Potter T.D., Colman B.R. (co-chief editors). The handbook of weather, climate, and water: dynamics, climate physical meteorology, weather systems, and measurements
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In Figures 4 to 6, contours of potential temperature are shown rather than
temperature. The fronts are indicated by the zones of sloping, tightly packed
contours of potential temperature. At any given pressure level, potential temperature
is proportional to temperature, so concentrated gradients of potential temperature
imply concentrated gradients of temperature. One advantage of using potent ial
temperature is that frontal zones consist of isentropes (lines of constant potential
temperature) packed horizontally or vertically. A second advantage is that mixing
can take place adiabatically along isentropes; cross-isentrope transport is only possi-
ble in conjunction with diabatic processes. Thus, the extent to which the front acts
like an impenetrable wall is given by the extent to which the frontal zone is defined
by a single isentrope.
In Figures 5 and 6, the frontal zone includes air from the stratosphere that has
been drawn downward into the troposphere in a structure known as a tropopause
Figure 5 Cross section through a strong upper-level front, showing potential temperature
(K, solid), observed wind (barbs, as in Fig. 4), and section-normal wind component (dashed,
m=s, positive into the page). (From Shapiro, 1991).
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LARGE-SCALE ATMOSPHERIC SYSTEMS
fold. Tropopause folds are common to upper-level fronts; because of the large
amounts of mixing that take place in the vicinity of upper-level fronts, tropopause
folds are a primar y mechanism for the exchange of air between the stratosphere and
troposphere. The mixing associated with upper-level fronts is also one manifestation
of clear-air turbulence. Extreme tropopause folding events can bring stratospheric air
all the way to the ground. This is presently not thought to be dangerous, although it
was a concern in the days of aboveground nuclear tests.
Dynamical Aspects of Fronts
Synoptic-scale surface fronts are often portrayed as zones of colliding air masses or
war zones where battles are fought within thunderstor ms. This portrayal is wrong,
primarily because of the influence of the Coriolis force. Beneath a cold air mass will
tend to be a region of high pressure, which in the absence of the Coriolis force would
accelerate the cold air in the direction of the warm air and directly cause vertical
motion. Instead, with the Coriolis force, the cold air comes into approximate balance
and the wind is ultimately directed parallel to the isobars, with higher pressure to the
right. Rather than two armies rushing forward toward each other, perhaps the appro-
priate image is of two armies nervously pacing sideways along a demilitarized zone.
Figure 6 Cross section through a strong merged front, as in Fig. 5. Dashed line shows path
of aircraft. (From Shapiro and Keyser, 1990).
5 FRONTS 517
In addition to the horizontal wind variations mentioned earlier, a front that is
nearly in geostrophic and hydrostratic balance will be associated with significant
vertical wind shear. Recall from the introduction to this section that because of the
relationships between pressure and temperature on the one hand and pressure and
wind on the other, a horizontal temperature gradient implies vertical shear of the
horizontal wind. Since a front is a zone of concentrated temperature gradient, the
vertical wind shear (or thermal wind) is large there too. The shear vector is oriented
such that winds aloft will tend to blow parallel to the front with cold temperatures to
the left and warm temperatures to the right. Because of the interrelationship between
wind shear and temperature gradient, an upper-level jet stream is often located
directly above a deep surface or upper-level front.
Because wind shear tends to improve the ability of midlatitude convection to
organize itself into convective systems or supercells, the wind shear of a front is one
reason severe weather tends to be locat ed near fronts. A second reason for convec-
tion, and clouds in general, near fronts is the vertical motion that tends to be
associated with fronts. This upward motion is not due to warm air being forced
upward by cold air; it is a consequence of the atmosphere attempting to stay within
balance.
In an intensifying synoptic-scale front, deformation and convergence is causing
the magnitude of the temperature gradient to increase. At the same time, by means
that will not be explained here, the deformation acts to reduce the vertical wind
shear, even though the vertical wind shear would have to increase to remain within
balance. The resulting imbalance between the pressure gradient force and Coriolis
force produces accelerations: At low levels the air accelerates from the cold side of
the front toward the warm side, and aloft the air accelerates from the warm side of
the front toward the cold side. An ageostrophic circu lation is thereby established, and
mass continuity demands upward motion on the warm side of the front and down-
ward motion on the cold side in order to complete the circulation cell . The horizontal
ageostrophic flow, once it reaches finite magnitude, causes an acceleration to the
right by the Coriolis force, eventually producing an increase in the vertical shear.
Meanwhile, the vertical motion is acting to cool the air through ascent on the warm
side of the front and warm it through descent on the cool side of the front, thereby
acting to reduce the horizontal temperature gradient. This vertical circulation is
called a direct circulation because relatively warm air rises and relatively cold air
sinks; the opposite circulation, which is found when fronts are weakening, is called
indirect.
