Reexamining the Cold Conveyor Belt

Transcription

SEPTEMBER 2001
SCHULTZ
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Reexamining the Cold Conveyor Belt
DAVID M. SCHULTZ
NOAA/National Severe Storms Laboratory, Norman, Oklahoma
(Manuscript received 18 September 2000, in final form 9 January 2001)
ABSTRACT
Despite the popularity of the conveyor-belt model for portraying the airflow through midlatitude cyclones,
questions arise as to the path of the cold conveyor belt, the lower-tropospheric airflow poleward of and underneath
the warm front. Some studies, beginning with Carlson’s analysis of the eastern U.S. cyclone of 5 December
1977, depict the cold conveyor belt moving westward, reaching the northwest quadrant of the storm, turning
abruptly anticyclonically, rising to jet level, and departing the cyclone downstream (hereafter, the anticyclonic
path). Other studies depict the cold conveyor belt reaching the northwest quadrant, turning cyclonically around
the low center, and remaining in the lower troposphere (the cyclonic path). To clarify the path of the cold
conveyor belt, the present study reexamines Carlson’s analysis of the cold conveyor belt using an observational
and mesoscale numerical modeling study of the 5 December 1977 cyclone.
This reexamination raises several previously unappreciated and underappreciated issues. First, airflow in the
vicinity of the warm front is shown to be composed of three different airstreams: air-parcel trajectories belonging
to the ascending warm conveyor belt, air-parcel trajectories belonging to the cyclonic path of the cold conveyor
belt that originate from the lower troposphere, and air-parcel trajectories belonging to the anticyclonic path of
the cold conveyor belt that originate within the midtroposphere. Thus, the 5 December 1977 storm consists of
a cold conveyor belt with both cyclonic and anticyclonic paths. Second, the anticyclonic path represents a
transition between the warm conveyor belt and the cyclonic path of the cold conveyor belt, which widens with
height. Third, the anticyclonic path of the cold conveyor belt is related to the depth of the closed circulation
associated with the cyclone, which increases as the cyclone deepens and evolves. When the closed circulation
is strong and deep, the anticyclonic path of the cold conveyor belt is not apparent and the cyclonic path of the
cold conveyor belt dominates. Fourth, Carlson’s analysis of the anticyclonic path of the cold conveyor belt was
fortuitous because his selection of isentropic surface occurred within the transition zone, whereas, if a slightly
colder isentropic surface were selected, the much broader lower-tropospheric cyclonic path would have been
evident in his analysis instead. Finally, whereas Carlson concludes that the clouds and precipitation in the cloud
head were associated with the anticyclonic path of the cold conveyor belt, results from the model simulation
suggest that the clouds and precipitation originated within the ascending warm conveyor belt.
As a consequence of the reexamination of the 5 December 1977 storm using air-parcel trajectories, this paper
clarifies the structure of and terminology associated with the cold conveyor belt. It is speculated that cyclones
with well-defined warm fronts will have a sharp demarcation between the cyclonic and anticyclonic paths of
the cold conveyor belt. In contrast, cyclones with weaker warm fronts will have a broad transition zone between
the two paths. Finally, the implications of this research for forecasting warm-frontal precipitation amount and
type are discussed.
1. Introduction
One approach to understanding midlatitude cyclones
is to consider the four-dimensional airflow through such
storms. Conceptual models of such airflow represent
kinematic descriptions of the paths of air parcels through
cyclones. Dynamics, however, cannot be entirely divorced from such models, as proposed airflow models
should be consistent with the current understanding of
the dynamics of midlatitude cyclones (e.g., polewardmoving, rising warm air is consistent with quasigeoCorresponding author address: Dr. David M. Schultz, NOAA/National Severe Storms Laboratory, 1313 Halley Circle, Norman, OK
73069.
E-mail: [email protected]
q 2001 American Meteorological Society
strophic reasoning and the conversion of available potential energy to kinetic energy within the cyclone).
Although models of airflow through midlatitude cyclones have existed since the nineteenth century (e.g.,
references within Kutzbach 1979; Cohen 1993; Wernli
1995), the essence of our modern interpretation was
perhaps best distilled by Carlson (1980) in his study of
the central U.S. cyclone of 5 December 1977. His conceptual model based on that storm, the conveyor-belt
model, describes three airstreams that affect the structure of clouds and precipitation: the dry airstream, the
warm conveyor belt, and the cold conveyor belt. Subsequently, the conveyor-belt model was applied to other
cyclones by other investigators, and the model became
quite influential and enjoyed popularity. Occasionally,
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criticisms that challenge aspects of the conveyor-belt
model have arisen. Sometimes these criticisms have
strengthened or expanded the scope of the conveyorbelt model; other times these concerns have been unheeded. As will be shown later in this paper, some troubling issues regarding the path of one of those airstreams
through the cyclone, the cold conveyor belt, remain
largely unappreciated.
The purpose of this paper is to explore the path of
the cold conveyor belt. As will be shown later in this
paper, studies prior to 1980 (e.g., Shaw and Lempfert
1906; Eliassen and Kleinschmidt 1957) found that the
cold conveyor belt turns cyclonically around the low
center, remaining in the lower troposphere. In contrast,
Carlson (1980) found that the cold conveyor belt turns
anticyclonically and rises to the mid- and upper troposphere. Perhaps as a consequence of the popularity
of Carlson’s conceptual model, studies subsequent to
1980 continued to identify the anticyclonic path of the
cold conveyor belt, even in the face of contradictory
evidence. Since the concept of the anticyclonically turning cold conveyor belt originated with the cyclone studied by Carlson (1980), a reinvestigation of this case from
a modern perspective should help to clarify this discrepancy.
In section 2, the literature on airflow through cyclones
is reviewed, including an extended discussion of the
conveyor-belt model. Issues related to the cold conveyor
belt are discussed in section 3. In section 4, an observational overview of the 5 December 1977 case, including isentropic analysis, is presented. Section 5 follows with a mesoscale model simulation of the event,
with numerically calculated air-parcel trajectories in
section 6. Section 6 also contains a discussion of the
generality of these results to cyclones other than the 5
December 1977 event. In section 7, the origin of the
cloud and precipitation poleward of the warm front in
the 5 December 1977 case is examined and the use of
conveyor-belt concepts in forecasting precipitation is
discussed. This paper closes with a summary of principal conclusions in section 8.
2. Previous literature
In this section, the development of the conveyor-belt
model is placed in historical perspective. First, airstream
models that preceded Carlson’s (1980) conceptual model are reviewed briefly in section 2a, followed by a more
thorough discussion of the elements of the conveyorbelt model portrayed by Carlson (1980) in section 2b.
Finally, the influence of the conveyor-belt model on later
research is discussed in section 2c, along with some
criticisms that have arisen in section 2d.
a. Early airstream models
Airflow models have a long history as reviewed by
Kutzbach (1979, chapter 6), Cohen (1993, section 2.8),
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and Wernli (1995, section 1.3). Surface air trajectories
were first computed by Shaw (1903) for the 26–27 February 1903 storm that devastated the British Isles. Shaw
laid basic principles for the construction and interpretation of air-parcel trajectories (Shaw 1903; Shaw and
Lempfert 1906; Shaw 1911, chapter 7). His work also
illustrated convergence of cold easterly flow encircling
the low center and poleward-moving warm air in the
northeastern quadrant of the storm [e.g., Shaw and
Lempfert (1906; figure reproduced in Kutzbach 1979,
p. 182)], questioning a previous hypothesis that midlatitude cyclones had symmetric inflow. Such surface
convergence implied ascent of the poleward-moving
warm air over the cold easterly flow, leading to precipitation. Bjerknes (1919) presented a schematic illustrating similar airflows (Fig. 1a).
A drawback to these early models is that they did not
depict airflow very far above the surface. The development of upper-air networks and isentropic analysis
allowed quantitative means to assess vertical motion
(e.g., Rossby et al. 1937; Danielsen 1961; Green et al.
1966), as reviewed in Gall and Shapiro (2000). Bjerknes
(1932, Fig. 13) computed storm-relative moist-isentropic flow rising anticyclonically into the base of an altostratus deck above a warm-frontal surface. He also
noted the apparent inconsistency of this airflow turning
anticyclonically despite being part of a larger cyclonic
circulation. Namias (1939) showed streamlines along
isentropic surfaces, showing the ascending warm air
over the warm front and the dry descending air behind
the cold front. The dry, descending air was further explored by Palmén and Newton (1951) in a cold-air outbreak over the central United States and by Danielsen
(1961, 1964, 1966) in extratropical cyclones over the
United States. Eliassen and Kleinschmidt (1957) presented isentropic streamlines relative to the moving surface low center (hereafter, storm-relative isentropic
analysis) at different levels through a column of points
within a warm-frontal zone: the air underneath the
warm-frontal surface (S1 and S 3 in Fig. 1b) gently rose
as it encircled the low center, while air above the warmfrontal surface (S 5 and S 7 in Fig. 1b) rose abruptly, as
described in earlier studies (e.g., Namias 1939).
