بررسي جزيره حرارتي شهر مشهد با استفاده از تصاوير ماهواره‌اي و نظريه فركتال

The Investigation of Mashhad’s Heat Island Using Satellite Images and Applying Fractal Theory

رشد جمعيت و توسعه شهرنشيني يكي از عوامل موثر بر افزايش دماي هوا در نواحي شهري است كه و موجب ايجاد جزيره حرارتي بر روي اين مناطق در مقايسه با محيط اطراف مي شود و اثرات ناشي از آن مي تواند نقشي اساسي و مهم در كيفيت هوا داشته و به تبع آن، سلامت عمومي ايفا كند. در اين پژوهش، تصاوير TM ماهواره لندست پنج در تاريخ 25 ژوييه 1992 و ETM+ ماهواره لندست هفت در تاريخ 6 اگوست 2002 براي بررسي جزيره حرارتي شهر مشهد مورد بررسي قرار گرفته و نقشه‌هاي دما و كاربري اراضي با استفاده از آن‌ها تهيه شده است. براي بررسي بهتر اين پديده، نيمرخ هايي در جهت شمالي- جنوبي، شرقي- غربي و شمال غربي- جنوب شرقي در نظر گرفته شده است. علاوه بر اين براي داشتن درك بهتري از رفتار حرارتي پوشش هاي مختلف و اثرات الگوي فضاي سبز بر دماي محيط، بعد فركتالي اين نيمرخ ها با استفاده از روش تقسيم كننده، محاسبه گرديده است. نتايج نشان داده دماي تابشي سطح مشهد به طور كلي در طي دهه 1992 تا 2002 افزايش داشته است و اين افزايش در مناطق مسكوني چشمگيرتر است. نقشه‌هاي كاربري اراضي نيز نشان مي‌دهند مناطق مسكوني در سال 2002 نسبت به سال 1992 توسعه يافته و بسياري از پوشش‌هاي گياهي از بين رفته و اين مطلب توسط بعد فركتالي محاسبه شده نيز مورد تاييد قرار گرفته است. نتايج همچنين نشان داده نيمرخ شمال غربي- جنوب شرقي به علت گسترش بيشتر شهر و عدم يكنواختي سطوح، نسبت به ساير نيمرخ‌ها بعد فركتالي بيشتري دارد.

Introduction The population growth and urban development are the effective factors of increasing the air temperature for urban areas, which may cause formation of heat island, which itself influences air quality and consequently, the public health. The heat island is one of the phenomena which effects the human beings’ living environment in urban areas on a large scale. The heat island occurs when an extra percentage of surface vegetation is wiped out and replaced with buildings, roads and other urban constructions. This problem causes the trammel of the ripe solar radiation into the urban structures during the day and its reflection at night. Thus, the natural process of earth surface getting cold during the night happens more slowly. Consequently, the air temperature of cities will be naturally higher than the temperature of suburb regions. Because of its important effects on environment and health, urban heat island was evaluated for Mashhad, as a major city in Iran, using satellite images and the fractal theory. Materials and Methods Mashhad is located at latitude 36ْ 17 ʹ45" -N and longitude 59ْ 36 ʹ43" –E. The population is 2410800, and it is one of the largest cities in Iran. An extra percentage of the surface natural covers is wiped out and replaced with urban constructions and many landscapes have changed into residential areas. Surface radiation emittance, as recorded by thermal infrared sensors, includes both topographically and non-topographically induced high frequency variations such as roads and edges which are caused by different spectral characteristics of different neighboring land covers. The use of fractals for analyzing thermal infrared images would improve our understanding of thermal behavior of different land-cover types as well as the effects of landscape pattern on thermal environmental processes. In this research, TM images of LANDSAT for June, 25th, 1992 and ETM+ of LANDSAT 7 for Aug, 6th, 2002 were used to study the urban heat island in Mashhad and also to obtain temperature and Land-use maps by using them. In addition, for better understanding of this phenomenon, the profiles in that direction of North-South, East-west and Northwest-Southeast were considered. Moreover, the fractal dimensions of these profiles were computed using the divider method, to as to have better understanding of thermal behavior of different coverings and the effects of land-space pattern on ambient temperature. Results and discussion The results showed that the surface radiant temperature of Mashhad during the decade 1992 to 2002 increased and this increase was remarkable in the residential areas. Land-use maps demonstrated developing of the residential areas for 2002 rather 1992, and many plant covers were destroyed and this subject was approved by calculation of the fractal dimensions. The relatively low values of fractal dimension suggested that the texture was less spatially complex. It means that the spectral responses to the thermal band along the line tend not to vary drastically. In urban areas due to the unsteady vegetation and roughness variability, the fractal dimension had high value. In northwest – southeast profile where urban or built-up cover had occupied the majority of the surface, the fractal dimension and temperature in both images were higher than other profiles. Because in east-west profile, urban area was developed more severely during 1992 to 2002, the fractal dimension increased more than other profiles. So, we concluded that, in Mashhad, the urban development resulted in increase of spatial variability, temperature and the fractal dimensions.