Numerical simulation diagnostic of a flash flood event in Jeddah, Saudi Arabia

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 that produced heavy rainfall in a short time period. This event caused at least 11 fatalities and more than 114 injured. 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 indicated that a stationary mesoscale convective system developed over the city of Jeddah and lasted for about 5 hours. It was the source of the huge amount of rainfall. The model generated a precipitation peak 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 (110mm). The amount of rainfall resultant from this storm event exceeded the total annual rainfall (typically between 50 and100mm). The simulation of this event showed that warm sea surface temperature, high humidity in the lower atmosphere, and a high convective available potential energy (CAPE) provided a very good environment for convection. It also showed the presence of a cyclonic system over the north and the east of the Mediterranean, and a subtropical anti-cyclone over Northeastern Africa that contributed to cold air advection bringing cold air to the Jeddah area. In addition, an anti-cyclone centered over the southeast of the Arabian Peninsula and the Arabian Sea produced a low-level jet over the southern part of the Red Sea and transported high water vapor amounts over Jeddah. Results showed that the main driver behind the storm was the interaction between these systems and Jeddah urban heat island, producing strong low-level convergence. The model results also showed that other variables would have contributed to storm severity. Those variables include but are not limited to the terrain properties, the efficiency of the low-level jet, and the urban environment.