Knowledge of Alpine Weather at a Peak

As hikers, skiers and pilots know from experience, alpine weather can be both extreme and unpredictable. For reasons that are still little understood, the topography of the world's mountainous regions can cause sudden, violent meteorological conditions __ from gusting winds and clear air turbulence to hailstorms and torrential downpours. These conditions can trigger flash floods, landslides and avalanches, and pose hazardous in-flight conditions for aircraft.

To learn more about the mechanics behind these systems and to improve the understanding and prediction of alpine precipitation and flooding, more than 200 scientists from Canada, the United States, Britain, France, Germany, Austria, Italy, Switzerland and Slovenia spent 10 weeks in the European Alps last fall as part of the decade's largest and most sophisticated mountain weather experiment. Known as the Mesoscale Alpine Programme, it focused on weather systems that are 10 to 100 kilometres across __ the ones that carry weather conditions you can feel, such as thunderstorms, hail and snow.

The Alps were chosen not only because they have more meteorological instrumentation than any other area of the world, but also because of the profound influence they have on regional weather. Most natural disasters in the Alps and the surrounding area are caused by alpine precipitation. Conditions peak in the fall, when warm air from the south absorbs moisture as it passes over the Mediterranean Sea and then cools suddenly over the mountains, sometimes dumping up to 30 centimetres of rain in a matter of minutes. Several sudden, major floods occur as a result each year.

A meteorologist taking readings at an Alpine weather station.

Three regions were equipped with additional weather stations and measurement instruments and monitored by instrumented research aircraft: part of Germany's Rhine Valley and Austria's Wipptal Valley, and the Lago Maggiore region of Italy. A team of Environment Canada meteorologists were stationed at project headquarters in Innsbruck, Austria, to provide detailed hourly weather forecasts using an ultra-high-resolution software they have been developing over the past eight years.

The MC2 software, which was run on a Swiss supercomputer, collected data in real time from across the entire Alpine massif, from Lyon to Vienna. More than once a minute, the program updated information from thousands of points exactly three kilometres apart on a horizontal grid 1 000 kilometres square. Readings for each of these points were taken at 50 different atmospheric levels (twice that of a normal forecasting model), making it the first time such a fine mesh has been used to forecast conditions over such a large area.

The detailed meteorological and topographical information collected using specialized instruments and research aircraft, coupled with the real time simulations carried out by the MC2, made it possible for the forecasters to detect even minute changes in cloud content and air movement, and forecast the strength of wind, precipitation and other weather parameters in the region to an unprecedented degree of accuracy. They were able to pinpoint the location of dangerous storms to within a few kilometres, and determine several hours in advance when they would start and stop. The state-of-the-art supercomputing technology used by the MC2, however, proved very expensive to use routinely over such a large area.

A second objective was to study and forecast gravity waves caused by changes in the density of the air. These highly localized oscillations often occur over mountainous terrain, and can cause extreme air turbulence for aircraft when they reach their peak amplitude and break, like ocean waves. Although the MC2 proved adept? at calculating the strength of these oscillations, it had difficulty determining when they would break, although simulations over a smaller area on an ultra-fine one-kilometre grid improved the model's success rate significantly.

Scientists estimate that it will take about five years to analyze and interpret the more than 10 gigabytes of new forecast data collected daily over the course of the experiment, but their first early results will be presented at a meeting in Slovenia this June. This work will greatly improve the understanding and forecasting of hazardous alpine weather conditions and, as a result, the safety of people and property in mountainous regions around the world.

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