Download Midlatitude Cyclones - The University of Manchester

344
cyclones Explosive Cyclones
Mullen, S. L., and P. Baumhefner. “Sensitivity of Numerical Simulations of Explosive Oceanic Cyclogenesis
to Changes in Physical Parameterizations.” Monthly
Weather Review 116, no. 11 (1988): 2289–2329.
Nuss, W. A. “Air-Sea Interaction Influences on the
Structure and Intensification of an Idealized Marine
Cyclone.” Monthly Weather Review 117, no. 2 (1989):
351–369.
Orlanski, I., and J. J. Katzfey. “Sensitivity of Model Simulations for a Coastal Cyclone.” Monthly Weather
Review 115, no. 11 (1987): 2792–2821.
Petterssen, S., and S. J. Smebye. “On the Development of
Extratropical Cyclones.” Quarterly Journal of the Royal
Meteorological Society 97 (1971): 457–482.
Roebber, P. J. “Statistical Analysis and Updated Climatology of Explosive Cyclones.” Monthly Weather
Review 112, no. 8 (1984): 1577–1589.
Rotunno, R., and M. Fantini. “Notes and Correspondence: Petterssen’s ‘Type B’ Cyclogenesis in Terms
of Discrete, Neutral Eddy Modes.” Journal of Atmospheric Sciences 46 (1989): 3599–3604.
Sanders, F. “Explosive Cyclogenesis in the West-Central
North Atlantic Ocean, 1981–1984. Part 1: Composite
Structure and Mean Behavior.” Monthly Weather
Review 114, no. 10 (1986): 1781–1794.
Sanders, F., and E. P. Auciello. “Skill in Prediction
of Explosive Cyclogenesis over the Western North
Atlantic Ocean, 1987/88: A Forecast Checklist and
NMC Dynamical Models.” Weather and Forecasting
4, no. 2 (1989): 157–172.
Sanders, F., and J. R. Gyakum. “Synoptic-Dynamic Climatology of the ‘Bomb.’” Monthly Weather Review 108,
no. 10 (1980): 1589–1606.
Uccellini, L. W., and P. J. Kocin. “The Interaction of Jet
Streak Circulations during Heavy Snow Events along
the East Coast of the United States.” Weather and
Forecasting 2, no. 4 (1987): 289–308.
Uccellini, L. W., et al. “The President’s Day Cyclone of
18–19 February 1979: A Subsynoptic Overview and
Analysis of the Subtropical Jet Streak Influencing
the Pre-Cyclogenetic Period.” Monthly Weather Review
112, no. 1 (1984): 31–55.
Uccellini, L. W., et al. “The President’s Day Cyclone of
18–19 February 1979: Influence of Upstream Trough
Amplification and Associated Tropopause Folding on
Rapid Cyclogenesis.” Monthly Weather Review 113, no.
6 (1985): 962–988.
Uccellini, L. W., et al. “Synergistic Interactions between
an Upper Level Jet Streak and Diabatic Processes
That Influence the Development of a Low-Level Jet
and a Secondary Coastal Cyclone.” Monthly Weather
Review 115, no. 10 (1987): 2227–2261.
Wash, C. H., et al. “Study of Explosive and Nonexplosive
Cyclogenesis during FGGE.” Monthly Weather Review
120, no. 1 (1992): 40–51.
Whitaker, J. S., I. W. Uccellini, and K. F. Brill. “A ModelBased Diagnostic Study of the Rapid Development
Phase of the President’s Day Cyclone.” Monthly
Weather Review 116, no. 11 (1988): 2337–2365.
Judah Cohen
Midlatitude Cyclones
Midlatitude cyclones (also called extratropical
cyclones, or simply cyclones in the rest of this article)
are nearly circular regions of reduced surface
pressure that generally range in diameter from a
few hundred to a few thousand kilometers and
occur in association with the jet streams in the
middle-latitude regions of the globe (roughly 30 –
70 latitude). Cyclones derive their energy from the
potential energy in the pole-to-equator temperature
gradient. This temperature gradient can become
concentrated within zones called fronts where the
temperature changes rapidly and the wind abruptly
shifts direction. Winds around a cyclone blow counterclockwise in the Northern Hemisphere and
clockwise in the Southern Hemisphere, transporting
warm air poleward and cold air equatorward. Consequently, cyclones are one means by which heat
is transported from the tropics to the poles. Because
cyclones are the primary source of most winter
precipitation in the midlatitudes, understanding
the structure and dynamics of cyclones can lead to
improved weather forecasts.
