Long-term changes and regional differences in temperature and precipitation in the metropolitan area of Hamburg

Climate changes and the urban climate of the 'green city' Hamburg and its metropolitan region are analyzed using observational data for temperature and precipitation. Values for Hamburg's synoptic site HH-Fuhlsbuttel start in 1891 and are used to determine climate changes. Additional data from up to 45 climate stations are used to analyze the different aspects of the regional climate and urban effects on the temperature [urban heat island (UHI)] and precipitation [urban precipitation impact (UPI)]. The analysis of the long-term data shows that the climate has already changed. Annual precipitation significantly increases similar to 0.8 mm/year when focusing on years 1891-2007 and similar to 1.3 mm/year for 1948-2007. Precipitation increases are largest in November through March and March as well as June for 1978-2007. For April and July of this period, a precipitation decrease is found. The precipitation distribution shows that moderate daily precipitation amounts (<= 10 mm/day) have increased by about 10% between 1948-1977 and 1978-2007. Precipitation amounts >10 mm/day have increased by 20% in the same period. Average temperatures significantly increase by 0.07 K/decade (1891-2007), 0.19 K/decade (1948-2007). 0.6 K/decade (1978-2007) with largest significant increases in fall. For the UHI, it is found that the average temperature is higher up to 1.1 K in the densely build-up city area than outside. Values are about halved for more green urban areas but also depend on more local impacts. The minimum temperatures are up to 3 K higher and maximum temperatures slightly lower in the inner city than in the rural during summer. The winter temperatures are higher throughout the urban area. The UHI differences depend on wind speed; this dependence is best described by using the inverse square root of the wind speed. Classification using different wind directions shows that the precipitation is significantly higher (5-20%) for downwind of urban areas compared with the upwind side. Copyright (C) 2009 Royal Meteorological Society