The Long-Term Changes of Urban Heat Island Intensity and Thermal Stress Conditions in Moscow Megacity

The Moscow megacity is the biggest monocentric agglomeration in Europe with population more than 17 million people, which undergo rapid growth and sprawl. The size of the city, its location in continental climate (Dfb type), the presence of pronounced urban heat island (UHI) with a mean annual intensity equal to 2 ⁰С and extremes up to 12-13 ⁰С [1] and city’s location within a flat and homogeneous terrain turn Moscow to a promising test-bed for urban climate studies.

The current study is devoted to the analysis of the long-term changes of the air temperature and thermal stress indices in Moscow megacity during 1977-2018 period, with a focus on the UHI intensity and urban-rural contrasts in terms of thermal stress indices. For the heat stress indices, we use the Heat Index, Humidex, Wind Chill Temperature (WCT), Physiologically Equivalent Temperature (PET), Universal Thermal Climate Index (UTCI) and the frequency of the so-called “hot nights”.

Among the considered trends, especially interesting is the intensification of summertime UHI, which results in the urban-induced amplification of global warming during the last decades. We show that such process is mostly driven by city growth and sprawl, and is additionally amplified by the changes of the meteorological drivers of the UHI (wind speed and cloudiness). UHI intensification is especially noticeable for the hot weather conditions. The heat stress frequency is growing faster in the city than in its rural surroundings. For the winter season, the situation is opposite. The mean winter temperature is growing, and the cold stress frequency shows significant negative trends. However, the mean winter UHI intensity is not growing, and winter urban-rural contrasts in terms of cold stress frequency are even decreasing. Such multidirectional dynamics could be explained by the changes of meteorological drivers of UHI development, which compensate the effect of the urban growth.

Further in-depth investigation of the impacts of the urban sprawl on the UHI intensity and heat stress conditions was carried out with application of high-resolution (1 km grid step) numerical simulations with a mesoscale model COSMO-CLM and TERRA_URB urban canopy scheme [2]. The model was equipped with realistic urban canopy parameters, obtained with application of the original GIS-based technique [3] and successfully verified in comparison with observations in previous modelling studies for Moscow [4]. Simulation have shown that urban sprawl on the edges of the city affects not only the local temperature conditions, but makes a noticeable non-local impact on UHI intensity and heat stress conditions for the rest part of the city.

Acknowledges: the research was supported by Russian Science Foundation, project no. 17-77-20070.

References:

1. Lokoschenko et al., 2014, Urb. , https://doi.org/10.1016/j.uclim.2014.01.008
2. Samsonov et al., 2015, Urb. Clim., https://doi.org/10.1016/j.uclim.2015.07.007
3. Wouters et al., 2016, Geosci. Model Development, https://doi.org/10.5194/gmd-9-3027-2016
4. Varentsov et al., 2018, Atmosphere, https://doi.org/10.3390/atmos9020050