C2 - Prediction of wet and dry periods of the West African monsoon

Principal investigators: Prof. Dr. Andreas H. Fink, Prof. Dr. Peter Knippertz, Prof. Dr. Tilmann Gneiting

Other researchers: Eva-Maria Walz (PhD)

Former researchers: Andreas Schlüter (PhD), Peter Vogel (PhD)

Despite their potential socio-economic importance, predictions of the onset and active/break spells of the West African Monsoon (WAM) rains currently have limited skill at forecast timescales between several days up to two weeks. The central question of this project is to what extent an influence of more predictable extratropical wave activity can enhance the predictability of convective rainfall and dry spells over tropical North Africa during the April-October season.

To address this question, an improved understanding of the involved physical mechanisms and an optimal statistical treatment of ensemble forecasts are required. In the long run, an objective identification of regimes with enhanced predictability will provide useful guidance to operational forecasters in order to realize the full potential of existing ensemble prediction systems.

The investigations will be split into two periods due to different climatological background settings in the WAM region:

• the pre- (April-June) and post-monsoon (October) periods, during which episodes of upper-level westerlies and extratropical trough penetration occur; and
• the full monsoon (June-September) season, during which upper-easterlies prevail and extratropical influences are manifested more indirectly by, e.g., dry air intrusions or synoptic-scale cold surges.

In either period, the large-scale extratropical wave forcing will be assessed against wave forcing from the tropical waveguide (e.g., Kelvin and equatorial Rossby waves) and the Madden Julian Oscillation (MJO).

The scientific analysis will be based on observational data from field campaigns, conventional networks, and satellites on one hand and on reforecasts from the ensemble prediction system of the European Centre for Medium-Range Weather Forecasts on the other hand. During the pre- and post-monsoon seasons, the effect of upper-troughs on the North African heat low will be investigated through, e.g., a modified version of the pressure tendency equation. Other diagnostic tools include moisture fluxes, shear and convective instability measures, as well as methods to identify the phase of convectively coupled equatorial waves and the MJO.

The equatorial wave phases and degree of extratropical wave forcing will be used to define four regime indices that allow the identification of conditional predictability, which can be applied to operational ensemble products in a postprocessing framework. This will be achieved by using ramifications of ensemble model output statistics, Bayesian model averaging, and ensemble copula coupling.

Novel aspects and particular challenges of this project lie in the identification of regimes and in the application of postprocessing techniques in tropical rather than midlatitude environments.