Terchi, A. (Office National de la Meteorologie, Alger, Algery)
Frontal precipitation is frequently organized into bands approximately parallel to the fronts. Heavy rainfalls are often related to this type of clod structure. Several mechanisms have been proposed to explain this kind of rainband. The concept of Conditional Symmetric Instability has been advanced by Bennets and Hoskins (1979) and Emanuel (1983) to account for this band organisation. CSI occurs as the Moist Potential Vorticity in a saturated atmosphere becomes negative. This instability manifests as hellicoidal roll perturbations with axes parallel to the thermal wind. The effect of CSI is to enhance the precipitation caused by the frontogenetical forcing in concentrating the ascending motion in a narrow region. The last stage is the creation of conditional gravitational instability at mid-levels in prefered linear regions which reinforces the banded nature of the rainfall. This theory is applied in our study as an attempt to explain the heavy rainfalls of November 1993 at the origin of devastating floods in south western Algeria. The synoptic situation was characterized by the existence of a deep cyclone over the area and the slow crossing of a cold front. The study showed the presence of atmospheric conditions that support the release of CSI slightly ahead the surface cold front. After the completion of this study, softwares have been developed to make operational the detection of CSI by using the NWP products received in GRID form.