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We present the development of the PathfinderTURB algorithm for the analysis\udof ceilometer backscatter data and the real-time detection of the vertical\udstructure of the planetary boundary layer. Two aerosol layer heights are\udretrieved by PathfinderTURB: the convective boundary layer (CBL) and the\udcontinuous aerosol layer (CAL). PathfinderTURB combines the strengths of\udgradient- and variance-based methods and addresses the layer attribution\udproblem by adopting a geodesic approach. The algorithm has been applied to\ud1 year of data measured by two ceilometers of type CHM15k, one operated at\udthe Aerological Observatory of Payerne (491 m a.s.l.) on the Swiss plateau\udand one at the Kleine Scheidegg (2061 m a.s.l.) in the Swiss Alps. The\udretrieval of the CBL has been validated at Payerne using two reference\udmethods: (1) manual detections of the CBL height performed by human experts\udusing the ceilometer backscatter data; (2) values of CBL heights calculated\udusing the Richardson's method from co-located radio sounding data. We found\udaverage biases as small as 27 m (53 m) with respect to reference method 1\ud(method 2). Based on the excellent agreement between the two reference\udmethods, PathfinderTURB has been applied to the ceilometer data at the\udmountainous site of the Kleine Scheidegg for the period September 2014 to\udNovember 2015. At this site, the CHM15k is operated in a tilted configuration\udat 71° zenith angle to probe the atmosphere next to the Sphinx\udObservatory (3580 m a.s.l.) on the Jungfraujoch (JFJ). The analysis of the\udretrieved layers led to the following results: the CAL reaches the JFJ\ud41 % of the time in summer and 21 % of the time in winter for a total\udof 97 days during the two seasons. The season-averaged daily cycles show that\udthe CBL height reaches the JFJ only during short periods (4 % of the\udtime), but on 20 individual days in summer and never during winter. During\udsummer in particular, the CBL and the CAL modify the air sampled in situ at\udJFJ, resulting in an unequivocal dependence of the measured absorption\udcoefficient on the height of both layers. This highlights the relevance of\udretrieving the height of CAL and CBL automatically at the JFJ

atmospheric chemistry and physics

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