Geoscience Reference
In-Depth Information
Air-mass boundaries give rise to baroclinic frontal zones a few hundred kilometers 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. Air masses and frontal zones move poleward (equatorward) in summer (winter).
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
favored 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 300-200mb, which
also follow the longwave 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 the warm-sector air is rising or not (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 kilometers 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.
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What are the essential differences between mesoscale and synoptic scale systems?
•
Using an appropriate website with synoptic weather maps (see Appendix 4D), trace the
movement of frontal and non-frontal lows/troughs and high pressure cells over a five-day
period, determining rates of displacement and changes of intensity of the systems.
•
In the same manner, examine the relationship of surface lows and highs to features at the 500mb
level.
•
Consider the geographical distribution and seasonal occurrence of different types of non-frontal
low pressure systems.
