COI Funded Project: Long Term SODAR Wind Profile Measurements Over the Coastal Ocean
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.