Just when I thought I'd mastered some of the maths involved in the
relationship between things like 850hPa and max temp etc today seems to have
thrown quite a big spanner in the works. At 2pm temp was 4.6C under full
cloud. There was some light rain about. According to the GFS hires the 850
hPA was -11C at that time for this location. I realise the lower atmosphere
is a 3D model but this does seem to be a bit extreme with no solar heating.
I have to presume that the slack NE flow is producing an unmixed layer very
close to the surface which is then giving a temperature reflective of the
current temperature of the North Sea rather than the air aloft which would
normally be used for a max temp calculation. This would also explain the
colder temperatures further west despite the higher 850 hPa's.

Dave,
S.Essex

On 13 Feb, 16:38, "Dave Cornwell"
wrote:
Just when I thought I'd mastered some of the maths involved in the
relationship between things like 850hPa and max temp etc today seems to have
thrown quite a big spanner in *the works. At 2pm temp was 4.6C under full
cloud. There was some light rain about. According to the GFS hires the 850
hPA was -11C at that time for this location. I realise the lower atmosphere
is a 3D model but this does seem to be a bit extreme with no solar heating.
I have to presume that the slack NE flow is producing an unmixed layer very
close to the surface which is then giving a temperature reflective of the
current temperature of the North Sea rather than the air aloft which would
normally be used for a max temp calculation. This would also explain the
colder temperatures further west despite the higher 850 hPa's.

Dave,
S.Essex

Could also be that the air is dry? i.e. perhaps a higher lapse rate
for at least part of the ascent? Dry air lapse rate is 1C per
100metres, so in theory you could get a 15c differences between sea
level and 1500metres.

Could it be, Dave, that the model data you quote is incorrect? The midday
ascents show 850 mbar temperature in the range -7 (Albermarle) to -9 (De
Bilt). The closest to your model value was Paris (-10), but the surface temp
there is -3.
Another thing, if the lapse rate is equal to the dry adiabat, it indicates a
well mixed layer, not the opposite. You can not have a shallow surface layer
warmed by the underlying sea, with colder air above, and also have an
unmixed layer at the surface. When cold air overlies a warm sea, there is a
flux of heat from the sea to the atmosphere, and this will lead to a well
mixed boundary layer up to where ever the capping inversion is. In today's
instance, using the ascent for Nottingham at midday, there is a dry lapse
rate up to about 950 mbar, then a saturated lapse up to a capping inversion
at 800 mbar, T= -10. The layer surface to 800 mbar presents a well mixed
layer with convection up to 800 mbar.
The GFS vertical charts for Europe (1800z) do not show 850 mbar temps
anywhere near -11 between 49N and 69N on the Greenwich meridian.
http://www.wetter3.de/vertikal.html

Watch for the line wrap!
http://weather.uwyo.edu/cgi-bin/soun...F%3ASKEWT&YEAR
=2010&MONTH=02&FROM=1312&TO=1312&STNM=03354





--
Bernard Burton
Wokingham, Berkshire, UK.

Weather satellite images at:
www.woksat.info/wwp.html

"Dave Cornwell" wrote in message
...
Just when I thought I'd mastered some of the maths involved in the
relationship between things like 850hPa and max temp etc today seems to
have
thrown quite a big spanner in the works. At 2pm temp was 4.6C under full
cloud. There was some light rain about. According to the GFS hires the 850
hPA was -11C at that time for this location. I realise the lower
atmosphere
is a 3D model but this does seem to be a bit extreme with no solar
heating.
I have to presume that the slack NE flow is producing an unmixed layer
very
close to the surface which is then giving a temperature reflective of the
current temperature of the North Sea rather than the air aloft which would
normally be used for a max temp calculation. This would also explain the
colder temperatures further west despite the higher 850 hPa's.

Dave,
S.Essex

"Bernard Burton" wrote in message
...
Could it be, Dave, that the model data you quote is incorrect? The midday
ascents show 850 mbar temperature in the range -7 (Albermarle) to -9 (De
Bilt). The closest to your model value was Paris (-10), but the surface
temp
there is -3.
Another thing, if the lapse rate is equal to the dry adiabat, it indicates
a
well mixed layer, not the opposite. You can not have a shallow surface
layer
warmed by the underlying sea, with colder air above, and also have an
unmixed layer at the surface. When cold air overlies a warm sea, there is
a
flux of heat from the sea to the atmosphere, and this will lead to a well
mixed boundary layer up to where ever the capping inversion is. In today's
instance, using the ascent for Nottingham at midday, there is a dry lapse
rate up to about 950 mbar, then a saturated lapse up to a capping
inversion
at 800 mbar, T= -10. The layer surface to 800 mbar presents a well mixed
layer with convection up to 800 mbar.
The GFS vertical charts for Europe (1800z) do not show 850 mbar temps
anywhere near -11 between 49N and 69N on the Greenwich meridian.
http://www.wetter3.de/vertikal.html

