Extratropical Cyclones: Their Mesoscale Structure, ...
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such cases reveal significant differences from the bulk approach; the proper evaluation of these
processes for suitable inclusion within models remains elusive. Remote sensing techniques such
as multi-parameter radar will be needed to better illustrate the nature of precipitation over the
needed wide areas to properly observe these critical portions of the water cycle.
It needs to be emphasized though that interpretations of the precipitation efficiency results have
to account for the fundamental changes that occur in the frontal cloud structure as a function
of initial conditions. Even though initial large scale forcing may be the same, varying initial
moisture and temperature conditions lead to differences in the latent heat exchange within the
cloud which in turn affect, for example, the strength of the frontal zones and vertical velocity
magnitudes. Such changes in the internal dynamic structure lead to changes in the degree
of saturation, abundance of cloud water and distribution of precipitation. A variable such as
precipitation efficiency is the net result of the complete system in which the moisture itself
can affect the nature of the system in highly non-linear ways. The proper observation of key
aspects of the water budget, including vertical velocity fields, is therefore quite critical in order
to properly validate such model predictions; remote sensing offers the best hope of achieving
this.
6.7 Concluding Remarks
Extratropical systems are very common features that playa role in the transport of water in the
atmosphere. These cloud systems produce much of the precipitation that falls on the Earth's
surface. Therefore, understanding regional water budgets must address these features. The
detailed nature of these systems needs to be well-understood however if their contribution to
the water budgets can be quantified under the wide variety of conditions that actually occur in
the atmosphere. Although this article showed that detailed calculations reveal some of the sensitivities that need to be considered, far more work is needed to develop an acceptable manner
of properly handling the crucial moisture re-distribution issues faced by climate modellers.
Programs being mounted to better simulate regional water budgets in different regions of the
world will be a major driving force for the continuing assessment of the impacts of these weather
systems. Efforts focussing on high latitude basins such as the Mackenzie Basin of Canada where
snow is prevalent should be linked with low precipitation efficiencies due to the dominance of
sublimation in low moisture conditions, although some instances in this environment may have
a high efficiency if the initial snowfall is able to saturate the sub-cloud region. Warmer areas
in which rain is more common, including the Baltic Sea region, should in general have higher
efficiencies due in part to the availability of surface moisture.
Remote sensing tools playa critical role within studies aimed at better understanding the effects
of extratropical cyclones on water cycles. Satellite-borne systems are very often used, radar
are needed for clouds and precipitation studies, and other instruments are being or need to be
used for observing other critical dynamic and moisture parameters.
Although the original focus of studying extratropical systems was their short-term weather
prediction, a separate thrust is now being driven by the requirements of climate studies to
better appreciate their role on regional and global water and energy cycles.
Acknowledgments
The author would like to thank Kit Szeto for his assistance in carrying out the simulations used
in this article. He would also like to thank Bob Crawford, John Hanesiak and Jason Burford
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such cases reveal significant differences from the bulk approach; the proper evaluation of these
processes for suitable inclusion within models remains elusive. Remote sensing techniques such
as multi-parameter radar will be needed to better illustrate the nature of precipitation over the
needed wide areas to properly observe these critical portions of the water cycle.
It needs to be emphasized though that interpretations of the precipitation efficiency results have
to account for the fundamental changes that occur in the frontal cloud structure as a function
of initial conditions. Even though initial large scale forcing may be the same, varying initial
moisture and temperature conditions lead to differences in the latent heat exchange within the
cloud which in turn affect, for example, the strength of the frontal zones and vertical velocity
magnitudes. Such changes in the internal dynamic structure lead to changes in the degree
of saturation, abundance of cloud water and distribution of precipitation. A variable such as
precipitation efficiency is the net result of the complete system in which the moisture itself
can affect the nature of the system in highly non-linear ways. The proper observation of key
aspects of the water budget, including vertical velocity fields, is therefore quite critical in order
to properly validate such model predictions; remote sensing offers the best hope of achieving
this.
6.7 Concluding Remarks
Extratropical systems are very common features that playa role in the transport of water in the
atmosphere. These cloud systems produce much of the precipitation that falls on the Earth's
surface. Therefore, understanding regional water budgets must address these features. The
detailed nature of these systems needs to be well-understood however if their contribution to
the water budgets can be quantified under the wide variety of conditions that actually occur in
the atmosphere. Although this article showed that detailed calculations reveal some of the sensitivities that need to be considered, far more work is needed to develop an acceptable manner
of properly handling the crucial moisture re-distribution issues faced by climate modellers.
Programs being mounted to better simulate regional water budgets in different regions of the
world will be a major driving force for the continuing assessment of the impacts of these weather
systems. Efforts focussing on high latitude basins such as the Mackenzie Basin of Canada where
snow is prevalent should be linked with low precipitation efficiencies due to the dominance of
sublimation in low moisture conditions, although some instances in this environment may have
a high efficiency if the initial snowfall is able to saturate the sub-cloud region. Warmer areas
in which rain is more common, including the Baltic Sea region, should in general have higher
efficiencies due in part to the availability of surface moisture.
Remote sensing tools playa critical role within studies aimed at better understanding the effects
of extratropical cyclones on water cycles. Satellite-borne systems are very often used, radar
are needed for clouds and precipitation studies, and other instruments are being or need to be
used for observing other critical dynamic and moisture parameters.
Although the original focus of studying extratropical systems was their short-term weather
prediction, a separate thrust is now being driven by the requirements of climate studies to
better appreciate their role on regional and global water and energy cycles.
Acknowledgments
The author would like to thank Kit Szeto for his assistance in carrying out the simulations used
in this article. He would also like to thank Bob Crawford, John Hanesiak and Jason Burford