Modeling the impacts of anthropogenic heating on the urban climate of Philadelphia: a comparison of implementations in two PBL schemes

Waste heat released from human activities (anthropogenic heating) can be a significant contributor to the urban energy balance, and can thus play an important role in affecting the urban thermal environment, ambient air quality, and other attributes of the urban climate system. To quantify the impacts of anthropogenic heating we have incorporated it as a source term in the near-surface energy balance within the MM5 mesoscale atmospheric model. This energy balance is calculated as part of the planetary boundary layer (PBL) module within the MM5. Because of the multiple PBL scheme options available within the MM5 and other atmospheric modeling systems we have enabled anthropogenic heating within two commonly used PBL modules-Blackadar (BL) and Gayno-Seaman (GS). Results from a case study series of simulations for Philadelphia suggest that anthropogenic heating plays an important role in the formation of the urban heat island, particularly during the night and winter. Control simulations (without anthropogenic heating) consistently underestimated urban air temperatures and the observed urban heat island effect. Simulations for winter suggest that anthropogenic heating contributes 2-3 °C to the nighttime heat island. In addition, anthropogenic heating is also found to have impacts on the nocturnal PBL stability and PBL structure during the morning transition. The choice of PBL scheme affects the magnitude of these modeled impacts. In winter, for example, the addition of anthropogenic heating in the BL scheme resulted in a 3 °C temperature increase at night compared with about a 2 °C temperature increase in the corresponding GS simulation.