Abstract. Mercury is a global pollutant due to its long lifetime in the atmosphere. Its hemispheric transport patterns and eventual deposition are therefore of major concern. For the purpose of global atmospheric mercury chemistry and transport modelling the ECHMERIT model was developed. ECHMERIT, based on the global circulation model ECHAM5 differs from most global mercury models in that the emissions, chemistry (including general tropospheric chemistry and mercury chemistry), transport and deposition are coupled on-line to the GCM. The chemistry mechanism includes an online calculation of photolysis rate constants using the Fast-J photolysis mechanism, the CBM-Z tropospheric gas-phase mechanism and aqueous-phase chemistry based on the MECCA mechanism. Additionally, a mercury chemistry mechanism that incorporates gas and aqueous phase mercury chemistry is included. A detailed description of the model, including the wet and dry deposition modules, and the implemented emissions is given in this technical report. First model testing and evaluation show a satisfactory model performance for surface ozone and mercury mixing ratios (with a mean bias of 1.46 nmol/mol for ozone and a mean bias of 13.55 fmol/mol for TGM when compared with EMEP station data). Requirements regarding measurement data and emission inventories which could considerably improve model skill are discussed.

[1] The Volta region is a climate-sensitive semiarid to subhumid region in West Africa. To investigate the impact of expected global climate change on regional water availability, regional climate modeling was performed. Two time slices (1991-2000 and 2030-2039) of the ECHAM4 scenario IS92a were dynamically downscaled with MM5 to a spatial resolution of 9 km. The quality of MM5 simulations in reproducing regional climate was assessed using reanalysis data for initial and boundary conditions. Although an underestimation of coastal rainfall was detected, sufficient accuracy in the Volta Basin could be achieved. The regional climate simulations show an annual mean temperature increase of 1.2-1.3°C in the Volta region. This temperature change significantly exceeds interannual variability. A mean annual change in precipitation from À20% to +50% ($ À150 to +200 mm) is simulated, with a spatial mean increase of 5% ($45 mm). In the rainy season, rainfall predominantly increases, whereas a strong decrease is found for April, which is connected to a delay in the onset of the rainy season. In addition, interannual variability in the Volta region increases in the early stage of the rainy season. The climate change signals in infiltration excess and evapotranspiration show a nonlinear response to precipitation change. Aridity, expressed by the de Martonne aridity index, does not change significantly. The change signal in precipitation predominantly lies within the range of interannual variability. In contrast, the decrease in April exceeds interannual variability in the Sahel region.Citation: Jung, G., and H. Kunstmann (2007), High-resolution regional climate modeling for the Volta region of West Africa,

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The Benguela Current, located o the west coast of southern Africa, is tied to a highly productive upwelling system 1 . Over the past 12 million years, the current has cooled, and upwelling has intensified 2-4 . These changes have been variously linked to atmospheric and oceanic changes associated with the glaciation of Antarctica and global cooling 5 , the closure of the Central American Seaway 1,6 or the further restriction of the Indonesian Seaway 3 . The upwelling intensification also occurred during a period of substantial uplift of the African continent 7,8 . Here we use a coupled ocean-atmosphere general circulation model to test the e ect of African uplift on Benguela upwelling. In our simulations, uplift in the East African Rift system and in southern and southwestern Africa induces an intensification of coastal low-level winds, which leads to increased oceanic upwelling of cool subsurface waters. We compare the e ect of African uplift with the simulated impact of the Central American Seaway closure 9 , Indonesian Throughflow restriction 10 and Antarctic glaciation 11 , and find that African uplift has at least an equally strong influence as each of the three other factors. We therefore conclude that African uplift was an important factor in driving the cooling and strengthening of the Benguela Current and coastal upwelling during the late Miocene and Pliocene epochs.

Atmospheric models such as the Weather Research and Forecasting (WRF) model provide a tool to evaluate the behavior of regional hydrological cycle components, including precipitation, evapotranspiration, soil water storage, and runoff. Recent model developments have focused on coupled atmospheric‐hydrological modeling systems, such as WRF‐Hydro, in order to account for subsurface, overland, and river flow and potentially improve the representation of land‐atmosphere interactions. The aim of this study is to investigate the contribution of lateral terrestrial water flow to the regional hydrological cycle, with the help of a joint soil‐vegetation‐atmospheric water tagging procedure newly developed in the so‐called WRF‐tag and WRF‐Hydro‐tag models. An application of both models for the high precipitation event on 15 August 2008 in the German and Austrian parts of the upper Danube river basin (94,100 km2) is presented. The precipitation that fell in the basin during this event is considered as a water source, is tagged, and subsequently tracked for a 40‐month period until December 2011. At the end of the study period, in both simulations, approximately 57% of the tagged water has run off, while 41% has evaporated back to the atmosphere, including 2% that has recycled in the upper Danube river basin as precipitation. In WRF‐Hydro‐tag, the surface evaporation of tagged water is slightly enhanced by surface flow infiltration and slightly reduced by subsurface lateral water flow in areas with low topography gradients. This affects the source precipitation recycling only in a negligible amount.

The Volta Basin in West Africa is a region sensitive to water shortage. Future climate conditions therefore may put additional stress on the competition for the scarce water resources between industry, agriculture, and households. For an investigation of the sensitivity of the hydrological regime to global climate change in the data-sparse and poorly gauged region of the Volta Basin, joint regional climate–hydrology simulations were performed. MM5 was used as a regional climate model to downscale two time slices of a global ECHAM4 simulation to a resolution of 9 km. These regional climate simulations were used to drive a physically based, distributed hydrological model at 1 km resolution. The performance of the model components and the joint model system was evaluated for different historical periods. Results show that discharge in the Volta Basin reacts highly sensitively to precipitation differences. The pronounced rainfall decrease at the beginning of the rainy season is not transferred to discharge changes. During the rainy season most of the surplus rainfall evaporates due to a strong increase in evaporation as a consequence of higher near-surface air temperatures. The average change signal in precipitation, as well as surface and subsurface hydrology variables, lies in most variables within the range of inter-annual variability, but regionally stronger signals are also observed.

Abstract. In the Volta Basin, infrastructure watershed development with respect to the impact of climate conditions is hotly debated due to the lack of adequate tools to model the consequences of such development. There is an ongoing debate on the impact of further development of small and medium scale reservoirs on the water level of Lake Volta, which is essential for hydropower generation at the Akosombo power plant. The GLOWA Volta Project (GVP) has developed a Volta Basin Water Allocation System (VB-WAS), a decision support tool that allows assessing the impact of infrastructure development in the basin on the availability of current and future water resources, given the current or future climate conditions. The simulated historic and future discharge time series of the joint climate-hydrological modeling approach (MM5/WaSiM-ETH) serve as input data for a river basin management model (MIKE BASIN). MIKE BASIN uses a network approach, and allows fast simulations of water allocation and of the consequences of different development scenarios on the available water resources. The impact of the expansion of small and medium scale reservoirs on the stored volume of Lake Volta has been quantified and assessed in comparison with the impact of climate variability on the water resources of the basin.

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