Changes in Rainfall Patterns Due to the Growth of an Urban Area

The changes in rainfall patterns, which evolve during the growth in size of a tropical urban area, are studied by means of model simulations. The urban area is located along a coastline. The
simulated rainfall is produced by mesoscale disturbances during the rainy season when the prevailing wind is weak and when the sky is not cloudy. These disturbances are generated by differential heating associated with differences in the thermal properties of the sea, urban and rural surfaces. Simulations of the rainfall during a 24-hour period are made for different sizes of the urban area corresponding to different stages in the growth of urban area. One of the simulations corresponds to a condition before the beginning of the urbanization (urban area is zero). Two sets of simulations are made. One set of simulations is made for flat terrain and another for a terrain with a mountain in the rural area. The results are then analyzed in order to determine the time evolution of the rainfall patterns associated with the growing urban area.

The results of the simulations for flat terrain indicate that the accumulated rainfall occurs in patterns of well-defined zones or bands. The horizontal profile of the accumulated rainfall in these
bands is approximately gaussian in shape, in the case of relatively small urban areas. For large areas, the shape tends to be bimodal. For relatively small urban areas, the total accumulated rainfall remains approximately the same for different sizes of the urban area. The corresponding total for large urban areas is greater due to the fact that the rainfall bands are wider. With regard to the time of occurrence, urbanization results in an earlier occurrence of rain, it also results in an earlier ending.

The simulations for cases with a mountain show that the generation of a mesoscale disturbance due to upslope motions during the daytime inhibits the development of mesoscale disturbances due to the existence of urban areas. Almost all of the rainfall occurs over the mountain and neighboring areas.