Geoscience Reference
In-Depth Information
2
WATER ALOFT: FLUID MECHANICS
OF THE LOWER ATMOSPHERE
On account of the short residence time and great mobility of water vapor in air, the lower
atmosphere is one of the critical pathways in the global hydrologic cycle; it transports
water and energy around the globe without regard to continental boundaries and thus links
the continents, the upper atmosphere, and the oceans. The transport and distribution of
water vapor in the lower atmosphere, where it is most abundantly present, are among the
main factors controlling precipitation and evaporation from the surface; these processes,
in turn, determine soil and groundwater storage and the different runoff phenomena.
2.1
WATER VAPOR IN AIR
2.1.1
Global features
The global amount of water vapor contained in the air is roughly (see Table 1.3) equivalent
with a layer of liquid water covering the earth, with a thickness of around 25 mm on
average. The thickness of this layer, which is the total liquid equivalent of water vapor
in the atmospheric column at a given location, is also called the
precipitable water
,
W
p
.
However, this quantity of water vapor is not distributed uniformly and it can greatly vary
over a wide range of scales in space and in time. For instance, the water vapor content
of the atmosphere, just like the temperature, generally tends to decrease with increasing
latitude. Available data (Randel
et al.
, 1996) show that the precipitable water is more
likely to be well below 5 mm near the Poles, and close to 50 mm near the Equator.
But this is not always the case; even at similar latitudes there can be huge regional
variations, the most extreme example being the warm dry deserts of the world. Most of
the atmospheric water vapor is found relatively close to the ground, and at any given
location water vapor decreases sharply with height; typically, about half the total water
vapor in the atmospheric column can be found below a height of 1 or 2 km.
Because the global annual evaporation is around
E
=
1 m, the average atmospheric
residence time of water vapor
W
p
/
E
is only about 9 days. This time scale governs
the hydrologic interactions and water transfers between the atmosphere and the other
two compartments of the global system, the oceans and the continents. This time scale
is especially fundamental to the transport of atmospheric water vapor from its source
regions - mainly evaporation from the oceans - to sinks in precipitating weather systems.
Indeed, the excess precipitation on the continents, which does not evaporate, ultimately
runs off to the seas and oceans of the world. A balance is maintained in the global system
by the fact that over the oceans the situation is reversed and that evaporation is generally
larger than precipitation, allowing the excess oceanic water vapor to be transported back
