STUDY OF DYNAMIC CHARACTERISTICS OF SQUALL LINES OVER THE SAHEL AND THEIR INTERACTION WITH THE ATMOSPHERE ENVIRONMENT

In this presentation the dynamic and structural characteristics of squall lines ( SL) over west Africa using satellite data and ECMWF analysis are examined. Tracking of SL using METEOSAT IR data from 1990 to 1998 has shown three categories of squall lines : short, medium and long life ( SLS, MLS, LLS respectively ). Space time evolution of these cloud clusters using their center of mass and size led to a determination of their genesis/decay locations, their trajectories and associated speed. Genesis of SL is observed between 10° and 17°N and decay preferentially west of the genesis region. SLS displacement has a north - east to south-west orientation. The other types of squall lines move zonally and expand between 5°E and 15°W. MLS and LLS exhibited approximately the same speed, but move faster than SLS. The size of the three types of cloud clusters is particularly important during the night between 1900 and 0600 local standard time (LST), with peaks sizes between 2200 and 0500 LST. Size evolutions of squall lines as function of their life cycle, have shown that SLS reached their maximum size near the midpoint of their life cycle between 55 and 60 % of their life time. Fully mature MLS reached their maximum size before the midpoint of their life cycle between 20 and 45 % of their life time and LLS are generally largest later than the other types. Interactions between SL and their environment, particularly easterly waves are studied using winds and potential vorticity computed from ECMWF analysis.