The western Mediterranean is an area that is characterised by complex terrain and as a result is prone to a variety of meteorological events on a broad range of scales, from synoptic to meso-gamma. The local atmospheric circulations in that area, develop in response to synoptic scale forcing but are also determined by the specific conditions.
The ANOMALIA project is an EC project within the objectives of area I, topic 3 (Climate change impacts), research task 8 (Storms and Floods) of the RTD Programme in the Field of Environment 1991-1994, Revised Workprogramme. The project is shared among four European Institutions (JCMM Reading, ISAO-CNR Bologna, UIB Palma de Mallorca, LA Toulouse).
The main objective of the project is the study of severe weather events in the western Mediterranean area, with particular emphasis to those events that produce torrential rain and floods. The project aims at providing a better understanding of the non-linear interaction processes that take place as a consequence of the close proximity of mountains and sea in the western Mediterranean region. The influence of the complex terrain is more pronounced in Autumn and Winter, because during those months the sea acts as a powerful source of latent and sensible heat.
Some of the objectives of the ANOMALIA project coincide with the objectives set for MAP. These include: the effect of orography on deep convection and frontal precipitation that lead to flash flooding, the numerical prediction of moist processes over complex terrain and the better understanding of local factors influencing severe and quasi-stationary events.
Five case studies have been selected for the ANOMALIA project. These are: (a) Libya case (22-25 Nov., 1990), (b) Menorca case (7-11 Oct., 1992), (c) Algeria case (1-7 Feb., 1993), (d) Brig case (21-25 Sept., 1993) and (e) Piedmont case (3-7 Nov., 1994). All of these events produced severe weather in the area of interest with strong winds, floods, damage to property and in some cases, loss of life. Despite their common effects, each case differs from the others in the synergistic factors that forced and influenced the weather system associated with the case. The last two of the ANOMALIA cases (Brig and Piedmont) are associated with the Alpine area and are case studies for MAP as well.
Piedmont case - Preliminary results by the Reading group
The results presented here, involve omega-equation attribution ideas and precipitation forecasts, using data from the UK Met-Office Unified Model, with a resolution of 40 km.
Sources of vertical motion can be studied solving the omega-equation numerically, and evaluating the different forcing mechanisms (synoptic, diabatic heating/cooling). Fig. 1 shows the vertical velocity field at 700 mb, for the 12Z of 5/11/94, as obtained from the forecast model. The solution of the omega-equation is shown in Fig. 2, with only synoptic forcing from all levels (b), and diabatic forcing only (c). Although the diabatic forcing part of the solution explains most of the observed vertical velocity field, it is not the driving mechanism. The maximum in the solution obtained with only synoptic forcing is just off the coast instead of further inland, nearer to the Alps, because the orographic forcing is not included in the solution. Further analysis (not shown) reveals that, for the Piedmont case, the synoptic forcing comes from upper levels.
Figure 1. w at 700 mb, as obtained from the model. Contours every 5 cm/s, first 5 cm/s. (12Z 5/11/94)
This is consistent with the upper level PV pattern over the western Mediterranean during that period, shown on the 315 K surface for the 12Z of the 5/11/94 in Fig. 3. The pattern consists of a high PV elongated strip oriented N-S that approached the area from the West, and became thinner and more intense in time. The vertical velocity at low levels (Fig. 1), consists of a strong localised updraft ahead of the upper PV anomaly, and the pattern looks very much as expected from theory.
The role of the Alps in the observed vertical velocity pattern though, should not be underestimated. The winds at low levels are strong, with a SE orientation over N. Italy and S. France. The Alpine orography, provide an additional lifting mechanism of low-level warm and moist air. In fact, it seems that it is the combined effect of orographic plus upper level forcing that result in the strong updrafts observed in the area.
Figure 2. Solutions to the omega equation. (a) Synoptic only. Contour interval 1 cm/s, first 1 cm/s (b) Diabatic forcing only. Contour interval 5 cm/s, first 5 cm/s. Date: 12Z of the 5/11/94.
Figure 3. PV on the 315 K surface. (12Z 5/11/94)
The 36 hr and 12 hr forecasts of the Unified Model (not shown) for the 12Z on the 5/11/94, show an extensive area of strong precipitation over N. Italy, S. France, with a maximum precipitation rate of about 10 mm/hr. The model also distinguishes between large scale stratiform precipitation and convective precipitation. Stratiform precipitation has a maximum over the land, whereas the maximum of convective type precipitation is shifted over the sea, off the coast.
Work in progress, including the solution of the omega-equation with orographic forcing, will help the better understanding of dynamical and thermodynamical aspects of the Piedmont as well as the other cases of the ANOMALIA project.
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