Analysis of the compound impact of heat waves and droughts in the urban environment

As a consequence of global warming, heat waves become longer lasting, more frequent, and more intense in extratropical regions. Summer heat stress often occurs simultaneously with drought events causing compound impacts in the affected region. In order to analyze such coincidences in a mid-latitude continental, Central/Eastern European city (i.e., Budapest, the capital of Hungary with 1.7 million inhabitants), local temperature and humidity conditions provided by satellite data, are evaluated. So, the unique 22-year-long time series of continuous measurements from the MODIS instrument on NASA's Terra and Aqua satellites was used to study the surface urban heat island (SUHI) pattern, surface temperature and humidity in detail.

A significant warming trend can already be identified in the surface temperature data during the period of 2001-2022. The analysis of summers shows that the SUHI intensity decreases as the rural area around the city becomes warmer, especially in July and August. When less water is available in the rural area in a drought event, the lack of latent heat thus facilitates the warming of the surface temperature in the rural area as well, as in the urban area, because these conditions are unable to reduce the surface temperature via the latent heat as usual. This way, the SUHI intensity is mainly determined by the rural surface temperature. The SUHI is very weak during summers with frequent and intense heat waves and droughts, due to the fact that the land surface temperatures are very high in both urban and rural areas resulting in very little difference between the built-up area and the vegetation-covered surrounding. This phenomenon is analyzed in the present study in detail for the years 2003, 2007, and 2022, when intense heat waves occurred in the region. Such detailed analysis aiming to understand the complex environmental processes in the urban environment is essential in order to develop effective adaptation strategies to the upcoming challenges of climate change, which will probably result in increasing frequency and persistence of heat waves and droughts in the future, with adverse effects to the quality of the life of the urban population.

Acknowledgements: Research leading to this study has been supported by the Hungarian National Research, Development and Innovation Fund (under grant K-129162), and the National Multidisciplinary Laboratory for Climate Change (RRF-2.3.1-21-2022-00014).