نوع مقاله : مقاله تحقیقی (پژوهشی)
مؤسسه ژئوفیزیک دانشگاه تهران، ایران
عنوان مقاله [English]
In this paper, the flow and turbulence structure in the boundary layer of an urbanized region with complex topography (Tehran) was studied using data from a meteorological station, Sodar (PA1 Model) for heights above 50m, on days 13 through 24 of August, 2002, and an ultrasonic anemometer, located in Tehran University Geophysics Institute, in August 2005. Days for observation were selected such that they were without any active synoptic system in the region, the skies were clear, wind speed at 10 m did not exceeded 5 meters per second, and the relative humidity was low. The data used for the vertical profiles are 12-day averages for 4 local hours, namely: 09:30, 15:30, 21:30 and 03:30.
The study of turbulence using the gradient Richardson number shows that, during the day, the boundary layer is generally turbulent while, at nights, in addition to the reduction of turbulence intensity, the depth of the turbulent region of the boundary layer also decreases. Tests done by Monti et al. (2002) have shown that theis not sensitive to the time of averaging in range. The Sodar data are also 15 minutes averaged of the measurements. Additionally, Sodar data are spatial average of 25-meter, and so this averaging may filter out some turbulent layers in the profiles. This averaging may cause large Richardson numbers.
Graphs of, and show that nearly continuous turbulence occurs during the night, but the values are weaker with respect to daytime turbulence. The diagram shows that this quantity has a daytime maximum value and a nighttime minimum, indicating the effect of stability on turbulence generation. Turbulent intensity components and are almost the same, and are also four times that of during the night and nearly three times of during the day. This shows that turbulent kinetic energy production is a combination of buoyancy and shear effects during the day and is mostly due to the effects of shear during the night.
A study of the behavior of turbulent quantity, in association with buoyancy and mechanical turbulent kinetic energy source production terms, explains the thermal and roughness effects on the characteristics of . Values of diurnal are reduced relative to height at different hours due to the increased production of mechanical and buoyancy turbulence in the surface layer. Values of these quantities are low even in the presence of wind shear that can be due to radiation-induced effects on these quantities during the day, especially in relatively low wind conditions.
Wavelet analysis of wind speed using ultrasonic anemometer data with one-minute averaging in the surface layer shows mostly wavy and continuous structures with periods of 6 to 90 minutes that are related to turbulent fluctuations and both regular and irregular internal waves.
It seems that the topography-induced flows (down slope, upslope and drainage flows) and urban effects (flows from thermal islands and their interactions with artificial topography such as high buildings, roads and vegetation) cause important changes in the circulation of the wind flows of the region when synoptic systems are absent. Local flows in the region with the effects of complex terrains are generated by pressure gradients and thermal forcing. Urban flows span a wide range of space and time scales. These factors change turbulence and vertical wind profiles.
The time series of various quantities show approximately diurnal variations. Vertical profiles of turbulent quantities show that the flow is stratified in the lower section of the boundary layer (under 500m). The depth over which the katabatic flow occurs reaches about 200 meters. This stratified lower section of the boundary layer possibly caused by the effects of complex topography, the urbanization of the region and their circulation interactions (especially during the night, when they reinforce each other). The height of this layer doubles during daytime. The layering of the wind profile may be due to air intrusion from various slopes originating from different sources according to Monti et al. (2002), or to the structure of generated internal waves.