Ocean–atmosphere–wave characterisation of a wind jet (Ebro shelf, NW Mediterranean Sea)

Abstract

Abstract. In this contribution the wind jet dynamics in the northern margin of the Ebro River shelf (NW Mediterranean Sea) are investigated using coupled numerical models. The study area is characterised by persistent and energetic offshore winds during autumn and winter. During these seasons, a seaward wind jet usually develops in a ∼ 50 km wide band offshore. The COAWST (Coupled Ocean-Atmosphere-Wave-Sediment Transport) modelling system was implemented in the region with a set of downscaling meshes to obtain high-resolution meteooceanographic outputs. Wind, waves and water currents were compared with in situ observations and remote-sensingderived products with an acceptable level of agreement. Focused on an intense offshore wind event, the modelled wind jet appears in a limited area offshore with strong spatial variability. The wave pattern during the wind jet is characterised by the development of bimodal directional spectra, and the ocean circulation tends to present well-defined twolayer flow in the shallower region (i.e. inner shelf). The outer shelf tends to be dominated by mesoscale dynamics such as the slope current. Due to the limited fetch length, ocean surface roughness considering sea state (wave-atmosphere coupling) modifies to a small extent the wind and significant wave height under severe cross-shelf wind events. However, the coupling effect in the wind resource assessment may be relevant due to the cubic relation between the wind intensity and power.

Shelf Circulation Induced by an Orographic Wind Jet

“…Table shows the main characteristics of the selected events at V3, which is approximately located in the wind‐jet axis, and some statistics comparing the WRF model data with buoy B2. Mean and maximum values show that during the warmer months, the intensities are lower than during the colder months, which is consistent with previous investigations (Grifoll et al, ). The statistics show that the WRF model is able to capture the wind intensity during the wind‐jet episodes fairly well, with biases that usually do not reach 1 m/s and RMSD values around 2 m/s.…”

“…Additionally, this behavior is linked with the two‐layer flow structure observed in our numerical results, where water from deeper layers must upwell near the coast to replace the water flowing offshore at the surface. The two‐layer flow structure has been previously observed in the Ebro Shelf by Grifoll et al () and is consistent with results of other investigations (Fewings et al, ; Grifoll et al, ; Horwitz & Lentz, ; Tilburg, ), where a two‐layer flow structure due to cross‐shelf winds has been found at the inner shelf. This two‐layer behavior was first investigated by Ekman (), who analytically found the existence of an upper current in the direction of the wind and a compensating, comparatively weak current running in the opposite direction below.…”

“…Regions with coastal wind jets induced by orographic effects are areas with complex dynamics in terms of water circulation response (Grifoll et al, ; Jordà, ; Trasviña et al, ), spatial and temporal wind variability (Jiang et al, ), and sharp wave height variations (Shimada & Kawamura, ). In this sense, several researchers have focused on the description of different aspects such as the wave generation/decaying process (Shimada & Kawamura, ), the surface water cooling effect due to upwelling (Trasviña et al, ), or the influence of the sea surface roughness variation due to the wave climate (Grifoll et al, ). However, these regions have still not been thoroughly investigated and some aspects of the water circulation response to wind jets remain unclear.…”

“…Horwitz and Lentz () noted that it is the cross‐shelf rather than vertical density gradient that is critical to predict transport driven by a cross‐shelf wind stress. To investigate these processes, the use of numerical modeling has been proven to be a powerful tool (e.g., Grifoll et al, ; Horwitz & Lentz, ; Tilburg, ).…”

Vertical Azimuth Display simulator for wind-Doppler lidar error assessment

“…The simulation frequency choosen is 0.3 Hz because is a typical figure measured in similar lidar buoys in the nearshore mediterranean sea [14]. The initial phase is 0 deg, corresponding to the lidar scanning cone pointing in the vertical direction (i.e.…”