Unusual line of very sharp showers shows up on the 1300
radar stretching across south Oxon, Bucks and Herts.

I've seen this before, though rarely, when vigorous
cumulus penetrates the overhanging altostratus shield
of an ana-coldfront.

Martin ... mechanism please !!

Philip Eden

"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...
Unusual line of very sharp showers shows up on the 1300
radar stretching across south Oxon, Bucks and Herts.

I've seen this before, though rarely, when vigorous
cumulus penetrates the overhanging altostratus shield
of an ana-coldfront.

Martin ... mechanism please !!

Philip Eden

Don't like to steal Martin's 'thunder' :-) but I was wondering whether it
was a split cold front with the showers mentioned developing on the surface
cold front and the main band on the forward upper front. Will be interesting
to see what's on the 12Z analysis.

Jon.

"Jon O'Rourke" wrote in message
...

"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...
Unusual line of very sharp showers shows up on the 1300
radar stretching across south Oxon, Bucks and Herts.

I've seen this before, though rarely, when vigorous
cumulus penetrates the overhanging altostratus shield
of an ana-coldfront.

Don't like to steal Martin's 'thunder' :-) but I was wondering whether
it
was a split cold front with the showers mentioned developing on the
surface
cold front and the main band on the forward upper front. Will be
interesting
to see what's on the 12Z analysis.


.... yes, it certainly is marked. All I can come up with atm is that the
00Z Watnall ascent (can't see the 12Z yet) which was in the warm air at
that time, *would* produce deep moist instability to the surface
temperature/dew point regime just ahead of the surface cold front - i.e.
Air temp ~19, dew point ~16 or 17. The key would be the surface temp
getting high enough as the AS thinned behind the upper cold (as Jon
noted above), allowing surface-based convection to be initiated and
'focussed' ahead of a PVA-maxima swinging around the upper trough (which
seems broad on the low-res charts I have available - so it will almost
certainly contain minor lobes of +ve vorticity) to both help initiation
and force the resultant convective elements to run 'in the flow'. The
thing we have here that _perhaps_ is not always present is the lobe of
notably high surface dew points that has been advected ENE'wards (as
noted in another thread).

That's my starter: now bring on the clever boys and girls!

Martin.

"Martin Rowley" wrote in
message ...


... yes, it certainly is marked. All I can come up with atm is
that the
00Z Watnall ascent (can't see the 12Z yet) which was in the warm
air at
that time, *would* produce deep moist instability to the surface
temperature/dew point regime just ahead of the surface cold
front - i.e.
Air temp ~19, dew point ~16 or 17. The key would be the surface
temp
getting high enough as the AS thinned behind the upper cold (as
Jon
noted above), allowing surface-based convection to be initiated
and
'focussed' ahead of a PVA-maxima swinging around the upper trough
(which
seems broad on the low-res charts I have available - so it will
almost
certainly contain minor lobes of +ve vorticity) to both help
initiation
and force the resultant convective elements to run 'in the flow'.
The
thing we have here that _perhaps_ is not always present is the
lobe of
notably high surface dew points that has been advected ENE'wards
(as
noted in another thread).

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?

pe

On Fri, 27 Aug 2004 15:34:29 +0100, "Martin Rowley"
wrote:

... yes, it certainly is marked. All I can come up with atm is that the
00Z Watnall ascent (can't see the 12Z yet) which was in the warm air at
that time, *would* produce deep moist instability to the surface
temperature/dew point regime just ahead of the surface cold front - i.e.
Air temp ~19, dew point ~16 or 17. The key would be the surface temp
getting high enough as the AS thinned behind the upper cold (as Jon
noted above), allowing surface-based convection to be initiated and
'focussed' ahead of a PVA-maxima swinging around the upper trough (which
seems broad on the low-res charts I have available - so it will almost
certainly contain minor lobes of +ve vorticity) to both help initiation
and force the resultant convective elements to run 'in the flow'. The
thing we have here that _perhaps_ is not always present is the lobe of
notably high surface dew points that has been advected ENE'wards (as
noted in another thread).

