Adjustment of the summertime marine atmospheric boundary layer to the western Iberia coastal morphology

Abstract

During summer (June, July, and August), northerly winds driven by the Azores anticyclone are prevalent over western Iberia. The Quick Scatterometer Satellite 2000 to 2009 summertime estimates reveal a broad high wind speed (≥7 ms−1) area extending about 300 km from shore and along the entire Iberian west coast. Nested in this large high‐speed region, preferred maximum regions anchored in the Iberian major capes, Finisterre, Roca, and S. Vicente, are found. Composite analyses of wind maxima were performed to diagnose the typical summertime synoptic‐scale pressure distribution associated with these smaller size high‐speed regions. The flow low‐level structure was further studied with a mesoscale numerical prediction model for three northerly events characterized by typical summertime synoptic conditions. A low‐level coastal jet, setting the background conditions to the marine atmospheric boundary layer (MABL) response to topography, was found in the three cases. The causes for wind maximum downwind capes were investigated, focusing on the hypothesis that western Iberia MABL responds to hydraulic forcing. For the three events supercritical and transcritical flow conditions were identified and expansion fan signatures were found downwind each cape. Aircraft measurements, performed during one of the events, gave additional evidence of the expansion fan leeward Cape Roca. The importance of other forcing mechanisms was also assessed by considering the hypothesis of downslope wind acceleration and found to be in direct conflict with soundings and surface observations.

Spatial and temporal variability of the Iberian Peninsula coastal low‐level jet

“…These higher IPCJ occurrence areas, where the wind speed at the surface and at the lower atmosphere is also highest, were also recently mapped by Monteiro et al . () using remote sensing QuickSCAT measurements.…”

Coastal Iberia Summertime Low‐Level Flow Assessed From Scatterometers

“…It is hypothesized that this maximum is due to the bend in Iberia coast in a similar way as reported by Edwards et al () for California and by Patricola and Chang () for the Benguela region. The southern regime is characterized by the smaller Roca and/or S. Vicente expansion fans that are a hydraulic response to topography, as described for the California coast by (Dorman et al, ; Dorman & Koračin, ; Winant et al, ) and previously for Iberian coast by (Monteiro, Santos, et al, ; Rijo et al, ; Soares et al, ).…”

“…ASCAT‐6.25 evening passes show wind intensification frequencies of about 21% and 18% at Finisterre and Roca, and close to 28% at S. Vicente. In a previous study using QuikSCAT during the summers of 2000 to 2009 Monteiro, Santos, et al () found that S. Vicente intensifications occur less often (18%).…”

“…The distance between Cape Finisterre (Finisterre) and Cape Roca (Roca) is about 480 km, sufficient to treat the two capes independently. Conversely, the distance between Roca and Cape São Vicente (S. Vicente) is only ~200 km, and in supercritical conditions an expansion fan triggered downwind Roca can be blocked by a compression jump or bulge forming upwind S. Vicente (Monteiro, Santos, et al, ). Similar interaction between closely spaced capes was previously reported by Haack et al () between Cape Blanco and Cape Mendocino for the U.S. west coast.…”

“…In addition to this complex atmosphere-ocean coupling, the near-shore high-speed flow is further modified by its interaction with the coastal topography. For regions, as the one studied here, several authors proposed that the MABL interaction with coastal topography is explained by hydraulic theory (Monteiro, Santos, et al, 2016;Patricola & Chang, 2016;Samelson, 1992;Winant et al, 1988). Within this framework the MABL flow can be treated as the flow in a channel, the walls of which are defined by the large-scale pressure gradient offshore, the sea surface below, and the coastal mountains inland.…”