Influence of cold pools on the circulation over West ... - Convection

School <strong>of</strong> Earth and Environment
INSTITUTE FOR CLIMATE AND ATMOSPHERIC SCIENCE
<strong>Influence</strong> <strong>of</strong> <strong>cold</strong> <strong>pools</strong> on the circulation
over West Africa and the Sahara:
Observations and convection-permitting
simulations
Doug Parker
Met Office Pr<strong>of</strong>essor <strong>of</strong> Meteorology
GFI, Bergen, 3 September 2013
Acknowledgements: Cathryn Birch, Cyrille
Flamant, Luis Garcia-Carreras, John
Marsham, Tim Wright

Outline <strong>of</strong> talk
1. Quick introduction the West African monsoon;
2. Observations <strong>of</strong> <strong>cold</strong> <strong>pools</strong> and circulation over West
Africa / Sahara: we have measurements <strong>of</strong>
a. New <strong>cold</strong> <strong>pools</strong>;
b. Aged <strong>cold</strong> <strong>pools</strong>;
c. Statistics <strong>of</strong> many <strong>cold</strong> <strong>pools</strong>.
3. Cascade modelling <strong>of</strong> <strong>cold</strong> <strong>pools</strong> and the circulation.

Typical West African Monsoon structure
Fink et al. (2014) Mean
climate and seasonal cycle <strong>of</strong>
the West African Monsoon. In
Meteorology and forecasting
for tropical West Africa; in
preparation. Parker and Diop-
Kane, Eds.

AMMA observations
5 aircraft,
3 ships
SOP3
SOP1&2
Around 800 scientists,
engineers and operational
specialists from 25
countries, 2004-2009

Atlantic
Sahara
The monsoon flow is active at night and in the
morning. By day the winds are light and the
buoyancy well-mixed in the vertical.
Pressure-latitude plots <strong>of</strong> potential temperature (~ buoyancy)
and winds, 0300-0300 UTC, 28-29 Aug 2000, from the Met
Office Unified Model over West Africa. Note we are not resolving
gravity currents explicitly here, but observations show that they
tend to form at the nose <strong>of</strong> this flow. Parker et al. (2005) QJR
Meteorol. Soc.

Monsoon diurnal cycle resembles
Linden+Simpson model
Linden and Simpson (1986): baroclinic zones strengthen when the daytime
turbulence dies down, and thermally-direct circulations (with gravity current
fronts) develop.
Bou Karam et al. (2010) show evidence <strong>of</strong> this using airborne lidar.
~ 1500 Z
~ 0300 Z

Cold pool interacting with
monsoon front (ITD): 5 June 2006
Meteosat split-window
image with flight-track
(above).
Agoufou surface
observations (left).
Flamant et al. (2007)

Cold pool interacting with
monsoon front (ITD): 5 June 2006
Lidar dust-loading measure (shading) and dropsondederived
potential temperatures (contours).
ITD / <strong>cold</strong> pool are basically
indistinguishable at this time.
Dust front / ITD
Flamant et al. (2007)

Following day … 6 June 2006
InSAR total column water
anomalies derived from
space-borne radar. ~20m
resolution!
Evidence <strong>of</strong> CBL
convective eddies within
aged gravity current flow.
Parker and Wright
(unpublished).
Also Flamant et al. (2009)

Fennec case: 21 June 2011
Deep convection over Atlas mountains generates <strong>cold</strong> <strong>pools</strong>
heading south into the Sahara, 1800 UTC – 0600 UTC, 20-
21 June 2011.
Airborne measurements ~0900 UTC, 21 June, when the <strong>cold</strong>
pool has aged.
Following sequence <strong>of</strong> plots shows SEVIRI dust product,
hourly starting 2100 UTC on 20 June 2011

B605: <strong>cold</strong> pool from Atlas
21 June 2011 [Ǝ F-F20 flight data also]
Top <strong>of</strong>
SABL
Sonde
s
Mixed
layer top
Cold pool
Dust layer /
edge, 11
kft, 23.0 N.
Dust
front
23.83 N
Plume <strong>of</strong>
“frontal”
ascent ~
23.5N
Lidar, North ---
South
Mixed
layer
top

Internal layer /
aged <strong>cold</strong>
pool?

23 – 23.5N
B605: Low level run (onboard
plots repeated in next slides)
Dust front
Gravity wave?
Shear line
Dewpoint
O3
Winds
(vectors
sketched)
Nephelometer

21 June 2011 case: Comments
Dust front has a significant, 2K temperature contrast but no
apparent wind structure: winds dominated by along-front
easterlies. Perhaps the turbulence at the surface is
suppressing the flow as in Linden and Simpson?
Does the penetration <strong>of</strong> the <strong>cold</strong> pool air accelerate the NE
jet? See next slide:

Interpretation / hypothesis 1 …
Intrusion <strong>of</strong>
<strong>cold</strong> air
“sharpens”
pressure
gradient here.
Intensification
<strong>of</strong> NE jet.
Lifting <strong>of</strong> PBL air
by <strong>cold</strong> pool
penetration.
Cold
pool
South
North
Separation <strong>of</strong> <strong>cold</strong> pool and mixed layer
observed by lidar / sonde 1.
Cold pool mainly mixed-out by the time <strong>of</strong>
the final ascent on B605.

