Abstract
ABSTRACT The impact of fog on relief modified urban heat island (RMUHI) in Krakow has been presented using fog observations at 06 UTC from two meteorological stations: a rural one (Balice, B) and an urban one (Botanical Garden, BG) from the period 2006–2015. Daily UHI magnitude for the valley floor for the same period was estimated as TminBG–TminB, while for the period 2010–2015, eight daily courses of UHI were available, for the urban areas in the valley floor and 50 m above it, together with air temperature inversion data. UHI data for days with various combinations of fog occurrence and weather conditions were compared using non-parametric statistical tests: Wald-Wolfowitz test, Kolmogorov-Smirnov test and U Mann-Whitney test. Data of 2010–2015 were also the subject of cluster analysis (k-means method). Fog is an important factor decreasing UHI magnitude by about 1 K but mainly during weather conditions with little or no cloudiness and small wind speed or atmospheric calm, during anticyclonic synoptic situations, and only in the valley floor areas. With an increase in cloudiness and wind speed, the role of fog decreases and is similar in all parts of the city.
Highlights
Urban heat island (UHI) is probably both the clearest and the best documented example of inadvertent climate modification
Fog and UHI Magnitude, 2006–2015 In the cold half-year of 2006–2015, there were 208 nights with radiative weather type (RWT) and in 27% of them fog was noted at 06 UTC, either at one or at both stations
From the remaining 1614 nights with NRWT, only in 9% did fog occur at 06 UTC (Table 2), as fog formation is most effective during nonadvective conditions, especially with a clear sky, which favors radiative cooling and water vapor condensation
Summary
Urban heat island (UHI) is probably both the clearest and the best documented example of inadvertent climate modification. The impact of fog on UHI, reported in most studies, is latent heat release during fog formation, which leads to an increase in minimum air temperature (e.g., Gough and He, 2015; Tam et al, 2015). Fog is often formed during similar weather conditions when the highest UHI magnitude values are observed, i.e., during windless nights or nights with low wind speed, local environments are of a high importance influencing turbulence and radiative processes (Gultepe et al, 2007). During the nights with low wind speed and little cloudiness, rural areas surrounding Kraków, due to the location in the area with diversified relief, experience katabatic flows and cold air reservoir formation, which favors fog formation, while the urban areas’ impact is the opposite. The influence is examined during different weather conditions in two vertical urban zones described: the valley bottom and the areas located 50 m above
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