The sensitivity of cold air pool evoloution in hilly terrain regions

Cold air pools (CAPs) over complex terrain have rarely been investigated for small scale terrain typical of many regions across UK. Presented here is a case study investigation from the COLPEX project of a CAP that formed during “ideal” synoptic conditions (i.e., clear skies and low winds). The maximum nighttime 2 m temperature difference observed across the valley depth (170 m) was 6.7°C, occurring close to sunrise. Despite the synoptic conditions being “ideal”, this investigation highlights how sensitive CAPs in hilly terrain are to disturbance.

Three episodes where the CAP growth is disturbed, but not eroded, are attributed to; (1) gravity wave activity, (2) an acceleration of the ambient wind, (3) the development of a nocturnal low level jet (NLLJ). The final breakup of the CAP occurred some 3.5 hours after local sunrise and the NLLJ appeared to play some role. Clearly there is a need to resolve and further understand such micro and mesoscale meteorological phenomena in order to accurately predict nighttime minimum temperatures over such scales. For this to be achieved an improvement in model resolution, parametrisation or downscaling techniques, may be needed.