تغییرات سالانه ارتفاع لایه مرزی شهر تهران

نوع مقاله: مقاله کامل

نویسندگان

1 پژوهشگر دورة دکتری گروه جغرافیای طبیعی، دانشکدة جغرافیا، دانشگاه تهران

2 دانشیار گروه جغرافیای طبیعی، دانشکدة جغرافیا، دانشگاه تهران

3 استادیار گروه جغرافیای طبیعی، دانشکدة جغرافیا، دانشگاه تهران

4 استاد دانشگاه کانتربوری زلاندنو

چکیده

لایة مرزی به‏علت پیوند مستقیم با زندگی انسان اهمیت بسزایی دارد. در این پژوهش با دریافت داده‏های ساعتی و میانگین روزانه از مرکز اروپایی پیش‏بینی میان‏مدت هوا و دپارتمان علوم جوی دانشگاه وایومینگ برای دورة 1988 تا 2017، با کاربرد نرم‏افزارهای Excel و R و با به‏خدمت‏گیری توابع محاسباتی روش بستة پیشرفته، مجموعه توابع چندبُعدی و ابزار ECMWF در محیط ArcMap تغییرات ارتفاعی لایة مرزی شهر تهران در بازة سالانه تهیه شد. نتایج نشان داد که سقف متوسط ارتفاع لایة مرزی در حدود 850متری سطح زمین قرار دارد و در شرایط شبانه به‏طور متوسط تا حدود 80متری و در شرایط روزانه تا حدود 2300متری سطح زمین پایین و بالا می‏رود. این متغیر در کل دورة سالانه در حدود 5 متر افزایش ارتفاع داشته است. به‏لحاظ الگوی فضایی، کمینه ارتفاع در هر یک از سه متغیر موردبررسی در شمال شرقی تهران و بیشینة آن در جنوب و جنوب غربی تهران رخ ‏داده است. ضمن اینکه این متغیر با برخی متغیرهای اقلیمی دیگر نظیر دمای سطحی رابطة معناداری (8143/0) دارد. همچنین، براساس یافته‏ها، روش بستة پیشرفته مقبولیت بسیار زیادی در محاسبة ارتفاع لایة مرزی با استفاده از داده‏های رادیوسوند دارد.

کلیدواژه‌ها


عنوان مقاله [English]

Annual Variation of the Urban Boundary Layer Height of Tehran

نویسندگان [English]

  • Mojtaba Mahdian Mahforouzi 1
  • Aliakbar Shamsipour 2
  • Mostafa Karimi 3
  • Peyman Zawarreza 4
1 PhD Student, Faculty of Geography, University of Tehran
2 Physical geography department, Geography faculty, Tehran university, Tehran, Iran
3 Assistant Prof. Faculty of Geography, University of Tehran
4 Professor of climatology, University of Canterbury, New Zeland
چکیده [English]

Extended Abstract:

1. 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 that the effect of friction is seen and the diurnal variation of temperature is witnessed. 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.

2. Data and methods
In order to conduct a thorough research, long term data was required. 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 Forecast (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 Cochran'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 using the ECMWF tools. This tool uses the R (or Rstudio) software and the RBridge to calculate integrated NetCDF data. The maps were drawn in ArcMap 10.

4. Results and discussion
Mean daily ABL height was located almost at 850 meters above ground level (AGL). Of course in the average state of the highest elevation in the layer were occurred on the south and southwest of Tehran with a height of 873 meter and the lowest elevation were in the northwest of Tehran city with an altitude of 745 meter. 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 Kan 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. The results indicated that there is some significant correlation between the ABL and climatic parameters. The highest correlation was calculated between the ABL variation and the surface temperature. Seemingly, the closer the measured parameters to the surface are, the higher the correlation coefficient will be.

5. Conclusion
The Boundary layer height fluctuates between 80 meters AGL at nights to 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 Alborz Mountain rang 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. Apparently, the closer to the surface is the measured parameters, the higher the coefficient. 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.

کلیدواژه‌ها [English]

  • Boundary Layer Height
  • Radiosonde
  • ECMWF
  • Advanced Box Method
  • Tehran
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