Investigating the Effect of Heat Waves on the Intensification of Thermal Islands in Sanandaj City From 1989 to 2018

 Heat waves (HWs) are one of the most important climatic disasters that have devastating environmental consequences in nature every year). The purpose of this study is investigation of the effect of heat waves on the intensification of thermal islands in Sanandaj city from 1989 to 2018. The constant rise in temperature of the city as an urban heat island and the sudden occurrence of HW's as one of the major climatic hazards, is an important concern of urban management policy makers; because intensify heat of this city and cause a lot of environmental damage.

 In order to identify HWs in Sanandaj city, from 1989 to 2018, by using Fumiaki Index and MATLAB software, days whit temperature above +2 standard deviation or above the mean Normalized Thermal Deviation (NTD) that lasted at least two days, were identified as the day with HWs and calculated by equation 1. (1) Where, T (i, j, n) is temperature of day ith from month jth in year nth indicates the average temperature of day i from month j. To eliminate the noise in the mean, a 9-day moving average filter was performed on these data three times and calculated by the following equation. (2) Where, ∆T= (i, j, n) indicates absolute deviation of temperature from the average on day jth of the month i th, in year n th compared to the average temperature of the same day. In order to the values of temperature deviation of different times and places to be comparable at a certain time and place, it is necessary to standardize these absolute values of temperature deviation by means of temperature diffraction. Like day-to-day changes, diffuse T∆ at 31 days for each day is calculated by the following equation. (3) The value is the average temperature deviation in 31 days that is calculated by the following equation. (4) Finally, (NTD) is calculated by the following equation. (5) Where .Then in MATLAB software, days with temperatures +2 above average (NTD) and lasting at least two days, were selected as the day with the HW. (6) Then the thermal island was calculated in Sanandaj city using Equation 7. SUHI= MLSTurban –FLSTrural (7) Where, SUHI is the island surface heat index, MLSTurban and FLSTrural are the average surface temperature of urban and rural areas, respectively.

 The results showed that, during the study period (1989-2008), the highest frequency of HW hazards in this city was in September, February, March, and October 1991. The maximum duration of HWs was 6 days, which occurred in December 2017 and 2005, therefore long-term HWs have been experienced in this city. Results also showed, in both HW and NHW conditions, in the hot and cold months of the year, often a cold island is formed in the city center during the day and a heat island is formed at night. Results also showed that short-time heat waves have been effective in intensifying heat islands. Examination of the intensity of thermal islands in this city showed that during the day in both HW and NHW conditions, which in the HW conditions dominance of the cold island compared to normal day, it has been reduced and in the last months of winter (February), even during the day, a heat island has been created in the center of the city. At night time, in both HW and NHW conditions, a heat island was created in Sanandaj center, but the intensity of night- time heat islands in HW conditions is often significantly higher than normal conditions especially in the winter. Investigation of the condition of thermal islands in the warm months of the year showed that in both HWs, a cold island has been created in the city center that the intensity of cold islands during the HW conditions, especially in the summer months, was often higher than NHW conditions. At night time, there was often a heat island in the city center that was more intense than normal day. Also, in HW conditions, wind speed and especially relative humidity has decreased significantly more than the cold months of the year.

 According to the results the highest incidence of HW hazards occurred in the winter and early spring. Also, long-term (6-days) HW occurred in this period. The increasing trend, frequency and continuation of HW, especially in the cold months of the year, can be the effects of climate change and global warming. Severe and continuous HWs occurred in Sanandaj city, especially in late winter, can cause early germination and flowering of crops and gardens and it will negatively affect agriculture and horticulture and will lead to great economic losses. The effects of HWs on heat islands occurred in the suburbs due to having a clear sky without pollution, with minimal vegetation and lack of surface water resources and ground with low heat capacity is affected by HWs faster than the city center and as the land surface around the city becomes warmer than its center, a cold island is formed in the city center. At night, the suburbs due to low heat capacity, lose absorbed heat faster and as a result, the heat island is formed in the city center. In general, the occurrence of heat waves in the intensification of thermal islands in the Sanandaj city, especially in the warm months of the year, has a significant effect, and it is likely to intensify in the coming decades, especially at night during the hot months of the year.