Geoscience Reference
In-Depth Information
the wind speeds remain below 40 km h
−
1
, they are referred to as
tropical depressions
; they
are called
tropical storms
for wind speeds between 40 and 120 km h
−
1
, and
hurricanes
for winds above that range. In the western Pacific Ocean they are known as
typhoons
.
Such systems have also generated some of the largest rainfalls ever recorded. Amounts
of 15-25 cm in a 24 h period are not uncommon over level land.
3.2.5
Orographic effects
The precipitation resulting from each of the general weather types discussed here can
be markedly affected by topographic features, such as elevation, slope and aspect of
the land surface. The result tends to be increased precipitation on windward slopes,
and smaller precipitation on leeward slopes, also called rain shadows. In some regions
with identifiable prevailing wind directions, such as the coastal ranges of western North
America or the foothills of Meghalaya in eastern India, the windward slopes can be
readily identified. On the other hand, as noted by Gilman (1964), in the Appalachian
mountains, the windward and leeward sides can be quite variable, depending on the
wind direction. Smith (1979) specified that there are three independent mechanisms
of orographic precipitation, as follows. (i) Large-scale upslope precipitation, which is
generated by forced vertical motion of the stratiform type or by triggered convection as
the air moves over rising terrain. (ii) Small-scale redistribution of precipitation from pre-
existing clouds by small hills; over the hill tops the precipitation is increased, because
their higher surface can intercept the falling drops before they evaporate, and apparently
also because the drops undergo increased accretion by washout of low-level clouds.
(iii) Generation of upslope winds in a conditionally unstable air mass as a result of slope
heating by the Sun; these develop into rising thermals, which in turn can grow into
cumulonimbus clouds above the lifting condensation level.
In general, because there are several other factors beside elevation, the effect of orog-
raphy by itself in causing increased precipitation is not always obvious; physically, its
main effect is as a trigger mechanism for convective activity. Accordingly, as observed by
Suzuki
et al
. (2002), the relationship between precipitation and elevation is usually more
pronounced for convective than for stratiform rainfall. The relationship is also stronger
and more apparent for larger accumulated rainfall amounts, for longer accumulation time
scales, and for larger rainfall intensities. For instance, in the analysis of hourly rainfall
data, the random effects of other factors may dominate the measured precipitation, so
that the effect of topography may go undetected. With daily rainfall data topographic
effects gradually emerge, albeit with large variations from one day to the next. With
monthly data the effects of the other factors tend to become averaged out and the effect
of elevation is more apparent. Numerous studies reviewed by Daly
et al
. (1994) have
reported linear relationships between precipitation and elevation, but other relationships,
such as loglinear functions have also been documented. In the mid-latitudes the climato-
logical precipitation maxima tend to occur at or near the crest of mountainous barriers.
However, in warmer regions (e.g. Hawaii), or in large-scale precipitation events (e.g.
the Sierras in California) the maximal precipitation may occur somewhat lower, ahead