Browning and Harrold (1969), Browning et al.
(1973), and Harrold (1973) focused on the warm ascending airstream. Specifically, they found that a drier
midtropospheric flow capped the warm ascending air,
generating potential instability as these two airstreams
ascended over the warm front. They also found that cool
dry air descended from the downstream anticyclone and
flowed underneath the warm-frontal zone. Palmén and
Newton (1969, Fig. 10.20) illustrated three airstreams
through midlatitude cyclones. They featured the warm
ascending air over the warm front, the dry descending
air behind the cold front, and midtropospheric air entering the cyclone from the west (reminiscent of the
midtropospheric cyclical trajectories of Whitaker et al.
1988). These studies provided the elemental airstreams
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that were integrated and extended in the development
of Carlson’s (1980) conveyor-belt model.
b. The conveyor-belt model
Integrating previous literature and storm-relative isentropic analysis of the central U.S. cyclone of 5 December 1977, Carlson’s (1980) conveyor-belt model
comprised three elemental airstreams: the dry airstream,
the warm conveyor belt, and the cold conveyor belt (Fig.
2).
1) DRY
AIRSTREAM
The dry airstream originates in the upper troposphere
and lower stratosphere and descends on the west side
of the upper-level trough (Fig. 2). Rounding the base
of the trough, this airstream fans out, ascending over
the warm/occluded front and descending behind the surface cold front as illustrated by Danielsen [1964; reproduced in Browning (1999, Fig. 9)] and Carlson
(1980, Fig. 10). Because of its importance in convective
destabilization and cyclogenesis, the dry airstream has
been the subject of explorations by, among others, Carr
and Millard (1985), Young et al. (1987), Durran and
Weber (1988), Gurka et al. (1995), Browning and Golding (1995), and Browning (1997).
2) WARM
FIG. 1. Early models of airflow through midlatitude cyclones: (a)
schematic airflow of warm air (open arrows) overriding cyclonically
turning cold air (solid arrows) (Bjerknes 1919, Fig. 6) and (b) stormrelative isentropic streamlines at different levels through a point
northwest of the warm front of a cyclone (Eliassen and Kleinschmidt
1957, Fig. 35). Notation along the streamlines is found in the legend
on the bottom right of the figure.
CONVEYOR BELT
As noted by Harrold (1973), the term conveyor belt
was first used in 1969 in a discussion at the Conference
on the Global Circulation of the Atmosphere specifically
to describe the warm, moist air from the warm sector
of a midlatitude cyclone that flows poleward and ascends over the warm-frontal zone, forming a cloud mass
called the polar-front cloud band. Carlson (1980) termed
this flow the warm conveyor belt and found that it started in the lower troposphere, ascended over the warm
front, and turned anticyclonically at jet level (Fig. 2).
More recently, Kurz (1988), Wernli (1995, 63–64; 1997,
p. 1694), Bader et al. (1995, section 5.2), and Browning
and Roberts (1996) have argued that, during the early
stages of cyclogenesis, if the upper-level flow is an open
wave (no closed flow), then the warm conveyor belt
rises to the upper troposphere and turns anticyclonically
downstream of the cyclone. Later in the evolution of
the cyclone (e.g., occlusion), if the upper-level flow is
characterized by closed flow, some of the warm conveyor belt air may turn cyclonically around the low
center [the so-called trowal airstream of Martin (1999)].
Thus, the deep cloud mass composing the head of the
comma-shaped cloud pattern, at least in some cases, is
due to the cyclonically turning portion of the warm conveyor belt [see also Reed et al. (1994)]. It is interesting
to note that Bjerknes (1932) suggested that, if the rising
warm sector air was unstable, cyclonic flow would be
favored over anticyclonic flow, perhaps indicating some
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FIG. 2. The conveyor-belt model of airflow through a northeast U.S. snowstorm (adapted from
Kocin and Uccellini 1990, Fig. 26), based on the Carlson (1980, Figs. 9 and 10) conceptual model.
measure of the depth of the cyclone (and, thus, closed
flow).
