Long Term SODAR Wind Profile Measurements Over the Coastal Ocean
Eugene Terray, Applied Ocean Physics and Engineering
Project Funded 2007:
We propose to investigate the performance of Doppler SODAR for profiling winds in the coastal marine atmospheric boundary layer (MABL) over a wide range of atmospheric stability. Measurements will be conducted for a one year from the MVCO Air-Sea Interaction Tower. The motivation is to provide a new tool for studying the complex interaction between the atmosphere and the coastal ocean arising from the differences in the surface heat flux and roughness between the land and water. In the case of on-shore flow, the stress at the surface is strongly correlated with the roughness due to the surface waves, and can be enhanced near the coast due to shoaling (Sun et al., 2001). The situation for off-shore flows is much more complicated. If the water is warmer than the air, an unstable internal convective boundary layer develops and wave growth and air-sea fluxes in general are enhanced (Attie and Durand, 2003; Sun et al., 2001). The dynamics in the case of warm air flowing over cooler water is, by contrast, poorly understood. The formation of a stable boundary layer lowers the surface stress and reduces wave generation (and hence the surface roughness - which in turn feeds back to damp the atmospheric turbulence), as well as imparting less momentum to the currents. The vertical structure of the MABL in this case also is complex (Mahrt et al., 2001). Because the surface layer turbulence is suppressed, the capping layer above can become partially decoupled and accelerate, leading to the formation of a low-level maximum in the wind - similar to the formation of a nocturnal jet over land (Fig. 1, Vickers and Mahrt, 2004). This process was recently modeled using both mesoscale (COAMPS) and LES models. Both models reproduced the qualitative features discussed above, but under-predicted the momentum flux at the surface. Although we have couched our discussion in terms of land/sea flows, similar effects are associated with temperature changes across fronts located well offshore (Mahrt et al., 2004), and have also been observed in the open ocean as a positive correlation between the sea surface temperature (SST) and wind stress derived from satellite observations (Xie, 2004). In contrast to the understanding of MABL development (which itself is not complete), there is even less theoretical understanding of the response of the ocean to these horizontal stress gradients. Spall (2006) has looked at the effect of SST/wind stress coupling on the growth rate of baroclinic waves. However, his specific mechanism is mainly relevant for low latitude, strongly stratified flows, and we are not aware of similar work in the coastal ocean - although there one might expect that bathymetry, the existence of strong temperature gradients and the presence of crossfrontal winds would lead to interesting effects in the coastal circulation. But in any case, advances in theoretical understanding are likely to be stimulated by advances in observation. In addition to its scientific interest, the vertical structure of MABL winds is an important factor affecting the performance and life expectancy of offshore wind turbines, where knowledge of the expected shear across the turbine blades (which can have a span of almost 150 m) is an essential design consideration. Despite their importance, acquiring long-term wind profiles at sea under a wide range of conditions has been a notoriously difficult problem.