Atmospheric Wake of Madeira: First Aerial Observations and Numerical Simulations

Abstract

The island of Madeira is well known for giving rise to atmospheric wakes. Strong and unsteady atmospheric wakes, resembling a von Kármán vortex street, are frequently observed in satellite images leeward of Madeira, especially during summer months, when conditions favoring the formation of atmospheric wakes occur frequently under the influence of the Azores high. Reported here is the analysis of the first airborne measurements of Madeira’s wake collected during the 2010 Island-induced Wake (I-WAKE) campaign. High-resolution in situ and remote sensing data were collected in the I-WAKE by a research aircraft. The measurements reveal distinctive wake signatures, including strong lateral wind shear zones and warm and dry eddies downwind of the island. A strong anticorrelation of the horizontal wind speed and sea surface temperature (SST) was found within the wake. High-resolution numerical simulations with the Weather Research and Forecasting (WRF) Model were used to study the dynamics of the wake generation and its temporal evolution. The comparison of the model results and observations reveals a remarkable fidelity of the simulated wake features within the marine boundary layer (MBL). Strong potential vorticity (PV) anomalies were found in the simulated MBL wake, emanating from the flanks of the island. The response of the wake formation within the MBL to surface friction and enhanced thermal forcing is explored through the model sensitivity analyses.

Lee Waves on the Boundary-Layer Inversion and Their Dependence on Free-Atmospheric Stability

“…The lee wave train is not visibly influenced by the prominent wake of Madeira (Grubišić et al, 2015) and extends over a distance of 40 km with approximately nine wave crests, hence λ ≈ 4.4 km.…”

Dynamics and oceanic response of the Madeira tip‐jets

“…A first direct observational experiment in the Madeira wake, based on airborne measurements, was described by Grubišić et al . (2015), revealing the presence of strong lateral shear zones and lee‐side atmospheric eddies. Isolated islands are important sources of ocean vorticity as shown by de Souza et al .…”

Modeling the air‐sea feedback system of Madeira Island

“…Early remote sensing studies revealed intense vortex shedding downwind of Madeira [ Chopra and Hubert , ]. Subsequent recent atmospheric wake numerical studies and targeted sampling refined the understanding of the complex wake region of Madeira [ Nunalee and Basu , ; Grubišić et al ., ]. Vortex shedding events are most frequent in June–August when the ABL depth is below mountain‐top (<1500) and capped by a strong inversion, thereby favoring resonant interactions with the terrain [ Grubišić et al ., ].…”

“…Reduced lee‐side winds due to terrain blocking are expected to suppress ocean mixing, further contributing to the near‐surface oceanic warming. Indeed, there is a sharp contrast between the calm winds in the lee and the strong winds at the island flank that manifests in the sea state and SST [ Grubišić et al ., ]. The correlation between high SST and low winds was robust during the Madeira measurement campaign, Island‐induced Wake (I‐WAKE).…”