Investigation of Periodic and Seasonal Variations of Urban Heat Island (UHI) at Night and Day Using Satellite Imagery in Kermanshah City

Urban Heat Island (UHI) as one of the factors influencing human activity within the cities that has become very important simultaneously with the physical development of cities in recent decades. With the advance of thermal satellite imagery in the last few decades and the possibility of retrieve of Land Surface Temperature (LST), these data have been used to assess the Surface Urban Heat Island (SUHI), which is directly linked to the UHI. The purpose of this study is to investigate periodic and seasonal variations of UHI in Kermanshah city using satellite imagery. Kermanshah is the most important city in the central region of western Iran and the capital of Kermanshah Province. The elevation of the city from the sea level is about 1322 meters and its area is approximately 96 square kilometers. Based on the results of the general census of population and housing, the population of this city in 2006, 2011 and 2016 were 784602, 851405 and 946651 respectively. The population of the city during the 60-year period from 1956 to 2016 has increased sevenfold. To study the SUHI in this city, the MODIS product images were used with a 1 km spatial resolution at 10:30 and 22:30 in the two 1-year periods: old period (2000-2001) and the new period (2015-2016), as well as Landsat images at three different times (May 9, 1987, May 4, 2000, and May 15, 2015) to retrieve land surface albedo and vegetation cover. In this way, the diurnal, seasonal and annual variations of the UHI were studied using the Surface Urban Heat Island (SUHI) index, which is difference between the surface temperature of urban and non-urban areas. This difference was studied in two ways: 1- Between the average temperature of all pixels inside and outside the city 2- Between the representative pixel temperature inside the city (near Azadi Square in the city center), and the representative pixel temperature outside the city (a point near the south of Shahid Ashrafi airport).
The results of the study showed that the urban areas during the day, especially in the warm period of the year, have a lower surface temperature than non-urban areas due to the role of the reflective roofs in increasing the surface albedo; hence, the Surface Urban Cool Island (SUCI) is formed and the value of the SUHI index is negative. In the new period (2015-2016), due to the further development of the city, the effect of reflective roofs on the city temperature reduction is intensified, and along with the deterioration of the vegetation cover outside the city and the rise of temperature, the SUCI is strengthened so that the average temperature in the city is 1.5 degrees lower than outside the city. In extreme cases, the value of the SUHI index reaches .65 and -3.44. At night, the situation is reversed and the temperature inside the city is about 1 to 2 degrees higher than the outside of the city mainly due to the anthropogenic heat. Therefore, the value of the SUHI index is positive and the UHI is formed. In the case of point comparison (the difference between the temperature of two representative pixels inside and outside the city), the differences inside and outside the city increase by about two-fold, i.e, 3 to 4 degrees. In extreme cases, the value of the night time SUHI index reaches .4 and -9.9