Preliminary numerical simulations of the Piedmont flood case of 4-6 November 1994 have been made, in the context of the ``ANOMALIA'' EU-project, in order to investigate the role of the orography, both local and large-scale, and of the Mediterranean sea. The simulations were based on the BOLAM model (Buzzi et al., 1994) at relatively low resolution (30 km in the horizontal with 24 s-levels), using ECMWF analyses as initial and boundary conditions.
A reference experiment allows us to compare precipitation results with available observations and to diagnose the relevant dynamical processes. The model predicted total precipitation, cumulated over the 24-hour period starting on Nov. 5 at 00 UTC, is shown in Fig. 1a. It compares well with the corresponding observed values shown in Fig. 3 of Buzzi et al. (1995). In particular, position and intensity of the double maxima of precipitation, one over the Ligurian Apennines - Maritime Alps and the other over the Italian flank of the Western Alps, with peak values comprised between 250 and 300 mm, are well captured by the model (though the former maximum is displaced a little to the south of the actual position). The simulated precipitation is of stratiform nature in the northern maximum and of mainly convective type (i.e. produced by the moist convective parameterization: see Fig. 1b) in the other near the coast.
Figure 1 24 hour cumulated total (a) and convective (b) model precipitation on 5 Nov. 1994 (reference experiment; integration starting on 4 Nov. at 12 UTC). Isolines drawn every 50 mm.
At low levels, two lines of pronounced convergence and shear appear on Nov. 5 over the Western Mediterranean (Fig. 2a). The one to the west is associated with the main cold front, while the other is apparently a pre-frontal rainband, preceded to the east by a strong south-easterly flow, which has the character of a low level jet (Buzzi et al., 1995). This jet flows over the Ligurian Apennines but is blocked at low levels by the Western Alps, where the strongest convergence and uplifting is simulated (Fig. 2a). Note also the confluence with a weak easterly low level flow over the Po Valley, that was actually observed (Buzzi et al., 1995).
The moisture budget has been computed in different boxes, one covering the area of strong precipitation and the other the entire Western Mediterranean basin. While in the former box precipitation was balanced almost totally by moisture influx due to the southerly flow, over the Mediterranean the lateral fluxes and the surface flux of latent heat contributed with comparable magnitude to the moisture budget.
Figure 2 Wind field at 10 m after 30 h of simulation (5 Nov. 1994, 18 UTC) for the reference experiment (a) and for the no orography experiment (b).
Sensitivity experiments allow us to appreciate the role of different topographical features characterizing the region. A partial removal of the orography in an area of about 400 km radius around Piedmont was sufficient to suppress almost all the precipitation in the area affected by flooding. Flattening of the orography over the entire domain weakened also the intensity of the low and of the associated front over the Western Mediterranean. In both cases, however, the precipitation peaks disappear, while weak precipitation is simulated all along a convergence line appearing from Tunesia to northern France (see Fig. 2b, which refers to the case in which orography was entirely suppressed). A comparison between Fig. 2a and 2b shows the importance of the Alps not only in blocking the flow upstream but also in preventing the organization of the convergence line downstream over Central Europe.
Model results without latent and sensible heat fluxes over the Mediterranean indicate that they contribute moderately to precipitation amounts in the later stage of integration, mainly enhancing convective rain over the Apennines.
Buzzi, A., M. Fantini, P. Malguzzi and F. Nerozzi, 1994: Validation of a limited area model in cases of Mediterranean cyclogenesis: surface fields and precipitation scores. Meteorol. Atmos. Phys., 53, 137-153.
Buzzi, A., N. Tartaglione, C. Cacciamani, T. Paccagnella and P.
Patruno, 1995: The Piedmont flood of November 1994: Preliminary
meteorological analysis. MAP Newsletter, 2, 2-6.
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