SUMMARYA reference case of a Sahelian weather system observed during the Hydrological Atmospheric Pilot Experiment, HAPEX-SAHEL, in August 1992, is described from a seasonal viewpoint as well as from synoptic and convective system viewpoints. It is shown that the case-study is representative of the climatology at all these scales and presents many interacting scales and physical processes. At intraseasonal scale, the monsoon onset is characterized by an abrupt shift of precipitation together with a latitudinal migration of the African easterly jet (AEJ) and convection. At the month and day scales, the convective activity occurs in an apparent zonal break of the tropical easterly jet. The month of August 1992 exhibits intense synoptic activity. The vorticity eld is characterized by northerly (dry) and southerly (wet) components located at 850 hPa on each side of the AEJ. Their intraseasonal modulation on a period of 20 to 40 days leads to active and break phases of the synoptic activity. Around 21 August, the 700 hPa vorticity eld features the propagation of a typical easterly wave with a westward propagation of a cyclonic circulation followed by an anticyclonic circulation. Convective activity occurs mainly ahead of the 700 hPa vorticity maximum with the formation of a squall line on Aṏ r mountains propagating south-westward at 15 m s ¡1 . The convective system propagates about twice as fast as the vortex core, in contrast with the convection in the European Centre for Medium-Range Weather Forecasts re-analysis which stays in phase with the vorticity. The squall line corresponds to the largest contributor to the systems passing in August 1992 over the HAPEX-SAHEL region; its environmental conditions and its effects on the atmosphere including the surface parameters are presented.

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SUMMARYA 24 h multi-scale simulation of the life cycle of a Sahelian squall line is presented. The initial and coupling elds have been taken from the European Centre for Medium-Range Weather Forecasts re-analysis (ERA-15) after humidity corrections to avoid spurious absolute and conditional convective instabilities. Comparisons of modelsynthetic and Meteosat infra-red radiances indicate that the simulated scenario of convection development and structure are realistic, though differences are found concerning the size of the squall line with a delay of 2-3 h. The simulated arc-shaped structure and surface signature t well with radar and ground station observations, respectively.The mature stage of the simulated squall line presents a cross-line structure similar to conceptual models. An intense rear-to-front ow is generated below the trailing stratiform part. An acceleration of the African easterly jet (AEJ) is simulated behind the system, occurring across its full width. This leads to the generation of a dipole of counter-rotating vortices. The resulting potential-vorticity signature leads to local reinforcement of the barotropic instability on the north ank of the AEJ.This simulation is used to analyse the impact of the convection at synoptic-scales and to compare it with ERA-15. It suggests that the convection contributes to reinforce the monsoon at least up to 500 km. The monsoon in ow moves eastward and westward with time-scales shorter and longer than 24 h, respectively. The tropical easterly jet intensi es to the south-west of the convection area up to 700 km away. The AEJ is strongly intensi ed ahead of the trough where active convection occurs. Organized convection developing in the vicinity of the AEJ core induces weakening of the AEJ ahead of the squall line and strengthening behind it. A common result in the simulation and in ERA-15 is that the AEJ core moves westward faster due to the convective activity.

Maybe a recount is needed, because the numbers do not add up to provide sufficient amounts of freshwater for all competing needs. Water covers about 70% of the Earth's surface, and is also abundant as groundwater and water vapor. However, less than 1% of water is available to people as freshwater for their use, and humanity already is utilizing more than half of the world's available surface water. A few more numbers: 1.2 billion people cur rently rely on contaminated water, 3 5 million people mostly children die each year due to preventable deaths from waterborne diseases, and one-third of the world's people suffer from water scarcity That last figure could double during the next generation and create a band of scarcity around the center of the world, according to Jonathan Lash, president of the Washington, D.C.-based World Resources Insti tute (WRI),who cited these numbers. "We are just using more water than the world can give us," Lash said in issuing an October 21 report on freshwater systems as part of the group's five-part Pilot Analysis of Global Ecosystems (PAGE). The report of freshwater systems which will be followed in coming months by reports on coastal, forest, grassland, and agricultural ecosystems is based on existing information that will support an upcoming Millennium Ecosystem Assessment,according to Lash. That assessment, which Lash said is an offi cially sanctioned United Nations process, will be launched in 2001 by WRI,the United Nations Environment Program, and other organizations. The 4-year project will cost an estimated $20 mil lion, with the Global Environment Facility putting up $7 million, according to Lash. Lash said freshwater resources provide trillions of dollars worth of ecosystem servic es and goods to people, including flood con trol, nutrient cycling, and carbon storage as