Impact of urban growth-driven landuse change on microclimate and extreme precipitation — A sensitivity study

More than half of the humanity lives in cities and many cities are growing in size at a phenomenal rate. Urbanisation-driven landuse change influences the local hydrometeorological processes, changes the urban micro-climate and sometimes affects the precipitation significantly. Understanding the feedback of urbanisation driven micro-climatic changes on the rainfall process is a timely challenge. In this study we attempt to investigate the impact of urban growth driven landuse change on the changes in the extreme rainfall in and around cities, by means of sensitivity studies. We conduct three sets of controlled numerical experiments using a mesoscale atmospheric model coupled with a land surface model to investigate the hypothesis that the increasing urbanisation causes a significant increase of extreme rainfall values. First we conduct an ensemble of purely idealised simulations where we show that there is a significant increase of high intensity rainfall with the increase of urban landuse. Then four selected extreme rainfall events of different tropical cities were simulated with first current level of urbanisation and then (ideally) expanded urban areas. Three out of the four cases show a significant increase of local extreme rainfall when the urban area is increased. Finally, we conducted a focused study on the city of Mumbai, India: A landscape dynamics model Dinamica-EGO was used to develop a future urban growth scenario based on past trends. The predicted future landuse changes, with current landuse as control, were used as an input to the atmospheric model. The model was integrated for four historical cases which showed that, had these events occurred with the future landuse, the extreme rainfall outcome would have been significantly more severe. An analysis of extreme rainfall showed that hourly 10-year and 50-year rainfall would increase in frequency to 3-year and 22-year respectively.