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IOP 4 – 5 March 2010

Investigating the typical occurrence of cold-air-pools during the COLd air Pool Experiment (COLPEX) Bradley Jemmett-Smith COLPEX meeting 15 October 2012. IOP 4 – 5 March 2010. Δ T/ Δ z = 7 o C/200 m Most within bottom 100 m. Temperature time series for the month of March

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IOP 4 – 5 March 2010

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  1. Investigating the typical occurrence of cold-air-pools during the COLd air Pool Experiment (COLPEX)Bradley Jemmett-SmithCOLPEX meeting 15 October 2012

  2. IOP 4 – 5 March 2010 ΔT/Δz = 7 oC/200 m Most within bottom 100 m • Temperature time series for the month of March • Green Δz = 95 m Blue Δz = 200 m

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

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

  5. IOP 4 – 5 March 2010 Radiosonde profiles Mixing ratio Potential temp Drying of layer

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

  7. IOP 4 – 5 March 2010 Sunset 18:00 CP formation 18:35 17:05 18:05

  8. IOP 4 – 5 March 2010 Sunset 18:00 CP evolution Valley Jet AWS 3 warming AWS 7, 5 warming 03:45 22:45 23:15 23:45 04:05

  9. IOP 4 – 5 March 2010 Anomalies LIDAR Backscatter Vertical velocities

  10. IOP 4 – 5 March 2010 Anomalies Tower

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

  12. IOP 4 – 5 March 2010 Anomalies Valley flow 50 m 21:15 Cooling at same rate, undisturbed growth. 01:45 Warmerflow 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!

  13. Mixing events - explanation • A mixing/warming events can: • occur across a number of sites, caused by an external source. • be caused internally, by a disturbance at a neighbouring site. • KH instability – potentially dependent on valley orientation and IVJ. • Change in wind direction will change the source of advection towards the valley.

  14. IOP 4 – 5 March 2010 Summary • CAP begins to form before sunset, with change in wind direction at valley sites. • Development of Internal Valley Jet (IVJ) soon after sunset. • Disturbances seen in the cold pool growth after 23:00 UTC: • Warming seen to coincide with a change in the wind-direction. • Layered erosion caused by Nocturnal Jet above the valley, which disrupts the IVJ. • Layered structure: • Very cold undisturbed layer; light winds, reversal in wind dir can occur. • Cold layer, higher 2 m wind speeds, susceptible to warming events. • IVJ/mixed layer region, very sensitive to changes in synoptic wind conditions. • Possible evidence for side valley drainage at Duffryn, layered structure. • Accelerated cooling at AWS 5, 05:30, accompanied with a reverse in wind dir. • CAP elevated during break-up, seen at Duffryn. Cold-air-pool persists up to 2 hrs after sunset at Clun Castle.

  15. Future work • Writing up March case at the moment. • Need to explain the rapid cooling towards the end of the night; synoptic, local or both? • Model comparison and investigations. • Climatology studys: • Wind climatology; forcing mechanisms. • Importance of; Nocturnal Jet, IVJ, diurnal change in atmospheric water vapour content (LW in), pressure situation (wind speed, direction), KH instability?

  16. Cold-air-pool occurrence Nights with an inversion >= 4oC occur 23% of the time.

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