On the self-ventilation of an urban heat island

Measurements of a new German Meteorological Service urban observation network in Hannover were used to analyse the urban heat island (UHI) and the dynamics of urban wind. The UHI was well pronounced on many nights of the year and had a maximum intensity during the observation period of more than 6 K. A strong correlation between the UHI intensity and wind speed difference between the urban and the rural station was found. During the night, wind speed inside the city was significantly higher than in the surrounding rural area for an increasing air temperature contrast. This effect was attributed to the weaker thermal stability inside the city when there was a well-developed heat island, which enhanced the turbulent momentum flux from above.