EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
< 0.1 Y blocked
U
(II-2-2)
> 0.1 Y no blocking
hm
where
1. U = wind speed
hm = height of the land barrier (in units consistent with U)
(b) An elevation of only 100 m will cause blocking of wind speeds less than about 10m/s, which includes
most onshore winds (Overland 1992). The horizontal scale of these effects is on the order of 50 - 150 km.
Another orographic effect called katabatic wind is caused by gravitational flow of cold air off higher ground
such as a mountain pass. Since katabatic winds require cold air they are more frequent and strongest in high
latitudes. These winds can have a significant impact on local coastal areas and are very site-specific
(horizontal scale on the order of 25 km).
(c) Another local process, the sea breeze effect, is air flow caused by the differences in surface
temperature and heat flux between land and water. Land temperatures change on a daily cycle while water
temperatures remain relatively constant. This results in a sea breeze with a diurnal cycle. The on/offshore
extent of the sea breeze is about 10 -20 km with wind speeds less than 10 m/s.
(d) Although understanding of atmospheric flows in complicated areas is still somewhat limited,
considerable progress has been made in understanding and quantifying flow characteristics in simple,
idealized situations. In particular, synoptic-scale winds in open-water areas (more than 20 km or so from
land) are known to follow relatively straightforward relationships within the atmospheric boundary layer. The
flow can be considered as a horizontally homogeneous, near-equilibrium boundary layer regime. As
described in Tennekes (1973), Wyngaard (1973,1988), and Holt and Raman (1988), present-day boundary
layer parameterizations appear to provide a relatively accurate depiction of flows within the homogeneous,
near-equilibrium atmospheric boundary layers. Since these boundary-layer parameterizations have a
substantial basis in physics, it is recommended that they be used in preference of older, less-verified methods.
d. Characteristics of the atmospheric boundary layer.
(1) Since the 1960's, evidence from field and laboratory studies (Clarke 1970, Businger et al. 1971,
Willis and Deardorff 1974, Smith 1988) and from theoretical arguments (Deardorff 1968, Tennekes 1973,
Wyngaard 1973, 1988) have supported the existence of a self-similar flow regime within a homogeneous,
near-equilibrium boundary layer in the atmosphere. In the absence of buoyancy effects (due to vertical
gradients in potential temperature) and if no significant horizontal variations in density (baroclinic effects)
exist, the atmospheric boundary layer can be considered as a neutral, barotropic flow. In this case, all flow
characteristics can be shown to depend only on the speed of the flow at the upper edge of the boundary layer,
roughness of the surface at the bottom of the boundary layer, and local latitude (because of the influence of
the earth's rotation on the boundary-layer flow). Significantly for engineers and scientists, this theory
predicts that wind speed at a fixed elevation above the surface cannot have a constant ratio of proportionality
to wind speed at the top of the boundary layer.
(2) Deardorff (1968), Businger et al. (1971), and Wyngaard (1988) clearly established that flow
characteristics within the atmospheric boundary layer are very much influenced by thermal stratification and
horizontal density gradients (baroclinic effects). Thus, various relationships can exist between flows at the
top of the boundary layer and near-surface flows. This additional level of complication is not negligible in
many applications; therefore, stability effects should be included in wind estimates in important applications.
II-2-8
Meteorology and Wave Climate
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