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Climate Model Berlin – Development of the Number of Climatological Threshold Days in the Future 2015

Introduction

Scientific research has long shown that the global climate is changing and will continue to change. The current state of research on the past and likely future global development is published in the framework of the reporting of the IPCC (Intergovernmental Panel on Climate Change, IPCC 2014). In this context, cities, in particular the large metropolises, are not only responsible for reducing greenhouse gas emissions; they are also affected due to increasing loads for different urban infrastructure sectors. According to current calculations of the Potsdam Institute for Climate Impact Research (PIK), in 2100 Berlin will have the climate, with respect to the annual variation of temperature and precipitation, that the southern French city of Toulouse has today (Reusswig, Becker et al. 2016). In particular, the increase in extreme weather events such as prolonged hot and dry periods or heavy precipitation presents enormous challenges to policymakers, the administration and the population.

The Berlin Senate has long recognised the urgency that these forecasts imply for the city, has responded with plans and measures both in the area of mitigation and in the area of adaptation to the impacts of climate change, and is continually developing them further. The legal framework for the field of action of climate protection with its various components was determined with the Berlin Energy Transition Act (EWG Bln 2016) and the strategies and measures for climate neutrality by 2050 in the Berlin Energy and Climate Protection Programme 2030 (Abgeordnetenhaus Berlin 2016).

Climate adaptation is adressed with shared responsibilities in Berlin.

The core elements of Berlin’s strategy for adaptation to climate change consist of three components:

  • the new three-part Planning Advices Urban Climate Map of the Environmental Atlas (SenStadtUm 2016b), which points out with high spatial resolution which areas of the city are already suffering from climatic loads and where Berlin has potential for relief.

The planning advices map is the final result of comprehensive urban climate modelling which allowed, besides the comprehensive simulations, also analyses of long-term time series at the Berlin stations Alexanderplatz, Dahlem, Grunewald, Tegel and Tempelhof as well as Potsdam (SenStadtUm 2015). In an excursus in the framework of the ERDF project GIS-based modelling of parameters relevant to urban climate on the basis of high-resolution data on buildings and vegetation” (SenStadtUm 2015a), a station-specific estimation of possible effects of climate change was already carried out with the regional climate model WETTREG2010 Scenario A1B (CEC-Potsdam n.d.), and the results were used also for the maps presented here.

The term “climatological threshold day” refers to a day on which a fixed threshold of a meteorological parameter is reached or exceeded. In the context of the subject of urban climate, the following threshold days are particularly relevant, as they are closely linked to the occurrence of bioclimatic loads in settled areas:

  • 04.12.01 Summer Days (maximum temperature ≥ 25 °C),
  • 04.12.02 Hot Days (maximum temperature ≥ 30 °C),
  • 04.12.03 Tropical Nights (minimum temperature ≥ 20 °C).

The long-term measurements of climate parameters exhibit a characteristic distribution of the minimum and maximum temperatures for the different measurement sites in the municipal area in the summer half-year. The distribution reflects the diverse heat behaviour of the city that results from the different use structures but also from the location of a site within the municipal area as a whole. Given otherwise comparable use, the spatial location within the city thus determines to what extent a site can profit from the cooler surroundings or is exposed to the influence of overheated adjacent districts. An open space under the influence of the urban heat island will exhibit a flatter diurnal variation than a comparable area outside the city. As the absolute level of the summer temperature is primarily determined by the predominant weather conditions and the location of the site in the municipal area merely leads to a modification, the characteristic temperature differences allow inferences to be drawn from the measured temperatures of one site about the level at a different site.

The exceedance of fixed values of the daily minima or maxima determines the occurrence of the so-called threshold days. Since especially the daily extrema and likewise the simultaneously measured temperature differences among the stations exhibit a characteristic distribution, knowing the temperature difference with respect to a reference site allows the probability that the threshold values are exceeded there, too, to be determined. Knowing the frequencies of the threshold days per year at a reference site thus allows the frequencies at another site to be estimated.