A numerical study of the mechanical and thermodynamic effects of urbanization on the climate of Las Vegas by dynamical downscaling

Conventional studies on the influence of land cover on urban climate have focused on the thermodynamic effects, especially those related urban heat island. The mechanical effect related to the change in the surface wind was often considered secondary and was explored using relatively simple models. For metropolitan areas in which the local climate is strongly controlled by topographic processes such as valley winds, the changes in the wind due to urbanization can play a role in modifying the local climate including its temperature structures. To explore this effect, a numerical study is conducted to study the climate of Las Vegas. It is one of the fastest growing desert cities which has seen a doubling of its developed areas in the last 30 years. An accurate prediction of the effect of urbanization on the climate of the city is crucial for resource management and planning. In this study, we use the Weather Research and Forecasting (WRF) model coupled with a land surface and urban canopy model to investigate the effects of urbanization on the climate of Las Vegas. High resolution numerical simulations are performed with a 3 km resolution over the metropolitan area. With identical lateral boundary conditions, three land-use maps, representing 2006, 1992 and hypothetical 1900 land coverage for Las Vegas, are used in multiple simulations. The differences in the simulated climate among those cases are used to quantify the urban effect. The simulated surface air temperature is validated against observational data from the Maccaran airport station. From the numerical simulations, it is found that urbanization produces substantial warming during the night but a minor cooling during the day. Detailed diagnostics of the surface energy budget are performed to help interpret this result. In addition, the emerging urban structures are found to have a mechanical effect of slowing down the climatological surface wind field over the urban area. This is likely due to the increased effective surface roughness with urbanization. The change in wind, in turn, leads to a secondary modification of the advection of temperature within the air shed of the metropolitan area which feedbacks to the change in air temperature. This finding suggests the need to combine the mechanical and thermodynamic effects in order to obtain a complete understanding of the influence of land cover on urban climate. In all cases of the simulations, it is also demonstrated that urbanization influences surface air temperature mainly only within the confine of the metropolitan area. This is broadly consistent with the consensus that urbanization has limited impact on the global mean surface air temperature over the vast non-urban areas.