Improving the urban weather generator with a physically based dynamic model of the nighttime urban boundary layer height

The urban weather generator (UWG) is a lightweight, free, and easy to use simulation tool that predicts the urban heat island intensity. One limitation of the UWG is that it takes as an input parameter the nighttime urban boundary layer (UBL) height, which is a time-varying physical quantity that is very little known. To overcome this limitation, a physically based dynamic model for the prediction of the nighttime UBL height is implemented in the UWG. The effect of this model implementation on the behavior of the UWG is analyzed in detail.

A primarily analysis aims at understanding deeply the behavior of the base UBL model under various weather conditions, with various nighttime UBL heights. It reveals that when the nighttime UBL height is high, the temperature predictions of the base UWG model do not depend on the characteristics of the urban layout, which renders the model inappropriate for assessing the performance of urban heat mitigation strategies. When the nighttime UBL height is low, spurious fluctuations occur in the predicted temperatures.  The dynamic nighttime UBL height model reconnects the air temperature predictions to the characteristics of the urban layout and regulates the thermal behavior of the UBL to avoid spurious fluctuations. The model predicts nighttime UBL height values that optimize the UWG prediction accuracy; however, these values remain considerably low compared with measured nighttime UBL heights.