The Mackenzie River is the largest North American source of freshwater for the Arctic Ocean. This basin is subjected to wide fluctuations in its climate and it is currently experiencing a pronounced warming trend. As a major Canadian contribution to the Global Energy and Water Cycle Experiment (GEWEX), the Mackenzie GEWEX Study (MAGS) is focusing on understanding and modeling the fluxes and reservoirs governing the flow of water and energy into and through the climate system of the Mackenzie River Basin. MAGS necessarily involves research into many atmospheric, land surface, and hydrological issues associated with cold climate systems. The overall objectives and scope of MAGS will be presented in this article.

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a field and modeling study aims to improve our understanding of boundary layer processes in the foothills of the alberta rocky mountains and how they relate to the initiation of severe thunderstorms. U nderstanding convective initiation (CI) and predicting severe thunderstorms remains a challenge for atmospheric scientists and forecasters. However, accurate forecasts of the timing and location of CI are critical to maximize the lead time and accuracy of severe weather watches and warnings. The small spatial and temporal scales (sometimes only hundreds of meters and minutes, respectively) on which atmospheric processes leading to CI occur make them difficult to measure observationally and simulate using numerical weather prediction (NWP) models. An attempt to resolve these processes via observations can be made using 

A five-year intensive measurement program of energy and water cycling in the Mackenzie River Basin revealed special attributes of the high-latitude climate system.

Diurnal changes in thé local atmospheric moisture budget over thé Canadian Prairies are computed using sequential radiosonde soundings from thé 1991 Régional Evaporation Study (RES-91). Previous attempts to estimate evapotranspiration with radiosonde data hâve used either similarity theory or a moisture budget, but hâve been confined to thé boundary loyer in either case. Thèse studies, as well as semi-empiric operational techniques which use surface-based data, exclude thé effects of moisture advection and energy exchanges between thé boundary loyer and thé free atmosphère, assuming negligible effects on evapotranspiration. The moisture budget method adopted hère includes horizontal advection explicitly, and treats vertical fluxes implicitly through a total tropospheric moisture budget. Comparison ofthe evapotranspiration estimâtes with those ofother techniques are positive only when results are averaged over several days to weeks. While thé advection estimâtes are a major source oferrorfor thé "daily" estimâtes in this particular study, it is shown that neither advection nor moisture flux through thé boundary loyer can be ignored in estimating daily evapotranspiration, regardless of thé technique used. The results also suggest that evapotranspiration is more variable on a daily basis than other techniques hâve indicated. With an improved synoptic database now availablefor advection estimâtes, thé moisture budget technique may provide an excellent ground-truth method for fine-tuning techniques for remote sensing of evapotranspiration, and could lead to improved parametrization schemes for both NWP models and GCMs. RÉSUMÉ On calcule les changements diurnes locaux du bilan de l'humidité atmosphérique sur les Prairies canadiennes à l'aide de radiosondages séquentiels obtenus lors de l'étude régionale sur l'évaporation (RES-91). Les essais antérieurs en vue d'estimer l'evapotranspiration au moyen des données de radiosondage ont employé soit la théorie de similitude, soit le bilan d'humidité, mais ils étaient limités à la couche limite dans chaque cas. Ces études ainsi que les techniques semi-empiriques opérationnelles basées sur les données de surface ne tiennent pas compte des effets de l'advection d'humidité et des échanges d'énergie entre la couche limite et l'atmosphère libre, leurs effets sur l'evapotranspiration étant supposés minimes. La méthode du bilan d'humidité adoptée ici comprend explicitement l'advection hori- ATMOSPHERE-OCEAN 35 (1) 1997, 29-63 BackgroundThis paper describes a simple technique to estimate areal evapotranspiration using an atmospheric moisture budget based on sequential radiosonde soundings from a single site. The method described hère considers thé moisture budget of thé whole troposphère, where analyses in other studies hâve been confined to thé atmospheric boundary layer (ABL). The method is tested using sounding data from thé Régional Evaporation Study (RES). RES was carried out over a 10,000 km area of southern Saskatchewan in 1991. The main goal was to improve estimâte...