THE SECLUSION INTENSIFICATION OF THE NEW-YEARS-DAY STORM 1992

The Bergen School meteorologists realized that not all cyclones follow their conceptual cyclone model. In particular they found cases with re-generation of occluded cyclones. In their literature, for instance as found in Godske and co-workers, 2 kinds of re-generation were considered challenging to weather forecasting: a thermodynamic intensification with similarities to the development of tropical cyclones, and intensification of what was called the non-frontal trough, or the back-bent occlusion, of strong cyclones. It has been believed that the latter kind is characteristic of the strongest surface winds observed in the Northwestern Atlantic. In this paper such a case, resulting in the strongest cyclone landfall on the Norwegian west coast this century, has been investigated from a simulation of a small synoptic-scale (similar to 1000 km), fast-moving extratropical cyclone (similar to 25 m s(-1)). It is found that the cyclone evolves as the conceptual frontal model of Shapiro and Keyser (1990), and that the strong winds are developed by a secondary, mesoscale (similar to 500 km) cyclogenesis closely linked to the seclusion process. The time scale of the intensification is 12 h, starting with what is called the seclusion trough at the tip of the back-bent warm front. As the cold air secludes the warm core, the disturbance develops into a separate low, here called the seclusion low. Release of latent heat connected to the back-bent warm front is found to play an important role in forming the seclusion. A part of the generated potential vorticity (PV) remains within the warm air in the seclusion process. Inversion of the low-level PV anomalies results in a low-level jet along the outer side of the seclusion trough. The strong winds are observed when the seclusion trough develops into the seclusion low and the low-level jet becomes parallel to the large scale westerly flow. A positive PV anomaly streamer, formed from a larger scale upper PV anomaly in phase with the surface low, takes part in this stage of the development.