Dynamics and microphysics of orographic precipitation during MAP IOP3
Résumé
A dynamical and microphysical four-dimensional study of an intense orographic precipitating system is carried out in the frame of MAP IOP3 (25–26 September 1999). High precipitation opportunely occurred in the range of the Swiss operational Doppler radar at Monte Lema (Switzerland), the US SPOL polarimetric Doppler radar and the French Ronsard Doppler radar both located near Lago Maggiore (Italy). Radar data have been combined to deduce four-dimensional precipitation and wind fields during the most intense precipitation period (1600–2000 UTC on 25 September). The organization and evolution of the microphysical field have been obtained through an analysis of SPOL polarimetric data: nine hydrometeor classes (light, moderate and heavy rain, hail, rain–hail and graupel–hail mixtures, dry and wet snow, and ice crystals) are inferred by means of a fuzzy-logic method initially developed by Vivekanandan et al.
The temporal mean study of reflectivity fields reveals that the precipitation presents a convective pattern and is primarily located on the foothills and over the first mountain. Mean microphysical fields reveal rain below the 0 °C level, wet snow, ice crystals and dry snow above with an embedded 2 km deep layer of graupel–hail mixture. This suggests convective microphysical processes.
Temporal series of precipitation and wind fields are then analysed in order to characterize in detail the organization and evolution of the system. Three stages were identified: first an elongated structure growing and intensifying over the lake while moving towards the Alps, then a spreading of the system over the mountains and finally a weakening over Lago Maggiore and the mountains. It has to be noted that the lake and the first mountainous peaks were important factors in the generation and the intensification of convective cells: the lake acts as a secondary moisture source which can favour local convection, and mountainous slopes favour updraughts which permit a downwind extension of the system by ejection of precipitating particles according to the fountain particles concept. Finally, through a temporal and spatial microphysical study, coalescence below the 0 °C level, riming and freezing above, are concluded to be the major processes in the formation of intense precipitation. The essential role of ice phase in the formation and enhancement of precipitation has been highlighted.
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