Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 106

Fig. 5.11 Frequency
distribution of wave age at
FINO1 for the year 2005. The
vertical line indicates a
breakpoint in bin width: to
the left: bin width = 5, to the
right: bin width = 50
Fig. 5.12 Wave age plotted
versus 100 m mean wind
speed at FINO1 for the year
2005
5.1.5 Impact of the Vertical Moisture Profile
Furthermore, the perfect moisture source of the sea surface has some additional
consequences. While turbulent heat and moisture fluxes are strongly correlated at
onshore sites, they are quite often uncorrelated at offshore sites. Turbulent heat
fluxes in the marine surface layer depend on the air-sea temperature difference
with upward fluxes when the sea is warmer than the air above. Turbulent moisture
fluxes are nearly always directed upward, because we nearly always have drier air
above more humid air directly over the sea surface. Since humid air is slightly
lighter than dry air (for a given temperature), these upward humidity fluxes always
contribute to a slight destabilization of the marine surface layer (Sempreviva and
Gryning 1996 ). Oost et al. ( 2000 ) also detected negative humidity fluxes together
with positive temperature fluxes in the MABL, which they could not explain with
classical Monin-Obukhov similarity. Edson et al. ( 2004 ) state that ''in fact, the
moisture flux component… provided more than half of the total buoyancy flux…,
and this component kept the surface layer slightly unstable.''Recently Barthelmie
et al. ( 2010 ) estimated that neglecting the humidity influence may lead to an
overestimation of the extrapolated mean wind at 150 m from low-level wind
speeds by about 4 %.