Geoscience Reference
In-Depth Information
with a limited depth as a result of the subsidence inver-
sions in both airmasses (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 throughout their length
or even at all levels in the troposphere. For this reason,
radar is being used increasingly 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 kilometres across and several
kilometres 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 25-30 m s
-1
at about 1 km 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 (Figure
9.12). This flow is termed a '
conveyor belt
' (for
large-scale heat and momentum transfer in mid-
latitudes). Broad-scale convective (potential) instability
is generated by the overrunning of this low-level flow
by potentially colder, drier air in the middle troposphere.
Instability is released mainly in small-scale convection
cells that are organized into clusters, known as
mesoscale precipitation areas (MPAs). These MPAs are
further arranged in bands 50 to 100 km wide (Figure
9.13). Ahead of the warm front, the bands are broadly
parallel to the airflow in the rising section of the
conveyor belt, whereas in the warm sector they parallel
the cold front and the low-level jet. In some cases, cells
and clusters are further arranged in bands within the
warm sector and ahead of the warm front (see Figures
9.13 and 9.14). Precipitation from warm front rain bands
often involves 'seeding' by ice particles falling from the
upper cloud layers. It has been estimated that 20 to 35
per cent of the precipitation originates in the 'seeder'
zone and the remainder in the lower clouds (see also
Figure 5.14). Orographic effects set up some of the cells
and clusters, and these may travel downwind when the
atmosphere is unstable.
Figure 9.10
Schematic model of a dry trough and frontogenesis
east of the Rocky Mountains. (A) Warm, dry air with low equiv-
alent potential temperature (
θ
e
) from the Rockies overrides warm,
moist, high
θ
e
air from the Gulf of Mexico, forming a potentially
unstable zone east of the dry trough. (B) Upward motion
associated with the cold front aloft (CFA). (C) Location of the CFA
rain band at the surface. (Equivalent potential temperature is the
potential temperature of an air parcel that is expanded adiabatically
until all water vapour is condensed and the latent heat released
then compressed adiabatically to 1000 mb pressure.)
Source
: After Locatelli
et al
. (1995), by permission of the American
Meteorological Society.
