AMMA-Model Intercomparison Project ´de ´ric Hourdin ∗ Fre LMD/IPSL, UPMC Ionela Musat LMD/IPSL, UPMC Franc ¸ oise Guichard ´te ´o-France) 42 avenue Coriolis 31057 Toulouse Cedex CNRM-GAME (CNRS and Me Paolo Michele Ruti ENEA Florence Favot ´ ´ CNRM-GAME (CNRS and Meteo-France) 42 avenue Coriolis 31057 Toulouse Cedex `le Filiberti Marie-Ange Deceased Ma˝i Pham SA/IPSL, UPMC Jean-Yves Grandpeix LMD/IPSL, UPMC Pascal Marquet, Aaron Boone, Jean-Philippe Lafore, Jean-Luc Redelsperger ´te ´o-France) 42 avenue Coriolis 31057 Toulouse Cedex CNRM-GAME (CNRS and Me Alessandro Dell’Aquila ENEA Teresa Losada Doval Departamento de Geofsica y Meteorologa, UCM, Madrid, Spain Abdoul Khadre Traore LPAOSF, UCAD, Dakar, Senegal ´e Hubert Galle LGGE, CNRS, Grenoble, France A cross-section analysis provides light but relevant framework to assess the model skill in terms of seasonal and intra-seasonal variations of West African monsoon [mechanisms rainfall ]. ∗ Corresponding author address: Fr´ ed´ eric Hourdin, Laboratoire de M´ et´ eorologie Dynamique du CNRS, UPMC, Tr 45-55, 3et, BP 99, 75252 Paris Cdex 05, France E-mail:
[email protected]
ABSTRACT AMMA-MIP is a Model Intercomparison Project developed within the framework of the African Monsoon Multidisciplinary Analyses project (AMMA). It is a relatively light intercomparison and evaluation exercise of both global and regional atmospheric models, focused on the study of the seasonal and intraseasonal variations of the climate and rainfall over the Sahel. Taking advantage of the relative zonal symmetry of the West African climate, one major target of the
these time scales, the African Monsoon is also strongly influenced by global patterns of the sea surface temperature and local coupling with surface processes, but the amplitude and mechanisms of those couplings are still very uncertain. An important goal of the African Monsoon Multidisciplinary Analyses project consists in addressing the main uncertainties in atmospheric processes controling the monsoon system and to contribute to the evaluation and improvement of climate and weather forecast models in that respect. The observational strategy included both reinforcement of the operational network of surface stations and soundings on a long term basis, and an intensive field campaign during the monsoon (northern summer) season in 2006 (Redelsperger et al. 2006). Recognizing the meridional stratification of the monsoon system, a large part of the observations were focused on a latitudinal transect located at approximately 0o longitude, in particular over Benin, Niger and Mali. Three ”mesoscale sites”, corresponding to three typical climates, were equipped in order to document the land-atmosphere processes along the transect. At each site, in addition to surface measurements (soil moisture, vegetation, crops, ... ) and additional soundings, three to four flux stations were deployed which represented the main land cover types of each site. Most aircraft measurements were also collected along the same transect. The coordinated intercomparison and evaluation of global and regional atmospheric models started at the beginning of the AMMA project. This led to the creation of AMMA-MIP, a light and focused intercomparison exercise. The models, global or regional, are evaluated in terms of their skill to reproduce the mean West African climate, and in particular, the seasonal and intraseasonal variations of rainfall and associated dynamical structures. The exercise is “light” in the sense that the modeling teams were only requested to provide relevant subsets of the full model outputs. A parallel effort is also carried out for observational data sets. A series of graphics are made available through a web interface (http://amma-mip.lmd.jussieu.fr). In this sense, AMMA-MIP is comparable to exercises organized within the framework of the GEWEX CloudSystem-Study group, such as the EUROCS cross-section over the Eastern Pacific (Siebesma et al. 2004), than to the classical MIPs developed in the climate community. It is also complementary to the efforts developed in the frame of WAMME (Xue et al. 2008). The model outputs can also be downloaded in NetCdF format. The design of the AMMA-MIP is presented herein. Next, some results for two contrasting years (2000 and 2003) are shown, which were selected prior to the cam-
exercise is the documentation of a meridional cross section made of zonally averaged (10W-10E) outputs. This paper presents the motivations and design of the exercise, and it discusses preliminary results and further extensions of the project.