Interpretation / hypothesis 2 …
Intrusion <strong>of</strong>
<strong>cold</strong> air
“sharpens”
pressure
gradient here.
Intensification
<strong>of</strong> NE jet.
Cold
pool
Internal mixed layer.
South
North
Sonde 1 observes remnant deep <strong>cold</strong> pool with
a shallow internal mixed layer – shallower than
southern sondes due to aerosol influence,
reducing surface fluxes.
Cold pool mainly mixed-out by the time <strong>of</strong> the
final ascent on B605.

View from the cockpit: Gust front over
northern Benin, 3 August 2006
Alan Foster, DirectFlight

MIT radar data from Niamey airport (13.6N, 2.17E)
3 August 2006
Flight B225: Run1, 500ft, 1336 – 1437
UTC

Over northern Benin, 3 August 2006
Gravity current in the “near field”.

The gust front can be associated with
severe winds, possibly up to 100 knots.
After …
Before …

NWP biases from <strong>cold</strong> <strong>pools</strong>: Central Sahara
(Garcia-Carreras et al. 2013, GRL)
Mesoscale flows
(<strong>cold</strong> <strong>pools</strong>) seem
to be important on
the large scale.
0-B: Obs – Model first
guess (solid)
O-A: Obs– Analysis
(dashed)
Night-time <strong>cold</strong>
<strong>pools</strong>: 13 pr<strong>of</strong>iles
Day-time <strong>cold</strong>
<strong>pools</strong> (07 to 18Z): 7
pr<strong>of</strong>iles
No <strong>cold</strong> <strong>pools</strong>: 104
pr<strong>of</strong>iles
• NWP model errors (solid lines) much larger when there are <strong>cold</strong> <strong>pools</strong>/nighttime
<strong>cold</strong> <strong>pools</strong> seen in Fennec data
• Assimilation impacts (solid versus dashed) small for no <strong>cold</strong> <strong>pools</strong>, large for
(day-time) <strong>cold</strong> <strong>pools</strong>
• Analysis errors (dashed) significant for temperature in night-time <strong>cold</strong> <strong>pools</strong>

Cascade-Africa UM simulations now
(and those originally planned).
Resolution
NWP and
Climate
Param.
convection
Long-term
Explicit
convection
40 km 100+ days
(6)
12 km 100+ days
(-)
100+ days
(6)
4 km 100+ days
(6)
Simulations performed by Grenville
Lister (Reading) on HECTOR national
supercomputer. Later simulations
conducted by partners on HECTOR.
1.5 km 12 days
(6)

Diurnal cycle rainfall rate
comparison
Plot is diurnal average
<strong>of</strong> mean rainfall rates
over 5-25N and 10W-10E
during 25 th – 29 th July
Parameterisation
scheme has maximum in
middle <strong>of</strong> day and little
overnight
Explicit model runs have
rainfall maxima around
18z like TRMM but peak
in model up to twice obs

Differences in mean state: 12km runs
• Different wind patterns transport different amounts <strong>of</strong> water northwards

Sahel - Sahara
geopotential difference
(m)
Meridional velocity
(ms -1 )
Impact on the diurnal cycle <strong>of</strong> the WAM
• Diurnal cycle in pressure gradient consistent with diurnal cycle in rain.
• Later rainfall in explicit run gives later minimum in Sahel-Sahara pressure
gradient.
Lots more details in Marsham et al., 2013, GRL

Convective <strong>cold</strong> pool outflows
Eddy heat fluxes composited around rain events
Exp
Param.
(note different colour-scale)
• In 12kmExp <strong>cold</strong> <strong>pools</strong> transport <strong>cold</strong> air north
• Six times more northwards flux than southwards transport
• Missing in 12kmParam
• 30% <strong>of</strong> cooling by meridional advection is within 250km <strong>of</strong> rain
in 12kmExp, only 10% in 12kmParam (Marsham et al., 2013, GRL)

v*q (m s -1 kg kg -1 )
v*q (m s -1 kg kg -1 )
Moisture flux (v*q) at 400m
Param. models over-cook monsoon
Climate
NWP
Mean diurnal cycle v*q at Niamey (14°N)
40km param
12km param
Time <strong>of</strong> day
Mean diurnal cycle v*q at 6°N
12km exp
4km exp
Time <strong>of</strong> day

Haboobs and dust uplift

Haboobs – density currents
• False colour image
• Dust is pink, deep cloud is red
• Example <strong>of</strong> a significant dust event on 9 th /10 th July 2011

