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

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.

A Physics-Aware Neural Network Approach for Flow Data Reconstruction From Satellite Observations

“…For this reason, we generate fluid flow configurations in a controlled manner suitable for supervised machine learning. Afterwards, we analyze the performance of our model on satellite wind retrievals in the wake of Guadalupe island by Horváth et al (2020) and compare those with linear reconstructions obtained via least-squares minimization. Our neural network has similarities with image inpainting approaches, mainly stemming from both sharing a scene completion process using partial observations.…”

“…We further evaluate the model on a real-world data set consisting of reconstructed velocity fields from satellite imagery of the atmospheric vortex street behind Guadalupe Island (Horváth et al, 2020). Figure 5 shows the satellite imagery on 9 May 2018 with the vortex generating structure behind the island.…”

“…Such flow structures have been studied based on satellite measurements since the 1960s (Hubert and Krueger, 1962;Chopra and Hubert, 1965;Young and Zawislak, 2006). Recent advances in remote sensing (Geerts et al, 2018) enabled the retrieval of high-resolution wind fields at kilometer-scale (Horváth et al, 2017(Horváth et al, , 2020, which is a necessary requirement for the analysis of atmospheric processes in turbulent environments. While operational wind products based on the Advanced Baseline Imager (ABI) onboard the Geostationary Operational Enrivonmental Satellite-R (GEOS-R) (Schmit et al, 2017) provide a 7.5 km resolution, Horváth et al (2020) utilized the internal wind vectors at 2.5 km resolution in their study of vortex patterns in the wake of Guadalupe Island off Baja California on 9 May 2018, which were combined with MODIS-GEOS wind products offering stereo cloud-top heights and semi-independent wind validation data (Carr et al, 2019).…”

“…Recent advances in remote sensing (Geerts et al, 2018) enabled the retrieval of high-resolution wind fields at kilometer-scale (Horváth et al, 2017(Horváth et al, , 2020, which is a necessary requirement for the analysis of atmospheric processes in turbulent environments. While operational wind products based on the Advanced Baseline Imager (ABI) onboard the Geostationary Operational Enrivonmental Satellite-R (GEOS-R) (Schmit et al, 2017) provide a 7.5 km resolution, Horváth et al (2020) utilized the internal wind vectors at 2.5 km resolution in their study of vortex patterns in the wake of Guadalupe Island off Baja California on 9 May 2018, which were combined with MODIS-GEOS wind products offering stereo cloud-top heights and semi-independent wind validation data (Carr et al, 2019). Utilizing such high resolutions is necessary for the analysis of small-scale structures, but comes at the price of an increased level of measurement noise, which is accompanied by general uncertainty in regions without or with only few clouds.…”

“…Disambiguation of possible flow configurations based on imperfect measurement data is thereby challenging. At present, such spaceborne measurements have been cleaned with median filters, smoothing, and thresholding of unreasonably large velocity components (Horváth et al, 2020). Thus, current approaches to reconstruct missing or uncertain information are based on assumptions about the signal smoothness, and do not yet incorporate that the wind fields are the result of fluid dynamical processes.…”