Geoscience Reference
In-Depth Information
At 21 UTC, one can note a profound reversal of the incident upslope-oriented flow, resulting
in the formation of a northwesterly flow over the slopes of the Massif Central. Such
pronounced return flow has already been observed over the Alps in the frame of the MAP
experiment from airborne and mobile radar systems in response to negative buoyancy
generated by both melting and evaporation of precipitation particles below the 0
o
C isotherm
(Bousquet and Smull 2003, Steiner et al. 2003). The process generating the observed downslope
flow by cooling from melting and evaporation of precipitation particles in the Massif Central is
likely similar to that observed over the Alps in the late nineties. This is supported by the fact
that stratiform precipitation lasted for several hours over the mountains before the formation
of this downslope circulation (Fig. 10g,i,k). In this particular case, however, it seems that the
downslope flow has had a strong impact on the convective activity by triggering new cells in
the Rhone valley (Fig. 11d), whereas its effect was not found significant in previous studies.
After the merging, the frontal system remained blocked over the Massif Central and the
eastern part of France for about 12 hours during which another 100 mm of rain felt over the
Cevennes area. It finally passed the Cevennes near 00 UTC, 8 September as it was swept
away by another frontal system approaching from the West. At this time, the low surface
pressure anomaly identified in Fig. 8c had reached the French territory and had started to
extend over a large part of the country. The associated cyclonic circulation (Fig. 10m) was
well captured by the French radar network.
More information about the vertical structure of precipitation can be inferred from Fig. 12,
which presents 6-hourly meridional cross-sections of the retrieved 3D composite reflectivity
pattern along a 1000 km line ranging from the Golfe du Lion, in the Mediterranean Sea, to
Belgium. These cross-sections provide a unique picture of the structure and evolution of
precipitation over the entire country and can be very useful to quickly identify regions of
intense rainfall, as well as to segregate between frontal and more convective precipitation. This
time series, extending from 00 UTC to 18 UTC on 7 September, thus confirms that convection
became significantly more intense after the frontal system reached the Massif Central
Mountains. At 12 UTC (Fig. 12c) and 18 UTC (Fig. 12d) one can see that very deep convection
was thus occurring over the Cevennes and the Rhone valley with convective cells reaching up
to 35 dBZ at a height of 11 km.
In addition to horizontal wind fields, the MUSCAT analysis used by the French weather
service also allows to retrieve accurate vertical velocities in the whole precipitating area within
the 1000 km x 1000 km domain of analysis. Retrieved vertical motion fields at a height of 6
km, which is the altitude at which maximum vertical motion was observed, are displayed in
Fig. 10. Overall, one can note a very good consistency between the location/intensity of
updrafts and the position of the most active convective cells (left panel). Upward vertical
motions are the most intense after the MCS has merged with the frontal rainband that is the
moment when low level convergence was the most important. This observation is consistent
with the vertical structure of precipitation deduced from Fig. 12 that indicates the presence of
deep convection and shows a particularly impressive vertical extension of the convective cells.
During the Klaus storm, getting real-time or quasi real-time information about wind
intensity would have been particularly useful to forecasters in order to trigger or cancel
alerts, as well as to precisely monitor the propagation of the strong wind swath. On
September 7 2010, a watch was ongoing for heavy precipitation and flash flood in the
Cevennes area but forecasters were more interested in getting high resolution radar
quantitative precipitation estimates in order to assess the hydrological risks.
