Some Prominent Flow Features
A prominent feature during the period was the convergence of the low-level
flow in the western part of the Po Valley (Fig. 4a) due to easterly winds
in the Po Valley and southeasterly winds blowing from the Gulf of Genoa
across the Apennines. This airflow characteristics existed during the
whole period until the end of the main precipitation period in the late
evening of 5 November . The vertical south-north cross section in Fig.
4b) shows that the easterly flow over the Po Valley was concentrated
essentially below 850 hPa and was predicted with a maximum wind speed of
more than 21 ms -1 . This low-level easterly jet is well confirmed by the
soundings of Milan. At upper levels the wind direction turned gradually
from east to south.
- FIGURE 4:Represented are equivalent potential temperature
(grey-shading and dash-dotted isolines) and the east component of the wind
(bold dashed lines). Negative values indicate that the wind blows into the
plane.
- Panel (a): Predicted wind at 10m valid for 12 UTC 5 November 1994
(12 h forecast).
- Panel (b): Vertical south (left) Ð north (right) cross section
at 9°ree;E longitude (Milan) through the Apennines and the Alps.
Vertical velocity fields give a good impression of the major processes
leading to precipitation in the SM. On 06 UTC 5 November three particular
areas of intense lifting at 700 hPa can be distinguished (
Fig. 5a): The vertical velocity band
extending in the north-south direction east of Marseille is indicative of
the warm conveyor belt ahead of the cold front. Upward motion is enhanced
by orographic lifting near the coast. In this band moist and warm air was
advected to the north. The second maximum of upward motion, elongated
along the coast at the southern end of the Alpine chain, is due to
orographically triggered convection in the southerly flow. In this area
the model produced a maximum of convective precipitation. Finally
significant orographic lifting is predicted at the southern slope of the
Alps at the northern edge of the Po Valley. Strong Foehn was also present
over the northern slope of the Alps (cf. Fig. 4a).
12 hours later (18 UTC 5 November) the warm conveyor belt was still well
marked by the vertical velocity band now located over Corsica and Sardinia
and reaching into the Po Valley. The zone of most pronounced orographic
lifting was then established at the western border of the Po Valley leading
to the extraordinary precipitation maximum of 14.0 m/s (
Fig. 2b). This shift of the main
precipitation area from the northern edge of the Po Valley into the western
bow of the Alps is well documented by the Monte Lema radar (
cf. Fig. 3). The dramatic intensification
of rainfall might be linked to the enhanced mass convergence in the western
Po Valley during this event. Convection along the coast resulted from the
advection of unstably stratified air over the sea in the general southerly
flow (Fig. 4b).
MAP Newsletter. October '00