3) COLD
CONVEYOR BELT
In contrast to the dry airstream and the warm conveyor belt, relatively little research has explored the cold
conveyor belt, its properties, and its path through the
cyclone. Air in the cold conveyor belt originates in the
lower troposphere of the downstream anticyclone and
passes underneath the warm-frontal zone (Fig. 2). Thus,
the warm front separates the warm and cold conveyor
belts. Potential vorticity is created in the cold conveyor
belt beneath the area of latent heat release in the ascending warm conveyor belt. The increasing potential
vorticity within the cold conveyor belt is transported
westward toward the lower-tropospheric cyclone center.
As demonstrated by Stoelinga (1996) and Rossa et al.
(2000), the diabatically generated potential vorticity can
enhance the cyclonic circulation about the surface cyclone without appreciably affecting the location and
overall structure of the cyclone.
Since any precipitation generated within the warm
conveyor belt must fall through the cold conveyor belt,
the temperature and humidity of the cold conveyor belt
can play an important role in controlling the type and
amount of precipitation reaching the surface. Three examples follow. First, if the cold conveyor belt is characterized by a shallow layer of subfreezing air, sleet,
and/or freezing rain may occur. Second, low-level cool-
ing in the cold conveyor belt due to melting of snowflakes may cause a changeover from rain to heavy snow
(e.g., Kain et al. 2000). Finally, if the cold conveyor
belt is relatively dry, evaporation/sublimation of hydrometeors will reduce the amount and intensity of precipitation reaching the ground; in some cases, only virga
may be observed. On the other hand, if the cold conveyor belt is nearly saturated, precipitation reaching the
surface could be substantial.
The literature suggests that the cold conveyor belt
takes one or both of two paths after passing underneath
the warm-frontal zone. Studies prior to 1980 show the
cold conveyor belt turning cyclonically around the low
center, remaining in the lower troposphere behind the
cold front (e.g., Fig. 1), hereafter the cyclonic path.
Carlson’s (1980) analysis departed from earlier research
in that the cold conveyor belt turns sharply anticyclonically and rises to the upper troposphere, hereafter the
anticyclonic path (Figs. 2 and 3). This rising air produces the structure termed the cloud head, which, in
some cyclones, emerges from underneath the polar-front
cloud band (e.g., Böttger et al. 1975; Monk and Bader
1988; Bader et al. 1995; Dixon 2000). Since 1980, conceptual models of the airflow through cyclones tend to
incorporate both interpretations (e.g., Carlson 1987;
Browning 1990, 135–136; Carlson 1991, 318–319).
Thus, the popularity of Carlson’s (1980) conceptual
model seems to have led to general acceptance of the
anticyclonic path of the cold conveyor belt, a concept
that was hitherto undescribed in the literature.
SEPTEMBER 2001
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FIG. 3. The cold conveyor belt as analyzed by Carlson (1980, his Fig. 5) for 1200 UTC 5 Dec
1977. Storm-relative isentropic streamlines (solid lines with arrows) on the u 5 297 K surface
and u w 5 128C surface (when saturated). Stippling indicates region of sustained precipitation
associated with the cloud shield. LSC identifies the limiting streamline of the cold conveyor belt.
Other notation as in legend.
c. Influence of the conveyor-belt model on later
research
A testament to the substantial value and influence of
Carlson’s (1980) conveyor-belt model, it has appeared
in review articles (e.g., Browning 1986, 1990, 1999)
and textbooks (e.g., Kocin and Uccellini 1990, 67–74,
78; Carlson 1991, section 12.4; Bluestein 1993, section
1.6.2; Bader et al. 1995, section 3.1). The conveyor-belt
model also has served as a starting point for exploring
cyclone structures and evolutions that do not fit into the
Norwegian cyclone model. For example, such airstream
analyses have been performed for cyclones conforming
to the evolution described by Shapiro and Keyser (1990)
(e.g., Kuo et al. 1992; Reed et al. 1994; Browning and
Roberts 1994), split fronts (e.g., Browning and Monk
1982; Young et al. 1987; Kuo et al. 1992; Mass and
Schultz 1993), cyclones affected by topography (e.g.,
Hobbs et al. 1990, 1996; Steenburgh and Mass 1994;
Bierly and Winkler 2001), and cyclones in different
large-scale flow regimes (e.g., Evans et al. 1994; Bader
et al. 1995, section 5.2). In addition, with the increasing
availability of mesoscale models, calculating air-parcel
trajectories has become a more accurate method than
storm-relative isentropic analysis to envision the airflow
through cyclones, thus giving the conveyor-belt model
a modern perspective.
