Evolution and assessment of urban heat island between the years 1998 and 2016: case study of the cities Bratislava and Trnava in western Slovakia

Abstract

The paper concerns research on urban heat islands (UHIs) and presents case studies of two Slovak cities of different sizes: the capital, Bratislava, and the regional centre, Trnava. The model MUKLIMO_3 was applied to the UHI modelling and a heat wave that occurred between 15 and 23 August 2018 was chosen. Land cover/land use (LC/LU) classes based on the local climate zone (LCZ) concept were used as input for the modelling. The model was validated by data taken from five stations in Bratislava and two in Trnava, and a good rate of agreement between the modelled and measured data was statistically proven. Manifestations of UHI were studied during the whole of the mentioned period by summing the hours when threshold temperature values exceeded 25 and 30 °C. The results for Bratislava show greater variability related to the size of the city and terrain dissection compared with Trnava, which is smaller and situated in a flat terrain. A single representative day, that is, 20 August 2018, was chosen, for which UHI was modelled with three inputs of LC/LU classes: situation in 1998, 2007, and 2016 in order to assess the effect of change of LC/LU classes on the distribution of temperatures. Spatial manifestation of UHI was assessed in the frame of LC/LU classes for 2016 at 12:00, 18:00, and 24:00 in Central European Summer Time. The greatest heat load was modelled in densely built-up LC/LU classes, while forest areas were coolest. In Bratislava, due to a combination of a dissected terrain and UHI, great variability was modelled, with maximum temperature differences of up to 12 °C and, in the territory with homogeneous elevation, up to 6 °C. The UHI field in Trnava is less variable, with modelled temperature differences of up to 3 °C. A comparison of individual time horizons proved the effect of the urbanization process on the change of air temperature by several tenths of a degree Celsius on average.