Watch for the line wrap!
http://weather.uwyo.edu/cgi-bin/soun...F%3ASKEWT&YEAR
=2010&MONTH=02&FROM=1312&TO=1312&STNM=03354


-----------------------
Thanks for the info Bernard - but am I mis-reading these two charts then?
One is the GFS ensemble 850 hPa data for London and the other the GFS hi-res
850 hPa chart at 18z showing -10C and the earlier cold pool having passed
through earlier in the afternoon to the continent. This cold pool
corresponding to my 4C temp at 2pm. I'm clearly a bit confused!
http://www.wzkarten3.de/pics/MT8_London_ens.png
http://www.wzkarten3.de/pics/Rmgfs002.gif

Cheers,
Dave

Dave. I agree that the model has temp of -10 over the Dover Strait and Kent
yesterday afternoon, and surface temperatures there were Langdon Bay 2C and
Greenwich LV 2C, over a sea temp of 7C. The nearest ascent we have is the
midnight for Herstmonceux, which has a -9.9C at 850, and a 1000 mbar temp of
0.0, equating to a max sea level temp of 2C at 1025 mbar. If the boundary
layer between the surface and 850 mbar were dry, the maximum temperature
difference between the surface and 850 mbar would be appx 14C. However, that
layer is not dry, and from cloud base at about 300m (Herstmonceux at 00z) up
to the inversion base at 850 mbar, the air is mostly saturated, with the
corresponding saturated temp lapse rate. For a saturated lapse rate from 850
mbar to the surface, and -10C at 850 mbar, the surface temp would be appx
+0.5 at 1025 mbar. So for an 850 mbar temp of -10, the msl temp would be
limited to the range +0.5 to +4, in the absence of other factors. I am not
sure what part of Essex you are in, but Stansted yesterday reported temps
generally in the range +1 to +2. They are 100m ASL, so you could add another
1C if you are at sea level. It is also possible, and you do not say how your
temp is measured, that your 4.6C is influenced by something local, like
buildings. Also, are you sure of the calibration of your instrument. Lastly,
short-wave radiation can be significant, even on overcast days, and the
satellite imagery for 13th showed considerable breaks in the cloud over the
SE. When the ground is heated by radiation, the boundary layer temperature
profile becomes out of balance, and over a shallow layer near the ground can
exceed the dry lapse rate. Balance is only approached by convective mixing,
which redistributes the surface heating over the entire boundary layer.

http://www.woksat.info/etcsb13/sb13-1330-b-uk.html

And a PS. The base of the capping inversion is at 850 mbar at Herstmonceux.
Slight changes in this height can result in large changes in 850 mbar temp.
Also, if there is cloud up to the capping inversion, long-wave radiation
from its top can produce the same sort of inbalance as heating from the
ground does, depressing the temperature at the inversion base so that the
lapse rate in the upper portion of the cloud exceeds the saturated rate.
Again, convective mixing tries to restore the balance, and the cooling is
redistributed over the entire boundary layer.

--
Bernard Burton
Wokingham, Berkshire, UK.

Weather satellite images at:
www.woksat.info/wwp.html

"Dave Cornwell" wrote in message
...

"Bernard Burton" wrote in message
...
Could it be, Dave, that the model data you quote is incorrect? The
midday
ascents show 850 mbar temperature in the range -7 (Albermarle) to -9 (De
Bilt). The closest to your model value was Paris (-10), but the surface
temp
there is -3.
Another thing, if the lapse rate is equal to the dry adiabat, it
indicates
a
well mixed layer, not the opposite. You can not have a shallow surface
layer
warmed by the underlying sea, with colder air above, and also have an
unmixed layer at the surface. When cold air overlies a warm sea, there
is
a
flux of heat from the sea to the atmosphere, and this will lead to a
well
mixed boundary layer up to where ever the capping inversion is. In
today's
instance, using the ascent for Nottingham at midday, there is a dry
lapse
rate up to about 950 mbar, then a saturated lapse up to a capping
inversion
at 800 mbar, T= -10. The layer surface to 800 mbar presents a well mixed
layer with convection up to 800 mbar.
The GFS vertical charts for Europe (1800z) do not show 850 mbar temps
anywhere near -11 between 49N and 69N on the Greenwich meridian.
http://www.wetter3.de/vertikal.html

