Extratropical Cyclones: Their Mesoscale Structure, ...
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common for only very low levels of liquid water to be present at temperatures below ODC so that
particle growth is dominated by sublimational growth onto ice crystals which can simultaneously
be undergoing aggregational growth to produce snowflakes. Within narrow banded regions, it
is not uncommon in contrast for significant liquid water to be present so that some degree
of accretional growth occurs that can lead to rimed crystals or perhaps graupel. Over most
regions and over much of the year, the final form of precipitation is rain that results from the
melting of these ice forms of precipitation.
It should be added though that sometimes winter forms of precipitation have to be considered,
above that typically associated with storms. Many of the winter forms of precipitation have
formed at least partially through melting and refreezing (such as freezing rain and ice pellets),
but it is not uncommon for precipitation to form through all liquid processes even when the
temperatures are all below freezing (Kajikawa et al., 1988; Stewart, 1992). In addition, many
of the forms of precipitation that can occur do so in close proximity to each other, or they can
even occur simultaneously.
In terms of the various forms of precipitation, it is also the case that some of the heaviest
precipitation linked with, for example, the warm front in such winter storms occurs in close
proximity to the wide variety of precipitation forms (Figure 6.7). This fact certainly poses
difficulties for properly measuring the actual rates under the harsh conditions that occur then,
including the use of remote sensing tools such as radar.
6.5 Estimates of Water Transports from Observations
6.5.1 Complete cloud systems
One of the most fundamental aspects of these systems is their water budget. Although the
assessment of the water budget appears to be rather straightforward, a major problem is the
lack of appropriate measurements. In general, sounding information is the basic measurement
that can be used but one has to be concerned with the locations of the soundings with respect
to the system and there are ongoing concerns with the accuracy of the humidity sensing devices.
In light of such concerns, it is essentially a requirement that satellite-based remote sensing is
needed to achieve the objectives of such studies.
It should be noted that the flux of moisture that is derived from soundings within the clouds
ignores contributions from the condensate itself. Especially when the condensate is snow (which
has a low terminal velocity) and the temperature is well below at ODC, the mixing ratio of the
condensate can easily exceed 20% of the water vapour saturation mixing ratio, even for low
precipitation rates. Moisture fluxes derived from the sounding information could therefore
underestimate the actual fluxes under such conditions (Figure 6.8).
There are consequently few articles addressing the water budgets associated with individual
weather systems, and, as indicated above, there are some uncertainties associated with their
findings. However, Houze et al. (1976) and McBean and Stewart (1991) estimated these budgets within storms over the west coast of North America and over the north Pacific Ocean
respectively (Figure 6.9). They both found that the main source of moisture was from the low
levels east of the storms with net inflow rates less than or of the order of 100 kg-1s-1m- 1 . At
levels above 50 kPa, there was net divergence.
There are also few articles that directly discuss the precipitation efficiency of the systems. Here,
precipitation efficiency is generally considered to be the ratio of surface precipitation to the
available water vapour. However, Wexler and Atlas (1958) pointed out that the precipitation
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common for only very low levels of liquid water to be present at temperatures below ODC so that
particle growth is dominated by sublimational growth onto ice crystals which can simultaneously
be undergoing aggregational growth to produce snowflakes. Within narrow banded regions, it
is not uncommon in contrast for significant liquid water to be present so that some degree
of accretional growth occurs that can lead to rimed crystals or perhaps graupel. Over most
regions and over much of the year, the final form of precipitation is rain that results from the
melting of these ice forms of precipitation.
It should be added though that sometimes winter forms of precipitation have to be considered,
above that typically associated with storms. Many of the winter forms of precipitation have
formed at least partially through melting and refreezing (such as freezing rain and ice pellets),
but it is not uncommon for precipitation to form through all liquid processes even when the
temperatures are all below freezing (Kajikawa et al., 1988; Stewart, 1992). In addition, many
of the forms of precipitation that can occur do so in close proximity to each other, or they can
even occur simultaneously.
In terms of the various forms of precipitation, it is also the case that some of the heaviest
precipitation linked with, for example, the warm front in such winter storms occurs in close
proximity to the wide variety of precipitation forms (Figure 6.7). This fact certainly poses
difficulties for properly measuring the actual rates under the harsh conditions that occur then,
including the use of remote sensing tools such as radar.
6.5 Estimates of Water Transports from Observations
6.5.1 Complete cloud systems
One of the most fundamental aspects of these systems is their water budget. Although the
assessment of the water budget appears to be rather straightforward, a major problem is the
lack of appropriate measurements. In general, sounding information is the basic measurement
that can be used but one has to be concerned with the locations of the soundings with respect
to the system and there are ongoing concerns with the accuracy of the humidity sensing devices.
In light of such concerns, it is essentially a requirement that satellite-based remote sensing is
needed to achieve the objectives of such studies.
It should be noted that the flux of moisture that is derived from soundings within the clouds
ignores contributions from the condensate itself. Especially when the condensate is snow (which
has a low terminal velocity) and the temperature is well below at ODC, the mixing ratio of the
condensate can easily exceed 20% of the water vapour saturation mixing ratio, even for low
precipitation rates. Moisture fluxes derived from the sounding information could therefore
underestimate the actual fluxes under such conditions (Figure 6.8).
There are consequently few articles addressing the water budgets associated with individual
weather systems, and, as indicated above, there are some uncertainties associated with their
findings. However, Houze et al. (1976) and McBean and Stewart (1991) estimated these budgets within storms over the west coast of North America and over the north Pacific Ocean
respectively (Figure 6.9). They both found that the main source of moisture was from the low
levels east of the storms with net inflow rates less than or of the order of 100 kg-1s-1m- 1 . At
levels above 50 kPa, there was net divergence.
There are also few articles that directly discuss the precipitation efficiency of the systems. Here,
precipitation efficiency is generally considered to be the ratio of surface precipitation to the
available water vapour. However, Wexler and Atlas (1958) pointed out that the precipitation