Abstract
25 November 2009 is an unforgettable day for the people in Jeddah, the second largest city in the Kingdom of Saudi Arabia (KSA). On that day, Jeddah turned into a disaster zone following a short heavy rainfall event that triggered flash floods leaving 122 fatalities and considerable losses. Numerical experiments using the Pennsylvania State University-National Center for Atmospheric research mesoscale meteorological model (MM5) have been performed to investigate the event. It was caused by a short quasi-stationary mesoscale convective system that developed over Jeddah and lasted for about 8 hours. Rainfall totals computed by the model exceeded 400 mmin some localities in the southern part of Jeddah city and to the north of Jeddah in Thuwal city. The limited available observed rainfall totals, atKingAbdulAzizInternationalAirportand wadiQaws rain gauges, and Jeddah’s weather radar observations corroborates the ability of the model to reproduce the spatial and temporal characteristics of the rainfall event. A synoptic environment characterized by warmRed Seasurface temperatures and high humidity in the low levels of the troposphere. A stationary anticyclone centered over the southeast of theArabian Peninsulaconcentrated the water vapour flow to a narrow passage over Jeddah. Simulation results suggested that the development of a mesolow by latent heat release, as well as cyclogenesis induced by Al Hejaz escarpments, could have played an important role in enhancing the event by providing low-level convergence and enhanced upslope winds, and upper level atmospheric instability.
Highlights
In less than fourteen months, the city of Jeddah in Kingdom of Saudi Arabia (KSA) went under water three times
This paper presented a mesoscale numerical study of November 25 in 2009 exceptional heavy rainfall event that triggered catastrophic flash floods in Jeddah city, Saudi Arabia
Special emphasis on the aspects of synoptic and mesoscale ingredients that could explain the reasons for the stationarity and the high efficiency of the mesoscale event were presented
Summary
In less than fourteen months (from November 2009 to January 2011), the city of Jeddah in Kingdom of Saudi Arabia (KSA) went under water three times. In the western KSA, the interplay of typical cold-season synoptic conditions with orographic and thermal forcing produces significant convection along the coast and the mountain range In this context, serious damage can occur when intense convective rainfall events combine with short hydrological response times, characteristics of steep streams and increasing urbanisation rates in coastal areas [2]. MM5 proved to be a suitable tool for hydrometeorological forecasting of flash floods in Catalonia region Following this introduction, we present a brief discussion of the climate conditions in Jeddah and an overview of the synoptic environment that took place during the event.
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