Watch for the line wrap!

http://weather.uwyo.edu/cgi-bin/soun...F%3ASKEWT&YEAR
=2010&MONTH=02&FROM=1312&TO=1312&STNM=03354


-----------------------
Thanks for the info Bernard - but am I mis-reading these two charts then?
One is the GFS ensemble 850 hPa data for London and the other the GFS
hi-res
850 hPa chart at 18z showing -10C and the earlier cold pool having passed
through earlier in the afternoon to the continent. This cold pool
corresponding to my 4C temp at 2pm. I'm clearly a bit confused!
http://www.wzkarten3.de/pics/MT8_London_ens.png
http://www.wzkarten3.de/pics/Rmgfs002.gif

Cheers,
Dave

"Bernard Burton" wrote in message
...
Dave. I agree that the model has temp of -10 over the Dover Strait and
Kent
yesterday afternoon, and surface temperatures there were Langdon Bay 2C
and
Greenwich LV 2C, over a sea temp of 7C. The nearest ascent we have is the
midnight for Herstmonceux, which has a -9.9C at 850, and a 1000 mbar temp
of
0.0, equating to a max sea level temp of 2C at 1025 mbar. If the boundary
layer between the surface and 850 mbar were dry, the maximum temperature
difference between the surface and 850 mbar would be appx 14C. However,
that
layer is not dry, and from cloud base at about 300m (Herstmonceux at 00z)
up
to the inversion base at 850 mbar, the air is mostly saturated, with the
corresponding saturated temp lapse rate. For a saturated lapse rate from
850
mbar to the surface, and -10C at 850 mbar, the surface temp would be appx
+0.5 at 1025 mbar. So for an 850 mbar temp of -10, the msl temp would be
limited to the range +0.5 to +4, in the absence of other factors. I am not
sure what part of Essex you are in, but Stansted yesterday reported temps
generally in the range +1 to +2. They are 100m ASL, so you could add
another
1C if you are at sea level. It is also possible, and you do not say how
your
temp is measured, that your 4.6C is influenced by something local, like
buildings. Also, are you sure of the calibration of your instrument.
Lastly,
short-wave radiation can be significant, even on overcast days, and the
satellite imagery for 13th showed considerable breaks in the cloud over
the
SE. When the ground is heated by radiation, the boundary layer temperature
profile becomes out of balance, and over a shallow layer near the ground
can
exceed the dry lapse rate. Balance is only approached by convective
mixing,
which redistributes the surface heating over the entire boundary layer.

http://www.woksat.info/etcsb13/sb13-1330-b-uk.html

And a PS. The base of the capping inversion is at 850 mbar at
Herstmonceux.
Slight changes in this height can result in large changes in 850 mbar
temp.
Also, if there is cloud up to the capping inversion, long-wave radiation
from its top can produce the same sort of inbalance as heating from the
ground does, depressing the temperature at the inversion base so that the
lapse rate in the upper portion of the cloud exceeds the saturated rate.
Again, convective mixing tries to restore the balance, and the cooling is
redistributed over the entire boundary layer.
--------------------
Thanks for going to the trouble of explaining that in detail Bernard. I more
or less understand it! Generally speaking my max temperature calculations
using :-
The relationship between 850-1000 hPa thickness (h*) and the unadjusted
maximum temperature (Tu) is given by: Tu = -192.65 + 0.156h* ... Eq(g)
(Courtesy of our FAQ !) Then modified using table for month and cloud cover.

It gives reasonable results as a rule.
My temperature set up is pretty good I think. AWS housed in a home made
Stevensons screen 1.25m above grass/soil, away from house although slightly
sheltered by garden fence. Can sometimes get slight solar elevation with
strong low late afternoon sun but only about 0.5C. No sun at all that day
which is what surprised me. It can be calibrated and has been in the past
against an NPL calibrated mercury thermometer. (borrowed from work!).
Compares well with other local stations and usually very well with official
one at Gravesend which is 20 miles away across the river. Same time there on
Sat was 3.9C according to UKMO so not too different.

Just one of those little atmospheric weather quirks I guess, Bernard.

Thanks, Dave.
www.laindonweather.co.uk