Geoscience Reference
In-Depth Information
Equations 6.51 and 6.52 are known as the
thermal wind equations
, which
is confusing because they are expressions for the wind shear and not the wind
itself. Equation 6.51 indicates that a negative meridional temperature gradi-
ent,
22
< 0, such as is found on average in Northern Hemisphere middle
latitudes where temperature decreases with increasing latitude, is associated
with winds that become increasingly westerly with elevation, that is,
/
Ty
22
> 0
uz
or / n
22
> 0, winds
also become more westerly with elevation. This relationship is borne out in
observations (see
Figs. 2.9 a
nd
2.10)
. The geostrophic relationship between the
wind velocity and meridional temperature gradients explains the location of
the westerly jets and their seasonal variation.
22
< 0. In the Southern Hemisphere, with
f
< 0 and
u
p
/
Ty
6.5 FRICTION
Frictional forces are important close to the surface and, in the oceans, close
to the coasts where they act opposite to the direction of the flow. To under-
stand one role of friction in atmospheric flows, consider a parcel of air in
geostrophic balance near a low in the Northern Hemisphere. Adding friction
to the balance of forces decreases the wind speed
(Figure 6.7)
and, as a conse-
quence, reduces the magnitude of the Coriolis force since it is proportional to
the velocity (Eq. 6.28). The result is that the pressure gradient force is stronger
than the Coriolis force, and the flow accelerates down the pressure gradient.
For this reason, the low-level low around a low-pressure system near the
surface has a component directed into the low. The air converges, and conser-
vation of mass requires upward motion. If the air is moist, this movement can
lead to clouds and precipitation. Similarly, it is common to find divergence in
the vicinity of a high due to the effects of friction, and the sinking air tends to
be clear and cool.
Frictional acceleration is also important in the ocean mixed layer, where the
effects of surface winds are strong. The horizontal equations of motion for this
case are derived in
chapter 8.
On or very close to the equator, the Coriolis force tends to zero and geo-
strophic flow is no longer sustained. Here, the main balance of forces is between
L
Figure 6.7 Low-level low around a low in the
Northern Hemisphere to illustrate frictional
convergence. Solid arrows indicate the direction
of the geostrophic flow; dashed arrows indicate
the direction of the flow when frictional
acceleration is added to the geostrophic balance.
