Measurement and modelling of temperature cold pools in Cerdanya

The Cerdanya valley in NE Spain experiences intense cold-air ponding (CAP) which decouples the valley atmosphere from the regional circulation, especially in winter. This makes air temperature prediction a challenge. A network of 40 temperature sensors was installed in 2012 along seven elevational transects to collect hourly temperatures throughout the cold pool, enabling measurement of the detailed cold-pool structure for the first time. Sensors were also installed in upper Conflent valley to the north-east for comparison, where previous research has shown that there is reduced CAP. Sensor data is validated against AWS observations at two locations. Through calculation of hourly lapse rates in various elevation bands for two years we show frequent inversions developing up to 1450 m, and sometimes extending much higher than this, concentrating in winter. Case studies of two intense episodes in December 2012 and January 2013 show that model simulations, despite being able to simulate broad mechanisms of CAP formation, underestimate the amount of cooling. This is due to over-enthusiastic simulation of a low-level jet in the first case, and mountain waves in the second. Solving these model problems has important consequences for future ability to predict episodes of extreme low temperatures and associated hazards (frost, fog) in Cerdanya and mountain valleys elsewhere.