PathfinderTURB: an automatic boundary layer algorithm. Development, validation and application to study the impact on in-situ measurements at the Jungfraujoch

Abstract

Continuous observations of the vertical structure of the planetary boundary layer are invaluable for the validation of atmospheric transport models on the micro and meso scale. Lidar and ceilometer backscatter observations offer a robust technique with growing spatial coverage, but the obtained backscatter profiles need to be carefully translated into boundary layer parameters. Here we present the development of the PathfinderTURB algorithm for the analysis of ceilometer backscatter data and the real-time detection of the vertical structure of the planetary boundary layer. Two typical aerosol layer heights are retrieved by PathfinderTURB: the Convective Boundary Layer (CBL) and the Continuous Aerosol Layer (CAL). PathfinderTURB combines the strengths of gradient- and variance-based methods and addresses the layer attribution problem by adopting a geodesic approach. The algorithm has been applied to one year of data measured by two CHM15k ceilometers operated at the Aerological Observatory of Payerne (491 m, a.s.l.) on the Swiss plateau, and at the Kleine Scheidegg (2061 m, a.s.l.) in the Swiss Alps. The retrieval of the CBL has been validated at Payerne using two reference methods: (1) manual detections of the CBL height performed by independent human experts using the ceilometer backscatter data of the year 2014; (2) values of CBL heights calculated using the Richardson's method from co-located radio sounding data. We found average biases as small as 27 m (53 m) with respect to reference method 1 (2). Based on the excellent agreement with the two reference methods, PathfinderTURB has been applied to the ceilometer data at the mountainous site of the Kleine Scheidegg for the period September 2014 till November 2015. At this site, the CHM15k is operated in a novel, tilted configuration at 71° zenith angle to probe the air next to the Sphinx Observatory (3580 m, a.s.l.) on the Jungfraujoch (JFJ). The analysis of the retrieved layers led to the following results: the CAL reaches the JFJ during 41 % of the time in summer and during 21 % of the time in winter for a total of 97 days during the two seasons. The season-averaged daily cycles show that the CBL height reaches the JFJ only during short periods (4 % of the time) on 20 individual days in summer and never during winter. Especially during summer the CBL and the CAL modify the air sampled in-situ at JFJ, resulting in an unequivocal dependence of the measured absorption coefficient on the height of both layers. This highlights the relevance of retrieving the height of CAL and CBL in mountainous regions.