The ALPEX experiment
THE unique character of theweather over and near the world's mountainous areas is well known, and the great diversity of weather events that may occur in this situation is nowhere better illustrated than by the meteorology of the Alpine region. There is an enormous variety of weather types in this region, and the rapidity of change from one type to another can be breathtaking.
In countries near the Alps some of themost significant meteorological effects are associated with the generation of deep depressions in the Gulf of Genoa which are known as "Genoa cyclones'. These depressions develop very rapidly and are often associated with strong winds, heavy rain, flooding and storm surges in the Mediterranean basin. The catastrophic floods in Florence in 1966 were linked with the development of one of these "Genoa cyclones', as are the periodic tidal surges which from time to time affect Venice. The characteristic local winds in the Alpine region, such as the Foehn and the Mistral, which occasionally blow with destructive force, are also well known phenomena.
In principle, the fact that extensivemountain ranges have major effects on the weather should cause no real surprise, since a mountain barrier will obviously deflect the atmospheric circulation both vertically and horizontally. The Alps straddle the path of the prevailing westerly winds and weather systems coming from the Atlantic, thus causing significant interruption to the natural air flow and leading to a variety of sometimes dramatic local effects. The sudden development of a Gulf of Genoa cyclone as an Atlantic weather system comes up against the Alps is vividly illustrated in the sequence of satellite pictures above.
The important role that mountainsplay in determining weather and climate over considerable areas of the globe was recognized from the outset of a major international meteorological research investigation, the Global Atmospheric Research Programme (GARP), whose overall objective was to study the dynamics of atmospheric phenomena in order to extend the range of useful weather forecasts.
The success of this fifteen-yearprogramme, jointly organized by the World Meteorological Organization (WMO) and the International Council of Scientific Unions (ICSU) in response to resolutions adopted at the 16th and 17th sessions of the General Assembly of the United Nations, has led to dramatic progress in meteorology as a whole. In particular, GARP included a major field investigation, the 1982 Alpine Experiment (ALPEX), the aim of which was specifically to understand the way in which air flows over or around mountains, the development of cyclones such as those in the Gulf of Genoa, and local mountain winds.
One of the main characteristics ofmountain weather is the small scale, meteorologically speaking, of the features involved and their sudden generation and disappearance. Accordingly, ALPEX was designed to gather sufficiently detailed information in space and time over the Alpine region. The meteorological services and scientific communities of twenty nations took part in the Programme, and several years of intensive efforts and detailed planning culminated in the implementation of a Special ALPEX Observing Period from 1 March to 30 April 1982.
For this, the existing network ofobserving stations was supplemented by thirty-four additional stations which provided many extra measurements of pressure and wind at all levels of the atmosphere. An array of sixty microbarographs, capable of tracing with great precision the slightest fluctuations in pressure, was set up along the St. Gotthard and Brenner sections of the Alps. Seventeen aircraft operating from Geneva undertook numerous sorties on predefined tracks, collecting many observations on wind speed and direction. In the Mediterranean itself, information was gathered from eleven research vessels and many buoys, field platforms and tide-gauges. …