Statistical Study of Heat Wave Characteristics in Isfahan Metropolis

Heat waves are one of the natural climate phenomena in different regions of the world, which have increased as a result of global climate fluctuations in recent years; which has negative effects on agriculture, fires of forests and pastures, water resources, energy consumption and human health.There is no comprehensive and clear definition of heat wave globally, so that even the World Meteorological Organization has not been able to provide a comprehensive definition of heat waves. Heat wave is variable in different situations and no specific criterion has been defined for it. On the other hand, the characteristics and intensity of heat waves are also variable in each place. But in general, heat waves can be considered as positive fluctuations above the average maximum daily temperatures, which have continuity over days and sometimes weeks.It seems that in each geographical region, based on its environmental and climatic characteristics, certain temperature thresholds can be considered to define the heat wave.Generally, a heat wave refers to intense periods of heat that last more than 5 days and the temperature is 5 degrees higher than normal and also creates a crisis for the environment and human health. In other words, intense heating of the air or the rule and invasion of very hot air in a wide area is called a heat wave.

In this research, the daily maximum temperature data of Isfahan synoptic station were used to identify heat waves in Isfahan. Identification of heat waves based on the 90th percentile was done in such a way that by considering the thresholds for each season and month, the periods in which the maximum temperature was higher than the desired threshold for at least three consecutive days (90th percentile) were considered as heat waves. . In the next step, the frequency of heat waves for different seasons and months was checked, and taking into account the difference between the 90th percentile threshold and the average temperature of the heat wave, the intensity of each heat wave was also calculated, and the trend of its changes in the scale of months and seasons was checked. In the next step, the continuity of each of the identified waves was checked for different months and seasons; in this way, the average continuity and the highest continuity of heat waves and the trend of their changes during the statistical period were evaluated. In this research, the daily heat wave magnitude index (HWMI) has been used. Extraction of heat waves in each season has been done based on a separate percentage threshold of that season. Annual values are also obtained from seasonal values. In this way, the sum of the number of waves of the seasons determines the number of annual waves and the average magnitude of all the waves of the seasons determines the average magnitude of the annual waves of each station. The magnitude of each heat wave is the sum of the magnitude figures of all the days of that wave. Also, the biggest wave of each year is considered as the indicator wave of that year and the biggest wave of each season is considered as the indicator wave of that season.

In the spring and summer seasons, in the first decades of the statistical period, durations of less than 5 days prevailed, but since the mid-90s, heat wave events have occurred in higher durations, with the highest duration in the spring season with 18 days for 2017 and the highest heat wave event. The summer season in 2003 was recorded with duration of 17 days. Spring, summer and autumn have not had much difference in the average duration of heat waves; in such a way that for these three seasons, heat waves continued for an average of 6 days; But for the autumn season, the lowest average duration was recorded with 4.35 days. In line with the trend of changes in continuities, a significant increase has occurred for a large index in the late 90s compared to the beginning of the statistical period. Its clear examples are in the spring season, the magnitude of 29.54 in 2017, in the summer season, the magnitude of 28.05 in 1977, and the highest average magnitude value, which is recorded in these seasons. Examining the changes in the heat wave intensity index, all seasons during the statistical period, from 1970 to 2021, shows that the most intense heat wave is related to the spring with an intensity of 3.88 in 2020, the summer season in 2012 with an intensity of 1.65, and the autumn season It occurred with a magnitude of 2.93 in 2017. The most severe heat waves related to the winter season occurred in three consecutive years between 2016 and 2018, and the intensity of these three waves was 3.31, 3.35 and 3.36, respectively.

There has been a significant increasing trend with a rate of more than 0.5 degree increase for each decade. By examining the frequency of occurrence of waves, we found that the occurrence of heat waves in all seasons of the year in the entire statistical period under study had an increasing trend. The highest frequency related to the spring season with 61 cases and the autumn season with 43 waves during the statistical period, have the lowest frequency of heat waves among other seasons. Examining the frequency of occurrence of heat waves on a monthly scale showed that the months of May and June had the highest frequency of heat waves with 21 and 20 occurrences, respectively, and July had the lowest frequency of heat waves with 13 occurrences in the statistical period studied. Examining the intensity of the waves showed that this index had an increasing trend in all seasons, and based on the results of the Mann-Kendall test, this increasing trend was significant for all seasons except summer. The changes in the highest duration of heat waves show that, in the first decades of the statistical period, durations of less than 5 days prevailed, but since the mid-90s, heat wave events have occurred in higher durations. The longest recorded duration of heat waves for spring and summer was 18 and 17 days, respectively. Since 1990, at the same time as the trend of global warming, the penetration of heat waves in Isfahan city has increased significantly, and the most heat waves were recorded in 2003 and 1999. On a monthly scale, the highest average persistence was in March and August. The highest duration recorded for heat waves in the months studied in the statistical period (1970-2021) was for April and July with 13 days and the smallest for January with 6 days. Along with the trend of changes in continuities, a significant increase has occurred for a large index in the late 90s compared to the beginning of the statistical period.