Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 126

Fig. 5.34 Principal sketch of
a land-sea wind circulation at
daytime (top) and at night-
time (below)
near the surface and a return flow emerges in the opposite direction aloft in order
to keep the mass balanced.
This leads to the well-known sea breeze during daytime and in the evening and
the land breeze at night and in the early morning (Fig. 5.34 ). This common feature
of a land breeze is the reason why in former times sailing ships left harbours in the
early morning and tended to return in the afternoon. The sea breeze front propa-
gates inland several tens of kilometres during the day and is—if enough moisture
is available in the air—often marked by a chain of cumulus clouds. In mid-
latitudes sea breezes tend to penetrate 20-50 km but in the tropics distances of up
to 300 km and over have been observed. The depth of this flow ranges from a few
hundreds of metres to one to two kilometres. Maximum wind speeds in sea breezes
can be around 10-11 m/s at about 100 m height (Atkinson 1981 ). Sea breezes
originate from a 100 to 120 km broad coastal zone over the water, detectable from
satellite images showing cloud-free conditions in this space (Simpson 1994 ). The
clouds are dissolved due to the sinking motion in this marine branch of the sea
breeze (see outer downward arrows in the upper frame of Fig. 5.34 ). Fewer
observations are available for the nocturnal land breeze, but it can be assumed that
the spatial extent of these winds is comparable to the extent of the sea breeze.
5.6.2 Low-Level Jets
The offshore vertical wind profile is not always a monotonically increasing
function with height. Sometimes wind speed maxima occur within or at the top of
the boundary layer. The formation of low-level jets over flat terrain requires a
diurnal variation in the thermal stratification of the surface layer. For details refer
to Sect. 3.4.2 . Although the necessary conditions for the formation of low-level
jets are usually absent over ocean surfaces, we sometimes observe low-level jets in
the marine boundary layer as well. For example, there are frequent reports from
the Baltic Sea and other coastal areas, especially in times when the sea surface is