History of Research. One of the earliest theories
of cyclone formation, the thermal or convectional
theory, was based on James Espy’s work in the
1840s. Espy argued that, as an organized mass of
clouds forms, the release of latent heat of condensation in the clouds causes warming, resulting in a
decrease in pressure within the air column. This
decrease in surface pressure leads to increased
inflow of warm, moist air in the lower troposphere
and then to further pressure falls upon condensation. Mounting observational evidence indicated
that many cyclones were not warm at mid-levels
cyclones Midlatitude Cyclones
verifiable representation of midlatitude cyclones,
something that had not been developed before.
Polar front theory held that the polar front, initially a straight (linear) feature, may spontaneously
produce small perturbations, or waves (Figure 1).
As the polar front becomes deformed by one of
these waves, a weak cyclonic circulation causes
warm tropical air to move poleward and cold polar
air to advance equatorward. The cold front rotates
around the cyclone more rapidly than the warm
front, eventually catching up to the warm front
and forming an occluded front. With the formation
of an occluded front, the cyclone center becomes
surrounded by cold polar air (also known as the
occlusion process). As development of the cyclone
is contingent upon the conversion of potential
energy in the temperature gradient to kinetic energy
of the cyclone, the cyclone weakens after occlusion.
Therefore, the occlusion process, J. Bjerknes and
Solberg argued, represents the beginning of the
decay phase of the cyclone. [See Occluded Fronts.]
Although polar-front theory was a monumental
advance, several aspects of the theory were not supported by observations of cyclones. First, cyclones,
especially those that deepen rapidly, often continue
to deepen after the occluded front forms. Thus, the
occlusion process is not the end of deepening, as
the Bergen meteorologists had described. Instead,
an explanation for cyclone development would
await further theoretical advances, described below.
as the thermal theory predicts, but cold. By the early
1900s, the stage was set for one of the most profound developments in meteorology—the polar
front theory of cyclones (also called the Norwegian
cyclone model).
The polar front theory for midlatitude cyclones
was developed at the Geophysical Institute in
Bergen, Norway, headed by Vilhelm Bjerknes. In a
series of landmark papers published just after World
War I, Jacob Bjerknes, Halvor Solberg, and Tor
Bergeron developed a model for cyclone structure,
based on data collected within numerous cyclones.
Their results built upon the work of Sir William
Napier Shaw, Max Margules, Felix Exner, and other
earlier researchers who recognized that cyclones
possessed discontinuities in wind and temperature
(later called fronts by Bjerknes’s group). Polar front
theory was an advance over previous models of
cyclones for three reasons. First, polar front theory
described for the first time the life cycle of cyclones
on the polar front, a globe-encircling boundary
between cold polar air and warm tropical air. The
Bergen meteorologists argued that cyclones are
not unchanging features moving across the Earth;
instead, they are born, mature, and die. Second,
polar front theory argued that the potential energy
in the temperature gradient across the polar front
provides the energy for cyclones, not the latent heat
release due to condensation. Third, polar front theory represented a simple, elegant, practical, and
Cold
L
(−20)
1
(−10)
500
millibars
H
Cold
(a)
Warm
Cold
Cold
Warm
500
millibars
500
millibars
Shortwave
trough
North
L
2
L
L
Warm
North
Surface
(b)
North
Surface
(c)
C Y C L O N E S : M I D L A T I T U D E C Y C L O N E S . Figure 1. The formation of a wave cyclone during selfdevelopment. (a) A short-wave trough (heavy dashed line) disturbs the flow aloft, enhancing
temperature advection. (b) The trough intensifies and provides the necessary vertical motions for
the development of the surface cyclone. (c) The surface cyclone occludes, and a cold pool of air remains
above it. (Adapted from Ahrens, 1988, p. 381. Copyright 1988 by West Publishing Company.)
