The Atmospheric Wake of Madeira: A simple air/sea coupled system

Air – sea coupled systems are best studied in turbulent regions of the ocean and atmosphere. In this work, we provide the first documentation of the atmospheric wake of Madeira employing observations from the I-WAKE campaign (Aug-Sep 2010). The I-WAKE measurement platforms include: (i) a sailing ship measuring sea surface temperature (SST) and vertical oceanic profiles, and (ii) the ATR-42 research aircraft, operated by SAFIRE (Meteo France).

Madeira is the main island of an archipelago consisting of four groups of islands (Madeira, Porto Santo, Desertas and Selvagens) located approximately 850 km offshore from the southwestern tip of Portugal. Madeira Island is 57 km long and 22 km wide, with a NW-SE orientation with a central mountain range reaching 1862 m at Pico Ruivo. Due to its subtropical location (32.7° N and 17° W), the summer conditions in Madeira are largely influenced by the Azores High, which establishes favorable conditions for wake formation. These include strong marine boundary layer (MBL) inversions located below the mountaintop and steady incoming trade winds that are perpendicular to the ridge orientation.

To help characterize the dynamics of a wake event, high-resolution numerical simulations using the regional Weather Research and Forecasting (WRF) model V3.3 were carried out. The comparison of the numerical simulations with the obtained in-situ data shows a remarkable agreement, enabling the use of the model for further investigating the wake dynamics. Results from the process studies suggest a strong SST influence on atmospheric eddy formation. The shallow convection in the MBL is a key factor of wake evolution. The best agreement between simulations and airborne data is achieved with a uniform increase of the model SST (from NCEP analysis) by 1 K.