Heavy Rainfall Predictability : the THORPEX West Africa case study
Abstract:
The African continent is afflicted by high-impact weather and climate extreme events with devastating consequences for local communities. A key objective of THORPEX is to understand what determines the predictability of these events, owing to case studies.The selected West African case concerns a wet spell that crossed the whole Sahel during the last decade of August and first decade of September 2009, marked with heavy precipitation leading to floods in many Sahelian countries. The event that occurred in Burkina Faso on September 1st has been the most extreme. In ten hours, Ouagadougou recorded 261 mm leading 150 000 affected people with 9 deaths and loss of properties and damaged roads. This wet spell was marked by back to back passage of deep African easterly waves embedding many mesoscale convective systems (MCS), what caused flooding in several locations. The MCS that caused flash-flooding at Ouagadougou was embedded within a well pronounced “breaking” easterly wave.
This extreme event has been analyzed following different complementary approaches. First a forecaster exercise has been replayed using the WASA/F forecasting method, to assess the degree of anticipation that forecasters can reach with the present numerical weather prediction (NWP) products and their experience. This step allowed a better understanding of this extreme event and to propose a conceptual model. Then we analyzed the skill of NWP global models, showing their ability to predict this event with a two-day lead time. A further modeling study at high resolution (5 km) with the French AROME operational model allowed a better forecast of this event both in term of intensity and location, up to three days in advance. Sensitivity tests and application of different diagnostics have been developed to assess the predictability and to better understand the occurrence of such extreme events over West Africa associated with a moist mesovortex on the southern flank of the African Easterly Jet, and forced both by midlatitude Rossby waves and equatorial waves.