Surface
345
346
cyclones Midlatitude Cyclones
Second, the catch up of the cold front by the warm
front does not occur in all cyclones, nor does it
explain the length of highly spiraled occluded
fronts. Instead, the occlusion process is best viewed
as the wrap up of the thermal pattern into a spiraled
front, a result of the deformation and rotation in
the flow around the cyclone center. Third, although
the Norwegian cyclone model advocates a close association between clouds/precipitation and surface
fronts, clouds and precipitation are often related to
processes occurring aloft, not to the surface fronts.
New theories to explain these and other discrepancies
between polar front theory and observations of
cyclones have been proposed and are being evaluated.
[See Occluded Fronts.]
The next major advance in understanding midlatitude cyclones occurred after the discovery of the jet
stream, a narrow region of high winds in the upper
troposphere. In the late 1930s, the global release
of instrumented weather balloons, which regularly
measure the temperature, humidity, and winds
above the surface, made it possible to analyze the
structure and motions within the jet stream. Disturbances in the jet stream, called jet streaks and
shortwave troughs (Rossby waves), are associated
with convergence and divergence. Regions of diverging air at the level of the jet stream are favorable
locations for surface cyclones to form owing to
evacuation of air in the column. Therefore, for a
surface cyclone to deepen, the divergence of air
aloft must be greater than the convergence of air
into the low-pressure center near the surface. Divergence aloft tends to occur on the east side of a
trough, making that region favorable for surface
cyclone development. [See Jet Stream.]
As the vertical structure of cyclones and their
relationship to the jet stream became better understood, practical means were explored for determining whether a cyclone would intensify or weaken.
The most significant contribution during the 1940s
and early 1950s came from two European meteorologists, Reginald Sutcliffe and Sverre Petterssen.
They found that as a trough in the jet stream
(a region of cyclonic vorticity advection aloft) and
its associated cyclonic flow move over a low-level
thermal gradient (a frontal zone), cyclonic flow is
induced at the surface. The weak circulation about
the frontal zone causes deformation of the frontal
zone, resulting in warm air advection ahead of the
surface cyclone and cold air advection behind. The
warm advection leads to decreasing surface pressure ahead of the cyclone, and hence the surface
cyclone propagates forward. The warming of the
air column ahead of the cyclone also builds the
downstream ridge and causes the wave to amplify,
thereby increasing the amount of cyclonic vorticity
advection aloft, leading to further warm advection,
and so on. This “bootstrapping” process is referred
to as self-development. Eventually, the strength a
cyclone can attain through self-development is limited by the opposing influence of vertical motion,
which cools the rising air ahead of the system and
limits the magnitude of the pressure falls. Sutcliffe
and Petterssen also showed that the strength of
cyclogenesis depends on the local static stability of
the atmosphere.
Yet another approach to understanding cyclogenesis was pioneered by Jule Charney in 1947 and Eric
Eady in 1949. This theoretical approach states that
cyclones are the result of an instability in the
jet stream called baroclinic instability. Baroclinic
instability theory links the observational approach
to understanding cyclones from polar front theory
and the practical approach of Sutcliffe and Petterssen. Baroclinic instability theory states that if the
temperature gradient is large enough (or equivalently, if the vertical shear of the horizontal wind
is large enough), then the jet stream will spontaneously break down into Rossby waves, resulting in the
formation of cyclones. In most observed cases, disturbances in the jet stream appear to be linked with
surface cyclogenesis, suggesting the validity of baroclinic instability as an explanation for cyclogenesis
in the atmosphere. In addition, baroclinic instability
theory is often used for theoretical studies of cyclogenesis, providing further support for its utility in
explaining observations of cyclogenesis. [See Baroclinic Instability.]
Another way of viewing the structure of cyclones
is to depict the different airstreams that flow
cyclones Midlatitude Cyclones
through the cyclone. This view, pioneered by Jerome
Namias in the late 1930s, became popular in the mid
to late 1960s. Instead of looking at discontinuities in
temperature (fronts), a more holistic view examines
the different source regions of the air flowing
through the cyclone. This airstream model yields
three main airflows in midlatitude cyclones: the
warm conveyor belt, the cold conveyor belt, and the
dry airstream.
