Analysis of surface urban heat island in Urmia and Tabriz cities and the relationship with water extent variation of Urmia Lake

Introduction:
Urban heat island monitoring with remote sensing data is increasing and one of the most important reasons is to provide more spatial information of urban temperature than terrestrial data. The heat island resulting from this data is called the surface urban heat island. Different methods can show the intensity of Surface urban heat island in a city differently. Furthermore, consider of the temporal and spatial variations in temperature can cause error in calculating urban heat island. The relationship between factors such as vegetation index, land use, altitude and meteorological factors with urban heat island has been investigated and proven in previous researches. In this regard, predicting the land surface temperature in and around the cities to simulate the intensity of the urban heat island in the coming years has been of interest to researchers because reliable predictions of the difference between urban and rural areas are essential for planning about cities. Different cities may affected by different factors depending on the climate in which they are established. Therefore, in the study of the heat island of Tabriz and Urmia, land use is investigated as a determining factor. In addition, temporal variations in the area of Lake Urmia will be studied to assess the relationship between the extent changes of this water body and the intensity of surface urban heat island in Tabriz and Urmia.
Material and Methods:
In this study, MODIS land surface temperature data (MOD11A1) in tile No. h21, v06 has been used to investigate the urban heat island. This tile covers northwestern Iran. The common time series used in MODIS Terra and Aqua is from 2003 to 2019. Terra and Aqua each monitor the entire earth twice a day. In this study, all four observations have been used in order to evaluate the diurnal variations of the urban heat island. Second MODIS data that used in this study is the land cover type (MCD12Q1). To identify the types of land cover, the FAO Land Cover Classification System has been selected among the existing classification layers, which has been generated by applying the supervised classification method to the MODIS reflectance data. Another MODIS product has been used to study the changes in the area of Lake Urmia. This product provides a time series of the world's lakes extent, depth and reservoir variations. The data were obtained from the detection of water and land pixels using a machine-learning algorithm.
Urban area and type of pervasive land cover around the city has been obtained using MODIS land cover type data. The pixels that belonged to a specific land cover in more than 75% of the study period (temporal frequency of land use species) were considered as the representative pixels of that land cover. To determine the urban and rural area, an area equal to the size of the urban extent around it has been selected as rural area. The land covers were examined among the rural pixels. The pixels that cover more than two thirds of the rural area have been identified (spatial frequency of land covers). The intensity of the heat island has been estimated according to each of the dominant land covers. Then, the intensity of surface urban heat island in relation to each of the land covers of rural district has been compared. This process also has been done once for the whole area of the rural pixels.
Results and Discussion:
Evaluations of land cover type in rural area of Urmia and Tabriz cities showed that land cover type of cropland and natural herbaceous has the largest area with more than 75% of land cover frequency. The surface urban heat island in Tabriz has annual cycles. In the warm period of the year, the cropland shows a more intense heat island rather than natural herbaceous and all rural area. In addition, at this time of the year, the estimate of urban heat island in relation to the area of natural herbaceous in most cases indicates the cold heat island in Tabriz. These conditions are inverted during the cold period of the year and the urban heat island in cropland and natural herbaceous shows the cold and heat island, respectively. The intensity of the urban heat island of Urmia in the land cover of cropland and natural herbaceous is well separated and show completely different annual cycles, but its annual variations is the same as in Tabriz. The use of natural herbaceous as a rural land cover in Urmia shows a more severe cold island in the warm period of the year than Tabriz.
The urban heat island of Urmia at night shows obvious differences compared to Tabriz. First, the annual in the Urmia heat island cycles are well seen, which indicates the increase of the night heat island in the warm period of the year and its decrease in the cold period. The second major difference is the urban heat island values related to different rural land cover type. The heat island of Urmia, although in smaller numbers, often shows more intensity than Tabriz. It may be due to the smaller size of Urmia city compared to Tabriz and its shorter distance from the lake that cause to more affect by water extent variation of the Urmia Lake. Because of this condition, the daytime urban heat island in Urmia occurred more frequently. In addition, there is a significant difference between urban heat island of rural land covers in Urmia. While this difference is less in Tabriz and less in nighttime than daytime.
Conclusion:
Calculation of the surface urban heat island with MODIS data showed that in some cases, especially during the daytime cold island occurs in some parts of the two cities of Tabriz and Urmia. The calculated heat or cold island was determined by selected type of land covers in rural area. In addition, the selected type of land cover in rural area has a great effect on estimating the intensity of the urban heat island. Cropland as a rural area during the night shows more intense heat island than natural herbaceous while during the daytime the opposite condition was happened. The use of all type land covers as rural area shows the intensity of the heat island between cropland and natural herbaceous as rural area.
Due to the large effect of heterogeneous surfaces on the measurement of surface temperature during the daytime, measurements at nighttime can provide the intensity of the urban heat island with better accuracy. In this regard, nighttime observations of MODIS land surface temperature, especially in Aqua, which is the closest observation to minimum temperatures, can be useful in monitoring the intensity of the urban heat island and its temporal-spatial changes, especially in warm period of the year.