Geoscience Reference
In-Depth Information
by horizontal gradients of atmospheric pressure. These barotropic components of
currents vanish when the pressure anomalies have been compensated by the sea
surface heights. Thus, neglecting the dynamic component of the IB correction is
only justified for processes slow enough compared to the isostatic compensation
(Schuh et al.
2003
).
3 Atmospheric Layers and Circulation
Large-scale features of the atmosphere impact geodetic properties also because of
its motions. To understand the overall structure, related to the dynamic impact of the
atmosphere on the solid Earth in the form of changes in the rotational motions of
the solid planet, it is necessary to understand the structure of the atmosphere and the
circulations within important layers. Then interactions at the interfaces between the
atmosphere, land and ocean below will be understood better.
The lowest layer, known as the troposphere, has most atmospheric mass. It
extends from the surface to roughly 10km, around the 200hPa level, but these limits
change somewhat with latitude and season, and it contains approximately 80% of the
atmospheric mass. The temperature decreases with height there, and it is typically an
unstable layer. The general instability of the troposphere means that active weather
phenomena occur there, with most of the water vapor residing there too. Clouds are
formed when water vapor condenses on particles in the atmosphere. In addition, the
wind circulation in the troposphere is quite complex.
The stratosphere is the layer above the troposphere, with the tropopause the sur-
face boundary between these two layers of markedly different characteristics. The
stratosphere is characterized by temperature increase with altitude, and so is a more
stable layer. Its upper limit, the stratopause, at roughly 30km in altitude, is a transition
to another region, the mesosphere where temperature again decreases with altitude
(see Fig. 2.2 by Salstein (
1995
)). Finally, the thermosphere, above roughly 85km is
a layer of again increasing temperature, due in large part by the strong absorption of
solar radiation by gases there. Table
3
gives some standard values of temperatures at
different altitudes, particular in these regions. Gases from the thermosphere in that
region are more easily ionized, and so the regions above are known as the ionosphere,
with details covered in later sections of this part.
The general circulation of the atmosphere includes the mean wind patterns and
anomalies about the mean. Near the surface, prevailing winds in the tropics are the
easterlies (from the east), also known as the trade winds, especially across regions
above the ocean, and these are fairly shallow. In the middle latitudes are westerly
winds, typically increasing with elevation, which achieve maximum values near the
top of the troposphere, culminating in a largely zonal (east-west) jet structure in the
mean in middle latitudes. This increase with height of the strength of the zonal winds
is related physically to the meridional gradients of temperature, and this balanced
portion is known as the thermal wind. In polar latitudes easterlies again occur, but
the overall mass of the atmosphere in the polar regions carrying this momentum is
