Post frontal sharp showers
 

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

Post frontal sharp showers
 

"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.

Post frontal sharp showers
 

"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.

Post frontal sharp showers
 

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

Post frontal sharp showers
 

"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

Post frontal sharp showers
 

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?

Post frontal sharp showers
 

"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

Post frontal sharp showers
 

Pity there's no Watnall sdg as that wld help a lot,Caistor Bay has dry air aloft which wld overrun
the frontal zone
http://weather.uwyo.edu/cgi-bin/soun...712&STNM=03918
the convection at Chilbolton was v shallow 1-2km ~16Z
http://www.chilbolton.rl.ac.uk/weather/cloudradar.htm
unfortunately there's no data from the Aberystwyth profiler for today ATM..
There is surface convergence at the rear of the front over southern England -
http://weather.ou.edu/~oscarvdv/maps...hetae_eur0.png

IMO insolation wld not be necessary,convergence and orography being enough to release potential
instabilty at the surface cold front underneath the overrunning dry air.The low cloud base and
turbulence of the light to medium showers was striking in SE london and a low level jet seemed
evident.
A later band of lovely Cu con did grow in a gash in the high overcast tho.IMO the low level forcing
was strong in the moist enviroment but capped off by strong descent in the dry air aloft (as I think
Martin suggests)

--
regards,
david
(add 17 to waghorne to reply)

Post frontal sharp showers
 

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

Post frontal sharp showers
 

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