The 9-10 November, 2001: algerian Flood. A Polar Low?

This paper presents the preliminary results of a study of the development of a devastating cyclonic storm that struck the Algerian coast on 9-10 November, 2001. The study uses a cloud resolving model to determine the processes leading to the heavy precipitation. Results suggest that the precipitation was not strongly orographic in character, but instead resulted from an unusually strong cyclone that developed just north of Algiers in the Mediterranean. The storm development was associated with a strong tropopause fold associated with the southward movement of a major trough in the Eastern Atlantic. The intense cyclone resulted when the tropopause fold moved around the base of the trough and interacted with a deep mixed layer moving north off the Sahara desert. Results suggest that the fold, viewed as a lobe of high potential vorticity (PV) air, broke off from its stratospheric origin forming a vorticity filament that then balled up into a strong vortex. At the surface the process was manifested as the warm occlusion of a developing cyclone and the subsequent isolation of a warm core vortex. This process was reminiscent of the classic development of a polar low. Similarities include: (a) The pre-existence of a major trough, (b) the occlusion of the frontal cyclone, (c) the isolation of the warm core low west of the frontal fracture, and (d) the growth of the warm core vortex. As it is recognized that the E-W oriented boundary of the ice shelf is important to the polar low, an analogous situation exists in the southern Mediterranean with the ice boundary is replaced by the southern Mediterranean coast and the Sahara desert to the south.