The warm conveyor belt originates in the tropical
air in the warm sector and rises up over the warm
front into the jet stream. The warm conveyor belt is
responsible for most of the clouds and precipitation
associated with cyclones. The cold conveyor belt
originates in the lower troposphere in the cooler
air ahead of the cyclone, travels westward underneath the warm conveyor belt, and then turns
cyclonically around the low center. The dry airstream originates in the middle and upper troposphere west of the cyclone and then descends
behind the cyclone. The dry airstream provides the
westward limit to most of the clouds and precipitation in a midlatitude cyclone.
Finally, a recent way of viewing the atmosphere is
to examine the structure of the potential vorticity
field. Sutcliffe–Petterssen self-development or baroclinic instability theory can be viewed in the framework of potential vorticity as a region of locally high
potential vorticity (a depression of the tropopause),
which approaches another region of high potential
vorticity (a lower tropospheric area of warm air).
The induced cyclonic circulation associated with
the tropopause depression causes the deformation
of the warm pool near the surface, in turn strengthening the tropopause depression. The cyclone therefore develops by mutual amplification of potential
vorticity anomalies on the tropopause and near
the surface. When moisture is present, a third
potential vorticity anomaly may form beneath regions of condensation. The formation of this anomaly and its associated cyclonic flow can enhance the
intensity of the surface cyclone. [See Potential
Vorticity.]
Life Cycle. The current view of the life cycle
of a midlatitude cyclone is illustrated in Figure 1.
Prerequisites for cyclone development include a
lower-tropospheric frontal zone and an upstream
upper-tropospheric disturbance, usually a jet streak
or a shortwave trough in the jet stream. The upperlevel disturbance generally moves faster than the
surface frontal zone, so the upper-level disturbance
will move over the frontal zone. Cyclonic flow associated with the upper-level disturbance will deform
the surface frontal zone, forming a weak surface
low-pressure system. The cyclonic flow induced
from the upper-level disturbance will cause warm
air to the south of the frontal zone to be advected
northward, east of the low center. The movement of
warm air replacing cold air forms a warm front.
Likewise, on the west side of the low center, cold
air to the north will be advected southward, replacing the warm air and forming a cold front. [See
Fronts.] If the cyclone is strong enough, the movement of air around the cyclone eventually stretches
the cold front and warm front, bringing them closer
together, just as ribbons of milk lengthen and merge
when stirred into coffee. As the air in the cold
conveyor belt wraps around the low center and the
air in the warm conveyor belt is lifted over the warm
front, the amount of warm air near the cyclone
center is reduced and the surface cyclone becomes
wrapped in cold air. Around this time or shortly
after, the upper-level disturbance catches up to the
surface cyclone, and the three-dimensional structure of the low-center becomes vertically stacked.
The cloud pattern of a midlatitude cyclone is
typically in the shape of a comma. The head of the
comma is nearly coincident with the low-pressure
center at the surface. Warm rising air in the warm
conveyor belt is responsible for most of the clouds
and precipitation in the comma head. Steady precipitation, often with embedded regions of heavier precipitation, falls out of the clouds ahead of the warm
front. As the warm front approaches, surface temperatures rise. In the warm air, skies may be clear or
partly cloudy, or they may have scattered showers
and thunderstorms. The tail of the comma is often
associated with convection that forms along a line
extending equatorward from the low center. This
line may sometimes be associated with the passage
347
348
cyclones Midlatitude Cyclones
of the cold front or occluded front, producing heavy
precipitation. Following the cold frontal passage,
skies clear and surface temperatures fall as the
winds shift from the south to the west and north.
Geographical Variability. Midlatitude cyclones
occur in many midlatitude locations around the
world, but they tend to move along preferential
routes called storm tracks. In the Northern Hemisphere, two primary storm tracks lie across the
North Atlantic Ocean and the North Pacific Ocean.
