Geoscience Reference
In-Depth Information
barrier wind
that may feature a low-level (850mb)
jet of 20m s
-1
. Such winds are common upstream
of the Sierra Nevada, California.
The displacement of air upward over an
obstacle may trigger instability if the air is
conditionally unstable and buoyant (see Chapter
5B), whereas stable air returns to its original level
in the lee of a barrier as the gravitational effect
counteracts the initial displacement. This descent
often forms the first of a series of
lee waves
(or
standing waves
) downwind, as shown in
Figure
6.13
. The wave form remains more or less
stationary relative to the barrier, with the air
moving quite rapidly through it. Below the crest
of the waves, there may be circular air motion in
a vertical plane, which is termed a
rotor
. The
formation of such features is of vital interest to
pilots. The presence of lee waves is often marked
by the development of lenticular clouds, and on
occasion a rotor causes reversal of the surface wind
direction in the lee of high mountains.
Winds on mountain summits are usually
strong, at least in middle and higher latitudes.
Average speeds on summits in the Colorado Rocky
Mountains in winter months are around 12-15m
s
-1
, for example, and on Mount Washington,
New Hampshire, an extreme value of 103m s
-1
has
been recorded. Peak speeds in excess of 40-50m s
-1
are typical in both of these areas in winter. Airflow
over a mountain range causes the air below the
tropopause to be compressed and thus accelerated
particularly at and near the crest line (the Venturi
effect), but friction with the ground also retards the
flow, compared with free air at the same level. The
net result is predominantly one of retardation,
but the outcome depends on the topography, wind
direction and stability.
Over low hills, the boundary layer is displaced
upward and acceleration occurs just above the
summit.
Figure 6.14
shows instantaneous airflow
conditions across Askervein Hill (relief
c
. 120m)
on the island of South Uist in the Scottish
Hebrides, where the wind speed at a height of
10m above the ridge crest approaches 80 percent
more than the undisturbed upstream velocity. In
360
(A)
12
270
DIR
8
180
SPD
4
90
0
7
9
11
13
15
17
19
21
15
(B)
24
13
T
20
11
Q
9
16
7
12
5
7
9
11
13 15
Time (PST)
17
19
21
Figure 6.12
The effects of a westerly sea breeze
on the California coast on 22 September 1987 on
temperature and humidity. Above: Wind direction
(DIR) and speed (SPD); below: air temperature (T)
and humidity mixing ratio (Q) on a 27m mast near
Castroville, Monterey Bay, California. The gradient
flow observed in the morning and evening was
easterly.
Source: Banta (1995, p. 3621, Fig. 8).
flat has a high albedo and the moist substrate
results in a high thermal conductivity relative to
the surrounding sandy terrain. The flows are
about 100m deep at night and up to 250m by day.
3 Winds due to topographic
barriers
Mountain ranges strongly influence airflow
crossing them. On the upwind side of mountains
perpendicular to the airflow, blocking may occur
when the airflow is stable and unable to cross the
barrier. As the flow approaches the barrier it slows
down, thus reducing the Coriolis force. Imbalance
with the pressure gradient force then causes the air
to turn poleward towards the lower pressure on
the left side of the flow. This sets up a low-level
