city itself on air movement (e.g., Chandler, 1965;Findlay and Hirt, 1969;Bomstein and Johnson, 1977;Shreffler, 1978Shreffler, , 1979a.…”

Section: Introductionmentioning

confidence: 99%

Patterns of weak, near-surface winds at Melbourne, Australia

Tapp

1985

Up to 30 months of near-surface anemograph records have been examined from 13 locations in and near Melbourne, Australia, to determine the wind patterns which existed during prolonged periods of light winds (at least 3 hours at 2 m s-' or less). A coherent katabatic wind system was found to develop in at least part of the monitored region on approximately 30% of nights. The flow broadly followed the slope of the basin surrounding the city, with a strong flow down the main river valley, and was partly reinforced by a land breeze in bayside areas. Other valleys also acted as channels for these winds. The general tendency of these katabatic winds was to converge towards the central business district and the northern part of Port Phillip Bay adjacent to the city centre. Where winds from different directions interacted, one of the winds dominated or successive replacement occurred causing the wind direction to vary considerably during a period.There were indications that in the presence of low-level stability with a synoptic gradient wind between east and north, the gradient flow may be deflected around the major topographic barrier to the northeast of the city. The existence of such a situation would have major implications in terms of air quality due to the possibility of pollutants being recirculated in conditions when vertical diffusion was very limited.

“…However, daytime circulations may also exist. Findlay and Hirt (1969) discuss observations taken on a sunny, midwinter day with a 13-tower network of anemometers in Toronto, Canada. An inward flow was evident at the time of maximum heating.…”

Section: Introductionmentioning

confidence: 99%

Detection of centripetal heat-island circulations from tower data in St. Louis

Shreffler

1978

Hourly averaged meteorological data gathered by a 2.5-tower network about St. Louis during 1976 are used in a search for centripetal circulations generated by the urban heat island. Considering data collected when the network resultant speed was less than 1.5 m s-', two data classes of several hundred hours each are formed. One class is associated with weak heat islands, daytime hours, and convective instability, while the other class is associated with strong heat islands, nighttime hours, and extreme rural stability. Mean centripetal flows are clearly discernible from data of both classes, but the convergence is stronger for the flows associated with the weaker heat islands. This unexpected result is explained in terms of the ease with which sustained vertical motions can be generated over the city by the available forcing under different stability regimes. The detectability of the heat-island influence diminishes very rapidly with increasing speed of the large-scale flow.