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 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

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

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

IOP 4 – 5 March 2010 Radiosonde profiles Potential tempMixing ratio Drying of layer

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

IOP 4 – 5 March 2010 CP formation 17:0518:05 18:35 Sunset 18:00

IOP 4 – 5 March 2010 CP evolution 22:4523:15 03:45 Sunset 18:00 AWS 3 warmingValley Jet 04:05 AWS 7, 5 warming 23:45

IOP 4 – 5 March 2010 LIDAR Anomalies Vertical velocitiesBackscatter

IOP 4 – 5 March 2010 Anomalies Tower

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

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

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.

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: 1.Warming seen to coincide with a change in the wind-direction. 2.Layered erosion caused by Nocturnal Jet above the valley, which disrupts the IVJ.  Layered structure: 1.Very cold undisturbed layer; light winds, reversal in wind dir can occur. 2.Cold layer, higher 2 m wind speeds, susceptible to warming events. 3.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.

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?

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