Mixed layer is a part of the atmospheric boundary layer in which pollutants are uniformly distributed. It is sensitive to the Earth’s surface, controls the flow of heat and momentum between the surface and the free atmosphere, thus playing a key role in atmospheric circulation. Most of atmospheric pollution models require the height of the mixed layer as an input to determine the depth of atmosphere through which surface emitted pollutants are well mixed. Thus, the study of the depth variation of this layer is important in the spatial and temporal distribution of air pollution. This can be estimated from direct measurement and also from numerical forecasting models (such as MM5) with a proper boundary layer scheme.
In this research, first we study the variations of the depth of the daytime mixed layer for the city of Tehran for two months, August and February 2005 which are representative of warm and cold seasons, respectively. Next, the maximum and minimum depths of the mixed layer for each month are selected. Then the affecting variables concerning these maximum and minimum amounts, such as synoptic conditions, temperature advection, humidity, surface fluxes, vertical motions and vertical wind shears, are examined.
We attempt to compare the results calculated by means of MM5 with the same height derived by radiosonde. Three domains of MM5 are defined using 9, 3 and 1 km resolutions. The smallest domain is centered at the radiosonde position. We use simple-ice for the explicit moisture scheme, cloud radiation for the radiation scheme, and B-M scheme for the cumulus parameterization. Boundary layer processes are calculated using the Medium Range Forecast (MRF) scheme.
In general, the simulated potential temperature, mixed layer depth, and specific humidity by the numerical model are consistent with the variables measured by radiosonde. However, the model is not able to capture the fine structure of the mixed layer. The results display considerable seasonal variability in the mixed layer depth. For instance, the midday mixed layer depth is found to be nearly 3 km in summer for calm weather conditions, and is nearly twice as that of the wintertime. It is also seen that the monthly trend of mixed layer depth variations in the two months are close to those of the surface heat flux. It seems the fluctuations in the depth variations of this layer result mostly from temperature advection in summertime, while in wintertime, the midday depth is mostly affected by weather systems and wind shear rather than other physical processes.