|地球流体力学课件【英】-Forced General Circulation
GFD-II-Ch.3. Forced Circulation. 2011-02-17 1
Ch.3: Forced General Circulation
Sec. 3.1: Atmospheric Circulation
On the horizontal plane (or isobar, isotropic surface), the atmospheric circulation is
relatively simple. At the leading order, it consists mainly of zonal flows. At the next order, the
zonal flow is distorted by stationary waves that are forced by localized surface heating (due to
land-sea contrast) and topography. (see Figs.3.1, 3.2). One may think that the reason for the first
order flow to be dominantly zonal U(y) is due to the solar radiation that is zonally uniform. This is
only half true.
The other fundamental reason for the presence of a dominant zonal flow U(y) is the rotation,
or more precisely the beta effect. The rotation of the earth restrains the fluid to flow along constant
potential vorticity lines (latitude circle) in the absence of strong damping and diabatic heating.
Furthermore, the absence of meridional boundaries allows the U(y) flow to be free modes in the
atmosphere (of zero frequency). These “free modes”, when forced by the zonally uniform heating,
becomes dominant due to resonant excitation.
The effect of beta can be seen easily in the context of the QG equation. The PV conservation
gives the equation for the steady state circulation as: