The 20th October 2000 case.
Damian Wilson
This is a descending bank of cloud and light precipitation, from a weakly
forced region associated with a nearby front. It could be considered simply
as a cloud system falling out of the sky.
Observational Data
Radar data,
from the Chilbolton radar, for this event can be found via the
University of Reading
Radar Group website.
Aircraft data should follow soon.
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The base of the cloud (from the radar) drops at a rate of around 0.8 m s-1,
which would be consistent with the ice simply falling downwards, rather than
it being a slanted system which is just advected across the radar site.
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There
is evidence of fall-streaks within the system. Since regions of high Z descend
at 3 m s-1 they must be associated with a slanting system which is being
advected. When such a streak reaches the melting level it produces rain. The
animations of this show that a single radar image cannot be considered to
be in equilibrium. This explains why sometimes large amounts of rain appear
to be falling from relatively low reflectivity ice.
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There is an interesting
signature of high ZDR at the base of the cloud, near the radar, in the
cross-sections taken at around 1055. I have not seen this signature elsewhere.
I might speculate it is to do with particles breaking up when they evaporate
but if so I would expect to see it more often. Differential reflectivities
slightly above 0dB exist in the higher cloud, suggesting crystals rather than
aggregates.
Model simulation
- A comparison of the model simulated precipitation with the Nimrod
analysis shows that there is a large problem. The region is far too active in
the model. However, the Nimrod analysis does show that the band was active
just four hours earlier. It could really be a case of cloud falling out of the
sky!
- At 1030 the model produces an anvil, and the lower level activity falls
off away from the radar, as observed, but the two are not really linked. There
is descent in the mid-levels, which keeps the system top relatively low.
Evaporation occurs below the anvil.
- As observed, there is a reasonable amount of embedded convection present.
This is characterised in the observations by the usual ZDR signature, like in
the 30/3/99 case.
Here the model cannot produce a reasonable forecast since the band of
precipitation is too active. However, it does capture the embedded convection
aspects.