In contrast, cyclones in the Southern Hemisphere
most commonly travel within a single storm track
around the Southern Ocean, best defined over the
southern Indian Ocean and least well defined over
the South Pacific Ocean. In the Northern Hemisphere, a large number of cyclones generally intensify at the entrance region (western end) of the
storm tracks off the east coasts of North America
and Asia, travel across the oceans, and weaken at
the end of the storm tracks over the eastern ocean
basins. Although most cyclones follow these storm
tracks and look like cyclones in the polar front
theory, individual cyclones may differ substantially
from this conceptual model. A few examples of
these differences are discussed next.
Since 1980, the meteorological community has
placed particular emphasis on understanding rapidly developing ocean cyclones, which have been
named bombs, and are often poorly forecast. In the
late 1980s, several field projects began discovering
unusual frontal structures in these cyclones. Cyclogenesis appears to be initiated much as described
above, but instead of the cold front rotating into the
warm front to form an occluded front, the cold front
breaks (or fractures) from the warm front and begins to move perpendicularly to the warm front, so
that it never catches up (Figure 2). The rapid movement of the surface low center also results in the
warm front being left behind the cyclone in the form
of a back-bent front. A region of strong localized
surface winds can sometimes occur in association
with the back-bent front and is called the sting jet.
When they occur, sting jets can cause extensive
wind damage, especially in the United Kingdom
and continental Europe. As the cyclone continues
to intensify, the back-bent front encircles the relatively warmer air behind the cold front, resulting in
a pool of warm air over the low center, known as the
warm seclusion. This cyclone evolution is called the
Shapiro–Keyser cyclone model. [See Cyclones, subentry on Explosive Cyclones.]
Modeling results suggest that the roughness of
the Earth’s surface may affect the types of frontal
structures that arise. For instance, when surface
friction is high, as it is over land, cyclones tend to
undergo an evolution more consistent with the
polar front cyclone model. When the surface friction
is lower, as it is over the ocean, the cyclone tends to
develop features more akin to the Shapiro–Keyser
cyclone model.
Research suggests that the shape of the jet stream
over the surface cyclone also affects the resulting
frontal structure. In cases where the jet stream is
diffluent, warm fronts are short and weak while cold
fronts are long and strong. These cyclones tend to
resemble the polar front cyclone model. In confluent flow, the warm fronts are long and strong and
the cold fronts are short and weak. These cyclones
tend to have structures like the Shapiro–Keyser
cyclone model.
Because of the Rocky Mountains, a developing lowpressure center in central North America may be
inhibited from developing in the same manner as
an ideal cyclone. Such cyclones develop most often
in Colorado or Alberta, where the slope of the Rockies
is steepest. The cyclones that develop here are likely
to exhibit certain structures (Figure 3). South of the
low center, a lee trough separates warm, moist,
southerly air to the east from warm, dry air that has
recently descended off the mountains. A lee trough
has a structure very similar to that of a warm front.
Depending on the amount of moisture ahead of the
lee trough, the lee trough may also resemble a dryline,
a type of air-mass boundary in the south-central
United States that is often a locus of severe weather.
Southwest of the cyclone, a cold front separates the
subsided air off the mountains from moist Pacific
Ocean air. Northwest of the low center, a cold front
occurs at the leading edge of southward-moving
arctic air trapped against the Rockies. North of the
cyclones Midlatitude Cyclones
IV
III
L
II
L
I
L
L
Warm
C Y C L O N E S : M I D L A T I T U D E C Y C L O N E S . Figure 2. An alternative model of frontal-cyclone
evolution: Incipient broad-baroclinic phase (I), frontal fracture (II), bent-back front and frontal
T-bone (III), and warm-core frontal seclusion (IV). Upper: sea-level pressure (solid), fronts (bold),
and cloud pattern (shaded). Lower: temperature (solid), and cold and warm air currents (solid and
dashed arrows, respectively). (From Shapiro and Keyser, 1990, p. 188. Copyright 1990 by the
American Meteorological Society.)
low center, an inverted trough separates easterlies
over the midwestern states from the northerly arctic
air against the Rockies. Often a quasi-stationary or
warm front south of the easterlies separates the
warm moist southerly air from the Gulf of Mexico.
