Episodes of heavy precipitations in Tunisia
Flash floods of September 2009 in Redeyef
Mohamed HAJJEJ, Directeur de la production ( )
National Institute of Meteorology, Tunisia
In Tunisia, September 2009 was marked by weather conditions often favourable to developments of intense thunderstorms over much part of the country especially during the days of 22, 23 and 24. Indeed, the annual change of the subtropical belt and intertropical low pressures are causing a rise in the intertropical convergence zone northward during this period of the year and an abnormal upper air potential velocity associated with divergent winds and a significant upward-divergent was reported on Central and West Africa during August 2009. So, a Southern regime are installed and has promoted masses of hot air and unstable to the Saharan countries and consequently our region. On the other hand, the low pressure system in mid-latitudes associated with a cold air unstable and dynamically active has moved slowly towards the Mediterranean West and North Africa and interests Tunisia.
Therefore, the encounter between the two kinds of unstable air masses has created local development and very intense storms with torrential rains interesting first the south and later the centre and the north of the country. Significant amounts of rain were collected during a very short time (204 mm in Mahdia and 150 mm in Redeyef) and caused flash floods particularly in Redeyef and a brief synoptic study of this situation will be presented.
It should be noted that: in Tunisia many episodes of heavy rainfall causing floods were observed during the twentieth century (1902, 1931, 1959, 1962, 1964, 1969, 1974, 1982 and 1990) and the frequency of those episodes was significantly increased at the beginning of this century (2003, 2006, 2007 and 2009) and more than half of recorded floods have occurred during the months of September and October. Understand of those situations causing meteorological extreme events requires more cooperation and studies to focalize efforts and participate more in different WMO research programs particularly in weather forecasting and environmental atmospheric.