Heat, Air and Water: How Cities Create Their Own Hydroclimates

Abstract

Metropolitan areas are usually hotter than their surrounding rural areas due to human activities and land-use change. This phenomenon, called the Urban Heat Island (UHI), significantly alters air and surface temperature in cities and subsequently changes the dynamics of the Atmospheric Boundary Layer (ABL). However, during rainfall, hot urban surfaces cool down rapidly due to runoff advection and infiltration processes. In this dissertation, we develop a complete prognostic model that can predict atmospheric energy fluxes and the temperature of ground surfaces and runoff during rainfall. In Chapter 2, details of the model for a laminar runoff, and experimental campaigns for model validation, are discussed. In Chapter 3, we apply the model to study the physical basis and important parameters of pavement-runoff heat transfer and to develop a reduced version that is implementable in large-scale weather models. The results of Chapter 3 underline the importance of pavement albedo and rain temperature in predicting runoff and surface temperatures and heat fluxes. In Chapter 4, an extended model is developed that can be used for a broader range of rain events, even extreme events where the runoff becomes turbulent and where the penetration of raindrops into the runoff significantly alters flow dynamics and heat transfer. In Chapter 5, we shift from surface temperature to air temperature UHI impacts, and study the large-scale circulations induced by urban heat on the ABL using large eddy simulations. The results indicate that circulations are controlled by two non-dimensional parameters: (1) the ratio of convective velocities associated with urban and rural surface heat fluxes, and (2) the ratio of bulk inflow velocity and rural convective velocity. Depending on the ratio of these two parameters, city-circulations fall into three regimes: (i) a bubble regime where the ABL is mainly driven by convection, (ii) a plume regime where inflow advection is the dominant process in the ABL, and (iii) a transition regime where mixed convection is the main process. In Chapter 6, we introduce a new method for measuring water depth that we devised for runoff experiments. Finally, in Chapter 7, the synthesis and future directions are discussed.