Computer Graphics Laboratory ETH Zurich

ETH

Evolution of an Atmospheric Kármán Vortex Street From High-Resolution Satellite Winds: Guadalupe Island Case Study

A. Horvath, W. Bresky, J. Daniels, J. Vogelzang, A. Stoffelen, J. Carr, D. L. Wu, C. Seethala, T. Günther, S. A. Buehler

Journal of Geophysical Research: Atmospheres, AGU, vol. 125, no. 4, 2020, pp. e2019JD032121

Abstract

Vortex streets formed in the stratocumulus-capped wake of mountainous islands are the atmospheric analogues of the classic Kármán vortex street observed in laboratory flows past bluff bodies. The quantitative analysis of these mesoscale unsteady atmospheric flows has been hampered by the lack of satellite wind retrievals of sufficiently high spatial and temporal resolution. Taking advantage of the cutting-edge Advanced Baseline Imager, we derived kilometer-scale cloud-motion winds at 5-min frequency for a vortex street in the lee of Guadalupe Island imaged by Geostationary Operational Environmental Satellite-16. Combined with Moderate Resolution Imaging Spectroradiometer data, the geostationary imagery also provided accurate stereo cloud-top heights. The time series of geostationary winds, supplemented with snapshots of ocean surface winds from the Advanced Scatterometer, allowed us to capture the wake oscillations and measure vortex shedding dynamics. The retrievals revealed a markedly asymmetric vortex decay, with cyclonic eddies having larger peak vorticities than anticyclonic eddies at the same downstream location. Drawing on the vast knowledge accumulated about laboratory bluff body flows, we argue that the asymmetric island wake arises from the combined effects of Earth's rotation and Guadalupe's nonaxisymmetric shape resembling an inclined flat plate at low angle of attack. However, numerical simulations will need to establish whether or not the selective destabilization of the shallow atmospheric anticyclonic eddies is caused by the same mechanisms that destabilize the deep columnar anticyclones of laboratory flows, such as three-dimensional vertical perturbations due to centrifugal or elliptical instabilities.