Climatological Changes in Rainfall due to Urbanization

A two-dimensional, theoretical model based on the fluid dynamical equations is developed to determine the effects of urbanization on rainfall. The model is applied to a case where the urban area is located between the sea and the rural area. The model incorporates explicitly the formation of cloud and rain and its subsequent time and space variations. This application is made because there are future plans of using the model to determine the effects of Metro
Manila urbanization on rainfall.

A control simulation corresponding to a case with no urban area is first done to serve as benchmark for determining the effects of urbanization on rainfall. This is followed by three additional simulations in which the size of the urban area is varied from 10, 20, to 30 km. The results of these simulations are compared to that of the control simulation. Characteristics of the simulation which are compared are time and location of occurrence, rainfall intensity, and accumulated rainfall.

The results show that there are important effects of urbanization on rainfall. The primary conclusion is that the size of the urban area is important in determining the time and location of the occurrence of rainfall. The largest urban area generates the earliest vertical circulation associated with the subsequent cloud formation. This, in turn, results in an earlier occurrence of cloud and rainfall. Also, the simulations show that most of the rains occur over the rural areas, located downwind of the urban area. Another notable effect seems to be an increase in the accumulated rainfall with an increase in the size of the urban area. However, this effect is quite complicated. The dependence of rainfall on the size of the urban area is non-linear.