Geoscience Reference
In-Depth Information
Frontal cyclones dominate the weather in the mid- and high latitudes, mainly in the
colder season, when the contrasts between the equatorial and the polar regions are the
most pronounced. In the warmer seasons, such cyclonic systems are generally weaker.
They typically involve length scales of the order of 10
3
km, also referred to as the
mesoalpha (see Table 1.5), macro, or synoptic scale and their highest incidence is around
55
◦
latitude.
3.2.2
Extratropical convective weather
Unstable atmospheric conditions have the potential of generating organized systems of
convective quasi-vortical movement of air, over a wide range of scales. Recall that an
ideal vortex is a flow in which the streamlines are concentric circles. In the atmosphere,
however, convective systems are considerably more complex than an inverted bathtub
vortex. Under the right moisture conditions of the atmosphere, these systems can develop
into thunderstorms, and may consist of a single storm cell, or of several cells as part of a
mesoscale convective system. The spatial extent of these systems tends to cover mainly
the mesobeta into the mesoalpha scales, typically ranging from about 50 to 500 km, but
individual cells can be as small as only a few kilometers. Individual cells are characterized
by strong local updrafts and downdrafts. Simply put, the updrafts are a manifestation of
the unstable conditions of the air (see Figures 2.2 and 2.4) and lead to condensation in the
cooling air resulting in precipitation. The downdrafts, on the other hand, result not only
from the entrainment by falling precipitation and some evaporative cooling, but also from
return flows required by continuity to compensate for the upward motions (Vonnegut,
1997); some of these are produced after updrafts reach their highest level and then fall
back as downward currents. Most systems of this type are accompanied by a specific
surface pressure pattern, first described by Fujita (1955) from time-to-space conversion
of barograph data. In brief, this pattern consists of a high-pressure zone or
mesohigh
,
trailed by a low-pressure zone, also called a
mesodepression
or
wake low
. Some of the
mechanisms involved have subsequently been further elucidated (see Johnson, 2001)
and are sketched in Figures 3.6 and 3.7. It is generally believed that the high pressure is
a result of evaporative cooling in precipitation downdrafts below cloud base; additional
effects may be caused by the impinging of the downdrafts on the ground, causing a
pressure
nose
, and by hydrometeor loading. Williams (1963) showed that the observed
pressure deficit could be the result of a descending, or subsiding, dry current to the rear of
the convective air, but the causes for this remain unclear. The nose, also known as a cold
air outflow leading edge or gust front, often assumes the form of a surge; some 20 cases
have been studied with a 461 m tower by Goff (1976) (see Figures 3.8 and 3.9), which
indicate that they have some features in common with the open channel surges discussed
in Chapter 7 (see also Simpson, 1997). At present, the details of storm development, and
the possible roles of gravity currents and gravity waves in the observed pressure patterns,
are still not completely understood.
Mesoscale convective systems of thunderstorms can be organized as squall lines
or as mesoscale convective complexes. Squall lines or instability lines are relatively
narrow bands of convective elements, like that illustrated in Figure 3.7; they are often
