Cross-valley vortices in the Inn valley, Austria: Structure, evolution and governing force imbalances

Abstract

Exchange of momentum and scalars in the mountain boundary layer is achieved through an interaction of meso-to-microscale motions, occurring to varying extents depending on the combined effect of thermally driven as well as dynamically driven forcings. One such motion, known as a secondary circulation, results from a horizontal force imbalance across a curved valley segment, wherein the centrifugal force towards the outside of the valley bend can create a pressure gradient force in the opposite direction. The lack of adequate measurement strategies capable of sampling such motions in curved mountain valleys explains the near-absence of any observational evidence of secondary circulations there. The goal of the CROSSINN (Cross-valley flow in the Inn valley investigated by dual-Doppler lidar measurements) campaign, conducted in a curved segment of the Inn valley, Austria, was to determine the character of the cross-valley flow by means of a coplanar retrieval applied to a multi-Doppler wind lidar configuration. A signature of a secondary circulation, hereafter referred to as a cross-valley vortex, stood out particularly during intense daytime upvalley flow episodes. Vortices were detected on 23 upvalley wind days, with a declining frequency of occurrence from August to October. Nearly all identified vortices were marked by a low-level upvalley jet, a clockwise wind direction turning with height, and a cessation of upvalley flow at the local ridgeline level. The routinely sampled coplanar-retrieved cross-valley wind field enabled the quantification of more advanced parameters based on vorticity, revealing a faster spin rate of the vortex around its streamwise axis given a stronger upvalley flow, and a period of revolution on the order of several tens of minutes. A detailed inspection of the lateral momentum budget and associated uncertainties confirmed the importance of the relationship between the centrifugal and the pressure gradient force for the cross-valley vortex occurrence in a curved valley.