Marchesi, Stefano (Dipartimento di Scienze dell'Ingegneria, Sezione Osservatorio Geofisico, Universita di Modena, MODENA)
Morelli, Sandra (same affiliation)
Stortini, Michele (same affiliation)
The event of intense precipitation called "Piedmont flood" has been simulated by means of the hydrostatic, limited area model "Eta" in order to try to point out the main forcing terms that characterised it. Among them, the complex orography of the region and the Mediterranean Sea certainly played an important role in determining the event both in space and time and the intensity of precipitation. Some parallel runs have been done in order to study the effect of these forcing terms and to recognize them, by modifying some aspects of the physical parametrization of the Eta model. The resolutions used in these runs are 25 by 25 km in the horizontal, while there are 17 levels in the vertical, from the surface to 100 hPa. The physical parametrization of the model in the control run consists of Mellor-Yamada level 2.5 scheme for the treatment of turbulence in the planetary boundary layer and in the free atmosphere and Mellor-Yamada level 2 scheme for turbulence in the surface layer, with a shallow logarithmic dynamical turbulence layer at the bottom and with a viscous sublayer at the air-sea interface where the vertical transport is determined entirely by the molecular diffusion; large scale precipitation according to a standard condensation criterion; largely revised Betts-Miller scheme for shallow (non-precipitating) and deep (precipitating) cumulus convection; surface processes; radiation.
In particular, concerning the sea, Eta model keeps the surface temperature and the corresponding saturation specific humidity constant during the simulation, as fixed lower boundary conditions. Two runs are compared, the control run and another one with sea surface temperature and specific humidity replaced by the corresponding values in the middle of the lowest atmospheric Eta model layer. Thus, surface sensible and latent heat fluxes are substantially reduced to zero, with the effect of almost completely eliminating the convective component of precipitation over the Mediterranean region. Also the large-scale component is modified in this run with respect to the control run and the pattern of the difference seems to suggest not only a difference in the intensity, but also in the position of the precipitation areas; over Piedmont there is a prevailing decrease of large-scale precipitation.
In addition, the effect of the presence of the viscous sublayer and of the original Betts-Miller scheme for the treatment of convective provesses over the sea is taken into account. Both runs show a general increase of the convective component of precipitation.
The most interesting situations from the point of view of the modification of the components of precipitation are studied into details using vertical profiles of temperature and specific humidity as well as other quantities related to them, such as equivalent potential temperature, which is linked to the potential instability of the atmosphere. These profiles are compared to the corresponding ones obtained from ECMWF initialised analysis and, eventually, with observed profiles.
These first comparisons of precipitation components seem to aim at a strong influence of the Mediterranean sea over this event of intense precipitation in the region. Possibly, other cases of this kind will be studied under this point of view, in order to try to point out some common aspects of such episodes.