Study of flow and turbulence in an atmospheric boundary layer in an area with complex topography (Tehran)

In this paper, the flow and turbulence structure in the boundary layer of an urbanizedregion with complex topography (Tehran) was studied using data from a meteorologicalstation, 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 anyactive synoptic system in the region, the skies were clear, wind speed at 10 m did notexceeded 5 meters per second, and the relative humidity was low. The data used for thevertical profiles are 12-day averages for 4 local hours, namely: 09:30, 15:30, 21:30 and03:30.The study of turbulence using the gradient Richardson number shows that, during theday, the boundary layer is generally turbulent while, at nights, in addition to the reductionof turbulence intensity, the depth of the turbulent region of the boundary layer alsodecreases. Tests done by Monti et al. (2002) have shown that the Rig is not sensitive to thetime of averaging in range30 s  Tav  900 s. The Sodar data are also 15 minutes averagedof the measurements. Additionally, Sodar data are spatial average of 25-meter, and sothis averaging may filter out some turbulent layers in the profiles. This averaging maycause large Richardson numbers.Graphs of u s, v s and w s show that nearly continuous turbulence occurs during the night,but the values are weaker with respect to daytime turbulence. The TKE diagram showsthat this quantity has a daytime maximum value and a nighttime minimum, indicating theeffect of stability on turbulence generation. Turbulent intensity componentsusu andusvare almost the same, and are also four times that ofusw during the night and nearly threetimes ofusw during the day. This shows that turbulent kinetic energy production is acombination of buoyancy and shear effects during the day and is mostly due to the effectsof shear during the night.A study of the behavior of turbulent quantity (sw) / z 3, in association with buoyancyand mechanical turbulent kinetic energy source production terms, explains the thermaland roughness effects on the characteristics of TKE. Values of diurnal z (sw) / 3 arereduced relative to height at different hours due to the increased production of mechanicaland buoyancy turbulence in the surface layer. Values of these quantities are low even inthe presence of wind shear that can be due to radiation-induced effects on these quantitiesduring the day, especially in relatively low wind conditions.Wavelet analysis of wind speed using ultrasonic anemometer data with one-minuteaveraging in the surface layer shows mostly wavy and continuous structures with periodsof 6 to 90 minutes that are related to turbulent fluctuations and both regular and irregularinternal 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 artificialtopography such as high buildings, roads and vegetation) cause important changes in thecirculation of the wind flows of the region when synoptic systems are absent. Local flowsin the region with the effects of complex terrains are generated by pressure gradients andthermal forcing. Urban flows span a wide range of space and time scales. These factorschange turbulence and vertical wind profiles.The time series of various quantities show approximately diurnal variations. Verticalprofiles of turbulent quantities show that the flow is stratified in the lower section of theboundary layer (under 500m). The depth over which the katabatic flow occurs reachesabout 200 meters. This stratified lower section of the boundary layer possibly caused bythe effects of complex topography, the urbanization of the region and their circulationinteractions (especially during the night, when they reinforce each other). The height ofthis layer doubles during daytime. The layering of the wind profile may be due to airintrusion from various slopes originating from different sources according to Monti et al.(2002), or to the structure of generated internal waves.