DIAMET project meeting
IOP2: B648A/B: 20 Sept 2011
Summary: Doug Parker, Leeds, followed by:
Chilbolton Observations: John Nicol, Reading
Cloud physics measurements: Jonny Crosier, Manchester
Latent heating in model/obs: Chris Dearden, Manchester
1200 UTC analysis, 20 Sept 2011
Frontogenetic strain – intensifies front and suppresses waves.
•Three distinct bands of precip oriented SW – NE (at a ~30 deg angle to
the surface front)
• Each band separated by a precip-free slot
• The precip bands are widest to the south; they narrow to a sharp tip
at their northern extremity
Jeffrey Chagnon
•None of the model forecasts indicate this precipitation structure --even at short lead times [Sue Ballard asks about the radar calibration /
rain reaching surface, relative to what the model represents].
B648A Track: High level sondes
Run 2
Run 3
0945 IR/VIS
Upper
level jet
Surface
cold
front
Run2: crossing into jetstream
Run 3: crossing a jetstream maximum (but note Ci
streaming to north of us in IR)
Above cloud deck …
B648A Dropsonde Run 2
Dropsonde data
SCF
Next cloud band approaching from SW?
SCF
Version sketched by
Geraint and me on the
aircraft (x-axis reversed)
0ºC line?
Dry slot
Cold air
SCF
Anticyclonic shear
B648A Dropsonde Run 3
N2 – but don’t trust
the contouring
0°C
B648A-B - track
Stacked
legs
Temperature –
All Runs
– theta-e
“High liquid water –
large drops”
Descending
legs
Descending
legs
Near
pseudoadiabatic
Surf.
Front
50.2N
Drier low
levels
Final profile ascent from 50ft:
Elevated neutral layers and strong inversions. Turbulence.
John Methven forecast trajectories:
“Frontal wave more obvious in forecast, but
passes through in morning.
“Low level PV strip/dry air on cold side of
front is most extensive at 850-800hPa
(not higher).
“Tropospheric fold lies just south of Irish
coast so should aim to turn dropsonde
legs at this location.”
Change in pressure along trajectories
(green=ascent over last day)
IOP2: B648A-B Issues/discussion
• Sequence of frontal waves on a cold front in
frontogenetic strain (suppressing wave growth)
• Reasonably comprehensive observations: we may
be able to make use of the apparent periodicity /
similarity of waves / rainbands on the front.
• What’s the story?
– Mesoscale variability in rainfall on a cold front with
waves
– Diabatic feedback on waves in frontogenetic strain
IOP2/B648 options
• Model / observational case study, describing the
frontal wave structures and associated banding. To
include detailed microphysics …
• Programme of modelling of waves, to investigate
3D structure and role of latent heating e.g.
– Semigeostrophic (Doug Parker / Reading)
– Idealised UM (Sue Gray)
– Full high-res UM (??)
• How did NAE model get waves right over
long range (7 days?)? Chance? (Tom Frame
/ John Methven)