Journal of Climate Research

Journal of Climate Research

Spatio-temporal changes and synoptic analysis of heat and cold waves over Hamedan province

Document Type : Original Article

Author
Mehdi Rezaei
Hamedan Research Center for Applied Meteorology, Hamedan, Iran
10.22034/jcr.2025.198804
Abstract
In recent years, the world has experienced an unprecedented number of extreme weather events, some causing major human suffering and economic damage. ‎The results of different investigations in ‎recent ‎decades‎ ‎‎‎indicate‎ that one of the major impacts of global warming on the earth’s surface ‎is‎ in the form of increasing frequency and intensity of extreme weather events ‎, from which heat (HW) and cold waves (CW) are the most attributable events to climate change. Their spatial characteristics at global, regional and national scales have been ‎widely studied‎ in the past, showing different patterns and diverse intensities.

HW and CW are considered ‎important‎ hazards ‎which‎ have different effects ‎on ‎our ‎life‎ depending on their duration, magnitude, ‎frequency, and the vulnerability of the territory based on its climatic resilience. These events may ‎be‎ continued ‎in ‎several-day episodes, ‎and thus can ‎lead‎ to ‎important ‎ impacts on human ‎health, ‎‎‎water resources, electricity ‎consumption‎, crop ‎yields‎ ‎and many other environmental and societal. In this research, we have studied the trend of temporal changes, spatial division, and synoptic analysis of these two important phenomena over Hamadan province in the thirty-year period of 1991-2021.



Materials Methods:

Our study is based on the meteorological data obtained from four synoptic stations of Hamedan Airport, Nojeh, Malayer, and Nahavand. In order to compute the duration and intensity of heat waves and cold waves for all of the selected observatories, we have used the method following the below steps. In this method, we: 1) defined a hot day as an exceedance of percentile 90th for maximum temperatures and a cold day as an exceedance of percentile 10th for minimum temperatures; 2) determine heat waves and cold waves as events with occurrence of 3 or more consecutive hot or cold days; and 3) calculate the intensity of all the events.

For each day of the study period, we have independently determined thresholds that identify hot and cold days. Daily maximum (TX) and minimum temperature (TN) were transformed into quantiles based on the calendar percentiles of each variable for all the investigated years, using a 7-day moving window.



Results & Discussion:

We observe an increment in the frequency, intensity, and duration of Heat waves over the study area, while for cold waves, we observe a decreasing trend in the frequency, intensity, and duration of activity of the waves. In the case of the spatial distribution of the waves shows that there is no meaningful correlation between the different features of the waves and the geographical coordinates of the studied stations. The analysis of the synoptic pattern indicated that hot days in Hamedan mainly were caused by the blocking systems at 500 hPa level which were made by expanding the Azores dynamic high pressure. We can observe the effect of the subtropical high pressure of the Azores on the west or south-west Iran in 500 hPa synoptic maps. At the same time, the high-pressure system in the subtropical region is expanding in North Africa and Iran is also affected by it. During the days of heat wave activity, the average height of the high-pressure subtropical system over Iran is higher than its long-term average height. So, at the pressure level of 500 hPa, the maps show a height of about 5900 meters in the studied area. We also found four different patterns driving cold wave occurrence. Here the maps at sea pressure level play the main role in determining the patterns which drive cold waves. The first pattern which made up about 40% of the cold waves is the European migrant high pressures. This pattern is associated with the formation of a cold high-pressure core at the sea level in the north and northeast of the Black Sea, and as a result, the cold air spreads to the south. The second main pattern, in this case, is the Siberian high which caused about 30% of the observed cold waves. Siberian high plays the main meteorological role at the surface level in Iran, especially in winter. With the expansion of this high pressure, cold air enters Iran through the north and northeast and its tongues advance to the Mediterranean coast. With the expansion of this high pressure to the southern latitudes, the flow of cold air will even cause a sharp drop in temperature in the south of Iran. The northern high pressure causes about 24% of the cold waves as the third main pattern followed by Mediterranean low-pressure systems which drive only 6% of the cold waves that have been observed in this analysis. In the pattern of northern high pressure, a thermal high pressure with a central pressure of 1028 hPa is located over northern Europe. This system transports cold air to eastern Europe and, when accompanied by a shallow trough in the mid-level, it transports cold air to Iran. In Mediterranean low pressure, we have a low-pressure center with a cold core in the east and northeast of the Mediterranean, which is formed by the fall of cold air from the northern latitudes. At the level of 500 hPa, a shallow trough will cause the rapid movement of currents to the east.



Conclusion:

The results show an increase in the frequency, intensity, and duration of Heat waves over Hamedan province. On the other hand, for cold waves in the region, we observe a decreasing trend in the frequency, intensity, and duration of activity of the waves. Examining the spatial distribution of the waves shows that there is no meaningful correlation between the different features of the waves and the geographical coordinates of the studied stations. Examining the synoptic patterns that cause the waves shows that the main pattern that caused, approximately all the observed heat waves is the blocking systems at 500 hPa level which are made by expanding the Azores dynamic high pressure. Furthermore, we found four different synoptic patterns that drive cold waves in our study area. Among these patterns the European migrant high pressures and the Siberian high drive 40% and 30% of the observed cold waves, respectively.
Keywords
Heat waves
Cold waves
Climate crises
Climate change
Hamedan
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Journal of Climate Research
Volume 1403, Issue 59 - Serial Number 59
Vol. 15, No. 59,, Autumn 2024
Winter 2025
Pages 75-89