Heavy Rainfall Predictability : the THORPEX West Africa case study

The African continent is afflicted by high-impact weather and climate extreme events with devastating consequences for local communities. Lives and property losses, damage to infrastructure (roads, bridges, and buildings) and resources (energy and water supply, health and emergency facilities) have been reported over the years because of high impact weather.

A key question is to understand what determines the predictability of these events, both for individual cases and statistically. In the framework of THORPEX (THe Observing Research and Predictability Experiment) www.wmo.int/thorpex, THORPEX-Africa regional committee have selected few sub-regional based case studies related to flooding events to study the predictability of high impact weather. To achieve this task, three steps have been defined: i) description and conceptual of the case study, ii) verification of forecast issued and assessment of model ability to predict such event , iii) modeling studies to further study the predictability of such event.

The West African case concerns a wet spell that crossed the whole African Sahel during the last decade of August and first decade of September 2009, marked with heavy precipitation leading to floods in many Sahelian countries (Niger, Senegal, Mauritania, Gambia, etc). The event that occurred in Burkina Faso on September 1st has been the most extreme. In ten hours, Ouagadougou, the capital city, recorded 261 mm from a quasi–stationary mesoscale convective system (MCS) leading 150 000 affected people (on 1.5 million people) with 9 death and loss of properties and damaged roads.

At seasonal timescale the SST pattern over the Atlantic as well as predicted Nino condition in the Pacific indicated wet over the Gulf of Guinea and dry over western Sahel. However, at intraseasonal timescale, in August, there was a reversal of the Atlantic dipole that favored further northward monsoon penetration and low-level westerlies enhancement. Then the Sahel experienced a wet spell during last part of August and first part of September. This wet spell was marked by back to back passage of deep synoptic AEWs embedding many MCSs, what caused flooding in several locations. The MCS that caused flash-flooding at Ouagadougou on September 1st was embedded to 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 and the Synergie forecasting system, to assess the degree of anticipation that forecasters can reach with the present 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 (4 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 (tracking, budgets...) are underway 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 Rossby midlatitude waves and equatorial waves.