Introduction
Atmospheric Boundary Layer (ABL) is the lowest layer of the Troposphere affected by the land surface. Unlike the free atmosphere above, the land surface has a significant effect on the ABL and this layer, is the only part of the atmosphere in which the effects of friction and the diurnal variation of temperature can be observed. In fact, the ABL plays the role of a dealer of energy and mass between the land surface and the free atmosphere.
Many efforts have been undertaken to understand the behavior of the ABL due to its importance since almost all human activity (except aviation) forms in this layer. This, however, should be mentioned that those efforts are in a strong correlation with the development level of the country and the methods to study the ABL varies country by country. The study methods for ABL observations are divided into three main branches: a) Modeling and Simulation (using dynamic and numerical models); b) Numerical estimation (using atmospheric profiles); and c) Using High-tech devices (Remote Sensing, RADAR, and LiDAR).
Like most climatology phenomena, the ABL probably owns some regular pattern in different time scales. In this research, the variation of the ABL over the city of Tehran is investigated in an annual time scale. The reason to choose the city is the high population, high urban concentration, and constant integration with inversion and air pollution, and above all, lack of knowledge about the ABL over the city.
Methodology
In order to conduct a thorough research, long term data were required from a variety of sources. Therefore, 30 years of data were gathered in daily time scale (at 00:00 GMT and 12:00 GMT equal to 03:30 and 15:30 Tehran Mean Time respectively). Total data records reached a sum of 10958 for each parameter. Data were collected from the European Center for Mid-range Weather Forcast (ECMWF) and the Atmospheric Science Department of the University of Wyoming. Data gathered from the ECMWF was in NetCDF format and the air profile gathered was in Notepad form.
To have solid results, the data should be of high certainty and reliability. To investigate the reliability of the ECMWF data, the ABL height was calculated for some days employing the Advanced Parcel Method which defines the top of the ABL as the height in which the virtual potential temperature is equal to that of the surface values. Using Kuchran’s method to estimate the volume of the test subject from a society, 372 days were randomly selected and the ABL height was calculated based on radiosonde profiles. Then, the correlation coefficient and the Root Mean Square Error (RMSE) between the two data sets were calculated and since the coefficients were significant, data was used further. As for the maps, data gathered from the ECMWF was averaged usnig the ECMWF tools. This tool uses the R (or Rstudio) software abd the RBridge to calculate integrated NetCDF data. The maps were drawn in ArcMap 10.
Results and discussion
Mean daily ABL height was located almost at 850 meters above ground level (AGL). While nighttime ABL descends to almost 80 meters AGL, the daytime ABL rises to 2300 meter AGL. The ABL height has experienced a total rise of 5 meters per day in the total time period of the research. Spatially, the lowest ABL heights were experienced in northeastern part of the city and the highest ABL were measured in south and southwestern part of the city. Also, the correlation between the mean daily data (the ECMWF uses eight data measurements to calculate the mean daily data) and the maximum and minimum averages were calculated. The results showed that the maximum values are of higher influence in mean daily data rather than the minimum.
The findings also indicate that, during all 6 periods of the study time scale, the position of the minimum and maximum boundary layer values are almost the same. High values occur at south and southwest of the city and low values occur at northeast of the city. There is also a midsection area that is usually extended from Ka valley to the east of the city. There is also a sign of some core area, especially the one located above the Pardisan Park that sometimes affects the ABL patterns.
Furthermore, the correlation between the ABL and some climatic parameters (e.g. sunshine hours, surface temperature, surface heat and moisture flux, relative humidity, air pressure, wind speed at 0, and 10 meter height AGL) was calculated for Tehran. The results indicated that there is some significant correlation between the ABL and climatic parameters. The highest correlation was seen between the ABL variation and the surface temperature. Seemingly, the closer the measured parameters to the surface are, the higher is the correlation coefficient.
Conclusion
The Boundary layer height fluctuates between 80 meters AGL at nights and 2300 meters in daylight. The average height has increased almost five meters per year. However, since 1988 to 2012 it has risen and after 2012 it experienced subsidence. Both in minimum and maximum, the highest boundary layer height has been measured in south and southwestern part of the city and the lowest values were measured in northeastern of the city. The iso-height lines are extended from northwest to southeast. This is probably due to the effect of Alburz Mountain range surrounding Tehran in northern edges. The ABL showed high correlation with some climatic parameters but the coefficient between the ABL height and some other parameters were insignificant. Moreover, regarding the correlation coefficient and the root mean square error results, the Advanced Parcel Method seem to be of enough reliability to calculate the boundary layer height based on radiosonde profiles.