Urbanization Leads to Changes in the Rainfall Timing: Attribution Experiments Using WRF and Urban Sprawl in Hyderabad, India

While the spatial changes in the rainfall characteristics upwind/downwind of cities has been well studied and established, here we show that urbanization can also lead to temporal changes in the rainfall in and around urban regions. Taking the case of a protypical rapidly growing city in Asia – Hyderabad in southern India, we show that the observed climatological shift in the heavy rains from afternoon to evenings can be explained as an urbanization signature.

Rapid urbanization and the associated Land Use Land Cover Changes (LULCC) affect the partition of surface energy fluxes. For an approaching storm system, the environment from rural to urban surfaces is influenced by differential latent and sensible heat fluxes and regions of heterogeneous surface energetics. Anthropogenic heat from the urban areas also contributes to the surface sensible heating and the regional aerosols. As a result, cities have altered local wind flow and circulation patterns in response to urban heat island effect. A major implication of this mesoscale modification is the possible impact on the urban hydrological cycle.

Changes in precipitation is a direct feature of this urbanization-based hydroclimatic changes. A growing body of literature suggests an increase in rainfall maxima typically downwind of urban areas. In this study, we investigate the impact of urbanization of the city of Hyderabad in India using a mesoscale numerical weather prediction model (WRF-ARW). LANDSAT satellite imagery spanning three decades are used to create LULC maps of the city’s urban footprint. These urban landcovers were provided as an input for a representative thunderstorm event that passed over the city. Ensemble members compare the effects of city footprint and anthropogenic heating individually and also when coupled together. Results of WRF model simulations indicate that urbanization indeed is affecting the temporal pattern of precipitation over the city by delaying the occurrence of the rainfall event (by about two hours) as the urbanization and anthropogenic heating impact increases. Spatial characteristics of rainfall are found to be varying with the changes in urbanization with intensification of precipitation upwind and in some instances downwind and storm dissipation as the system passes over the city. Interestingly, there was also some impact on the precipitation intensity, which seemed to be decreasing with increasing urbanization. The findings add one more important dimension to the possible impact of urbanization leading to delayed rains from afternoon to evening/night. The implications of these hydroclimatic changes if seen globally are profound and will be a subject of a follow up study. It is also recommended that urbanization impact studies using numerical models should be done within a probabilistic rather than a deterministic framework involving various ensembles to get a more comprehensive understanding of the uncertainty involved with the results.