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Archive

3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Analysis of the West African Monsoon water cycle with the help of Numerical Weather Prediction models and observational products from AMMA.


Olivier Bock (IGN/LAREG); O. Bock, R. Meynadier, S. Gervois, F. Guichard, A. Boone and J.-L. Redelsperger

This study investigates the regional-scale atmospheric water cycle over West Africa in the framework of the AMMA programme. It aims at characterizing the scales of variability (from seasonal to diurnal timescales) and inter-relations of atmospheric water budget terms and the underlying processes. The surface moisture budget is first examined using elaborate products TRMM 3B42v6 satellite precipitation (P) estimates and evapo-transpiration (E) estimates produced by land surface models (ALMIP project). These products allow for a comprehensive description of the water budget in terms of E-P. When combined with precipitable water (PW) tendency from Numerical Weather Prediction (NWP) models, the vertically integrated moisture flux divergence (MFD) can be derived as a residual from the water budget equation. Hence, all four terms of the budget can be analysed. We focus here on the seasonal and intra-seasonal timescales during the AMMA Extended Observing Period (2005 to 2007).
NWP models provide also their own complete description of the water cycle but with limitations due to deficiencies in model physics and inconsistencies introduced by the assimilation process. We inter-compared the water budgets from ECMWF-IFS operational analysis and forecasts, NCEP/NCAR reanalysis I and NCEP/DOE reanalysis II. Precipitation and evapo-transpiration simulated by the NWP models are assessed using the elaborate products. Differences are highlighted such as a too southerly ITCZ, i.e. an underestimation of P in the Sahel, and a surprisingly too strong E in the same region (mostly during the dry season) indicating poor coupling between these too parameters in the models. These deficiencies are though to be linked with limitations in the model parameterizations and impact of data assimilation.
Moisture flux divergence terms are also computed from NWP model analyses which are thought of higher quality than forecasts. They are evaluated with the help of the hybrid MFD estimates. However, large uncertainties in MFD are observed too, revealing namely differences in the representation of the West African Monsoon between the NWP models and errors linked with the computation of MFD (due to coarse horizontal, vertical and time sampling resolution). Using model fields at higher spatial and temporal resolution from the special AMMA reanalysis provides more consistent results which are presently analysed.
Finally, the GPS PW estimates available from the special AMMA GPS stations are also used in this study. They are combined with the above-mentioned E and P products to derive a hybrid MFD estimate with 3-hourly resolution. This dataset allows studying the water cycle of individual rain events at the level of the diurnal cycle. A composite analysis reveals coherent features over all sites, namely a proper time phasing between moisture storage and depletion and strong moisture flux convergence associated with the peak in precipitation. This hybrid dataset is also used to assess time sampling issues in the NWP model derived water budget.
This work conducted over West Africa could be extended to the North of the continent in the framework of HYMEX and the methodology could be more generally used for the study of the atmospheric component of the hydrological cycle over continental surfaces.