Rapid Modification of Land Surface Temperature during Rainfall

Ground heat storage and the earth surface temperature have important impacts on many atmosphere characteristics such as earth’s surface energy budget, the static stability of and turbulence intensity in the atmospheric boundary layer, and the development of phenomena such as urban heat islands and sea breezes. Because of the rapid growth in urbanization over the past few decades, the average surface temperature in big cities is increasingly higher than rural areas, also implying a larger heat storage in the urban fabric.  When rain falls on a hot urban surface, it cools down the surface very quickly. The fast initiation of the runoff on the impervious urban surface, which will advect heat away, is one of the key factors in this cooling process (since the rain temperature is expected to be less than the surface temperature). Therefore, the thermal energy of the runoff increases, and when the runoff eventually merges with the streams, it induces intensive thermal pollution that can severely impact the ecology and health of these streams. Furthermore, the rapid cooling induces a rapid change in atmospheric stability and influences the development of the storm, but these changes are not currently captured in models. Hence, understanding how the advective, radiative, and conductive processes of heat transfer during a precipitation event helps us to predict how the surface and runoff temperature change during rainfall. These processes are poorly studied due to the difficulties in capturing all the important physical processes and parameters in the experiments and models.

In this study, a model is first developed to study dynamics and heat budgets of a runoff layer during a rainfall event. A traditional kinematic wave approach is used to model the runoff dynamics, and then the solution is coupled with the runoff and subsurface heat budget equations, which are solved numerically. Next, the proposed model is validated using data form a series of experiments conducted on the Arizona State University campus over different pavements (e.g. impervious and pervious asphalt and concrete). In these experimental campaigns, artificial rain is generated and a suite of novel sensors are developed specifically to measure the key variables needed to understand the physics for validation of the model. Using the verified model, we investigate how the various hydrological and thermal properties of the pavements, as well as ambient environmental conditions, modulate the surface heat storage and runoff temperatures.