Wind Regimes - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 15

q

u g þ v g

¼

v g ð 2 : 7 Þ

The geostrophic wind blows parallel to the isobars of the pressure field on
constant height surfaces. Following Eqs. ( 2.5 ) and ( 2.6 ), a horizontal pressure
gradient of about 1 hPa per 1,000 km leads to a geostrophic wind speed of about
1 m/s. In the northern hemisphere, the geostrophic wind blows counter-clockwise
around low pressure systems and clockwise around high pressure systems. In the
southern hemisphere the sense of rotation is opposite.
Term VI in Eqs. ( 2.2 - 2.4 ) is not negligible in case of considerably curved
isobars. The equilibrium wind is the so-called gradient wind in this case:

oy qu !

qfu ¼ o p

ð 2 : 8 Þ

r

ox qv !

qfv ¼ o p ð 2 : 9 Þ

r

Once again, the upper sign is valid for flows around lows, the lower sign for
flows around high pressure systems. The gradient wind around low pressure sys-
tems is a bit lower than the geostrophic wind (because centrifugal force and
pressure gradient force are opposite to each other), while the gradient wind around
high pressure systems is a bit higher than the geostrophic wind (here centrifugal
force and pressure gradient force are unidirectional).
Sometimes, in rare occasions, the curvature of the isobars can be so strong that
the centrifugal force in term VI is much larger than the Coriolis force in term V so
that an equilibrium wind forms which is governed by pressure forces and
centrifugal forces only. This wind, called cyclostrophic wind by meteorologists,
is found in whirl winds and tornados.
The geostrophic wind and the gradient wind are not height-independent in
reality. Horizontal temperature gradients on levels of constant pressure lead to
vertical gradients in these winds. The wind difference between the geostrophic
winds or gradient winds at two different heights is called the thermal wind.

2.4 Thermal Winds

We introduced in Sect. 2.3 the geostrophic wind as the simplest choice for the
governing large-scale forcing of the near-surface wind field. The geostrophic wind
is an idealized wind which originates from the equilibrium between pressure
gradient force and Coriolis force. Until now we have always anticipated a baro-
tropic atmosphere within which the geostrophic wind is independent of height,
because we assumed that the horizontal pressure gradients in term III of ( 2.2 ) and
( 2.3 ) are independent of height. This is not necessarily true in reality and the