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stratiform precipitation, associated with older cells, could be seen to the North. The vertical
velocity field (Fig. 13, bottom right) in the convective region also shows intense updraft up
to 8 m/s, which is consistent with observed reflectivity cores up to 60 dBZ. Note that
retrieved vertical velocities were significantly higher than those retrieved within the 7
September 2010 system due to the much higher horizontal resolution of the wind field (1 km
vs. 2.5 km).
A new precipitation event occurred over the same area the day after as a new stationary
MCS formed at about the same location, along the southern flank of the Massif Central
Mountains (Fig. 14). This time, however, the situation was quite different as a rainband
associated with a southeastward propagating cold front merged with the isolated MCS at
the end of the day and eventually swept it away. In essence, this new event is thus
relatively similar to the 7 Sept 2010 case described previously. Figure 15 shows the evolution
of the MCS between 0 UTC and 8 UTC on the 22 nd of October. This time series begins
slightly after the frontal system reached the Massif Central. The location of the cold front can
be identified by the wind shift seen at both low and mid-levels. Note that the strong low-
level southerly flow impinging on the barrier at 0 UTC quickly weakened as the front
approached the eastern flank of the Massif, which acted to cut the feed of moisture
originating from the Mediterranean Sea and prevented the formation of new convective
cells. Convection over the area thus died very quickly and the MCS was rapidly swept away
by the cold front. A major difference with the 2010 case is that the frontal system did not
remain blocked over the relief and stratiform precipitation over the slopes thus only lasted
for a short period of time. This is likely the reason why no reversal flow could be observed
over the slopes of the Massif Central on this day as the cooling resulting from melting and
evaporation of precipitation particles, which is responsible for the formation of the
downslope flow, was not sufficient for this phenomenon to occur.
Fig. 14. French operational radar reflectivity mosaic outputs on 21 October 2008 at 00, 12,
and 23 UTC. Red circle indicate the location of the isolated MCS.
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