Geoscience Reference
In-Depth Information
Most are minor outbreaks, but on 23 November 1981,
102 were reported during southwesterly flow ahead of
a cold front. They are most common in autumn, when
cold air moves over relatively warm seas.
SUMMARY
Newer cyclone theories regard fronts as rather
incidental. Cloud bands and precipitation areas are
associated primarily with conveyor belts of warm air.
Divergence of air in the upper troposphere is essential for
large-scale uplift and low-level convergence. Surface
cyclogenesis is therefore favoured on the eastern limb of
an upper wave trough. 'Explosive' cyclogenesis appears to
be associated with strong wintertime gradients of sea-
surface temperature. Cyclones are basically steered by the
quasi-stationary long (Rossby) waves in the hemispheric
westerlies, the positions of which are strongly influenced
by surface features (major mountain barriers and land/sea-
surface temperature contrasts). Upper baroclinic zones
are associated with jet streams at 300 to 200 mb, which
also follow the long-wave pattern.
The idealized weather sequence in an eastward-
moving frontal depression involves increasing cloudiness
and precipitation with an approaching warm front; the
degree of activity depends on whether or not the warm-
sector air is rising (ana- or kata-fronts, respectively). The
following cold front is often marked by a narrow band of
convective precipitation, but rain both ahead of the warm
front and in the warm sector may also be organized into
locally intense mesoscale cells and bands due to the
'conveyor belt' of air in the warm sector.
Some low-pressure systems form through non-frontal
mechanisms. These include the lee cyclones formed in the
lee of mountain ranges; thermal lows due to summer
heating; polar air depressions commonly formed in an
outbreak of maritime Arctic air over oceans; and the upper
cold low, which is often a cut-off system in upper wave
development or an occluded mid-latitude cyclone in the
Arctic.
Mesoscale convective systems (MCSs) have a
spatial scale of tens of kilometres and a timescale of a few
hours. They may give rise to severe weather, including
thunderstorms and tornadoes. Thunderstorms are
generated by convective uplift, which may result from
daytime heating, orographic ascent or squall lines. Several
cells may be organized in a mesoscale convective complex
and move with the large-scale flow. Thunderstorms
associated with a moving convective system provide an
environment for hailstone growth and for the generation
of tornadoes.
Ideal airmasses are defined in terms of barotropic
conditions, where isobars and isotherms are assumed to
be parallel to each other and to the surface. The character
of an airmass is determined by the nature of the source
area, changes due to airmass movement, and its age.
On a regional scale, energy exchanges and vertical mixing
lead to a measure of equilibrium between surface
conditions and those of the overlying air, particularly
in quasi-stationary high-pressure systems. Airmasses
are conventionally identified in terms of temperature
characteristics (Arctic, polar, tropical) and source region
(maritime, continental). Primary airmasses originate in
regions of semi-permanent anticyclonic subsidence over
extensive surfaces of like properties. Cold airmasses
originate either in winter continental anticyclones (Siberia
and Canada), where snow cover promotes low
temperatures and stable stratification, or over high-latitude
sea ice. Some sources are seasonal, such as Siberia; others
are permanent, such as Antarctica. Warm airmasses
originate either in shallow tropical continental sources
in summer or as deep, moist layers over tropical
oceans. Airmass movement causes stability changes by
thermodynamic processes (heating/cooling from below
and moisture exchanges) and by dynamic processes
(mixing, lifting/subsidence), producing secondary airmasses
(e.g. mP air). The age of an airmass determines the degree
to which it has lost its identity as the result of mixing with
other airmasses and vertical exchanges with the underlying
surface.
Airmass boundaries give rise to baroclinic frontal zones
a few hundred kilometres wide. The classical (Norwegian)
theory of mid-latitude cyclones considers that fronts are a
key feature of their formation and life cycle. Newer models
show that instead of the frontal occlusion process, the
warm front may become bent back with warm air
seclusion within the polar airstream. Cyclones tend to
form along major frontal zones - the polar fronts of
the North Atlantic and North Pacific regions and of the
southern oceans. An Arctic front lies poleward and there
is a winter frontal zone over the Mediterranean. Airmasses
and frontal zones move poleward (equatorward) in
summer (winter).
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