Explorative Observations of the Urban Climate System in the Rotterdam Metropolis

Until recently, the Urban Heat Island (UHI) effect was thought to be relatively unimportant for Dutch conditions since The Netherlands are located in the mild Cfb climate, close to the sea. This view altered after the heat waves of 2003 and 2006 that caused an excess mortality between 1400 and 2200. The excess mortality was found in the cities in particular. Climate projections show that also in The Netherlands heat waves will likely become more frequent in the next decades, which may form an important threat to the livability in urban areas. However, information regarding UHI in Dutch cities is completely lacking, both from observational and model perspective, which hampers the design of suitable adaptation and heat mitigation strategies. In this study, an assessment of the intensity of UHI in the city of Rotterdam is made. The main aim is to assess whether or not heat stress already is or will be a critical issue in the Rotterdam area. Traverse measurements were performed in the city using two cargo bicycles as a mobile platform, equipped specifically for urban meteorology measurements. The cargo bicycles allow manoeuvring easily through the narrow streets in the city. They are equipped with a thermometer, a humidity sensor, a 2-dimensional sonic anemometer and a set of radiation sensors to measure solar radiation and infrared radiation exchange from six directions. The data are recorded at 1 Hz, and connected with concurrent GPS readings. The instruments are powered by a solar panel mounted on the baggage carrier. Measurements were performed along two previously determined routes through a number of characteristic urban districts, including an industrial area, an older residential area, a city park and the harbour area. The routes were photographed at fixed intervals from 50 cm above the ground with a fisheye lens pointing upwards. The observations were carried out during four 1-2h time intervals on 6 August 2009, a warm day with a maximum temperature of near 30°C. Data from the traverse measurements were compared with recordings from Zestienhoven, a nearby synoptic weather station outside the urban area. Satellite images and city maps were used to support the analyses. Furthermore, mesoscale model simulations for the Rotterdam area were performed. Preliminary results show that during daytime the city centre was 2 K warmer than Zestienhoven while the city park along the route was 2.4 K cooler. The air temperature perceived by humans was estimated to be 28°C at Zestienhoven and about 34°C in the city centre. During the late evening (22-24 LT), the city centre was ~5 K warmer than Zestienhoven and again, some locations near green spaces in the city were about 2 °C cooler. The difference in temperature between the city and the countryside amounted to more than 7 K during nocturnal hours. The preliminary results clearly demonstrate the existence of a considerable UHI in the city of Rotterdam and confirm the important role of green spaces in mitigating heat stress. The presentation will further highlight the measuring strategy and results from our analyses.