Geoscience Reference
In-Depth Information
z
C
C
B
z C
B
D
A
D
A
D
T,q
Fig. 3.10 Moist air passing over rising terrain may result in a downslope wind which is warmer and drier. The
graph on the right shows the changes in temperature T (solid line) and specific humidity q (dashed
line) of a parcel of air with height z ; the lapse rate of the solid line segments AB and CD is very close to
dry adiabatic, and that of the segment BC is close to saturated adiabatic. The height z C is the
condensation level, where precipitation starts and the rate of temperature decrease changes from dry
adiabatic to saturated adiabatic.
of the topographic barrier; this may be due to upwind “rainout.” On the other hand, in
some situations of steep and narrow mountain ridges, the precipitation maxima may be
delayed, and may occur downwind from the crest of the barrier.
Example 3.1. Passage over mountain barriers
In the previous chapter the concepts of adiabatic lapse rate and the resulting atmospheric
stability were in introduced. Figure 2.4 illustrates how air, which is initially stable,
can be made unstable by being forced to rise. This is called conditional instability.
As another illustration, consider this time an air mass that is forced to rise by moving
over rising terrain, as sketched in Figure 3.10. Again, initially the temperature of an
air parcel will decrease at a rate roughly equal to the dry adiabatic lapse rate. Above
the condensation level this rate will become smaller, and roughly equal to the saturated
adiabatic lapse rate. As the saturated air continues to rise, it cools further down and its
moisture gradually precipitates out. After it passes the peak, and goes back down, through
its descent the air warms up at the dry adiabatic lapse rate and gradually becomes less
and less saturated. Finally, after its passage downwind from the mountain the air is both
warmer and drier than it was originally upwind. This type of phenomenon is variously
referred to as Fohn in the Alpine regions of Europe, Santa Ana in southern California
and Chinook east of the Rocky Mountains of North America. Again, as was the case
with frontal systems, when the entire incoming air mass is initially stable and moves
Search WWH ::




Custom Search