Urban air temperature anomalies and their relation to soil moisture observed in the city of Hamburg

The spatial variability of the urban air temperature for the city of Hamburg is analyzed based upon a one-year dataset of meteorological and pedological measurements. As local air temperature anomalies are subject to land-use and surface cover, they are monitored by a network of measurement stations within three different urban structures. Mean annual temperature deviations are found to be +1.0 K for inner city sites and +0.25 K to -0.2 K for suburban sites compared to a rural reference. The nocturnal urban heat island (UHI) is identified and averages +1.7 K at the inner city stations, +0.7 K at a suburban district housing area and +0.3K at a nearby green space. The observed UHI effect is most prominent when the wind speed is low ( 2ms -1 ) and the sky is only partly cloudy ( 6/8 th ). In spring 2011 an average inner city UHI of up to +5.2 K is observed during situations matching these conditions, while the extraordinary dry fall of 2011 lead to remarkably high air temperature differences at all observed stations. As expected, no evidence for a significant impact of topsoil moisture on nighttime UHI effect is found. The analysis of air temperature anomalies during daytime results in an annual mean deviation of -0.5 K above unsealed, vegetated surfaces from a sealed site during days with a turbulent mixing induced by wind speed > 2ms -1 . Here, there is an indication for a relation between the water content of upper soil layers and the warming of air: 11 to 17 % of the variance of the diurnal air temperature span is found to be explained by the soil water content for selected relevant days.