sensitivity of cold air pool evolution in hilly terrain regions
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Sensitivity of cold air pool evolution in hilly terrain regions Bradley Jemmett-Smith 1 , Andrew Ross 1 , Peter Sheridan 2 , John Hughes 1 21 st Symposium on Boundary Layers and Turbulence Leeds, UK 9 June 2014. 1 Institute of Climate and Atmospheric Science , University of Leeds, UK - PowerPoint PPT PresentationTRANSCRIPT
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Sensitivity of cold air pool evolution in hilly terrain regions
BRADLEY JEMMETT-SMITH1, ANDREW ROSS1, PETER SHERIDAN2, JOHN HUGHES1
21st Symposium on Boundary Layers and Turbulence Leeds, UK
9 June 2014
1Institute of Climate and Atmospheric Science, University of Leeds, UK2Met Office, Exeter, UK
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Jeremy Price (Met Office)
Clun Valley, Shropshire, UK
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Nocturnal Boundary Layer• Develop around sunset, forming in dips,
hollows, valleys or basins etc.
• Cold air pools (drainage flows/katabatic
winds) and/or cools in-situ.
• Break-up following sunrise as CBL
establishes.
• Ideal conditions; low wind speeds, no
cloud cover, stable atmospheres. (Whiteman, 2000)
Cold air-pools (CAPs) in complex terrain
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Why?
• Hazardous driving conditions; frost, fog, persistence of lying snow.
• Health risks; pollution episodes.
• Representation in Numerical Weather Prediction models; unresolved
scales, SBL parameterizations, surface energy balance.
• Improve understanding of SBL processes; better representation of
minimum temperature, fog, frost etc.
• Recent capabilities allow high resolution modelling of scales <1km.
• Application of downscaling techniques, e.g., for improved road
temperature forecasts.
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• Comprehensive field campaign, June 2009 to April 2010.
• University of Leeds, Met Office, NCAS.
• Satellite weather stations (HOBOS, AWS), wind, temperature and pressure.
• Large flux towers; components of the energy budget – see Price et al, (2011).
• Valley depths ~200m, widths typically 1km.
COLd air Pooling EXperiment (COLPEX)Unique data set of SBL observations
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9–month climatology
• Nights with temperature inversions greater than
4oC occur 23% of the time.
• Strong CAPs frequent during October and April –
periods of prolonged high pressure.
• Strongest CAP observed 7-8th January, exceeding
9.9oC: ~5oC/100m. Tmin = -18.2oC.
How often do they occur?
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IOP 16 4-5 March 2010
• High pressure
• Low wind speeds
• Clear skies
IR sat image 00:00 UTC 5 March 2010
Analysis 00:00 UTC 5 March 2010
“Ideal” case study – Synoptic conditions
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4–5 March 2010
WarmingCooling
Theta and ELR evolution
Potential temp Env. Lapse rate
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Episodes of evolution disruption
LIDAR vertical velocity 4-5 March 2010
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Episode 1 – Gravity wave activity?
LIDARVertical velocities
Radiosondeascent rate
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Wind speed t-series
Episode 2 – Momentum mixed down?
Hill topValley Bottom
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Episode 3 – Nocturnal Low Level Jet (NLLJ)
Duffryn sondes
Nocturnal Jet Drainage flow
Supergeostrophic
Hill tops
Veering
Wind directionWind speed
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Bulk Richardson number
Evolution of valley stability
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Model representation
Momentum mixed down
Gravity wave activity
Higher sustained
wind speeds
Wind speed and direction vertical profile, from valley bottom, Duffryn
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• CAPs observed frequently throughout COLPEX.
• Relatively small changes in synoptic wind affect CAP evolution during ‘’ideal’’ case study.
• CAP evolution was disturbed, and characterised by three episodes:• Gravity wave activity; possibly caused by wind speed/directional shear.
• Downward momentum transport of the developing NLLJ, accelerating hill top wind speeds.
• Established NLLJ, with hill top winds of 4—5 m/s.
• The developing NLLJ appears to be the overriding factor.
Summary
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Thank you for listening. Any questions?
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Publications
Vosper, S., Hughes, J., Lock, A., Sheridan, P., Ross, A., Jemmett-Smith, B., and Brown, A. 2013: Cold pool formation in a narrow valley. Quarterly Journal of the Royal Meteorological Society. doi: 10.1002/qj.2160
Price J. D. , Vosper S., Brown A., Ross A., Clark P., Davies F., Horlacher V., Claxton B., McGregor J. R., Hoare J. S., Jemmett-Smith B., and Sheridan P. 2011: COLPEX: Field and Numerical Studies over a Region of Small Hills. Bull. Amer. Meteor. Soc., 92, 1636–1650. doi: http://dx.doi.org/10.1175/2011BAMS3032.1
Sheridan, P. F., Vosper, S. B., and Brown, A. R. 2013. Characteristics of cold pools observed in narrow valleys and dependence on external conditions. Quarterly Journal of the Royal Meteorological Society.
Price J. D. , Vosper S., Brown A., Ross A., Clark P., Davies F., Horlacher V., Claxton B., McGregor J. R., Hoare J. S., Jemmett-Smith B., and Sheridan P. 2011: COLPEX: Field and Numerical Studies over a Region of Small Hills. Bull. Amer. Meteor. Soc., 92, 1636?1650. doi: http://dx.doi.org/10.1175/2011BAMS3032.1
COLPEX Publications
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Momentum mixed down
Nocturnal Low Level JetGravity wave
Hill top level
Sunrise Final break-up of CAP
m/s
NLLJ the overriding factor?
LIDAR vertical velocity 4-5 March 2010
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Recurrence of the NLLJ
Mean of 5 profiles on separate CAP nights
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Recurrence of the NLLJ?
Consistent
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Strong CAPs in the Clun Valley…….
Conditions ideal for CAPs to form
Ideal conditions: Ambient wind <3 m/s Msl pressure >1029 hPa Wind dir from N Low values of Flw, i.e.,
0.80.
Preferentially form when: Ambient wind <7 m/s Msl pressure >1008 hPa Wind dir from N or NW Dry, clear (Flw < 0.91)
Results from 9–month climatology: July 2009 – April 2010