- Autor/Doctorand: LAURA TRAPERO BAGUÉ
- Dia: 3 de desembre de 2014
- Hora: 12:00
- Lloc: Sala de Graus Eduard Fontseré
- President: Dr. Ricardo García Herrera
- Secretari: Dra. Samiro Khodayar
- Vocal: Dr. Bernat Codina Sánchez
- Suplents: Dr. Josep Calbó Angrill, Dr. Didier Ricard, Dr. Angel Redaño Xipell
- Directors/Tutors: Dr. Jeronimo Lorente Castello, Dr. Joan Bech Rustullet, Dr. Jeronimo Lorente Castello (Tutor)
- Resum: Heavy precipitation events over complex terrain are often favoured by enhancement mechanisms of precipitation. The windward slopes of the Eastern Pyrenees, as other mountainous Mediterranean regions, are regularly affected by heavy precipitation events (HPE). This present work benefits from high resolution numerical simulations of several case studies to investigate the synoptic and mesoscale environment favourable to precipitating systems development over the Eastern Pyrenees. The mesoscale models WRF and MESO-NH run at horizontal grid-lengths of 2.5-2 km have reproduced realistic precipitation structures over the Pyrenees but the uncertainty associated to sensitivities to initial conditions and microphysics has also been highlighted. The analysis has been done for both stable and conditionally unstable conditions.
The former part of the analysis has explored the application of the results from purely idealized simulations or previous studies in other mountainous regions to the case study simulations. For conditionally unstable events with dynamic evolving conditions (Mediterranean cyclogenesis), the analysis of a triggering term and the moist Froude number reflects the complexity of applying theoretical results. On the other hand, for winter northerly stable flows the precipitation field reveals a dependency on the upstream Froude number. These patterns are synthesised in a conceptual model. Backward trajectories based on Eulerian on-line passive tracers indicate that the direct orographic lifting is the main forcing mechanism which triggers precipitating systems over the Pyrenees. For stable conditions, the simulations capture the formation of an along barrier flow in the north slope due to the blocking effect exerted by the Pyrenees. Under specific mesoscale configurations, a distinct confluence zone is also formed in the easternmost part of the massif, which in turn favours the generati-on of a well developed density current. The understanding of the microphysical processes involved in precipitation enhancement has
been examined by means of the hydrometeor vertical distribution. The dominant mechanisms for intense precipitation are associated with graupel.
From the lagrangian analysis of the feeding flow in the conditionally unstable episodes, the simulations indicate a marked dependence of the precipitation intensity over the Pyrenees on two factors: the cross-barrier wind intensity and the moisture advection towards the Pyrenees in the lower atmosphere. Different ranges have been suggested to discriminate the orographic precipitation intensity over the Pyrenees. In general, the stronger the cross-barrier component wind and the water vapour flux the more intense the orographic precipitation will be. During the most intense winter event, the detection of a narrow corridor of strong water vapour transport (atmospheric river; 550 kg/m·s) contributed to the generation of persistent precipitation. A similar situation occurred during the 1982 HPE when the Atlantic moist and warm conveyor belt combined with a LLJ (30 m/s) impinged over the Eastearn Pyrenees more than 10 h. For this last episode, it has been also proved the limited role of the Mediterranean as local moisture source (2-3 g/kg) as the moisture coming from remote source regions (subtropical Atlantic and Africa) is larger (7-9 g/kg).
Divendres, 21 Novembre, 2014