1. The AMMA-MIP background As emphasized in the introductory paper (Xue et al. (2008), this issue) global coupled models fail to simulate properly the West African climate (IPCC 2007; Cook and Vizy 2006). Atmospheric models forced by observed sea surface temperatures do simulate many features of the West African Monsoon. However, they fail to represent the full complexity of the monsoon as will be illustrated herein. The African Monsoon is characterized by a well defined meridional structure of surface albedo and vegetation (Fig. 1 A), with relatively weaker longitudinal variations. This structure is tightly connected to that of the mean rainfall (Fig. 1 B), with maximum rainfall occuring in the Sudanian region (8N-12N) during the northern summer. In addition, there is a sharp transition over the Sahel (12N-20N), which is a particularly sensitive region that experienced a significant drought in the late 70s and 80s. The meridional structure of the mean rainfall is itself related to the mean meridional circulation (Fig. 2), characterized by a near surface monsoon flow which brings water evaporated over the Guinea gulf over the African continent. This monsoon flow converges with a southerly dry air flow coming from Sahara at the ”intertropical discontinuity” in the region of the Saharan heat low where dry convection occurs. The return branch of this Hadley circulation at around 600 hPa is associated through the angular momentum budget and thermal wind balance with the African Easterly Jet which, in turn, transports additional moisture from the Indian ocean. The Inter Tropical Convergence Zone is positioned at around 10N, where most of the convective rainfall occurs, with a mean upward motion which reaches 200 hPa where the Tropopause Easterly Jet is located. The relative zonal symmetry of the climate means do not account for the strong longitudinal variations taking place on a daily basis. The cumulated rainfall is the result of successive convective events, which are local or organized as meso-scale convective systems or squall lines (Mathon et al. 2002). The interaction between the tropical waves and the convection plays a dominant role at both synoptic (the main convective activity typically develops ahead of or within the trough of the African Easterly Waves) and intraseasonal (very important for agriculture) time scales. At
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Fig. 1. A satellite-based image of West African surface albedo (A) (source EUMETSAT/GEM, http://wwwgem.jrc.it/stars/albedo.htm) and GPCP cumulated rainfall (B) for the year 2000 (mm). The red rectangle corresponds to the zone retained for the AMMA-CROSS section and the green rectangles to the meso-scale AMMA sites.
Fig. 2. Mean meridional circulation (stream lines) and associated mean zonal wind (m s−1 , contours). Mean JAS conditions are from the NCEP reanalyses. 3
paign. This first intercomparison also is aiding in the preparation for the next phase of this project which will include the year of the intensive field campaign experiment.
(zoom), which provides another approach to regional climate modeling. Two configurations of this zoomed version are tested at IPSL and LPAOSF.2 Two models have contributed with ensemble simulations of, respectively, 5 (LMDZ/IPSL) and 10 (UCLA/UCM) members. Two teams have also provided sensitivity experiments to model parameterized convection (LMDZ/IPSL, Tiedtke versus Emanuel) and vertical resolution (ENEA/ECHAM4, 42 versus 19 layers).
2. The AMMA-MIP status AMMA-MIP design The AMMA-MIP is made of two parts, corresponding to two types of output files, both provided at daily frequency for a full seasonal cycle. 1 The first part, AMMA-CROSS, is a latitudealtitude cross-section made of 10W-10E zonally averaged variables (red boxes in Fig. 1). AMMA-CROSS is focused on the latitudinal extent of the West African monsoon system, jumps and breaks of the monsoon rainfall, and their relation with the mean meridional circulation, penetration of the monsoon flow, strength of the Saharan heat low, surface fluxes, etc. The idea of the cross-section is inherited from the Pacific crosssection mentioned above. It also has been found to be a suitable framework for more academic investigations (e. g. Zheng and Eltahir 1998; Peyrill´e et al. 2007). In the second part, AMMA-MAPS, a subset of variables (on a few standard pressure levels for 3D fields) are provided over the region 10S-30N and 35W30E. The focus of AMMA-MAPS is on the African easterly jet and easterly waves, in conjunction with rainfall, the location of convection, surface fluxes, orography, etc. The exercise is focused on the atmospheric component and its coupling with continental surfaces. Only atmospheric models with imposed sea surface temperatures are considered, since the biases of coupled models are generally related to large biases of the sea surface temperature so therefore they are not suitable for local studies over west Africa. Years 2000 (a dry summer during which the Jet2000 campaign was conducted Thorncroft et al. 2003) and 2003 (a wetter year) were selected for a first approach.
