Wind is the carrier of pollutants and any other gaseous or particle matters in the atmosphere. Stable atmosphere with low wind provides favorable conditions for high contamination of pollutants in urban areas. The importance of mesoscale atmospheric flows in air pollution dispersion has been recognized in the past three decades and has been the focus of intensive research; both observational and numerical. Mesoscale or local scale circulations are more prominent when the synoptic pressure gradients are weak, allowing horizontal temperature contrasts to develop, which in turn lead to mesoscale pressure perturbations.
Tehran, a city which is situated at the southern foothills of the Alborz Mountain chain has an average elevation of 1500m, and covers an area of 864 km2. Alborz Mountains have a significant influence on the dynamics and thermodynamic modification of air movement over the city. At the same time, the Urban Heat Island effect (UHI) can cause its own mesoscale flow, complicating an already complex local scale flow. The topography and the urban fabric can cause slope flows, mountain flows, and valley flows amongst many others factors.
This paper focuses on the use of state-of-the-art atmospheric numerical model – The Weather Research and Forecasting (WRF) – in an ideal situation to study the characteristics of the mesoscale flow systems that prevail over Tehran when air quality is unfavourable. So the average of soundings of Radiosonde at Mehrabad station, for the almost all fair days of cold seasons of 2005 to 2008 has been selected as ideal initial condition and boundary condition with 10 × 10 km spatial and 12-hour temporal resolutions. The simulations were carried out for a 3-day period in December 2005 when an aircraft, due to low visibility caused by high concentration of air pollution, crashed in 2 miles away from the end of runway into an inhabited area.
Three simulations are prepared. For the first experiment, called control run, we used the default urban setting of Tehran. In the second simulation urban properties of Tehran was removed completely from land-use fed to the model to investigate the effect of urban area on thermal induced circulation of Tehran. This simulation is called NoURB simulation. To investigate the role of urbanization the 3rd simulation was prepared. In this simulation which will be referred as Urban simulation, three urban categories are used; class 31 of USGS land use/land cover used for “Low Intensity Residential” which includes areas with a mixture of constructed materials and vegetation. These areas are most commonly included as single-family housing units in which the population densities is lower than that in high intensity residential areas. Class 32 of USGS represented as “High Intensity Residential” which includes highly developed areas where people reside in high numbers. Finally class 33 of USGS used for “Commercial/Industrial” which includes infrastructure (e.g. roads, railroads, etc.) and all highly developed areas not classified as High Intensity Residential.
The Noah LSM provides surface sensible and latent heat fluxes, and surface skin temperature as lower boundary conditions. It has asingle vegetation canopy layer and the following prognostic variables: soil moisture and temperature in the soil layers, water stored on the canopy, and snow stored on the ground. It includes: 1) increasing the roughness length from 0.5 m to 0.8 m to represent turbulence generated by roughness elements and drag due to buildings; 2) reducing surface albedo from 0.18 to 0.15 to represent the shortwave radiation trapping in the urban canyons; 3) using a larger volumetric heat capacity of 3.0 J m-3 K-1 for the urban surface (walls, roofs, and roads) which is usually consisted of concrete or asphalt materials; 4) increasing the value of soil thermal conductivity to 3.24 W m-1 K-1 to parameterize large heat storage in the urban surface and underlying surfaces, and 5) reducing green vegetation fraction over urban city to decrease evaporation.
In order to better represent the physical processes involved in the exchange of heat, momentum, and water vapor in urban environment in mesoscale model, an UCM is coupled to the WRF model. The main purpose of the coupled model is to improve the description of lower boundary conditions and to provide more accurate forecasts for urban regions. The UCM is a single layer model which has a simplified urban geometry. Some of the features of the UCM include, shadowing from buildings, reflection of short and longwave radiation, wind profile in the canopy layer and multi-layer heat transfer equation for roof, wall and road surfaces.
The basic function of an UCM is to take the urban geometry into account in its surface energy budgets and wind shear calculations. The urban model is based on the urban canopy model which includes: 1) 2-D street canyons that are parameterized to represent the effects of urban geometry on urban canyon heat distribution; 2) shadowing from buildings and reflection of radiation in the canopy layer; 3) the canyon orientation and diurnal cycle of solar azimuth angle, 4) man-made surface consists of eight canyons with different orientation; 5) Inoue’s model for canopy flows; 6) the multi-layer heat equation for the roof, wall, and road interior temperatures; 7) anthropogenic heating associated with energy consumption by human activities; and 8) a very thin bucket model for evaporation and runoff from road surface.
The main limits in this kind of study over Tehran metropolitan are lack of observation data beside lack of documentation. The previous studies over Tehran indicate a significant increase in minimum temperatures in 50-year trend especially in cold seasons. This indicates that the artificial and anthropogenic heating leading to urban heat island in Tehran have been significant in this period. These studies also indicate that nocturnal drainage flows in Mehrabad Int. Airport synoptic station has also been weakened, in this 50-year period. This paper focuses on the use of state-of-the-art atmospheric numerical model – The Weather Research and Forecasting (WRF) – in an ideal situation to study the characteristics of the mesoscale flow systems that prevail over Tehran when air quality is bad. The results indicate that urban areas near complex topography can increase transfer of material (pollution) and energy in the boundary layer and from this layer to the free atmosphere. Other results from this study show UHI induces rural-urban flows which significantly reduce drainage wind speed.