d. Criticisms and ambiguity of the conveyor-belt
model
Despite enjoying popularity, the simple picture of the
conveyor-belt model has been criticized in the past for
the following reasons. Schultz (1990, p. 219), Kuo et
al. (1992), and Reed et al. (1994) argued that airstreams
in cyclones do not resemble flat conveyors or threedimensional tubes with well-defined boundaries. Instead, Wernli (1997) described airstreams as coherent
flexible tubes that evolve over time rather than being
steady-state entities. Mass and Schultz (1993), Reed et
al. (1994), and Bader et al. (1995, p. 213) showed that
the three-conveyor-belt model may be an oversimplification of the airflow through some cyclones because
more than three airstreams may be needed to describe
the variety of individual air-parcel trajectories. Grotjahn
and Wang (1989) showed that surface fluxes of heat and
moisture can modify the airflow through marine cyclones; specifically, the cold lower-tropospheric air from
the downstream anticyclone (i.e., cold conveyor belt air)
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can gain heat and moisture by traveling over warmer
water, thus becoming ‘‘warm conveyor belt air,’’ a process recognized by Shaw (1911, p. 213). Some studies
of marine cyclones were unable to identify a well-defined cold conveyor belt (e.g., Reed et al. 1994; Browning et al. 1995) or found no sharp boundary between
the warm and cold conveyor belts (e.g., Kuo et al. 1992;
Browning and Roberts 1994). Tung (1990) and Mass
and Schultz (1993) found no evidence for the anticyclonic path of the cold conveyor belt. A final concern
is ambiguity about what the conveyor belts actually represent: streamlines, trajectories, streak lines, or some
combination of the above.
Classically, airflow is represented by one of three
methods: streamlines, trajectories, or streak lines. A
streamline is a line tangent to the instantaneous velocity
of the fluid at every point, a trajectory is the path of a
material parcel of fluid through some time period, and
a streak line is a line along which the fluid elements
constituting the line have passed through a particular
point in space over some time period (e.g., Huschke
1959). These three are equivalent only when the flow
is steady state.
In schematic airflow models, it may not be clear that
what is being depicted is uniquely associated with
streamlines or trajectories. A steady-state assumption is
typically invoked implicitly in conveyor-belt studies to
allow for the simplification that isentropic streamlines
can be considered trajectories. The steady-state assumption, however, requires that the cyclone is neither
changing its intensity nor its structure, generally an unlikely prospect. In addition, storm-relative isentropic
analysis assumes that the local vertical velocity of the
isentropic surface is zero, which may not be the case,
particularly in a rapidly evolving situation. Although it
is easier to calculate streamlines than trajectories, strictly, streamlines can represent trajectories only approximately.
Whereas numerically calculated trajectories appear to
resolve the dilemma of objectively determining the conveyor belts, spatial and temporal resolution limits the
accuracy of the calculations. Moreover, not all of the
airflow in a cyclone will coincide with these aggregations of trajectories; it may be possible to find trajectories that do not belong to any ensemble of trajectories.
Nevertheless, it is plausible to assume that it is possible
to determine collections of sufficiently similar trajectories that their aggregation into conveyor belts will both
represent the airflow properly and encompass the majority of a cyclone’s airflow. Thus, conveyor belts can
be considered equivalent to the coherent ensembles of
trajectories defined by Wernli and Davies (1997) and
Wernli (1997), separated by the airstream boundaries of
Cohen and Kreitzberg (1997). For the present study, I
am assuming that the conveyor belts can be represented
by collections of trajectories that follow similar paths.
The ambiguity in what airflow models represent is
not limited to schematic depictions of airflow through
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synoptic-scale cyclones. For example, an airflow model
by Lemon and Doswell (1979), a revision of an earlier
such model developed by Browning (1964), was used
to represent airflow in supercell thunderstorms. Although not described as such originally, the airflow
models used by Browning (1964) and Lemon and Doswell (1979) for supercells are similar in spirit to the
conveyor-belt model for midlatitude cyclones (e.g.,
Carlson 1991, section 12.5; Lemon 1998).
