Impact of Urbanization on Predicting Extreme Rainfall Events over Jeddah, Saudi Arabia

The Kingdom of Saudi Arabia is characterized by a hot and arid desert climate. Occasionally, however, extreme precipitation has led to flooding events that caused extensive damage in terms of human life and infrastructure. The 25 November 2009 and 30 December 2010 flash floods in the city Jeddah have for instance caused heavy damages in urban areas. Mesoscale convective systems associated with strong moisture convergence ahead of an upper-level trough that merged with the Red Sea Trough were the major initial features behind the occurrence of these intense rain events.

Complex geographical features around Jeddah, such as Hijaz mountains to the East of the city and a land-ocean boundary to the West, enhance low-level moisture convergence and convection activities. The mountain ranges act as a natural barrier to passing convective storms, resulting in large rainfall bursts that occur over a short time period. Rapid development of runoff due to intense rainfall flow from the foothills directly into the city causing important damages.

This study investigates the effect of incorporating an urban canopy model and urban land cover within the Jeddah simulating severe weather events with the Weather Research and Forecasting model at a convective-permitting scale (1 km grid spacing). The goal is to study the effect of urbanization on the intense precipitation production around the periphery of the Jeddah area. The urban experiment incorporates the urban canopy model and 2006 land cover classifications. The control simulation replaces the city area with its pre-settlement, natural land cover. The roles of sub-grid land cover and anthropogenic fluxes are also investigated. The changes in model simulated precipitation are largely tied to differences in sensible and latent heat fluxes that affect the height of the planetary boundary layer and atmospheric instability.