Here, then, is the true ‘‘battle’’ within a front: The balanced flow is acting in one
sense, and the unbalanced (ageostrophic) flow is acting in precisely the opposite
sense! The net effect is that both the vertical wind shear and horizontal temperature
gradient increase. By assuming that the ageostrophic circulation is precisely as
strong as it has to be to maintain thermal wind balance, the horizontal ageostrophic
and vertical motions can be diagnosed purely from the rate of frontogenesis
(strengthening of the tem perature gradient across an air parcel), using the so-
called Sawyer–Eliassen equation. This constant adjustment, as the air attempts to
keep up with thermal wind balance, is the primary cause of the vertical motion and
518 LARGE-SCALE ATMOSPHERIC SYSTEMS
resulting clouds and precipitation mainly on the warm side of active zones of strong
temperature gradient.
The ageostrophic circulation also modifies the frontal structure. Since the vertical
motions are constrained to be near zero at the ground and in the upper troposphere,
the vertical motion serves to weaken the horizontal tem perature gradient primarily in
the middle troposphere, between 3 and 6 km, thus explaining the paucity of strong
fronts at this level. Meanwhile, the low-level ageostrophic flow is directed from cold
air toward warm air, where it ascends. The convergence on the warm air side of the
front enhances the temperature gradient there, even as the gradient on the cold side
of the front is being weakened by the same mechanism. This explains the tendency
for surface fronts to have their strongest temperature gradient, or perhaps even a
discontinuity, right at the warm-air edge of the frontal zone. By similar arguments,
upper-level fronts ought to be strongest along the cold-air edge.
6 EXTRATROPICAL CYCLONES
Extratropical cyclones (also known as low-pressure systems) are large-scale circula-
tions that develop spontaneously in midlatitudes. The sense of the circulation is
cyclonic, or in the same direction as Earth’s rotation, which is counterclockwise
in the Northern Hemisphere and clockwise in the Southern Hemisphere. Extratro-
pical cyclones have scales ranging from hundreds to thousands of kilometers and can
be associated with widespread rain and snow and high winds. Extratropical anti-
cyclones, or high-pressure systems, are just as common and form by similar means,
but receive less attention since they tend to be associated with fair weather and light
winds.
Observed Characteristics
The life cycle of an extratropical cyclone has been shown in Figure 2. The general
evolution shown in Figure 2 is typical for extratropical cyclones forming over the
ocean and associated with a single upper-tropospheric mobile trough. Over land,
warm fronts tend to be weaker and the presence of mountain ranges and coastlines
strongly alters the structure of extratropical cyclones and their associated precipita-
tion.
Rather than simply discuss the typical distribution of weather about a developing
extratropical cyclone, the wide range of cyclone characteristics will be illustrated
here. Weather maps and infrared satellite images showing four different extratropical
cyclones affecting North America are shown in Figures 7 to 10. The first cyclone
(Fig. 7) is a mature extratropical cyclone striking the coast of the northwestern
United States, the second (Fig. 8) is a typical cyclone in the central United States,
the third cyclone (Fig. 9) is of a type known as an Alberta clipper, and the fourth
(Fig. 10) is a cyclone redeveloping along the East Coast of the United States.
Wind Typically, the strongest winds and pressure gradients associated with an
offshore extratropical cyclone are in the northwest quadrant (southwest in the
6 EXTRATROPICAL CYCLONES 519
Southern Hemisphere). Regional exceptions include the Pacific Northwest, where
the strongest winds tend to be southerly ahead of the cyclone as it reaches the coast
while high pressure remains in place inland (Fig. 7a), and the Northeast, where
strong winds can often be found between the low-pressure center and the anticyclone
in place ahead of it (Fig. 10a). Extratropical cyclones do not tend to have a tight
Figure 7 Mature extratropical cyclone in northwest United States, 0000 UTC Feb. 19, 1997.
(a) Surface weather map with fronts (bold), sea level pressure (solid), temperature (
F, red),
and areas of snow (light ¼light blue; moderate to heavy ¼dark blue), rain (light ¼light green;
moderate to heavy ¼dark green), and sleet or freezing rain (light red). Surface data is plotted
using conventional station model. Station temperatures and dewpoints are reported in Celsius
in Fig. 7 and in Fahrenheit in Figs. 8 to 10. The temperature and precipitation analyses have
been added by the author to the operational surface analyses generated by the National Centers
for Environmental Prediction. See ftp site for color image.
520
LARGE-SCALE ATMOSPHERIC SYSTEMS
inner core like hurricanes, particularly over land, and the radius of maximum winds
tends to be 100 to 500 km from the center.