Despite the criticisms summarized in this section, the
conveyor-belt model survives. This paper will focus on
just a small number of these issues, those related to the
cold conveyor belt. In particular, what is the path of the
cold conveyor belt after it passes underneath the warmfrontal surface?
3. The path of the cold conveyor belt
Although it is not unreasonable to postulate the cold
conveyor belt splitting and taking two different paths,
in much the same manner as the dry airstream fans out
behind the cold front or the warm conveyor belt splits
when rising over the warm front, evidence described
below questions the existence of the cold conveyor belt
splitting and rising anticyclonically. Additionally, evidence suggests that the bulk of the cold conveyor belt
remains in the lower troposphere and travels cyclonically around the low center. Four arguments describing
the evidence for these two claims are advanced in this
section.
a. Lack of evidence for the anticyclonic path
Table 1 lists 19 studies in which only the cyclonic
path of the cold conveyor belt was identified. Previous
literature with quantifiable airstreams or calculated trajectories that demonstrate the anticyclonic path, in fact,
are rare. Many schematic diagrams of storm-relative isentropic flow exist, but these do not show the actual
wind observations used to draw such anticyclonically
curving streamlines (e.g., Fig. 5 in Carlson 1980; Fig.
4 in Kurz 1988; Fig. 12.18a in Carlson 1991; Fig. 8c
in Browning and Roberts 1994).
In all the literature surveyed, only three studies possess numerically calculated air-parcel trajectories purported to follow the anticyclonic path of the cold conveyor belt. First, trajectory 5 in Whitaker et al. (1988,
Fig. 18b) undergoes anticyclonic turning as the parcel
approaches the low center and rapidly ascends to jet
level. Hibbard et al. (1989) also present trajectories calculated from a model simulation of the same storm. In
their Figs. 3 and 4, yellow-colored trajectories (representing those that originate in the ‘‘ocean-influenced
planetary boundary layer east of the cyclone,’’ which
suggests that they belong to the cold conveyor belt) are
shown, some of which originate in the warm sector and
rise over the warm front, taking a path similar to that
of trajectory 5 in Whitaker et al. (1988, Fig. 18). That
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TABLE 1. Previous literature illustrating the cyclonically turning branch of the cold conveyor belt, but not the anticyclonically turning
branch.
Shaw and Lempfert (1906)
Bjerknes (1919)
Eliassen and Kleinschmidt (1957)
Browning and Harrold (1969)
Golding (1984)
Carr and Millard (1985)
Iskenderian (1988)
Hibbard et al. (1989)
Stewart and Macpherson (1989)
Tung (1990)
Kuo et al. (1992)
Reed et al. (1992)
Cohen (1993)
Mass and Schultz (1993)
Schultz and Mass (1993)
Reed et al. (1994)
Browning and Roberts (1994)
Browning et al. (1995)
Browning and Roberts (1996)
Fig. 40 in Kutzbach (1979, p. 182)
Fig. 6
Pp. 134–137 and Fig. 35
Fig. 2a
Fig. 2
Figs. 5, 6, 9, 10, 13, 14
Figs. 7, 8
Fig. 5a
Fig. 5b
Fig. 7
Figs. 3d, 4d
Fig. 4
Fig. 9
Figs. 4.5, 4.11a, 5.6
Fig. 8
Pp. 911–912 and Fig. 16
Fig. 6b
Fig. 2.1
P. 910 and Figs. 16, 18a, 22
Figs. 10, 11a,c, 12a
P. 2699 and Fig. 10
Pp. 2699–2701 and Fig. 11c
Fig. 8d
Figs. 5, 9a
Fig. 11
such air parcels originate in the warm sector indicates
that they might best be labeled as belonging to the warm
conveyor belt, not the cold conveyor belt. The second
study showing anticyclonic cold conveyor belt trajectories examined model simulations of two oceanic explosively developing cyclones (Liu 1997). Liu found
that trajectories from a narrow range of initial altitudes
rose rapidly and turned anticyclonically, carrying much
of the moisture in the storm. Because these air parcels
entrained moisture and sensible heat fluxes, they did not
remain on a moist-isentropic surface, making Liu (1997)
reluctant about equating them to the cold conveyor belt.
