Dynamical aspects of the life cycle of the winter storm 'Lothar' (24-26 December 1999)

Dynamical aspects of the life cycle of the winter storm 'Lothar' (24-16 December 1999) are investigated with the aid of the European Centre for Medium-Range Weather Forecasts analysis data and mesoscale model simulations. Neither of these datasets capture the full amplitude of the observed extreme pressure fall and surface wind speeds, but they do help identify a range of key dynamical and physical features that characterize the development of this unusual event. The analysis and interpretation is primarily based upon the evolution of the lower- and upper-level potential vorticity (PV) field complemented by three-dimensional trajectory calculations. 'Lothar' originated in the western Atlantic and travelled as a shallow low-level cyclone of moderate intensity towards Europe. This translation took place below and slightly to the south of a very intense upper-level jet and was accompanied by continuous and intense condensational heating that sustained a pronounced positive low-level PV anomaly (not unlike the concept of a 'diabatic Rossby wave'). No significant PV anomalies were evident at the tropopause level during this early phase of the life cycle. The surface cyclone intensified rapidly when the shallow cyclone approached the jet-stream axis. The circulation induced by the diabatically produced low-tropospheric PV anomaly on steeply sloping isentropic surfaces that transect the intense upper-level jet contributed significantly to the rapid formation of a narrow and deep tropopause fold. This stratospheric PV anomaly virtually merged with the diabatically produced ephemeral PV feature to form a vertically aligned tower of positive PV at the time of maximum storm intensity. A sensitivity study with a dry adiabatic hindcast simulation shows no PV-tower configuration (and only a very weak surface development) and confirms the primary importance of the cloud diabatic heating for the tropopause fold formation and the rapid 'bottom-up' intensification of 'Lothar'. A comparison of the anomalously high Atlantic sea surface temperatures in December 1999 with the water-vapour source regions for the latent-heat release that accompanied the rapid intensification phase of 'Lothar' shows a close relationship. This is of importance when discussing the possible implications of climate variability and change on the development of North Atlantic winter storms.