The Madeira's Atmospheric Wake: Results from the I-WAKE Campaign

The 57 km long and 22 km wide NW-SE oriented island of Madeira lies in the western Atlantic, 950 km SW from the southern tip of Portugal. The highest island peak, Pico Ruivo, reaches 1862 m above sea level (ASL). Due to its size, atmospheric stability structure and winds upstream, wake formation in the downstream region of this mountainous island is expected, in particular in summer when the most sustained winds below the trade wind inversion come from NE, the direction perpendicular to the island orientation.

In situ and remote sensing measurements were obtained during the i-WAKE airborne campaign (Aug-Sep 2010) in Madeira. SAFIRE's ATR 42 research aircraft collected data both upstream and downstream of the island. In the event that is analyzed herein, data along ten flight legs downstream was collected on 2nd Sept. 2010. Wake signals such as abrupt wind jumps at the flanks of Madeira and warm cores of eddies are evident in the aircraft in situ data. A series of high-resolution simulations using the WRF-ARW model was carried out to investigate the nature of wake behind Madeira. At the beginning of the analyzed period (1st –3rd Sept. 2010), the lower troposphere was continuously stratified. During the course of the event, the atmospheric vertical structure became layered with a well-mixed boundary layer capped by a strong inversion below the mountain top (“shallow-water regime”). The model simulated wind components along the flight legs agree well with the aircraft observations. To investigate the main source of vertical vorticity in the wake, a vorticity budget analysis based on simulation data was performed. The dominant term in the vorticity equation was found to depend on vertical stratification. In the continuously stratified regime, friction was found to be the dominant term; in the shallow-water regime both stretching and friction terms appear equally large. The same numerical modeling framework is used to explore the sensitivity of the atmospheric wake to variation of the sea surface temperature.