Numirical Simulation Diagnostics of a Flash Flood Event in Jeddah, Saudi Arabia

December 18, 2013

Ahmad Samman

Committee: Bill Cotton (advisor) Russ Schumacher Darrel Fontane (Civil and Environmental Engineering)

Abstract

On 26 January 2011, a severe storm hit the city of Jeddah, the second largest city in the Kingdom of Saudi Arabia. The storm resulted in a flash flood, which produced heavy rainfall in a short period of time. This event caused at least 11 fatalities and more than 114 injuries. Although the observed rainfall data are limited to the weather station at King Abdul Aziz International airport north of the city, the extreme precipitation occurred over the southern part of the city.

The Regional Atmospheric Modeling System (RAMS) developed at Colorado State University was used to study the storm event. RAMS simulations indicted that a quasi-stationary Mesoscale convective system developed over the city of Jeddah and lasted for several hours. It was the source of the huge amount of rainfall. The model computed a total rainfall of more than 110 mm in the southern part of the city, where the flash flood occurred. This precipitation estimation was confirmed by the actual observation of the weather radar. While the annual rainfall in Jeddah during the winter varies from 50 to 100 mm, the amount of the rainfall resulted from this storm event exceeded the total annual rainfall. The simulation of this event showed that warm sea surface temperature, high humidity in the lower atmosphere and a large amount of convective available potential energy (CAPE) provided a favorable environment for convection. It also showed the presence of a cyclonic system over the north and the east of the Mediterranean Sea, and a subtropical anti-cyclone over Northeastern Africa contributed to cold air advection bringing cold air to the Jeddah area. In addition, an anti-cyclone (blocking) centered over east and southeast of the Arabian Peninsula and the Arabian Sea produced a low level jet over the southern part of the Red Sea which transported large water vapor amounts over Jeddah. The simulation results showed that the main driver behind the storm was the interaction between these systems over the city of Jeddah (urban heat island) that produced strong low-level convergence. Several sensitivity experiments were carried out which showed that other variables could have contributed to storm severity. Those sensitivity experiments included several simulations in which the concentration of cloud condensation nuclei was changed, and physiographic properties were altered by removing the water surfaces, and the urban heat island environment from the model. The results of these sensitivity experiments showed significant effects on the storm formation and severity.