10B.4 Processes involved in the Planetary Boundary Layer in the frame of the West African Monsoon Marie Lothon * (1) , Guylaine Canut (1) , Fr ´ ed´ erique Sa¨ ıd (1) , Franc ¸oise Guichard (2) , Fleur Couvreux (2) , Fabienne Lohou (1) and Bernard Campistron (1) (1) Universit´ e de Toulouse, Laboratoire d’A´ erologie - CNRS UMR 5560, Toulouse, France (2) Centre National de Recherche M´ et´ eorologique, M´ et´ eo-France, Toulouse 1. INTRODUCTION Although generated and ruled at the global scale, the West African Monsoon shows a very striking diurnal cy- cle through most of its components (Peyrill´ e and Lafore (2007), Parker et al. (2005)) : the thermal heat low in the Saharian desert (Lavaysse et al., 2008), the Inter Tropi- cal Front (ITD)—lower interface of the moist southwest- erly monsoon flow and the dry saharian air— (Lothon et al., 2008). Even the African Easterly Jet, thermal wind at around 500 hPa produced by the temperature gradi- ent between the West African continental surface and the Atlantic Ocean to the south, shows a diurnal cycle (Lee et al., 2007) in addition to periodicities of a 3-4 days as- sociated with the African Easterly Waves (AEW) (Burpee, 1972) and larger periods. This is due to the strong role played by the surface- atmosphere interaction within the WAM system and the very large contrasts of surface from the north of the region to the south. As an interface between the surface and the free atmosphere, the planetary boundary layer (PBL) is inherently involved in the processes of the WAM diur- nal cycle. The Sahel is of special interest in the context of the WAM, because of its sensitivity in various ways— from climate, hydrology, agriculture, economy, health to sociology—, due to its location at the transition between vegetated areas to the south (Benin, Togo, Nigeria) and the desert to the north (Niger, Mali, Algeria). The diurnal oscillation of the low troposphere in this area is manifested by a low level jet in the night (Lothon et al., 2008) and a deep PBL developing during the day for most of the year. However, during the wettest months, both dry convection and nocturnal jet are weakened, and deep convection takes place instead. Depending on the position of the ITD, which shows a seasonal migration northward before the monsoon onset and southward for its retreating (Sultan et al., 2007), the wind in the lower layers is easterly (dry season) or southwesterly (wet sea- son). Here we use observations of 15 flights during the * corresponding author address: Marie Lothon, Centre de Recherches Atmosph´ eriques, 8 route de Lannemezan, 65300 Camp- istrous, France; email: lotm@aero.obs-mip.fr AMMA (African Monsoon Multidisciplinary Analysis) Spe- cial Observing Periods (SOP) that took place during the summer of 2006 (Lebel et al., 2007) to study the PBL structure in this area and context and its role in the WAM. We especially focus on the nature of the interaction be- tween the moist monsoon flow and the overlying dry Sa- harian Air Layer (SAL) through PBL processes and trans- ports, in the transition time of the monsoon setting (dry or shallow cumulus convection, troposphere moistening), and during its active phase when deep convection occurs. 2. DATASET In Fig. 1, the water vapor mixing ratio observed at the ground at Niamey (13 ◦ 29’N, 2 ◦ 10’E, 205 m a.s.l.) during the entire year 2006 shows how marked the seasons are in this area, with very dry air from November to mid April, moistening during 2 to 3 months before a quite short wet season in August and September, and a short drying tran- sition in October. During the dry season, the ITD is south to the area, and the wind at the ground is the northeast- erly wind locally called ‘Harmattan’. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan 0 5 10 15 20 WVMR gkg -1 Dry Moistening Wet Drying Dry Figure 1: (*) Night-time and (◦) daytime averaged water va- por mixing ratio measured at surface at Niamey airport in 2006. Shaded area stand for SOPs considered here. The moistening transition starts when the ITD has crossed the area in its migration northward. The wind at the ground can be either monsoon—which progressively moistens the low troposphere—or Harmattan (ususally during the day, in the first part of that period). The abrupt shift of the Intertropical Convergence Zone (ITCZ) put into