Initiation of an African Squall Line adn its interactions with an Easterly Wave

The relationship between Squall Lines (SLs) and African Easterly Waves (AEWs) is one of the key factors for understanding the West African Monsoon. Up to now these interactions have been mainly studied owing to composite analysis of observations and to idealized model studies. On the other hand SLs have been simulated using Cloud Resolving Models (CRMs) but starting from homogeneous basic state and artificial perturbation. Recent progress in modelling and computers allow us now to treat more explicitely these scale interactions.

Here as a first stage, a CRM is used to simulate the genesis and the propagation of a Squall Line observed on the 21-22 August 1992 during Hapex-Sahel Experiment in Niger. At the same time ECMWF re-analys exhibits a well develloped AEW propagating westward. Simulations are initialized from ECMWF re-analysis (on a domain of 4500 km x 36000 km) and fine scale orography resolution. First, to allow a better boundary layer, we perform explicit simulations starting at sunrise with a resolution of 5 km over a domain of 450 km x 900 km. Second, we use these simulations to force and initialize simulations with finer resolution (2.5 km) over a domain of 360 km x 480 km. We succeed in simulating explicitly the formation of the SL, starting only with large scale fields, without any initial artifices. Trigger area, structure and intensities found are closed to satellite imagery and ground stations measurements. Detailed diagnoses are used to explore the mechanisms of triggering. The second phase will consist in simulating the entire lifecyle of the SL and to study interactions between the SL and the AEW by nested models.