Observations An effort has been carried out in parallel with respect to observational products, and the most relevant sources of data have been identified. Because of the importance of rainfall estimation and the uncertainties in rainfall climatologies, multiple datasets are used here including: GPCP 1-Degree Daily Precipitation Data; TAMSAT (resolution: 10-day and 0.05 deg); CPC Merged Analysis of Precipitation (CMAP) (resolution: pentad, monthly and 2.5); AGRHYMET compilation of rain-gauges data over the Sahel (Ali et al. 2005). Various satellite and reanalysis products were also considered. The atmospheric fields consist in analyses and/or re-analyses, and the project also takes advantage of available satellite products for atmospheric radiation such as ISCCP, CERES, or more specifically over the Sahel using RADAGAST (Miller and Slingo 2007). As was done for the simulation outputs, the observations are pre-processed in the form of mean cross-section (10W-10E) on the one hand and 2D horizontal maps on the other. Data are made available on a web site in the form of figures and NetCdF files, together with summary information about the nature and source of the data. Validation of surface fluxes will be one of the main outcomes of the AMMA field campaign. In addition to direct observations, the ALMIP database is also utilized (Aaron (2008), this issue). This collection of simulated surface fields was built using SoilVegetation-Atmosphere-Transfer (SVAT) models forced by a combination of observed, satellite-based and forecast meteorological fields.
Models involved So far, six teams have contributed to the AMMAMIP. Four global models are involved: ARPEGEclimat run at CNRM, ECHAM-4 run at ENEA (Ruti et al. 2006), the UCLA GCM (Mechoso and Arakawa 2000) run at UCM and the LMDZ4 run at IPSL (Hourdin et al. 2006). A limited area simulation with MAR (Gall´ee et al. 2004) was provided by LGGE. The LMDZ model is also run with a refined grid over West Africa
3. AMMA-MIP : first results The results for years 2000 and 2003 are presented which illustrate 1) the current skill of atmospheric models to reproduce the African monsoon, 2) the relevance of the AMMA-MIP framework and 3) the strategy foreseen for the use of the AMMA observations. Additional results are available on the AMMA-MIP
1 For those who would like to contribute, a full description of AMMA-MIP is available at http://ammamip.lmd.jussieu.fr/description.html
2 Descriptions of the various models are available on the web http://amma-mip.lmd.jussieu.fr/MODELS/Welcome.html
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number of layers 31 19 42 29 19 19 19 19 40
details 1 member 1 member 1 member 10 members 5 members Tiedtke convection scheme instead of Emanuel zoomed over West Africa zoomed over West Africa Limited area model
Table 1. AMMA-MIP model configurations. web site.
soon season. In more detail, the latitudinal extension over the Sahel (beyond 15N in Fig. 4) is very different in the various models. No systematic conclusion can be drawn. Some models predict a monsoon rainfall band which is too narrow in latitude (IPSL) and for others it is too broad (UCM and LGGE). Note that even the reanalyzes (NCEP) show important deficiencies in this respect. This characteristic seems to be quite sensitive to the model configuration as seen when comparing the IPSL, IPSLWA and LPAOSF simulations run with the same LMDZ model. Also note that the CNRM model, which represents the AEJ rather poorly, performs quite well for rainfall. All of the models show a significant intraseasonal variability with active sequences and breaks and, once again, the interannual variability is probably too weak in some models (LPAOSF) and too strong in others (CNRM). The annual cumulated rainfall over the Sahelian part of the cross-section (10W-10E, 13N-18N) is shown in Fig. 5 for the year 2000 (x-axis) and 2003 (y-axis). Note that, in the simulations, the only difference in the setup between year 2000 and 2003 comes from the specification of boundary conditions, and, in particular, of sea surface temperature. Only the ”forced” part of the observed variability can hopefully be reproduced by the climate models while the internal variability can be checked when ensemble simulations (performed with the same boundary conditions but a different – random – initial state) are available. A different symbol is used for the various models. For UCM and IPSL, the ensembles are shown with the same symbol. All the points above the straight oblique line correspond to a larger rainfall in 2003 than in 2000. The GPCP observation shows that rainfall was about 20% stronger in 2003. Globally, the models also predict a larger rainfall in 2003. It is clear, however, from the ensemble sets that the internal variability is significant, compared with the forced variability for those two years. Note also that the sim-
