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

Abstract

Abstract. We present the development of the Pathfinder-TURB algorithm for the analysis of ceilometer backscatter data and the real-time detection of the vertical structure of the planetary boundary layer. Two 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 variancebased methods and addresses the layer attribution problem by adopting a geodesic approach. The algorithm has been applied to 1 year of data measured by two ceilometers of type CHM15k, one operated at the Aerological Observatory of Payerne (491 m a.s.l.) on the Swiss plateau and one 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 human experts using the ceilometer backscatter data; (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 (method 2). Based on the excellent agreement between 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 to November 2015. At this site, the CHM15k is operated in a tilted configuration at 71 • zenith angle to probe the atmosphere 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 41 % of the time in summer and 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), but on 20 individual days in summer and never during winter. During summer in particular, 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 automatically at the JFJ.

New type of evidence for secondary ice formation at around −15 °C in mixed-phase clouds

“…The size of our crystals was determined by using ImageJ (Rueden et al, 2017;Schindelin et al, 2012). Images were later analysed visually and not by machine-learning methods, such as those developed by Praz et al (2017), for the habit, including the degree of riming both categorised according to the latest ice crystal classification scheme, as presented by Kikuchi et al (2013). The scheme catalogues solid precipitation particles into a total of 121 categories and provides for each category a representative image.…”

Atmospheric boundary‐layer characteristics from ceilometer measurements. Part 1: A new method to track mixed layer height and classify clouds

“…Martucci et al, ), or some combination of these (e.g. Lammert and Bösenberg, ; Martucci et al, ; Haeffelin et al, ; Poltera et al, ). For example, STRAT‐2D (Morille et al, ; Haeffelin et al, ) uses the variance field to determine which wavelet‐detected negative gradient is likely associated with Z ML , whereas pathfinderTURB (Poltera et al, ) combines gradient and variance field diagnostic before tracing Z ML .…”

“…Martucci et al, ). The recent pathfinderTURB (Poltera et al, ), based on the pathfinder algorithm (de Bruine et al, ), applies a graph theory approach to track Z ML through the course of the day, while COBOLT (Geiß et al, ) uses a time–height‐tracking approach with moving windows.…”

“…Focusing on daytime conditions, Poltera et al . () find pathfinderTURB is least accurate during afternoon transitions.…”

“…While adequate overlap correction can improve applicability of measurements in this region (Hervo et al, ), the nocturnal mixed layer near the ground is still often undetectable (e.g. Poltera et al, ). As optical overlap, instrument noise, and instrument‐related background are generally sensor specific, the performance of an ALC model may vary between individual instruments.…”