Finally, Kuo et al. (1992, their Figs. 9 and 12) examine
an oceanic cyclone in which several trajectories begin
in the lower troposphere, poleward of the warm front,
and rise to the mid and upper troposphere while turning
anticyclonically. In this case, however, strong frontogenesis in a region of strong baroclinicity occurs poleward of the original warm front. Thus, it may be argued
that this air did not originate within the cold air underneath the warm-frontal zone either. These three studies indicate that defining what exactly is the cold conveyor belt in certain instances may be problematic (e.g.,
Grotjahn and Wang 1989).
b. Unknown mechanism to force ascent of cold stable
air
For the air within the cold conveyor belt to rise to
the upper troposphere would imply that air would have
to pass through the stable layer that characterizes the
warm-frontal zone. It seems unlikely that such cold lower-tropospheric air moving underneath the warm-frontal
Surface trajectories
Schematic
Storm-relative isentropic
Schematic
Calculated trajectories
Isentropic trajectories
Schematic
Schematic
Schematic
Schematic
Schematic
Schematic
streamlines
streamlines
streamlines
streamlines
streamlines
streamlines
streamlines
zone would ascend in such a rapid manner. As remarked
by Shaw (1911, p. 213), ‘‘the cold air of the easterly
current is very unpromising material out of which to
make a rising current.’’ If the depiction of the anticyclonic path of the cold conveyor belt is to be taken
literally, strong ascent—perhaps the strongest associated
with the cyclone—must be present in this region.
Browning and Mason (1981, p. 579) attribute the ascent
to frictional convergence in the sheared part of the
boundary layer just ahead of the surface warm front,
but this hypothesis has not been tested rigorously.
Therefore, the process forcing this supposed rapid ascent in the cold conveyor belt has not been elucidated.
Golding (1984), Carr and Millard (1985, p. 386),
Browning et al. (1995, p. 1245), and Wernli and Davies
(1997, Fig. 9), among others, show no large ascent associated with trajectories in the cold conveyor belt. Consequently, there appears to be a question about the magnitude of the vertical motion in this region of the cold
conveyor belt.
c. Ambiguity of streamline analyses
Carlson’s (1980) isentropic analysis, reproduced in
Fig. 3, is based on a storm-relative streamline analysis
of the available radiosonde data on the u 5 297 K
surface when unsaturated and on the u w 5 128C surface
when saturated. The sharp anticyclonic ascent of the
cold conveyor belt as depicted occurs over Illinois, Indiana, and Michigan, in a region of strong shear of the
horizontal wind on the isentropic surface. Isentropic surfaces within the cold conveyor belt from other storms
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also show this region of strong shear (e.g., Fig. 12.18a
in Carlson 1991; Fig. 8 in Browning and Roberts 1994).
As discussed by Schaefer and Doswell (1979) and
Doswell and Caracena (1988), analysis of a vector field
from sparse data can be ambiguous, especially in regions
of strong shear. This statement is also confirmed by
Cohen and Kreitzberg (1997, their Fig. 5) who show
that their integrated contraction rate, a measure of the
rate at which nearby air-parcel trajectories approach
each other, will be large in a region of shear. Consequently, interpretating the path of the cold conveyor belt
based solely on isentropic streamline analysis can be
risky.
d. Complexity and confusion
The discrepancy over the path of the cold conveyor
belt leads to potential complexity and confusion. In their
discussion of the variety of cyclogenesis events, Bader
et al. (1995, p. 213) state: ‘‘In the cyclogenesis types
described in this section, it has been found necessary
to introduce a second warm conveyor belt and to reserve
the term ‘cold conveyor belt’ for a third conveyor belt,
confined to low levels, which appears later in the cyclogenesis.’’ This complexity leads to potential confusion over airstream classification (i.e., cold conveyor
belt vs warm conveyor belt) and under what conditions
the cold conveyor belt clearly exists and is well defined.
In hopes of clarifying this situation, the 5 December
1977 cyclone studied by Carlson (1980) is revisited
from a modern perspective.
4. Case study: Analysis of observations
To reexamine the storm studied by Carlson (1980),
an observational analysis is performed first. At 1200
UTC 5 December 1977 (hereafter, 5/12), a surface cyclone with a central pressure around 986 hPa was centered over western Kentucky (Fig. 4a). At 500 hPa, a
broad planetary-scale trough was situated over the central United States, with embedded shortwave troughs
located over Minnesota and slightly to the west of the
surface center (Fig. 4b).
