Geoscience Reference
In-Depth Information
(A)
(B)
Convective
100
Orographic
COSTA RICA
50
Cachi
0
0
200
0
24
48
72
km
Time (hrs)
(C)
100
30
Convection
80
25
60
20
Days with rain
15
40
Orographic
20
10
0
5
J
F
M
A
M
J
J
A
S
O
N
D
Months
Figure 5.16
Orographic and convective rainfall in the Cachi region of Costa Rica for the period
1977-1980. A: the Cachi region, elevation 500-3000m. B: typical accumulated rainfall distributions for
individual convective (duration 1-6 hours, high intensity) and orographic (1-5 days, lower intensity except
during convective bursts) rainstorms. C: monthly rainfall divided into percentages of convective and
orographic, plus days with rain, for Cachi (1018m).
Source: From Chacon and Fernandez (1985), by permission of the Royal Meteorological Society.
Doppler radar studies of the motion of falling
raindrops became feasible, the processes respon-
sible for such effects were unknown. A principal
cause is the 'seeder-feeder' ('releaser-spender')
cloud mechanism, proposed by Tor Bergeron and
illustrated in
Figure 5.14
. In moist, stable airflow,
shallow cap clouds form over hilltops. Precipita-
tion falling from an upper layer of altostratus (the
seeder cloud) grows rapidly by the washout of
droplets in the lower (feeder) cloud. The seeding
cloud may release ice crystals, which subsequently
melt. Precipitation from the upper cloud layer
alone would not give significant amounts on the
ground, as the droplets would have insufficient
time to grow in the airflow, which may traverse the
hills in 15-30 minutes. Most of the precipitation
intensification happens in the lowest kilometer-
layer of moist, fast-moving airflows.
G
THUNDERSTORMS
1 Development
In mid-latitudes the most spectacular example of
moisture changes and associated energy releases
in the atmosphere is the thunderstorm. Extreme
