Numerical Simulations of the 1994 Piedmont Flood: Role of Orography and Moist Processes

Abstract

The intense precipitation event that occurred between 3 and 6 November 1994 and caused extensive flooding over Piedmont in northwestern Italy is simulated and tested with respect to various physical aspects, using a meteorological mesoscale model (BOLAM). The period when the most intense rain occurred, mainly covering the second half of 4 and all of 5 November, is examined. A control experiment, starting at 1200 UTC 4 November, simulates the two observed precipitation peaks and captures the magnitude and timing of the most intense precipitation well even at relatively low horizontal resolution (about 30 km). The European Centre for Medium-Range Weather Forecasts analyses are used to provide the initial and boundary conditions. Model output diagnostics and comparison with observations indicate that most of the precipitation is associated with a prefrontal low-level jet, ahead of the cold front, impinging upon the orography of the region (Alps and Apennines). The model simulates a multiple rainband and frontal structure whose evolution determines both intensity and location of the prefrontal warm and moist flow. Almost all of the simulated precipitation over the Alps forms in the middle–low troposphere through forced ascent, whereas part of the secondary maximum, observed over the Apennines, is of convective type. Sensitivity experiments have been conducted to investigate the effects of orography, surface fluxes, and latent heat exchange processes in the atmosphere. The role of the orography is crucial in determining distribution and amount of precipitation, whereas sensible and latent heat fluxes from the Mediterranean Sea (over the period considered) enhanced only the convective precipitation. Distinct dynamical effects, important for the amount and the spatial distribution of precipitation, are found to be associated with warming due to condensation and cooling due to evaporation and melting of precipitation. The latter process seems to be responsible for the simulated formation of rainbands and complex evolution of the cold front over the western Mediterranean. The multiple front life cycle and propagation feeds back on the simulated precipitation distribution, affecting the location of the prefrontal moist flow. Condensation affects the atmospheric effective stratification where the flow impinges on the orography, determining the flow regime (orographic lifting vs blocking and flow around), which, in turn, has an important impact on precipitation.