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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Observations and simulations of dynamic and thermodynamic processes of Mediterranean cyclones leading to high impact weather

Norbert Kalthoff (Karlsruhe Institute of Technology, KIT); Dominique Lambert, Ulrich Corsmeier, Christoph Kottmeier, Evelyne Richard

Synoptic-scale weather systems, in particular cyclonic vortices, most efficiently transport water, energy, and momentum in mid-latitudes and thus critically affect the regional and global water cycles and energy redistribution. They also play a crucial role in the transport of trace gases and aerosols. Under certain conditions of the atmosphere (shear, stability, baroclinicity) and of the geographical factors they can develop to high impact weather (HIW) systems causing damage due to high wind speed, heavy precipitation, hail, and associated floods. One has to expect that a significant part of the man-made impact on climate change is experienced through changes in such tropospheric weather systems. A critical region in this aspect may be the western Mediterranean Sea, where lower tropospheric cyclones develop regularly, being associated with large amplitude upper-level troughs that extend southwards enforcing transport between the mid-latitudes and the subtropics.
The Mediterranean cyclones are often leading to enforced vertical moisture transport and embedded convection accompanied with subsequent heavy precipitation. These cyclones are difficult to analyse and to predict and there are many factors associated with their development, evolution, and predictability that are incompletely investigated and poorly understood. This results in imprecise forecasts, which might lead to economic as well as ecologic losses in the areas affected by such weather.
The scale of the systems to be investigated ranges from mesoscale γ (convection, embedded convection, squall lines) to the synoptic scale (MCCs, Mediterranean depression). That means the cyclones and convective systems cover a wide range from about one km to several thousand km as well as forecast times from nowcasting to medium-range. Process studies measuring the decisive parameters therefore have to be made on equivalent scales. It is foreseen to concentrate on four typical synoptic scenarios: (i) high amplitude trough approaching the Mediterranean from the west, (ii) remnant circulation in the lower troposphere reactivated by an upper tropospheric trough, (iii) streamer reaching the Mediterranean without cyclogenesis, and (iv) lee-cyclogenesis south of the Alps, in the Gulf of Genoa.
In this area (the most cyclogenetic area of the western Mediterranean basin), south of the Gulf of Genoa, Corsica is regularly affected by intense meteorological events: windstorms, heavy precipitation, waves and coastal erosion, droughts, forest fires, and lightning. All these events have a great impact on the economic activity of Corsica, which is mainly based on tourism, nautical activities, agriculture and farming. Corsica is also relevant for the study of most of intense precipitation cases affecting continental south eastern France and northern Italy, which are generally characterized by low level warm and moist air mass transport from the south channelled not only by the continental orography but also by the Corsica-Sardinia island ridges. In this case, Corsica is located in the upstream area of the flow finally generating the cyclogenesis and subsequent HIW over the Gulf of Genoa. Its geographical situation would allow Corsica to play the role of a “sentry”, not yet exploited for these events.
Within this scope, the scientific goal of the project is to investigate the essential dynamical and thermo-dynamical mechanisms of HIW-cyclones by measurements with two German research aircrafts and a meteorological observatory (KITCube) from the Institute for Meteorology and Climate Research of Karlsruhe Institute of Technology (Germany), based in Corsica, in close cooperation with other measurement platforms performed in the framework of HYMEX and CHARMEX.
Within the overall goals, specific emphasis is put on the contribution of turbulent fluxes from the sea surface to the development of atmospheric instability and convection as well as on the impact of orography on the initiation and/or modification of convective systems and cyclones.
This work will be accompanied with high-resolution numerical studies aiming at (i) providing some guidance of the experimental strategy and the conduct of aircraft missions, (ii) the understanding and the partitioning of the different physical processes at work, and (iii) the assessment of the predictability of the meteorological events leading to HIW in the vicinity of the Gulf of Genoa.