The spatial distribution of summer heat wave characteristics in Tehran Province from 1981 to 2020

Introduction

Temperature has undeniable effects on human activities and natural processes. In particular, an increase in temperature in the form of heatwaves affects all aspects of human life, including architecture, comfort, transportation, agriculture, and industry. Recently, both the frequency and intensity of heatwaves have increased globally, making them a major cause of weather-related deaths in many countries. The characteristics and intensity of heatwaves vary across locations. A heatwave is typically defined as positive fluctuations or peaks above the average daily maximum temperature that persist for several days or even weeks in specific geographical areas.

Several studies have analyzed long-term extreme warm events in Tehran Province, mostly at weather stations. However, no studies have focused on assessing the different aspects of heatwaves (HWs) using percentile indices (EHF, TX90, and TN90) as well as interpreting their climatological and spatiotemporal variations. Therefore, this study aims to (i) assess the decadal changes in the different HW aspects over Tehran Province during the last four decades, from 1981 to 2020, and (ii) interpret the spatiotemporal patterns and anomalies in the different HW aspects over the study area.

Materials and Methods Data

In this study, daily minimum and maximum temperatures (°C) from the ERA5-Land dataset have been used to calculate the aspects of heatwaves over Tehran province (Muñoz-Sabater et al., 2021).

Methods

Heat waves can be calculated based on three definitions (indices). These definitions are 1- based on the 90th percentile of TN (daily minimum temperature), which is called Tn90 heatwaves; 2- the 90th percentile of TX (daily maximum temperature), which is called Tx90 heatwaves; and 3- EHF, which are heatwaves calculated based on the Excess Heat Factor EHF. The number of heatwaves is calculated for the summer season (May to September). On days (at least three consecutive days) when one of the following conditions is observed, the phenomenon of heat waves has occurred:

TN greater than the 90th percentile of the minimum temperature (TN90)

TX greater than the 90th percentile of maximum temperature (TX90)

The positivity of EHF (EHF)

EHF is calculated daily and is a combination of two Excess Heat Indices (EHI) that indicate the potential for heat adaptation and the climatic significance of heat on a specific day.

Results

In this study, the decadal trends, regional distribution, and anomalies in the heatwave aspects (HW) in Tehran Province over the previous 40 years (1981-2020) were calculated.

Across all indices, the decadal sum and standard deviation of the five HW characteristics in Tehran Province gradually increased from the first decade (1981–1990) to the highest values in the last decade (2011–2020). The results indicated that the decadal total number of summer heat waves based on the EHF index in the fourth decade (2011–2020) ranged from a maximum of 39 events to a minimum of 19 events across Tehran Province, whereas this characteristic in the first decade ranged between 8 and 16 events. In the last decade, 65.8%, 85.5%, and 44% of the province's area experienced 25-35 events in each of the three indices EHF, TX90, and TN90, respectively. The magnitude and amplitude (highest recorded temperature during the most intense heat wave) also increased in the later decades according to all three indices, reflecting hotter heat waves with increased duration and frequency.

Using the EHF index, higher values were obtained for most characteristics (including HWN, HWD, and HWF) compared to the other two indices, indicating its greater sensitivity in detecting heat waves. The lowest values for HW characteristics were derived from the TN90 index.

Conclusion

The analysis of heatwave characteristics in Tehran Province over four decades (1981–2020) showed that not only the frequency but also the intensity, duration, and spatial extent of heatwaves have significantly increased over the four decades. A significant increase in the frequency and duration of heatwaves in the southern and southwestern regions of Tehran Province was observed in the recent decade (2011–2020).

Overall, ERA5-Land data, despite some limitations in high-altitude areas, are considered an effective tool for analyzing climatic trends and especially for assessing the spatiotemporal patterns of heatwaves on a regional scale, as mentioned in previous studies. Examining the spatial distribution, frequency, intensity, and duration of heatwaves and identifying the more vulnerable areas to these events is crucial for informing public health policies, agricultural planning, and crisis management strategies. The results of this study emphasize the need for special attention to the vulnerable areas of Tehran Province (the southern and southwestern regions) regarding heatwaves under global warming conditions.