Temperature The overall distribution of temperatures tends to be consistent with
the locations of fronts. The warm sector, where the highest surface tem peratures are
found, is between the warm front and the cold front, generally the southeastern
quadrant of the storm. Warmest temperatures are not in the center of the cyclone but
tend to increase toward the southeast and south, with the warmest temperature at any
given location typically occurring just prior to cold front passage. These patterns are
modified by the history of air parcels within the extratropical cyclone. For example,
cyclones in the northeastern Pacific tend to have small temperat ure variations at the
surface (Fig. 7a), since the low-level air has been exchanging heat and moisture with
the relatively uniform oceanic waters. Alberta clippers (Fig. 9a) and other cyclones
close to the eastern edge of the Rocky Mountains often have their warmest
temperatures to the southwest of the low in air that has descended over the
mountains and warmed adiabatically. Along the East Coast (Fig. 10), relatively
cold air ahead of the low often becomes trapped between the Appalachian Mountains
to the west and the warm Atlantic Ocean to the east. The boundary between the cold
air and the marine air is known as a coastal front and is analyzed as a trough in
Figure 10.
Figure 7 (continued )(b) Infrared satellite image, 2100 UTC Feb. 18, 1997, remapped to a
polar stereographic projection similar to the surface weather map, with coldest temperatures
(i.e., highest clouds) enhanced.
6 EXTRATROPICAL CYCLONES 521
Figure 8 Typical cyclone in central United States, 0000 UTC Feb. 4, 1997. (a) Surface
weather map. (b) Infrared satellite image, 0015 UTC Feb. 4, 1997. See Fig. 7 for details. See
ftp site for color image.
522 LARGE-SCALE ATMOSPHERIC SYSTEMS
Figure 9 Alberta clipper cyclone, 1800 UTC Feb. 15, 1997. (a) Surface weather map.
(b) Infrared satellite image, 1815 UTC Feb. 15, 1997. See ftp site for color image.
6 EXTRATROPICAL CYCLONES 523
Figure 10 Redeveloping East Coast cyclone, 0000 UTC Dec. 23, 1997. (a) Surface weather
map. (b) Infrared satellite image, 0015 UTC Dec. 23, 1997. See ftp site for color image.
524 LARGE-SCALE ATMOSPHERIC SYSTEMS
Precipitation The distribution of precipitation is the most variable aspect of
extratropical cyclone structure. The typical cyclone (Fig. 2) has extensive precipita-
tion north of the warm front and in the vicinity of and behind the low, with the
heaviest precipitation just ahead of the low center. Showers and rain bands tend to be
present along the cold front, with scattered or widespread precipitation possible in
the warm sector, especially near the low. The rain–snow line tends to be oriented
roughly parallel to the track of the low, with a bulge northward near the low center,
but its exact position depends on the temperature of the environment within which
the extratropical cyclone is forming.
Precipitation requires both a moist air mass and a mechanism for lifting the air
mass. The classical distribution of precipitation described above assumes a ready
supply of moisture and a pattern of vertical motion deter mined by the dynamics of
the cyclogenesis itself. The patterns can be strongly modified by both the air mass
characteristics and the distribution of orography. Over water, where orography is
nonexistent and moisture is readi ly available, precipitation tends to conform to the
classical picture. However, the three cyclones over land in Figures 8 to 10 have
markedly nonclassical precipitation patterns. The cyclone in Figure 8 possesses a
rain band well ahead of the prim ary surface cold front, because air closer to the cold
front has a history of descent over the Rocky Mountains and is relatively dry. Farther
north of the cold front, another region of precipitation is being produced by upslope
flow as air approaches the higher elevations of the Rocky Mountains and interacts
with an inverted trough. Both of these areas of precipitation are common to cyclones
in the central United States. The Alberta clip per in Figure 9 possesses no precipita-
tion whatsoever withi n the warm sector or along the cold front because the entire air
mass in the warm sector is dry after descending the mountains or originated as a cold
air mass that has not passed over a large body of water and therefore remains dry.
The storm along the East Coast in Figure 10 has an adequate supply of moisture
from the adjacent Atlantic Ocean but is being affected by the Appalachian Moun-
tains. Precipitation is relatively widespread on the eastern slope of the mountains
where the air is warm and moist and is being forced to ascend over the mountains
and over the cooler air trapped against them. Within the mountains, the trapped air
often creates the necessary low-level temperature inversion for sleet or freezing rain.
On the lee side of the mountains, precipitation is lighter.
The observations plotted in Figure 7 are too sparse to show the effects of the
orography of the western Uni ted States on the distribution of precipitation. However,
on scales of tens to thousands of kilometers, the orography modulates the precipita-
tion by mechanically forcing ascent and descent and by altering the structure and
tracks of weather systems. Prec ipitation tends to be high on the windward side of
mountains and low on the leeward side. As air passes over successive mountain
ranges, progressively less precipitation is produced.
The cloud patter ns associated with extratropical cyclones are broadly similar to
the preci pitation patterns, but the determination of cloud top heights possible with
infrared satellite imagery makes satellites a useful tool for diagnosing cyclone
structure, particularly over oceans where other forms of data are scarce. Most
common is a southwest to northeast oriented (in the Northern Hemisphere) band
6 EXTRATROPICAL CYCLONES 525