Regional climate model simulation of the West African Monsoon precipitation during Abstract This study presents the ability of the Hadley Centre regional climate model HadRM3P to simulate the West African monsoon precipitation. The model is evaluated with 1978– 1990 continuous baseline integration, at 50 km spatial resolution, driven by the European Centre for Medium- Range Weather Forecasts reanalysis ERA-15. The causes for major model biases (differences from observations) are examined through a supplementary experiment where HadRM3P is nested into the global climate model HadAM3P. The results are compared against both the surface observations of the climate research unit (CRU) and the driving atmospheric conditions. HadRM3P demonstrates pronounced rainfall downscaling skills in complex costal and orographic locations. The seasonal cycle of precipitation is simulated realistically, in space and time. The rain installation phase in the south, the high rain phase in the north, and the retreat of rain southward are well reproduced. On the other hand, HadRM3P shows some biases common to the driving model in this region. This suggests some deficiencies in the model’s representation of rainfall processes over West Africa. 1. Motivation West Africa is a vulnerable region where the scientific community is facing challenges in estimating the timing and the magnitude of climate changes, and their environmental and socioeconomic impacts. The interactions between continental surfaces and atmospheric forcing are crucial drivers for it climate, and the variability of rainfall is a key issue. However, the global climate models (GCMs) which traditionally provide useful climate projections at continental scale of several thousand kilometres, lack the regional scale details needed for adequate predictions of precipitation. Atmosphere regional climate models (RCMs) have shown promising performances in reproducing observed regional surface climate characteristics for many tropical regions, but to date have not been widely used over West Africa. 3. Experimental setup 3.1 Regional climate model  Horizontal resolution: 0.44° x 0.44° (~50 km) grid spacing  Vertical resolution: 19 hybrid levels, from ~50 m to 0.5 hPa  Time step: 5 minutes  Duration: 12 years, 1 December 1978 to 31 December 1990  Spinup time: two years  Experiment 1: HadRM3P driven by ERA-15 reanalysis and observed sea-surface temperatures (SSTs)  Experiment 2: HadRM3P driven by HadAM3P, both driven by observed SSTs 3.2 Observational data The data used for the validation of the RCM are mainly extracted from the Climatic Research Unit (CRU) analyses and the IRD/ASECNA/CIEH where available. Additionally, ECMWF ERA40 reanalysis are also used for the validation of the atmospheric circulation in the interior domain. 4. South westerly monsoon flow Figure 2 illustrates the seasonal excursion of the horizontal wind at 925-hPa averaged over March-May (MAM), June-August (JJA) and September-October (SON) in both the reanalysis and RCM. The black solid line represents the zero isoline of the zonal wind component, so as to delineate the south-westerly monsoon (SWM) flow and detect the location of the inter-tropical front (ITF). The RCM captures reasonably well the location of the confluence along the ITF of the moist SWM wind and the north-easterly dry wind (Harmattan). The RCM simulated SWM flow is also consistent with the results of HadAM3P GCM (not shown). Figure 3 depicts the vertical cross-section of the mean JJA air temperature and specific humidity, associated with the SWM, for the reanalysis and the RCM. The thermodynamic vertical structure of the simulated atmosphere is similar to the observations. The main discrepancies are the northward displacement of the surface maximum of moisture and the overestimation of its magnitude. Sahel (most prominent in MAM) and a dry bias over the highlands of Cameroon and Guinea. Table 1 summarises the seasonal means, biases, and standard correlations of simulated precipitations over the land points. Although the RCM overestimates rainfall, the bias in JJA and SON is less than 10%. The main deficiencies of the RCM are present and exacerbated in HadAM3P (not shown) implying they are in part locally forced due to deficiencies in the model formulation. 5.2 Interannual variability The skills of the RCM at simulating the interannual variability of rainfall is illustrated through the computation of seasonal (JJA) rainfall indices over the Sudan and Guinea coast regions (Fig. 5). The RCM captures the main features of the interannual variability in the Sudano-Sahelian and Guinean coast regions including reasonable simulations of the extreme years 1982, 1984 and The first two of these years are markedly dry, whereas 1988 is the wettest observed year over the Sudan. As expected, the RCM performs better than the GCM, as the latter is only constrained by observed SSTs. 5.3 Intraseasonal variability The performances of HadRM3P for simulating the intraseasonal variability of West African rainfall are highlighted by the 1988 time- latitude diagram of daily precipitation averaged over 10° W–10° E, where a meridional land-sea contrast exists (Fig. 6). Although the RCM simulates the monsoon preonset and onset stages, there are some important discrepancies in the timing and location of rainfall maxima. 6. Conclusions The HadRM3P RCM has demonstrated good skill in simulating the climatology of the south-westerly monsoon flow and its associated thermodynamic vertical structure of the atmosphere over West Africa. When driven by ECMWF reanalysis, HadRM3P simulates reasonably well the seasonal cycle and the interannual variability of rainfall. In comparison to its parent GCM, HadAM3P, HadRM3P simulates mean precipitation better (and the evolution of seasonal anomalies due, in part, to its use of reanalysis boundary conditions). However, these two models share some common biases over the Sahel region. 5. Precipitation 5.1 Seasonal cycle Figure 4 shows the mean climatological spatial distribution of seasonal rainfall simulated and observed. The RCM reproduces the northward and the southward movement the band of high rainfall (i.e. > 5 mm/day) associated with the ITCZ, from March to October. The main deficiencies of the model include a wet bias over the eastern part of the SeasonAverageBias (RCM – CRU)Correlation MAM JJA SON Figure. 1: Model domain including the relaxation zone Figure 2: Climatology of seasonally averaged 925-hPa wind fields in HadRM3P(bottom) and ERA40 (top) Figure 3: Mean JJA vertical cross section of temperature (top) and humidity (bottom), averaged from 20° W–15° E Table 1: 1981–1990 basic statistics of simulated rainfall over land points Figure 5: Time-series of JAS rainfall anomalies for CRU dataset (blue), HadRM3P (white) and HadAM3P (red) Figure 4: Observed and simulated mean MAM and JJA precipitation References: Le Barbé, L., T. Lebel, and D. Tapsoba, 2002: variability in West Africa during the years J. Climate, 15, Gallé, H., W. Moufouma-Okia, P. Bechtold, O. Brasseur, I. Dupays, P. Marbaix, C. Messager, R. Ramel, and T. Lebel, 2004: A high-resolution simulation of a West African rainy season using a regional climate model. J. Geophys. Res., 109, D05108, doi: /2003JD Jones, R.G., Noguer, M., Hassell, D.C., Hudson, D., Wilson, S.S., Jenkins, G.J., Mitchell, J.F.B.: 2004, ‘Generating high resolution climate change scenarios using PRECIS’, Met Office Hadley Centre, Exeter, UK/UNDP, New York, USA: 35pp. Sultan, B. and S. Janicot, 2003: The West African Monsoon dynamics. Part II: The preonset and the onset of the summer monsoon. J. Climate,16, Rowell, D., C.K. Folland, K. Maskell and M.N. Ward, 1995:Variability of summer rainfall over tropical north Africa ( ): Observation and modelling. Q. J. R. Meteorol. Soc., 121, over West Africa Figure 6: 1988 Time-latitude diagrams of daily rainfall, filtered to remove variability lower than 10 days IRD data HadRM3P Wilfran Moufouma-Okia, David Hassell and David Hein Hadley Centre for Climate Prediction and Research © Crown copyright /0356 Met Office and the Met Office logo are registered trademarks Met Office Hadley Centre FitzRoy Road Exeter Devon EX1 3PB United Kingdom Tel: Fax: