In message , Martin Rowley
writes

"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...

snip
What might have been the impact of convection cells reaching the
moist layer (i.e. the frontal surface), say somewhere between 600
and
700 mbar?

... I would normally expect some signs of those towers both feeding that
layer (as per Sc cugen), and being constrained (by slightly more stable
temperature lapse), or perhaps opposed by the general descent of air
that might be implied by this type. In this situation that you, Norman
(and to a lesser extent I) observed threw up highly vigorous convective
towers that could 'punch' through the residual overhang without
impacting upon it in any marked manner. I would however have liked to
see an ascent that was representative of that particular bit of the
airmass and I'm not sure we have one - the Watnall was the best I could
offer.

Following from this last, I'm not sure that we are dealing with a
'standard' (if there is such a thing) mid-latitude depression and it's
fronts: I have just had a very crude 'shoofty' back at the charts over
the past 5 or so days, and I suspect that there are some tropical
remnants engaged (ex-Danielle possibly coming around the western leg of
the mid-Atlantic high): the description of the cloud is very like the
classic 'eyewall' convection buried amongst extensive layer cloud so
beloved of the text books - yes I know, the text books aren't always
that good, but the high dew points we experienced suggests to me that
the analysis was somewhat complex!

Martin.

The convective towers developed here very quickly. After the frontal
rain stopped there was a bit of weak sunshine through some thinner parts
of the altostratus overhang. This quickly pushed the temperature up to
about 20c, still with the relatively high dewpoint mentioned by Martin.
This rise in temperature seems to have been at least partially
responsible for triggering the development. Within only a few minutes
the cumulus towers pushed through the altostratus overhang. I didn't
keep a check on the timing but my guess is that we had a torrential
shower not much more than 15 minutes after the cumulus towers first
started to develop. It certainly wasn't an everyday situation.

Norman.
(delete "thisbit" twice to e-mail)
--
Norman Lynagh Weather Consultancy
Chalfont St Giles
England