Investigation of the Planetary Boundary Layer in the Swiss Alps Using Remote Sensing and In Situ Measurements

Abstract

The development of the planetary boundary layer (PBL) has been studied in a complex terrain using various remote sensing and in situ techniques. The high-altitude research station at Jungfraujoch (3,580 m a.s.l.) in the Swiss Alps lies for most of the time in the free troposphere except when it is influenced by the PBL reaching the station, especially during the summer season. A ceilometer and a wind profiler were installed at Kleine Scheidegg, a mountain pass close to Jungfraujoch, located at an altitude of 2,061 m a.s.l. Data from the ceilometer were analyzed using two different algorithms, while the signal-to-noise ratio of the wind profiler was studied to compare the retrieved PBL heights. The retrieved values from the ceilometer and wind profiler agreed well during daytime and cloud-free conditions. The results were additionally compared with the PBL height estimated by the numerical weather prediction model COSMO-2, which showed a clear underestimation of the PBL height for most of the cases but occasionally also a slight overestimation especially around noon, when the PBL showed its maximum extent. Air parcels were transported upwards by slope winds towards Jungfraujoch when the PBL was higher than 2,800 m a.s.l. during cloud-free cases. This was confirmed by the in situ aerosol measurements at Jungfraujoch with a significant increase in particle number concentration, particle light absorption and scattering coefficients when PBL-influenced air masses reached the station in the afternoon hours. The continuous aerosol in situ measurements at Jungfraujoch were clearly influenced by the local PBL development but also by long-range transport phenomena such as Saharan dust or pollution from the south.

Investigation of the effective peak supersaturation for liquid-phase clouds at the high-alpine site Jungfraujoch, Switzerland (3580 m a.s.l.)

“…1c and d). To the southeast, the Great Aletsch glacier declines gradually from the JFJ (1500 m of altitude decrease over 18 km) while the northwestern side drops steeply, descending 1500 m over a horizontal distance of 4800 m (Ketterer et al, 2014).…”

Surface-to-mountaintop transport characterised by radon observations at the Jungfraujoch

“…1). Below station elevation, winds over the Swiss Plateau are channelled parallel to the mountain range, whereas above mountaintops winds are most frequently from the north-west with a broad unimodal maximum (Furger, 1992;Ketterer et al, 2014). At Jungfraujoch itself, however, terrain channels the wind into a bimodal distribution with maxima towards the north-west and south-east.…”

“…In order to feed the vertical transport, boundary-layer air is drawn towards the base of the mountains from up to 80 km away over the course of a day (Weissmann et al, 2005). The export of mass to the troposphere is significant at a regional scale and boundary-layer air is exported beyond the peak height even when the top of the boundary layer is lower over the Plateau (Ketterer et al, 2014). Henne et al (2004) show a schematic in their Fig.…”

“…Yet another approach is to relate synoptic weather classifications to the occurrence of vertical transport (Collaud Coen et al, 2011). Data augmenting in situ measurements has included lidar, used to de-tect the top of aerosol layers Gallagher et al, 2012;Ketterer et al, 2014), and radiosondes, which are launched from near the mountain and used to define the unperturbed free troposphere (Weiss-Penzias et al, 2006).…”

Sensitivity estimations for cloud droplet formation in the vicinity of the high-alpine research station Jungfraujoch (3580 m a.s.l.)

“…1. To the SE, the Great Aletsch glacier declines gradually from the JFJ (1500 m of altitude decrease over 18 km) while the NW side drops steeply, descending 1500 m over a horizontal distance of 4800 m (Ketterer et al, 2014).…”