Dust-uplift as a function <strong>of</strong> process
resolution.
LLJ
Haboobs
•Averaged over 1.5-km
domain
• Dashed lines:
parameterised
convection.
Marsham, et al. (2011), Geophys.
Res. Lett., 38, L16803.
• All models show similar peak from nocturnal low-level jets (LLJ)
• Only models with explicit deep convection show large peak from haboobs (dust
storms) (~50% <strong>of</strong> uplift potential) .
•At climate resolutions, LLJ is there, but almost non-existent haboobs and monsoon
uplift.
• “Tuning” cannot correct for this missing process

Conclusions
1. NWP biases and Data Assimilation stats from the Sahara
suggest that the absence <strong>of</strong> <strong>cold</strong> <strong>pools</strong> leads to a
substantial part <strong>of</strong> the model temperature and humidity
bias for the Sahara (Garcia-Carreras et al. 2013)
2. Cascade model suggests that <strong>cold</strong> <strong>pools</strong> are a significant
part <strong>of</strong> the monsoon moisture flux: absence <strong>of</strong> this flux in
parametrised-convection models in part leads to overactive
monsoon in compensation (Marsham et al. 2013).
3. We observed <strong>cold</strong> <strong>pools</strong> when they are “fresh”, and when
they have interacted with the ambient flow and land
surface. Gravity currents are modified; there is evidence <strong>of</strong>
gravity wave interactions. Several interesting cases are not
yet analysed …

Some questions
1. Many important processes, like triggering within MCSs,
occur in the gravity-current “near-field” (i.e. O(h) distance
from <strong>cold</strong> air source). The far-field is also important.
2. What determines decay <strong>of</strong> <strong>cold</strong> <strong>pools</strong>: surface fluxes (e.g.
Ross et al. 200x); ambient dynamics (e.g. Flamant / Bou
Karam papers); <strong>cold</strong> air run-out?
3. How do some <strong>cold</strong> <strong>pools</strong> persist for long distances
O(1000km)?
4. Gravity currents and gravity waves; what is their interaction
and role in convective triggering?

Future plans / opportunities
1. Observations <strong>of</strong> <strong>cold</strong> <strong>pools</strong> and circulation over West Africa /
Sahara: we have measurements <strong>of</strong>
a. New <strong>cold</strong> <strong>pools</strong>;
b. Aged <strong>cold</strong> <strong>pools</strong>;
c. Statistics <strong>of</strong> many <strong>cold</strong> <strong>pools</strong>.
Lots <strong>of</strong> these cases are not analysed or published. We’d welcome
collaboration! Lots <strong>of</strong> good MSc / PhD student projects!
2. Cascade modelling <strong>of</strong> <strong>cold</strong> <strong>pools</strong> and the circulation. Miroslav;
Birch; new Met Office PDRA; new NERC proposal.
3. NERC proposal (Met Office, Leeds, Cambridge).
4. EPSRC proposal on severe weather also planned (Ross, Leeds).

Other West African <strong>cold</strong>-pool
references
• COPT-81 case well observed: see Chalon et al. (1988?).
• Flamant papers / Bou Karam papers: the ITD can behave
as a gravity current; <strong>cold</strong> <strong>pools</strong> reinforce its structure.
• Marsham et al. 200x: in situ aircraft measurements <strong>of</strong> aged
<strong>cold</strong> <strong>pools</strong> within the ITD, including dust uplift and chemical
composition.
• Marsham et al. 2013: Descriptions <strong>of</strong> <strong>cold</strong>-pool events in
BBM observations in central Sahara
• Taylor et al. 2010 / Birch et al. 2013 : Cold pool and gravity
waves implicated in secondary convective initiation
(actually, they don’t interact much in this case).

The end
Thanks for your attention.

Bias in global UM NWP 925-hPa potential temperature
(Luis Garcia-Carreras)
• UM too warm (MSLP too low) at ~20N, too <strong>cold</strong> (MSLP too high) at coast (~8N)
• Too strong a pressure gradient – too strong a monsoon

Diurnal cycle in bias
• Mid levels (~6800m): model too warm at 18Z and too <strong>cold</strong> at 00Z,
consistent with early parameterised convection
• PBL: warm PBL at 15N at 18Z, with a cool bias at 06Z, consistent
too weak <strong>cold</strong> <strong>pools</strong> and an overactive monsoon.

Gust front over northern Benin,
3 August 2006
v-wind (top) and u-
wind (bottom)
Nephelometer : gives dust
loading (and insects?).
Plumes on edges <strong>of</strong> <strong>cold</strong> pool
associated with uplift).

Nephelometer data (dustloading)
in pr<strong>of</strong>ile ascent
through <strong>cold</strong>-pool front.
Well mixed in “<strong>cold</strong> pool”
up to 900 hPa. Suggests
well-mixed through PBL
at this time.
Gravity
waves / K-H
in <strong>cold</strong> pool?