Geoscience Reference
In-Depth Information
tion occurring, is generally designated as
nimbostratus. Patches of fractostratus may also
form in the cold air as rain falling through this air
undergoes evaporation and quickly saturates it.
The descending warm air of the kata-warm
front greatly restricts the development of
medium- and high-level clouds. The frontal cloud
is mainly stratocumulus, with a limited depth as
a result of the subsidence inversions in both air
masses (see
Figure 9.11B
). Precipitation is usually
light rain or drizzle formed by coalescence.
At the passage of the warm front the wind
veers, the temperature rises and the fall of pressure
is checked. The rain becomes intermittent or
ceases in the warm air and the thin stratocumulus
cloud sheet may break up.
Forecasting the extent of rain belts associated
with the warm front is complicated by the fact that
most fronts are not ana- or kata-fronts through-
out their length or even at all levels in the
troposphere. For this reason, radar is increasingly
being used to map the precise extent of rain belts
and to detect differences in rainfall intensity. Such
studies show that most of the production and
distribution of precipitation is controlled by a
broad airflow a few hundred kilometers across
and several kilometers deep, which flows parallel
to and ahead of the surface cold front (see
Figure
9.12
). Just ahead of the cold front, the flow occurs
as a low-level jet with winds of up to 25-30m s
-1
at about 1km above the surface. The air, which is
warm and moist, rises over the warm front and
turns southeastward ahead of the front, merging
with the mid-tropospheric flow (B in
Figure 9.13
).
This flow is termed a '
conveyor belt
' (for large-scale
Cirrus cloud
boundary
0
200
C
km
C
4b
3b
4a
Low
B
3a
5
X
N
5
B
A
3a
A
3b
2
Y
1b
A
Conveyor belt of ascending air
1a
0
300
B
Mid-troposphere flow
Subsiding flow ahead of occluded front
km
C
SYNOPTIC FEATURES
TYPES OF
MESOSCALE RAINBANDS
1
2
3
4
5
Precipitation (including bels of higher intensity)
Surface
Warm frontal
Warm sector
Cold frontal
Prefrontal cold surge
Postfrontal
Cold front
Warm front
Warm occluded
front
Figure 9.12
Model of the large-scale flow and
mesoscale precipitation structure of a partially
occluded depression typical of those affecting the
British Isles. It shows the 'conveyor belt' (A) rising
from 900mb ahead of the cold front over the warm
front. This is overlaid by a mid-tropospheric flow (B)
of potentially colder air from behind the cold front.
Most of the precipitation occurs in the well-defined
region shown, within which it exhibits a cellular and
banded structure.
Source: After Harrold (1973). By permission of the Royal
Meteorological Society.
Aloft
Cold front
Prefrontal
cold surge
Figure 9.13
Fronts and associated rain bands
typical of a mature depression. The broken line X-Y
shows the location of the cross-section given in
Figure 9.14.
Source: After Hobbs; from Houze and Hobbs (1982). By
permission of Academic Press.
