The Structure and Evolution of Midlatitude Cyclones

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The descriptive cyclone model originated by the Norwegian school of meteorologists in the second decade of the 20th century has long served as a cornerstone of weather forecasting in the middle latitudes. In this paradigm, a word-girdling boundary zone between polar and tropical air (the polar front) is the focal line of action, with waves on this front forming, evolving and dying; producing typical weather patterns and changes during their lifetime. The later development of observational networks extending through the troposphere, dynamical theorys of jet streams and planetary scale waves, and the advent of Numerical Weather Predic- tion models have both complicated and overshadowed the frontal cyclone model. This is due, in part, to the fact that in each case, the treatment is too coarse-meshed in the horizontal to capture the narrow frontal zone. More recently, radar and satellite-image depiction of mesoscale features associated with cyclones, and the development of numerical research models with mesoscale resolution, offers the possibility of describing mid-latitude frontal cyclones in much finer detail. Mass proposes to use a widely-used mesoscale model (the MM4 model), together with conventional coarse-grained operational data sets, to generate detailed quantitative four- dimensional descriptions of a sample set of five or six cases of such cyclones (explosively developing and normally developing marine cyclones, a continental cyclone over level terrain, and cyclones moving across and to the lee of the Rocky Mountains). Analyses of these data sets will include air trajectory computations and sophisticated three-dimensional graphics. Mass expects that these analyses will contribute to the establishment of a new paradigm to modify the useful, but incomplete, Norwegian model. This will contribute to the improvement of short-range, local weather forecasts and to the development of operational versions of numerical mesoscale models.