Chapter 6
Extratropical Cyclones: Their
Mesoscale Structure, Precipitation
and Role in the Transport of Water
Ronald E. Stewart
Climate Processes and Earth Observation Division
Atmospheric Environment Service
Downsview, Ontario
Canada
Abstract
The focus of this article is the nature of extratropical cyclones and in particular their effects on
the water cycle. These cloud systems produce much of the precipitation found in mid-latitudes.
Embedded features within the overall storm, such as precipitation bands, account for the greatest precipitation and these features are often, but not always, linked with frontal surfaces.
Depending on the background large scale forcing, stability and moisture availability, the degree to which these systems produce precipitation from the available vapour (the precipitation
efficiency) varies widely. Other roles played by these systems are, for example, the transport
of moisture to high regions of the troposphere and the moistening of low levels through the
evaporation or sublimation of falling precipitation, but little quantitative work on such aspects
has been conducted. Extratropical cyclones therefore directly affect the large scale water cycle
through their impact on precipitation and the re-distribution of vapour throughout the troposphere. Remote sensing plays a critical role in advancing the study of extratropical cyclones and
their impacts by providing quantitative observations of critical features that often cannot be
measured by any other means.
6.1 Introduction
Extratropical cyclones with their wide expanse of layered cloud contribute substantially to the
global water and energy cycles. They account for a major fraction of the precipitation in midlatitudes and they are associated with some of the main avenues for the latitudinal transport
of moisture through the atmosphere (see for example Lambert, 1988).
In order to fully appreciate their role on the water cycles globally and regionally, attention
has to as well be paid to their internal structure. Organized embedded circulations, mesoscale
NATO AS! Series, Vol. I 45
Radiation and Water in the Climate System:
Remote Measurements
Edited by Ehrhard Raschke
© Springer-Verlag Berlin Heidelberg 1996
Extratropical Cyclones: Their
Mesoscale Structure, Precipitation
and Role in the Transport of Water
Ronald E. Stewart
Climate Processes and Earth Observation Division
Atmospheric Environment Service
Downsview, Ontario
Canada
Abstract
The focus of this article is the nature of extratropical cyclones and in particular their effects on
the water cycle. These cloud systems produce much of the precipitation found in mid-latitudes.
Embedded features within the overall storm, such as precipitation bands, account for the greatest precipitation and these features are often, but not always, linked with frontal surfaces.
Depending on the background large scale forcing, stability and moisture availability, the degree to which these systems produce precipitation from the available vapour (the precipitation
efficiency) varies widely. Other roles played by these systems are, for example, the transport
of moisture to high regions of the troposphere and the moistening of low levels through the
evaporation or sublimation of falling precipitation, but little quantitative work on such aspects
has been conducted. Extratropical cyclones therefore directly affect the large scale water cycle
through their impact on precipitation and the re-distribution of vapour throughout the troposphere. Remote sensing plays a critical role in advancing the study of extratropical cyclones and
their impacts by providing quantitative observations of critical features that often cannot be
measured by any other means.
6.1 Introduction
Extratropical cyclones with their wide expanse of layered cloud contribute substantially to the
global water and energy cycles. They account for a major fraction of the precipitation in midlatitudes and they are associated with some of the main avenues for the latitudinal transport
of moisture through the atmosphere (see for example Lambert, 1988).
In order to fully appreciate their role on the water cycles globally and regionally, attention
has to as well be paid to their internal structure. Organized embedded circulations, mesoscale
NATO AS! Series, Vol. I 45
Radiation and Water in the Climate System:
Remote Measurements
Edited by Ehrhard Raschke
© Springer-Verlag Berlin Heidelberg 1996