A case‐study of cold‐air pool evolution in hilly terrain using field measurements from COLPEX

• Published in: Quarterly journal of the Royal Meteorological Society 2019-04, Vol.145 (720), p.1290-1306
• Main Authors: Jemmett‐Smith, Bradley C., Ross, Andrew N., Sheridan, Peter F., Hughes, John K., Vosper, Simon B.
• Format: Article
• Language: English
• Subjects: Air; Cold; cold‐air pools; COLPEX; complex terrain; Cooling; Evolution; Gravitational waves; Gravity waves; High pressure; Instability; Kelvin–Helmholtz instability; Low-level jets; nocturnal low‐level jet; stable boundary layer; Sunrise; Sunset; Synoptic conditions; Valley winds; Valleys; Weather; Weather forecasting; Wind; Winds
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.3499

A case‐study investigation of cold‐air pool (CAP) evolution in hilly terrain is conducted using field measurements made during IOP 16 of the COLd‐air Pool EXperiment (COLPEX). COLPEX was designed to study cold‐air pooling in small‐scale valleys typical of the UK (∼100–200 m deep, ∼1 km wide). The synoptic conditions during IOP 16 are typical of those required for CAPs to form during the night, with high pressure, clear skies and low ambient winds. Initially a CAP forms around sunset and grows uninterrupted for several hours. However, starting 4 hr after sunset, a number of interruptions to this steady cooling rate occur. Three episodes are highlighted from the observations and the cause of disruption attributed to (a) wave activity, in the form of gravity waves and/or Kelvin–Helmholtz (KH) instability, (b) increases in the above‐valley winds resulting from the development of a nocturnal low‐level jet (NLLJ), (c) shear‐induced mixing resulting from instability of the NLLJ. A weakly stable residual layer provides the conditions for wave activity during Episode 1. This residual layer is eroded by a developing NLLJ from the top down during Episode 2. The sustained increase in winds at hill‐top levels – attributed to the NLLJ – continue to disrupt the CAP through Episode 3. Although cooling is interrupted, the CAP is never completely eroded during the night. Complete CAP break‐up occurs some 3.5 hr after local sunrise. This case‐study highlights a number of meteorological phenomena that can disrupt CAP evolution even in ideal CAP conditions. These processes are unlikely to be sufficiently represented by current operational weather forecast models and can be challenging even for high‐resolution research models. Even on clear‐sky, low‐wind nights which are “ideal” for cold‐air pooling, various small‐scale processes can disrupt cold‐pool growth. This observational case‐study shows evidence of gravity waves, Kelvin–Helmholtz instability and turbulence induced by a nocturnal low‐level jet, all affecting cold‐pool development over the course of the night.