Finally, a squall line in the warm southerly air is often
associated with an upper-level frontal zone advancing above; the term cold front aloft has sometimes
been applied to this feature. These cyclones differ
substantially from the polar front and Shapiro–
Keyser cyclone models presented earlier.
In desert areas during the summer, intense solar
heating and the lack of moisture available for evaporation can lead to very high surface temperatures
(higher than about 35 C). As the air warms, it
expands, and compensating circulations arise that
remove mass from the column of air. As a result, the
pressure falls. These low-pressure centers are not
associated with the polar front and jet stream and
are usually not migratory. They are called thermal
lows or heat lows to indicate their method of formation, and they are distinct from midlatitude cyclones.
349
350
cyclones Midlatitude Cyclones
Fresh
polar/artic
H
air
H
H
50
50
Inverted
trough
H
Older
modified
polar
air
Surge
line
Inverted
trough
L
Fresh
Polar/arctic
air
Jet
axis
Older
modified
polar
air
Jet axis
Pacific
air
Cyclone
track
Pacific
cold
30
front
-110
Subsided
(mixed)
air
Dry
line
Gulf
air
Developing
warm front
30
-90
(a)
-110
Polar/arctic
front
Subsided
pacific
air
Pacific
cold front
Gulf
air
Squall
line
-90
(b)
C Y C L O N E S : M I D L A T I T U D E C Y C L O N E S . Figure 3. Schematic of cyclogenesis east of the Rockies
when an inverted trough is present for (a) the initial time and (b) some later (about 24 hours) time.
Solid lines denote mean sea-level isobars. Principal frontal boundaries are denoted by conventional
symbols with the inverted trough appearing as a dashed line. Approximate position of the jet
stream is shown by the dotted line in (b). The squall line is indicated by a dash–dot line. Estimated
surface air trajectories within the labeled air masses are denoted by the hatched arrows.
As their name suggests, midlatitude cyclones
(whose energy is derived from the pole-to-equator
temperature gradient) are typically distinct from
cyclones in the tropics (whose energy is derived
from the release of latent heat of condensation).
Sometimes, however, tropical cyclones may transition into midlatitude cyclones as they move poleward. [See Cyclones, subentry on Tropical Cyclones.].
Because of the variety of topography and geography on the Earth, midlatitude cyclones across the
world possess a great variety of frontal structure
and evolutions. For example, the Gulf Stream and
Kuroshio ocean currents are an important source of
the low-level temperature gradients and low static
stability needed for rapid cyclone development.
In another example, off the south coast of Australia
and in the center of the Pacific Ocean, cyclones
usually develop without strong warm fronts. Much
remains to be learned about how midlatitude
cyclones vary around the world, and more
important, about the causes of these structural
and developmental differences.
[See also Storms.]
BIBLIOGRAPHY
Ahrens, C. D. Meteorology Today: An Introduction to
Weather, Climate, and the Environment. 3d ed.
St. Paul, Minn.: West, 1988.
Bluestein, H. B. Synoptic-Dynamic Meteorology in Midlatitudes. New York: Oxford University Press, 1993.
Carlson, T. N. Mid-latitude Weather Systems. Boston:
American Meteorological Society, 1998.
Davies, H. C. “Emergence of the Mainstream Cyclogenesis Theories.” Meteorologische Zeitschrift 6, no. 6
(1997): 261–274.
Friedman, R. M. Appropriating the Weather: Vilhelm
Bjerknes and the Construction of a Modern Meteorology. Ithaca, N.Y.: Cornell University Press, 1989.
Keshishian, L. G., L. F. Bosart, and W. Bracken. “Inverted
Troughs and Cyclogenesis over Interior North America:
A Limited Regional Climatology and Case Studies.”
Monthly Weather Review 122, no. 4 (1994): 565–607.
cyclones Subtropical Cyclones
Kocin, P. J., and L. W. Uccellini. Northeast Snowstorms
(Volume I: Overview, Volume II: The Cases). Boston:
American Meteorological Society, 2004.
Kutzbach, G. The Thermal Theory of Cyclones: A History
of Meteorological Thought in the Nineteenth Century.
Boston: American Meteorological Society, 1979.
