A case-study of cold-air pool evolution in hilly terrain using field measurements from COLPEX: JEMMETT-SMITH et al..

A case-study investigation of cold-air pool (CAP) evolution in hilly terrain is conducted using field measurements made during IOP16 of the COLd-air Pool EXperiment (COLPEX). COLPEX was designed to study cold-air pooling in small-scale valleys typical of the UK (approximate to 100-200m deep, approximate to 1km wide). The synoptic conditions during IOP16 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 4hr 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 Episode1. This residual layer is eroded by a developing NLLJ from the top down during Episode2. The sustained increase in winds at hill-top levels - attributed to the NLLJ - continue to disrupt the CAP through Episode3. Although cooling is interrupted, the CAP is never completely eroded during the night. Complete CAP break-up occurs some 3.5hr 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.