ABSTRACT25 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 UniversityNational 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 mm in some localities in the southern part of Jeddah city and to the north of Jeddah in Thuwal city. The limited available observed rainfall totals, at King Abdul Aziz International Airport and 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 warm Red Sea surface temperatures and high humidity in the low levels of the troposphere. A stationary anticyclone centered over the southeast of the Arabian Peninsula concentrated 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.

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Different configurations of the Weather and Research Forecasting (WRF-ARW) regional climate model, centered over the Eastern Nile Basin, have been investigated. Extensive sensitivity analyses were carried out to test the model performance in simulating precipitation and surface air temperature, focusing on the horizontal extent of the simulation domain, the mesh size and the parameterizations of the boundary layer, radiation, cloud microphysics, and convection. A simulation period of 2 years (1998–1999) was used to assess the model performance during the rainy season (June–September) and the dry season (December–March). Three sets of numerical experiments were conducted. The first tested the effects of changing the horizontal extent of the simulation domain; three domains have been examined to investigate, e.g., the effect of including a larger part of the Indian Ocean, for which no significant impact was found. The second set of experiments tested the sensitivity of WRF to the horizontal mesh size (about 16, 12, and 10 km). It was found that increased resolution results in a more accurate simulation of precipitation and surface temperature. The third set of experiments was designed to select the optimal combination of physics parameterizations. All simulations were forced by ERA-Interim reanalysis data to provide initial and boundary conditions, including sea surface temperature, and the Noah land surface model (NPAH) was used to simulate land surface processes. To rate the model performance, we used a range of statistical metrics, summarized with a scoring technique to obtain a single index that ranks different alternatives. The simulated precipitation was found to be much more sensitive to the choice of physics parameterization compared to the surface air temperature. Precipitation was most sensitive to changing the cumulus and the planetary boundary layer schemes, and least sensitive to changing the microphysics scheme. Modifying the long-wave radiation scheme led to more significant changes compared to the short-wave radiation scheme

st century resulting in deficit in water availability for the rainfed agriculture. Deficit periods in which potential evapotranspiration exceeds precipitation will be extended to 10 months at some parts in the country instead of 4 months at present.

Tropical cyclone Phet is the second strongest tropical cyclone ever recorded in the Arabian Sea. Phet made landfall in the northeast mountainous area of Oman in early morning on 4 June in 2010, causing a breaking record rainfall in this arid region of 488 mm/48 h. The cyclone heavy rainfall triggered flash floods causing enormous losses in lives and infrastructure in northeast Oman. The state of the art Advanced Research WRF model is used to study the atmospheric circulation and to reproduce the heavy rainfall over Oman. Three one-way nested domains with 32 vertical layers with terrain following sigma coordinate are used to setup eight numerical experiments aiming to investigate the effect of initialization time, horizontal grid resolution and terrain elevations on reproducing the cyclone track, intensity and heavy rainfall. Simulation results show negligible effect of model initialization time on cyclone track, intensity and rainfall. In contrast, the orographic effect played a substantial role in rainfall simulation over northeast Oman. The heavy rainfall was a combination of the cyclone circulation effect and the orographic lifting in the mountains. The northeasterly cyclone moist-warm wind was lifted in the Omani mountains releasing its potential energy and enhancing further thermal convection. The numerical experiment with the highest terrain elevation (RUN3.3-C) resulted in overestimation of observed rainfall due to the enhanced topographic lifting of the saturated cyclone wind. Experiment with similar horizontal grid resolution but smoother terrain elevation (RUN3.3-TER) resulted in much less rainfall amount comparable to the observed values. The increased precipitation in RUN3.3-C is due to the increase in the rainwater and cloud water and graupel of the explicit moisture scheme.