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

Abstract

AbstractExchange 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.