Mean dynamical structure The post-onset conditions are presented first (15 July - 15 August, 2000) which consist in the AMMACROSS mean zonal wind for all the AMMA-MIP models and for NCEP and ERA40 reanalysis. The various models capture the main elements of the zonal circulation such as the westerlies (positive contours and red colors) near the surface within the monsoon flow (Eq-20N) or the predominance of easterlies in the mid troposphere. However, important differences are observed. The monsoon flow is too strong for some models (IPSLTI, CNRM, UCM and LGGE) and maybe somewhat too weak for others (ENEAL19 and ENEAL42). ENEA, IPSL and LGGE correctly simulate an isolated AEJ. The position of the Sub-Tropical Jet also varies in latitude from one model to the other. The comparison between the two ENEA simulations gives an idea of the impact of the vertical resolution; the comparison IPSL1/IPSLTI documents the major impact of parameterized convection while that of UCM run1 and UCM run2 illustrates the internal (not forced by SST variability) inter-annual variability as produced by GCMs. LPAOSF, IPSLWA and IPSL1 are run with model versions which are very similar to LMDZ but with different horizontal grids. The LPAOSF and IPSLWA grids are refined over West Africa. Rainfall A preliminary comparison of the seasonal cycle of rainfall over the 10W-10E cross section is shown in Fig. 4 for the year 2000. The simulations displayed are the same as in Fig. 3. The models generally capture the latitudinal migration of the intertropical convergence zone from south of the Equator over the ocean (the Guinean coast is located at about 5N) during northern winter, and to West Africa during the mon5
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Fig. 3. Latitude-pressure cross-section (averaged between 10W and 10E) of the zonal wind (m/s) for the various configurations (see Tab. 1) and for ECMWF and NCEP reanalyzes ; year 2000, after onset (15 July-15 August)
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Fig. 5. 2003 versus 2000 cumulated rainfall averaged over a sahelian box (10W-10E, 13N-18N) for all of the AMMA-MIP simulations, for the GPCP and TAMSAT observations and for the NCEP and NCEP2 reanalyzes. ulations which are the most realistic in terms of cumulated rainfall (LGGE, CNRM, IPSLWA, LPAOSF, ENEA) do not systematically show a stronger rainfall in 2003. Of course, in view of the IPSL and UCM results, ensemble simulations would be required to determine whether it is a real difference or not. Interestingly, the model with finest vertical resolution (ECHAM4, run 2: indicated by the upper of the two grey triangles) shows results which are quite close to GPCP. Note also that the NCEP reanalysis shows a rather poor score in terms of the simulation of the cumulated rainfall over the Sahel and that the TAMSAT observations do not agree with GPCP for year 2000, although the increase in 2003 is quite comparable. An important conclusion is that the difference between observations and models is generally much larger than the forced and internal variability. This justifies a posteriori the simple and light framework chosen for the AMMA-MIP project, with only a few years selected without requiring ensemble runs.
discrepancy between the various models. Surface hydrology also plays a key role through the partitioning of sensible and latent heat fluxes (e. g. Fontaine et al. 2002). As said in the introduction of this paper, validation of climate models in terms of surface fluxes is one of the expected important outcome of the campaign. Since data are not yet fully processed in a form usable for model validation, a comparison is presented of the latitudinal variations of the latent heat flux of a subset of simulations for year 2000 and 2003 (red and blue curve respectively, Fig. 6) with the same outputs from the ALMIP database. The dispersion of the models is very large in this graph. Only ECHAM-4 seems to be able to capture the basic latitudinal structure. Note that the errors obtained may be the reflection of biases of other variables (models with less rainfall have less water available for evapotranspiration).