Neiman, P. J., and M. A. Shapiro. “The Life Cycle of an
Extratropical Marine Cyclone. Part 1: Frontal-Cyclone
Evolution and Thermodynamic Air–Sea Interaction.”
Monthly Weather Review 121, no. 8 (1993): 2153–2176.
Newton, C. W., and E. O. Holopainen, eds. Extratropical
Cyclones: The Erik Palmén Memorial Volume. Boston:
American Meteorological Society, 1990.
Palmén, E., and C. W. Newton. Atmospheric Circulation
Systems: Their Structure and Physical Interpretation.
New York: Academic Press, 1969.
Sanders, F., and J. R. Gyakum. “Synoptic-Dynamic Climatology of the ‘Bomb’.” Monthly Weather Review 108,
no. 10 (1980): 1589–1606.
Schultz, D. M., D. Keyser, and L. F. Bosart. “The Effect of
Large-Scale Flow on Low-Level Frontal Structure and
Evolution in Midlatitude Cyclones.” Monthly Weather
Review 126, no. 7 (1998): 1767–1791.
Schultz, D. M., and G. Vaughan. “Occluded Fronts and
the Occlusion Process: A Fresh Look at Conventional
Wisdom.” Bulletin of the American Meteorological
Society.
Shapiro, M. A., and S. Grnäs, eds. The Life Cycles of
Extratropical Cyclones. Boston: American Meteorological Society, 1999.
Shapiro, M. A., and D. Keyser. “Fronts, Jet Streams, and
the Tropopause.” In Extratropical Cyclones: The Erik
Palmén Memorial Volume, edited by C. W. Newton
and E. O. Holopainen, pp. 167–191. Boston: American
Meteorological Society, 1990.
Steenburgh, W. J., and C. F. Mass. “The Structure and
Evolution of a Simulated Rocky Mountain Lee Trough.”
Monthly Weather Review 122, no. 12 (1994): 2740–2761.
David M. Schultz
Subtropical Cyclones
Subtropical cyclones have characteristics similar to
those of extratropical and tropical cyclones, but
unlike true tropical storms, subtropical storms can
occur at any time of the year. Because they are
hybrid storms, it is difficult to define consistent
physical characteristics for them.
Most subtropical storms have their maximum
intensity of rain and wind approximately
420 kilometers (300 miles) from the center. Unlike
tropical cyclones, subtropical cyclones often
have large centers, as much as 140 kilometers
(100 miles) in diameter. Within this zone, precipitation is light and pressure gradients are weak.
While tropical cyclones depend on latent and
sensible heat as driving mechanisms, subtropical
storms develop from cold upper-level polar troughs
(as do extratropical storms). Occasionally the southern portion of an upper-level trough “cuts off ” and
develops an upper-level cold-core low. If this circulation extends to the surface, the development of a
subtropical storm is initiated. Although the original
polar trough from which a subtropical storm develops has most of its precipitation east of its axis,
subtropical storms themselves are marked by a
high degree of symmetry.
Once formed, these storms are noted for their high
level of persistence, a result of their being well developed at upper levels (for example, a closed cyclonic
circulation at 500 millibars) while becoming progressively weaker toward the surface. Thus, the effect of
friction is small; in tropical cyclones, by contrast,
friction plays a major role in dissipation over land.
Rather than dissipating, subtropical storms are often
absorbed into advancing polar troughs.
In some regions, subtropical storms are an integral
part of the hydrological cycle. For example, in Hawaii
the subtropical storm known locally as the Kona
storm provides a large portion of the winter rainfall.
Most subtropical storms form from upper-level
cold-core lows, but there are also other modes of
formation. For example, a hurricane that moves
inland can change into a subtropical storm as part
of its decay process. This often produces more prolonged and intense rainfall than would a dissipating,
purely tropical system. A subtropical storm can also
become converted to a tropical system when warm,
moist air flows closely around the center. Rainfall,
which had been heavy on the storm’s periphery, slackens as a new enhanced area of rainfall develops
close to the center. Fluxes of latent heat now increase
near the center. The net result is an increase in temperature in the center and conversion to a warm-core
tropical system. For this reason, subtropical
351