1. Investigating the typical occurrence of cold-air-pools during the COLd air Pool Experiment (COLPEX) Bradley Jemmett-Smith 15 th Conference of Mountain Meteorology 21 August 2012 Andrew Ross 1, John Hughes 1, Jeremy Price 2, Peter Sheridan 2, Simon Vosper 2 University of Leeds 1, Met Office 2

2. Overview  Introduction to cold-air-pools:  When and why do they occur?  Why is the research needed, who benefits?  The COLd air Pooling EXperiment (COLPEX)  Case study IOP 4 – 5 March 2010  Overview  Evolution of the cold-air-pool  Summary and future work  Questions

3. When and why do cold-air-pools occur? (Borgren et al, 2000)  Temperature inversion within the valley  Develop around sunset, breakdown following sunrise.  Ideal conditions are:  A stable/static atmosphere, no cloud cover, high atmospheric pressure.  Low winds speed above the valley.  Driven by cooling of air above the surface, which pools or cools in-situ. (Whiteman, 1982)

4. Why is the research needed and who benefits?  Poor prediction of cold-air-pools in Numerical Weather Prediction (NWP) models.  Improve understanding for timing of formation and break-up of cold-air- pools.  Very few investigations on scales < 1km (exceptions include Bodine et al (2009), Whiteman et al (2008)).  Scientific interest.  Hazardous driving conditions; frost, fog, persistence of lying snow.  Health risks; pollution episodes.

5. Cold air pooling put in to perspective Met Office operational forecast models: UK4 – 4 km outer UKV – 1.5 km inner

6. England Shropshire Clun Valley (Map images ©2010 Google) COLPEX field site = 4x4 km

7. The COLd air Pooling Experiment (COLPEX) Collaboration between the University of Leeds, the Met Office and NCAS. Comprehensive field campaign from Jan 2009 – Apr 2010, based in the Clun Valley region Shropshire, UK. Instruments include; 3x flux tower sites, 10x AWS (developed by University of Leeds), 20x HOBOs, Lidar, radiometers (FGAM) and radiosonde launches during Intense Observation Periods (IOP’s).

8. Cold-air-pool occurrence Nights with an inversion >= 4 o C occur 23% of the time.

9. IOP 4 – 5 March 2010 Temperature time series for the month of March Green Δz = 95 mBlue Δz = 200 m ΔT/Δz = 7 o C/200 m Most within bottom 100 m

10. IOP 4 – 5 March 2010 Synoptic conditions IR sat image 00:00 UTC 5 March 2010 Analysis 00:00 UTC 5 March 2010

11. IOP 4 – 5 March 2010 Potential temp Valley Env. Lapse rate RH

12. IOP 4 – 5 March 2010 CP formation 16:3517:3518:35 Sunset 18:00

13. IOP 4 – 5 March 2010 Radiosonde profiles Potential tempRH Drying of layer

14. IOP 4 – 5 March 2010 Radiosonde profiles Wind DirectionWind Speed Nocturnal Jet formed SW wind seen

15. IOP 4 – 5 March 2010 LIDAR Anomalies Vertical velocitiesBackscatter

16. IOP 4 – 5 March 2010 Anomalies Tower

17. IOP 4 – 5 March 2010 Anomalies 21:15 Cooling at same rate, undisturbed growth. 01:45 Warmer flow at 50m Duffryn from the NNW. 04:45 Warming at AWS 3. SSW flow seen at 50m Duffryn. 05:45 ‘Extra’ cooling at some AWS. Up-valley flow seen! N flow seen at Duffryn 50m! Valley flow 50 m

18. IOP 4 – 5 March 2010 Potential tempEnv. Lapse rateRH Warming Cooling

19. IOP 4 – 5 March 2010 Summary  Clear and distinct change in wind direction at valley sites before sunset.  Cold-air-pool begins to form before sunset. Rapid cooling similar across all sites.  Development of a nocturnal jet above the valley.  Disturbances seen in the cold pool growth after 23:00 UTC: 1.Coincides with change in the wind-direction at 23:00. 2.Layered erosion of cold-pool top, continued cooling in some lower regions 05:30 onwards.  Layered structure, evidence suggests drainage current from side valley at Duffryn.  Cold-air-pool persists up to 2 hrs after sunset. Continuing IOP 4 – 5 March investigation  Explain the rapid cooling towards the end of the night; synoptic, local or both?  Model comparison and investigations, i.e. Nocturnal Jet, flow structure at Duffryn,  Climatology study and comparison to other cases.