Geoscience Reference
In-Depth Information
Figure 5.14
Schematic diagram of
T. Bergeron's 'seeder-feeder' cloud
model of orographic precipitation
over hills.
Note
: This process may also operate in
deep nimbostratus layers.
Source
: After Browning and Hill (1981).
Reprinted from
Weather
, by permission
of the Royal Meteorological Society.
Crown copyright ©.
Figure 5.15
Mean annual precipita-
tion (1951 to 1980) along a transect of
the South Island of New Zealand,
shown as the solid line in the inset map.
On the latter, the dashed line indicates
the position of the Godzone Wetzone
and the figures give the precipitation
peaks (cm) at three locations along the
Godzone Wetzone.
Sources
: Chinn (1979) and Henderson
(1993), by permission of the New
Zealand Alpine Club Inc and T.J. Chinn.
processes involved, Tor Bergeron proposed using the
term 'orogenic', rather than orographic, precipitation
(i.e. an origin related to various orographically produced
effects). An extreme example of orographic precipi-
tation is found on the western side of the Southern Alps
of New Zealand, where mean annual rainfall totals
exceed 10 metres! (Figure 5.15).
In mid-latitude areas where precipitation is pre-
dominantly of cyclonic origin, orographic effects tend
to increase both the frequency and intensity of winter
precipitation, whereas during summer and in continental
climates with a higher condensation level the main
effect of relief is the occasional triggering of intense
thunderstorm-type precipitation. The orographic influ-
ence occurs only in the proximity of high ground
in the case of a stable atmosphere. Radar studies
show that the main effect in this case is one of redis-
tribution, whereas in the case of an unstable atmosphere
precipitation appears to be increased, or at least redis-
tributed on a larger scale, since the orographic effects
may extend well downwind due to the activation of
mesoscale rain bands (see Figure 9.13).
In tropical highland areas there is a clearer distinction
between orographic and convective contributions to
total rainfall than in the mid-latitude cyclonic belt.
Figure 5.16 shows that in the mountains of Costa Rica
the temporal character of convective and orographic
rainfalls and their seasonal occurrences are quite
