STRUCTURE OF A FRONTAL CYCLONE

A diagnostic study of a mid-latitude cyclone has been carried out using routinely available numerical weather-prediction model products and imagery. The cyclone was intense but not exceptionally so, and the results are believed to have some generality. The detailed description of the cyclone structure, just before it began to deepen rapidly, integrates concepts from a number of researchers into a common framework. A dominant feature of the cyclone, close to its centre, was the 'cloud head': a region of cloud with a sharp convex outer edge, which formed poleward of the main polar-front cloud band. The cloud head was caused by two flows that entered it from the east, ascending and fanning out within it. One flow (the 'cold conveyor belt') brought low wet-bulb potential temperature (theta(w)) air back into the cloud head from low levels ahead of the warm front. The other flow was due to high-theta(w) air that peeled off from the base of the main warm-sector airflow (i.e. part of the 'warm conveyor belt') and travelled in the boundary layer back towards the cyclone centre, first undercutting dry air that had earlier descended from the upper troposphere (called a 'dry intrusion'), and then ascending at the upper boundary of the cloud head, above the cold conveyor belt. The transverse circulation that gave the ascent within the cloud head also led to the cold front fracturing along its length into two separate sharp surface cold fronts, with a more diffuse frontal region in between ('frontal fracture'). The two sharp surface cold fronts were associated with narrow cold-frontal rainbands ('line convection'), one of the line-convection segments forming the southern edge of the cloud head. The overrunning of the dry intrusion in the region of the frontal fracture led to a structure (known as a 'split front') in which an upper-level humidity front began to run ahead of the position of the surface cold front. A feature of the cyclone at this intermediate stage in its evolution was that a large proportion of the precipitation was being generated by ascent of relatively cold air (the cold conveyor belt) in the cloud head as part of the thermally indirect circulation at the left exit of a developing secondary upper-level jet.