Synoptic and Numerical Studies for Simulating Weather Instabilities that Affect The Kingdom of Saudi Arabia in Winter: I. The Synoptic Case Study@@@دراسة سينوبتيكية وعددية بغرض محاكاة عدم استقرار جوي يؤثر على أجواء المملكة العربية السعودية

A study of a weather phenomenon was carried out, where inter- action between middle latitude and tropical waves had occurred. The goal of this study was to simulate the interaction using a numerical model. The phe- nomenon occurred under winter synoptic regime and could lead to wide spread showers over a band that cover most of the kingdom. A detailed synoptic de- scription of a case, where the interaction had occurred over west Africa, was discussed. It was shown that the invasion of cold air to the south of 30oN was a necessary trigger action for such interaction. In this case a south wind com- ponent was developed due to air mass thermal differences. The developed wind component caused a change in the direction of the subtropical jet from westerly to south-west. This caused northerly advection of moist air from the Inter Tropical Convergence Zone (ITCZ) where deep convective cells existed. When a polar frontal system passed through the subtropical region and came in phase with the axis of the advected moisture tongue, rapid development of the system had occurred. In this case an active cloud band was formed and ex- tended from the tropics to the middle latitude wave.