Study of the occurrence of rain in the Tunis area in a mono-fractal framework

Abstract

International audienceLess study concern the scale variability of the rainfall field in North Africa (Mediterranean climate), where the climate differ significantly from the Sahelian Africa (monsoon climate). This paper undertakes a study of the occurrence of rain in the region of Tunis in a mono-fractal framework. The box-counting method is applied to four series of observations of a continuous period of two and a half years, based on a minimum resolution of 5 min and belonging to the semi-arid bioclimatic stage. These series are characterized by strong intermittency. Using the sensor detection threshold, two self-similar structures were detected: micro-scale (5min- 2 days) with fractal dimension 0.44 and a synoptic-scale (one week - eight months) with fractal dimension of 0.9. This last value is probably overestimated by the presence of the saturation of the available space by rain (fractal dimension equal to one) for period longer than eight month. Due to the length of the dry period observed, the length of this saturation period differs from other studies performed in other area. Between the two self-similar structures a transitional regime corresponding to a meso-scale (2 days- one week) could be distinguished. The increase of the threshold would allow to 'filter' the frontal structure so as to keep only the convective structures, a sub micro-scale structure (5mn - 1h 20) has been detected with 0.3 mm/5mn intensity threshold. These results may reflect the influence of two distinctive types of convective showers and original front controlling these series