Evolution of an Atmospheric Kármán Vortex Street from High-resolution Satellite Winds: Guadalupe Island Case Study
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 state-of-the-art imaging capabilities of GOES-16 ABI and the nested tracking algorithm, we derived 2.5-km cloud-motion winds every 5 minutes over an 8-hr daytime period for a vortex street in the lee of Guadalupe Island on 9 May 2018. A novel MODIS–GOES joint wind product provided accurate stereo cloud-top heights and semi-independent wind validation data. The time series of geostationary winds, supplemented with snapshots of 6.25-km ocean surface winds from ASCAT, allowed us to capture the wake oscillations and measure vortex shedding dynamics for the first time from spaceborne observations.
The vortex street developed under atmospheric conditions conducive to coherent vortex shedding. The marine boundary layer had a well-mixed subcloud layer capped by a strong temperature inversion with a weaker stably stratified layer above. The Froude number related to the dividing streamline was typically below the critical value of 0.4, corroborating previous findings. The derived wind field around Guadalupe exhibited characteristics expected from laboratory flows past bluff bodies: flow splitting with deceleration on the windward side, lobes of acceleration on the flanks, and an oscillating wake with transverse jets at quasi-regular intervals set by a vortex shedding period of 2–4 hr. The retrievals revealed a markedly asymmetric vortex street, with cyclonic eddies having larger peak vorticities than anticyclonic eddies at the same downstream location. Vorticity generally decreased with time, that is with downstream distance, due to viscous diffusion but the rate of decrease was a factor of two higher for anticyclones. Drawing on the vast knowledge accumulated about laboratory bluff body flows, we argue that the asymmetric island wake arises due to the combined effects of Earth’s rotation and Guadalupe’s non-axisymmetric shape resembling an inclined flat plate under low angle of attack. The asymmetric vortex decay implies a three-dimensional wake structure where centrifugal or elliptical instabilities selectively destabilize anticyclonic eddies by introducing edge-mode or core-mode vertical perturbations to the clockwise-rotating vortex tubes.
