Lidar Observations of Mountain Waves During Bora Episodes

Abstract

<jats:p>Airflows over mountain barriers in the Alpine region may give rise to strong, gusty downslope winds, called Bora. Oscillations, caused by the flow over an orographic barrier, lead to formation of mountain waves. These waves can only rarely be observed visually and can, in general, not be reliably reproduced by numerical models. Using aerosols as tracers for airmass motion, mountain waves were experimentally observed during Bora outbreak in the Vipava valley, Slovenia, on 24-25 January 2019 by two lidar systems: a vertical scanning lidar positioned just below the peak of the lee side of the mountain range and a fixed direction lidar at valley floor, which were set up to retrieve two-dimensional structure of the airflow over the orographic barrier into the valley. Based on the lidar data, we determined the thickness of airmass layer exhibiting downslope motion, observed hydraulic jump phenomena that gave rise to mountain waves and characterized their properties.</jats:p>

Bora Flow Characteristics in a Complex Valley Environment

“…Air flow properties and atmospheric structures appearing above the Vipava valley were, up until now, studied using the ground-based measurements from Mie scattering and Raman lidar, which use aerosols as traces of air mass motion and cover the tropospheric region above a ≈200 m height above the valley floor. The airflow in the lower part of the atmospheric boundary layer (ABL) was regularly monitored at the bottom of the valley using anemometer point measurements at ≈10 m. Based on these measurements, the airflow characteristics above and within the Vipava valley were studied and various atmospheric phenomena were detected, such as hydraulic jump in the Bora flow and mountain waves above the valley [14][15][16][17]. However, there is a gap in the vertical range of the atmosphere, between 10 and 200 m above the valley floor, which, up until now, remained uninvestigated.…”