Miejska wyspa ciepła w Warszawie – próba oceny z wykorzystaniem Local Climate Zones
Urban heat island in Warsaw – an attempt at assessment with the use of Local Climate Zones method

Summary/Abstract: The Urban Heat Island is defined as a relative increase in the temperature of the near-surface atmosphericlayer within cities relative to the surrounding rural areas (Oke 1987; Błażejczyk 2002; Fortuniak2003). UHI is observed in many big cities all over the world (Oke 1973; Park 1987; Stewart2011a) but also in small cities, with population of less than 3,500 people (Błażejczyk, Kunert 2006).There are many causes of the UHI, but the most important one is the change of land use and highratio of impervious surface fraction and this is why over the last decade there were many attempts tobind specific urban features with different climatic conditions which exist within the city (Kuchcik2003; Taniguchi et al. 2008; Erell et al. 2011; Szulczewska 2015). The effort to find a universal definitionof the mosaic of city climates lately resulted in the Local Climate Zones method established onthe example of Canadian cities. It relied on examining many urban features such as building and impervioussurface, terrain roughness, sky view factor etc. (Stewart 2011a, b; Stewart, Oke 2012).The basis for the analysis of the urban heat island (UHI) was data from the monitoring network ofthermal conditions in the Warsaw metropolitan area operated by IGSO PAS over the years 2011–2012. It comprises 28 stations complemented by one station of the Institute of Meteorology and WaterManagement (Okęcie) (Fig. 1). Each measurement point is defined by the distance from the city centre,ratio of biological vital areas, the index of building intensity and type of the Local Climate Zone(Tab. 1).The data was collected as 10-minute averages. The lowest daily 10-minute average values of airtemperature were accepted as minimum daily temperatures. The UHI was defined as the differencebetween the minimum daily temperature at the measurement points and at the Okęcie airport station(No. 21), hereafter referred to as the UHI-index.The highest UHI-indices were generally observed in compact midrise land-use types (up to2.5°C), however, it can reach 7.6°C. Slightly lower UHI-indexes were found inside open midrise andopen high-rise settlements (up to 1.7°C). In compact low-rise and open low-rise, the UHI was weak(0.5–1.0°C). Within sparsely built districts and settlements with scattered trees, the UHI-index occasionallyreached even 5.6°C as an effect of the high sky view factor but the average UHI-index hadnegative values. The only site which presents heavy industry is Żerań (No. 6), situated next to the ŻerańPower Station in the northern part of the city and the UHI-index there is distinctly higher than at theother site situated nearby (No. 4) (Figs 2, 3).On average, the greatest range of air temperature deviations from that of Okęcie airport stationwas recorded in Michalin (No. 28) and small cities outside of Warsaw, e.g., Tłuszcz and Sulejówek.The lowest deviations were in Bokserska (No. 22), Langego (No. 24) and Kolo (No. 13), which are oldhousing estates on the left side of the Vistula with extensive green areas. This clearly shows the positiveimpact of vegetation on city climate.UHI-index values are significantly dependant on the distance from the city centre. The highest areobserved at a distance no higher than 1 km, the lowest about 14.5–17 km from the centre. This characteristicgroups sites in a different way than the Local Climate Zones. On average, 13 km is the distanceof disappearance of the urban heat island in Warsaw (Fig. 4).Another type of a characteristic of the urban heat island is to compare the courses of the meanUHI-index for each Local Climate Zone taking into account also the synoptic situation. Four months arepresented: January and February, July and August 2012. There were weeks in which the urban heat islandin Warsaw was hardly observed (especially in winter) and the weather was steered by a cyclonicsynoptic situation. The thermal differentiation was rising while a change into anticyclonic situation wasestablished. The highest amplitude of the UHI-index was 8.4°C in winter and 8.7°C in summer and appearedbetween the compact midrise in the city centre and sparsely built areas (Fig. 5a, b).To illustrate diurnal changes of UHI and its characteristics in areas at a varying distance from thecity centre and Local Climate Zones, we examined daily courses of hourly air temperature for 5-dayperiods in various seasons. Three stations were chosen: Hoża (No. 17) – from the exact city centre, thepoint is situated in a small courtyard surrounded by 6-8 floor buildings; Orzycka (No. 23) – housingestate half way between the outskirts and the city centre as well as Powsin (No. 29) – situated in theclearing in a botanical garden, on the border of the city (Tab. 2, Fig. 1).The figures show well developed urban heat islands during the winter and summer season and thesituation without UHI (Fig. 6). As for the time of occurrence of the UHI, we have found the hours tobe from 8–10 PM to 4–7 AM, depending on the season.In winter, the air temperature on Hoża and Orzycka are similar and Powsin is outstandingly cooler(up to 4.5C). Such values persisted usually from 10 PM to 3 or even 7 AM local time (UTC+1.00)(Fig. 6a).In May, the UHI reached 9.9C under one air mass and clear weather. The differentiation betweenthe city centre and the housing estate placed equidistant from the outskirts and the city centre is growing.The most intensive UHI occurred from 2 AM to 6 AM. During the day, between 9 AM and 5 PMin the shaded areas of the city centre, the air temperature was even 2.2C lower than in a sunny clearingon the outskirts. At compared stations, the rises of air temperature are of similar shape and inclination, however, the falls of air temperature on the outskirts are much steeper than in the city centre. Thisphenomenon is an example of a delay of cooling-down of a densely built, ‘concrete’ city centre incomparison with the periphery (Fig. 6b).

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