The urban wind island from a three-dimensional perspective

The Urban Wind Island (UWI), a small but persistent positive wind anomaly over the city as a whole, has previously been revealed using a simplified conceptual model of the convective atmospheric boundary layer. However, the urban boundary layer is strongly heterogeneous and complex, and many interactions with surrounding land-use are not taken into account with the conceptual model used. Additionally, the transition to a stable or neutral nocturnal boundary layer substantially influences wind speed, for instance leading to nocturnal jets, which could also lead to UWI formation. This study extends the UWI research into less idealised cases by using the 3D WRF mesoscale model for Amsterdam (the Netherlands) and its surroundings, at 500m resolution. Two summers of forecast results for in total 173 days are used to identify whether the UWI persists in a 3-dimensional modelling environment, and which conditions are optimal for its formation and persistence. In order to focus only on wind modified by surface processes, large-scale influences which modify wind speed, such as frontal passages, are identified and eliminated from the dataset. We find that a positive UWI is present roughly half the time, with an order of magnitude that is similar to the previous work (~ 0.5 m/s). In addition we find an evening UWI that is caused by the delayed onset of the transition from an unstable to a stable or a neutral boundary layer in the urban area, while the rural area is already stable and calm.