A study of the structure of mid‐latitude depressions in a numerical model using trajectory techniques. I: Development of ideal baroclinic waves in dry and moist atmospheres

Trajectory‐tracing techniques are used to investigate the structure and air flow of developing baroclinic waves simulated by a numerical atmospheric forecast model. Three stages are described. The structure of the linear stage is explained with reference to the normal mode theory of baroclinic instability. In the mature stage, flow separation zones form which are identified as warm and cold fronts. At the occluded stage, circulation of cold air round the centre of the depression leads to changes in the structure of the cold front. When the initial state is nearly saturated, the overall development is speeded up, and at the mature stage the rate of deepening of the depression is doubled at the surface. In the occluded stage formation of a warm air conveyor belt is observed. Comparisons are made with previously published integrations by specially constructed models in order to establish a basis for comparison of forecasts from real data which will appear in a future paper.