Dynamical aspects of the life cycle of the winter storm ‘Lothar’ (24–26 December 1999)

Abstract

SUMMARYDynamical aspects of the life cycle of the winter storm 'Lothar' (24-26 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) eld 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 signi cant PV anomalies were evident at the tropopause level during this early phase of the life cycle. The surface cyclone intensi ed 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 signi cantly 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 con guration (and only a very weak surface development) and con rms the primary importance of the cloud diabatic heating for the tropopause fold formation and the rapid 'bottom-up' intensi cation of 'Lothar'.A comparison of the anomalously high Atlantic sea surface temperatures in December 1999 with the watervapour source regions for the latent-heat release that accompanied the rapid intensi cation 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.

Quantifying the climatological relationship between extratropical cyclone intensity and atmospheric precursors

“…Statospheric PV anomalies can then merge with the diabatically produced midlevel PV features to form a vertically aligned tower of PV. Davis [1992] and Wernli et al [2006] both showed that the superposition of stratospheric PV anomalies and moisture-induced midlevel PV could enhance cyclone intensity. Finally, Plant et al [2003] showed that midlevel PV anomalies, generated in response to latent heating, were important for the development of two socalled type C cyclones.…”

Mesoscale Convective Vortex Formation in a Weakly Sheared Moist Neutral Environment

“…Building off of Montgomery and Farrell (1992), MM04 and MM05 conducted numerous idealized model simulations (using both a simplified balance model and a full-physics mesoscale model) to describe and better understand the mechanism in a moist, baroclinic wintertime environment. These coherent structures have been shown to play a role in such varied atmospheric processes as explosive cyclogenesis (Gyakum et al 1992;Wernli et al 2002), squall lines (Parker and Thorpe 1995), and polar lows Farrell 1991, 1992;Fantini and Buzzi 1993;Mak 1994). The present study seeks to extend the MM05 investigation of the DRV dynamics into convectively less stable environments.…”

Objective identification, typing and tracking of the complete life-cycles of cyclonic features at high spatial resolution

“…Lothar that hit France on 26 December 1999; see Wernli et al, 2002). As a result they can move very rapidly, and can also accelerate and decelerate rapidly (in the jet entrance and jet exit regions).…”

“…Similarly, the parameter also acts as a useful alarm bell for forecasters to occasions when the behaviour of the feature in question can be more prone to error. Recall, for example, that the cyclonic storms Lothar and Martin that hit France in December 1999 developed in the presence of very strong upper jets (Ulbrich et al, 2001;Wernli et al, 2002), and were not well predicted by most operational models. As all the parameters depicted typically exhibit less bias than 10 m wind, they are one way in which the feature-based approach is able to address some of the deficiencies of the lower resolution ensemble forecast models, and provide useful information on the potential of a feature to cause high-impact weather.…”