Surface fluxes
The AMMA-MIP exercise is now in place. First results emphasize the difficulty of current climate models to correctly simulate the African Monsoon. Some models perform better for the dynamics, others for the representation of the seasonal and intraseasonal variations of surface rainfall (ECHAM-4 shows probably the best compromise at this stage). A light exercise with a focus on a few years only, appears to be relevant in order to assess the model skill in terms of seasonal and intraseasonal variations of rainfall and
4. Conclusions and extension
The latitudinal gradients of moist static energy have been shown to play a key role in the control of the African monsoon (Eltahir and Gong 1996). This latitudinal gradient is forced at first order by the thermal contrasts between the Guinea gulf (relatively cool at that time) and the relatively hot Sahara. The aerosols and clouds significantly modulate the latitudinal contrasts of top of the atmosphere and surface radiative fluxes. Even the surface albedo can be a source of 8
Fig. 6. Comparison of the surface latent heat flux (10W-10E average, W m−2 ) for various simulations and for the various models involved in the intercomparison exercise of surface schemes ALMIP. The AMMA-MIP simulations correspond to year 2000 (red) and 2003 (blue) while various models and years 2004 to 2006 are shown for ALMIP to give an idea of the uncertainty. All the flux values are averaged over June-July-August-September. in order to evaluate model improvements which can be obtained, for instance, by changing the horizontal or vertical resolution or modifying the parametrized physics. First results confirm that parametrized convection is a key issue here. The AMMA-MIP exercise is currently extended to the period of the intensive phase of the AMMA field experiment in 2006 (Janicot et al. 2008). 2005 is considered as well, both because the observing network were partially deployed at that time and because the 2006 monsoon onset occurred much later than in 2005, together with a late setting up of the SST cold tongue along the Guinean coast. In order to fully exploit the field campaign observations, and to allow the study of the diurnal cycle, additional 3D outputs at a 2-hour frequency are requested for the summer, 2006. Preliminary model results are now being collected. Note that this framework may help evaluate the biases of numerical weather forecast models as well. ECMWF has already provided time series according to the AMMA-MIP requirements for year 2006, consisting in forecasts at various time ranges. In terms of using campaign data, particular care will be given to the compilation of soundings (classical soundings, drop sounds sent from aircrafts or balloons, Parker et al. 2008). Boundary layer flights
were also dedicated to the documentation of the transect, from the ocean (with a rendez-vous with scientific ships over the Guinea gulf) to the Sahara. They will also serve as a basis for comparison. The use of the turbulent fluxes recorded at the meso-scale sites is a more complicated matter. However, combining the ALMIP approach with AMMAMIP model analysis (looking, for instance, at the correlation of the radiative or latent heat fluxes, rainfall, surface temperature, etc) should yield some clue to assess the representation of coupled processes at the surface. In order to try to separate the role of atmospheric processes from that of surface feedbacks, additional simulations will be encouraged in which surface evaporation will be computed by specifying the ratio of evaporation to potential evaporation (that of a free surface of water) as a function of latitude and season, thus cutting the feedback loop through surface moisture. An additional interest of the AMMA campaign for model evaluation is the joint observation of atmospheric composition and dynamics. The links between composition and dynamics is illustrated based on results of the LMDZ model with the two deep convection schemes (similar to simulations IPSL1 and 9
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Fig. 7. Illustration of the impact of parametrized convection – Tiedtke (1989) (left) versus Emanuel (1993) (right) scheme – in the AMMA-cross framework for July. A : mean meridional circulation (stream lines) and mean zonal wind (colors, m/s). B : relative humidity (%, colors) and parameterized convective heating rate (contours, K/day). C : CO concentrations (contours, ppmv) and idealized tracer emitted in the boundary layer over the African continent, south of 10N (colors, arbitrary units).
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IPSLTI in Fig. 3). With the Emanuel convective schemes (Fig. 7), the convective heating (panels B, contours, K/day) peaks higher up in the atmosphere, with stronger heating rates than for the Tiedtke scheme. The cooling (dashed contours) in the lower atmosphere over Sahel due to the evaporation of convective rainfall is also stronger with Emanuel. Those differences have a strong impact on the large scale dynamics : the African Easterly Jet (panel A, colors) is much better represented with the Emanuel scheme (as already shown in Fig. 3); the mean meridional circulation (stream lines) shows a well marked ITCZ only in the Emanuel case. The relative humidity (panel B, colors) shows a signature of this different transport with, in particular, a local maximum which is stronger and higher in the ITCZ for the Emanuel scheme. Similar differences are obtained for CO with the climate-chemistry model LMDZ-INCA (Hauglustaine et al. 2004) as well as with an idealized tracer emitted in the boundary layer (between surface and 850hPa) over the African continent South of 10N, experiencing a radioactive decay with a life time of a few days (panel C). Based on those considerations, it was decided to extend the AMMA-CROSS framework to the intercomparison of chemistry transport models and coupled chemistry climate models, comparing both realistic chemical tracers and tracers emitted in latitude bands, close to the surface, either over the continent or ocean. The AMMA-MIP project, and in particular the AMMA cross-section analysis, provides a relevant framework for focusing on the climate feedbacks and on the interactions between climate components, such as atmosphere, land-surface, chemistry. It is a unique framework allowing the study of the impact of different climatic components on the hydrological cycle, and it could be a good candidate for a GEWEX-GCSS case study.
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