Staggers back confused by all the two and three letter acronyms, camly
opens another stella, and says

whats all this mean in laymans terms?

"Keith Wassell" wrote in message
...
On Fri, 27 Aug 2004 15:34:29 +0100, "Martin Rowley"
wrote:

... yes, it certainly is marked. All I can come up with atm is that
the

snip

Staggers back confused by all the two and three letter acronyms, camly
opens another stella, and says

whats all this mean in laymans terms?

.... he, he: sorry about that! We're just trying to decide on the
'where's and 'why,fors' of the discrete line of, for a time, intense
convection that broke out across the north & NW Home Counties, well to
the north of the main rain area (and underneath that latter's cloud
shield) that slowly came south over southern England today.
You can find a simple (I hope) explanation of some of the terms in the
FAQ/Glossary (see my sig file), but in broad terms, I was trying to
imply (subject to correction), that the showers were set off because the
upper cloud thinned enough to allow the still strong sun to warm the
ground, lifting the temperature high enough to start cumulus clouds
growing, there being high humidity [i.e. dew points] (i.e. plenty of
'fuel' to let the cloud/rain droplets grow) available. The +ve PVA stuff
is short-hand to indicate that the windflow in the air above this area
is such that it encourages these things to both grow (vigorously) in an
organised fashion - as opposed to a 'pulse' (or 'here and there') way,
and become strung out in the manner that they did along the flow.

HTH [ & no doubt the Stella will make all clear ;-), though I prefer a
Guinness ]

Martin.


--
FAQ & Glossary for uk.sci.weather at:-
http://homepage.ntlworld.com/booty.weather/uswfaqfr.htm

In message , Philip Eden
writes
Unusual line of very sharp showers shows up on the 1300
radar stretching across south Oxon, Bucks and Herts.

I've seen this before, though rarely, when vigorous
cumulus penetrates the overhanging altostratus shield
of an ana-coldfront.

Martin ... mechanism please !!

Philip Eden

Had a short burst of torrential rain in Chalfont St Giles from one of
these showers. The raindrops were about as big as any I have seen.

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

Having only just got my hands on the Nottingham ascent for 1200, it seems
that the ingredients for the rapid convective development lay in a
combination of potential instability and low level sensible heating and
convergence.
The theta w for the ascents 03354 and 03882 are given below, together with
an interpolated intermediate location. Although the low level high theta w
air was well capped by post frontal subsidence at 03354, the strength of
this capping decreased southwards across the frontal band, although the
potential instability also decreased southwards, it would still have been
significant at the intermediate location. It is likely that very high theta
w air had detrained to the north of the frontal band in the boundary layer,
and insolation through the thinning upper cloud would have increased this
further. Local topographic forcing and flow convergence, possibly at post
frontal discontinuities parallel to the front, acted as 'micro fronts'
providing further positive feedback on convective initiation process.
Seeding from the overhanging upper cloud may have been a modulating
influence on the shower intensity.

Theta w
03354 03882 INTER
P
940 12.5 15.9 19.0 (after heating)
900 11.8 16.2 14.0
850 12.0 16.2 14.1
800 10.1 15.7 12.9
700 10.8 17.7 14.2
600 12.0 17.1 14.5
500 13.7 18.1 15.9
400 15.9 18.4 17.1


--
Bernard Burton
Wokingham, Berkshire, UK.

Satellite images at:
www.btinternet.com/~wokingham.weather/wwp.html



"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...
Unusual line of very sharp showers shows up on the 1300
radar stretching across south Oxon, Bucks and Herts.

I've seen this before, though rarely, when vigorous
cumulus penetrates the overhanging altostratus shield
of an ana-coldfront.

Martin ... mechanism please !!

Philip Eden