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Ronald E. Stewart
features, with their enhanced vertical motions redistribute and alter the vertical profiles of
momentum and moisture. There are a wide variety of such mesoscale features that need to be
accounted for.
In addition, these cloud systems are responsible for much of the production of the precipitation
that falls over vast regions of the world. This precipitation is produced under a wide variety of
conditions, it occurs with a variety of rates and even forms. The exact processes contributing
to this precipitation, including their often mesoscale origin, need to be addressed in order for
this critical parameter, precipitation, to be properly simulated within climate models of the
water cycle.
As well, the interaction between these layer cloud systems and orography often leads to regions
of enhanced precipitation. Because mountains are the source regions for many of the world's
rivers, the role of orography on the small scale distributions of precipitation from these clouds
is a critical problem.
For many reasons then, the nature of cloud systems affects water cycles in the atmosphere. The
purpose of this article is to summarize some of the key attributes of these cloud systems, with
the emphasis being on their role on water cycles, and to point out some of the ways in which
remote sensing contributes to the study of these systems and their impacts.
6.2 Mesoscale Structures
6.2.1 Occurrence and general nature
The overall attributes of extratropical cyclones are well appreciated and have been the focus
of numerous studies. The founding of the Norwegian School of Meteorology some 75 years
ago represented a major step in this regard (Bjerknes, 1919; Bjerknes and Solberg, 1922).
Much subsequent work has been done and has for example clarified their thermal (Palmen
and Newton, 1948) and kinematic "conveyor belt" structure (Harrold, 1973; Carlson, 1980;
Browning, 1985). Figure 6.1 shows a typical example of such a situation; most of these studies
have relied heavily on satellite information to provide an appropriate perspective.
A great deal of mesoscale organization occurs within the overall cloud regions (see for example Hobbs, 1978). These features are typically organized into quasi-two-dimensional structures
(precipitation bands) near the surface fronts and these are often well-observed with radar systems (Browning and Harrold, 1970; Hobbs, 1978; Heymsfield, 1979; Saarikivi and Pukakka,
1990). A typical example of such a band within a winter storm is shown in Figure 6.2. Since
these sub-structures interact with the larger scales, good physical understanding of these mesoscale features is essential for the accurate representation of the larger scale layer cloud field
in global models.
As recently pointed out by Ryan (1995), the current classification of mesoscale structures within extratropical cyclones is largely based on the observational studies in the United Kingdom
and the west coast of the USA. Regions of enhanced precipitation tend to occur within banded
structures (see for example Houze and Hobbs, 1982; Browning, 1985). The bands have furthermore been classified by width into narrow or wide bands, although it should be noted that
these studies will be biased to situations of maritime weather systems at mid-latitudes striking
the western portion of continents.
Wide bands range in width up to about 150 km and are found embedded within and parallel
to the frontal zone. These bands have been further classified by the storm sector in which they
are located. An important characteristic of the warm frontal and warm sector clouds is the
Ronald E. Stewart
features, with their enhanced vertical motions redistribute and alter the vertical profiles of
momentum and moisture. There are a wide variety of such mesoscale features that need to be
accounted for.
In addition, these cloud systems are responsible for much of the production of the precipitation
that falls over vast regions of the world. This precipitation is produced under a wide variety of
conditions, it occurs with a variety of rates and even forms. The exact processes contributing
to this precipitation, including their often mesoscale origin, need to be addressed in order for
this critical parameter, precipitation, to be properly simulated within climate models of the
water cycle.
As well, the interaction between these layer cloud systems and orography often leads to regions
of enhanced precipitation. Because mountains are the source regions for many of the world's
rivers, the role of orography on the small scale distributions of precipitation from these clouds
is a critical problem.
For many reasons then, the nature of cloud systems affects water cycles in the atmosphere. The
purpose of this article is to summarize some of the key attributes of these cloud systems, with
the emphasis being on their role on water cycles, and to point out some of the ways in which
remote sensing contributes to the study of these systems and their impacts.
6.2 Mesoscale Structures
6.2.1 Occurrence and general nature
The overall attributes of extratropical cyclones are well appreciated and have been the focus
of numerous studies. The founding of the Norwegian School of Meteorology some 75 years
ago represented a major step in this regard (Bjerknes, 1919; Bjerknes and Solberg, 1922).
Much subsequent work has been done and has for example clarified their thermal (Palmen
and Newton, 1948) and kinematic "conveyor belt" structure (Harrold, 1973; Carlson, 1980;
Browning, 1985). Figure 6.1 shows a typical example of such a situation; most of these studies
have relied heavily on satellite information to provide an appropriate perspective.
A great deal of mesoscale organization occurs within the overall cloud regions (see for example Hobbs, 1978). These features are typically organized into quasi-two-dimensional structures
(precipitation bands) near the surface fronts and these are often well-observed with radar systems (Browning and Harrold, 1970; Hobbs, 1978; Heymsfield, 1979; Saarikivi and Pukakka,
1990). A typical example of such a band within a winter storm is shown in Figure 6.2. Since
these sub-structures interact with the larger scales, good physical understanding of these mesoscale features is essential for the accurate representation of the larger scale layer cloud field
in global models.
As recently pointed out by Ryan (1995), the current classification of mesoscale structures within extratropical cyclones is largely based on the observational studies in the United Kingdom
and the west coast of the USA. Regions of enhanced precipitation tend to occur within banded
structures (see for example Houze and Hobbs, 1982; Browning, 1985). The bands have furthermore been classified by width into narrow or wide bands, although it should be noted that
these studies will be biased to situations of maritime weather systems at mid-latitudes striking
the western portion of continents.
Wide bands range in width up to about 150 km and are found embedded within and parallel
to the frontal zone. These bands have been further classified by the storm sector in which they
are located. An important characteristic of the warm frontal and warm sector clouds is the