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Typical Diurnal Cycle of Wind Shear
Because atmospheric stability is governed by solar radiation with its diurnal (daily)
cycle, we can expect that wind shear will also exhibit a diurnal cycle. This is typically the
case, as illustrated in Figure 8-11 by wind shear variations with time-of-day, measured for
a year at Clayton, New Mexico [Spera 1991]. Here wind shear is given by the difference
between hourly-average wind speeds at elevations of 9.1 and 45.7 m, normalized by the
wind speed at 30.7 m, and then averaged over 12 months. The highest wind shears occur
during stable conditions at night, and the lowest during unstable and neutral atmospheric
conditions near mid-day.
The atmospheric behavior that leads to these changes in wind shear can be explained
in qualitative terms by following a typical diurnal cycle of heating and cooling. Before
sunrise, heat flux is negative and the air temperature increases with elevation. An element
of air forced upward will sink back to its original elevation, so the atmosphere is stable.
Without mixing, wind shears can be high. A positive heat flux occurs at sunrise from solar
thermal radiation, and the temperature gradient near the ground reverses, so that air
temperature decreases with increasing elevation up to a level referred to as the inversion
height. Between sunrise and noon the inversion height grows rapidly in response to the
steadily increasing surface heat flux. In our typical diurnal cycle, we will classify this
period as unstable, because of the large amount of mixing that often occurs accompanied
by decreasing wind shear.
Growth of the so-called convective boundary layer below the inversion height slows
down between 1300 and 1600 hours (when the solar heat flux reaches it maximum value)
and levels off to a value on the order of 1 to 2 km, which is maintained even after the
earth's surface begins to cool by radiation. From 1200 to about 1800 hours in a typical
day, this layer of the atmosphere can be classified as neutral. Approximately one hour prior
Figure 8-11. Diurnal cycle of wind shear at Clayton, New Mexico, during 1979, showing
the effect of atmospheric stability conditions. [Spera 1991]
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