To illustrate the cold conveyor belt, a storm-relative
isentropic chart was constructed from the available
soundings at 5/12. An average velocity for the surface
cyclone center of 13.2 m s 21 in the direction of 478
(northeast) was determined from 5/03 to 5/18. This vector storm motion was subtracted from the total wind
speed at each level for each sounding to determine the
storm-relative wind. A dry-isentropic chart along the
u 5 297 K surface, the same dry-isentropic surface
selected by Carlson (1980, Fig. 5) for the cold conveyor
belt, then was constructed (Fig. 5).
Note that this methodology differs slightly from that
of Carlson (1980). His storm-relative isentropic chart
was constructed from the u 5 297 K surface, when the
flow was unsaturated, but along the u w 5 128C (u e ø
VOLUME 129
310 K) surface, when saturated. Because the mixing
ratio (and hence u w and u e ) varies horizontally along
the u 5 297 K surface (e.g., mixing ratios vary from
2.0 g kg 21 at Cape Hatteras, NC, to 15.7 g kg 21 at
Tallahassee, FL, in Fig. 5), a single value of u w /u e cannot
be assigned to a given value of u. Thus, Fig. 5 avoids
this ambiguity and is consistent with the more traditional
method of constructing isentropic maps that follow a
single dry-adiabatic surface. Nevertheless, the difference between these two approaches is reconsidered in
section 6a of this paper. Another difference is that Carlson (1980, p. 1502) uses the phase speed of the long
wave to determine the storm motion (6.7 m s 21 to the
east), whereas the present analysis uses the motion of
the surface cyclone (13.2 m s 21 to the northeast). Since
this paper concerns the cold conveyor belt, a lowertropospheric airstream, it is more appropriate to consider
the flow relative to a lower-tropospheric feature like the
center of the surface cyclone rather than a midtropospheric feature like the long wave.
Figure 5 shows the 297-K isentropic surface sloping
upward toward the north: the pressure of the surface
was greater than 900 hPa over the southern United States
and less than 400 hPa over much of Minnesota and the
Dakotas. Relative humidities were greater than 80%
throughout Alabama, Tennessee, and the Ohio River
valley. Storm-relative airflow on this isentropic surface
consisted of southeasterlies and easterlies undergoing
gentle ascent over the depth of 1000–740 hPa over the
southeast United States, mid-Atlantic, and upper Midwest. Farther west, northwesterly flow descended down
the isentropic surface from 500 hPa in Wyoming, Nebraska, and western Iowa to 980 hPa over Texas, Louisiana, and Mississippi. Another branch of the northwesterly flow curved cyclonically toward the east over
Minnesota, and then spread anticyclonically toward the
south over the Great Lakes, descending and meeting the
southeasterly flow along a confluent asymptote. Finally,
an approximately closed cyclonic circulation was centered over southern Indiana.
Other than schematic streamlines, Carlson’s (1980)
analysis (his Fig. 5 or Fig. 3 in this paper) displays no
data and was constructed differently from Fig. 5, so a
direct comparison is not possible. In Carlson’s analysis,
the low-level easterly flow turns into upper-level southwesterly flow, making the abrupt anticyclonic ascent
from 700 hPa to above 400 hPa over Illinois. In Fig. 5,
my analysis, albeit on a different isentropic surface,
shows two adjacent airstreams. None of the upper-air
data indicate any appreciable ascending motion over
Illinois. Thus, there is considerable ambiguity in the
path of the cold conveyor belt that cannot be resolved
without further information (i.e., section 3c).
Although the storm-relative isentropic method provides an easy way to assess streamlines through cyclones, the usefulness of this method is limited by the
availability of upper-air data, the steady-state assumption, and the conservation of potential temperature. Un-
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FIG. 4. (a) Surface chart at 1200 UTC 5 Dec 1977. Analysis by the National Meteorological Center.
Notation is standard. Temperature and dewpoint are reported in 8F. (b) The 500-hPa chart at 1200 UTC
5 Dec 1977. Notation is standard. Temperature and dewpoint depressions are reported in 8C.
fortunately, the sparsity of the sounding stations limits
unambiguous determination of the path of the cold conveyor belt in the crucial region of interest northwest of
the surface low center. Also, in this case, the storm
deepens and evolves from 5/12 to 6/12 and precipitation
occurs, violating the steady-state and dry-isentropic assumptions. To make a more accurate determination of
the path of the cold conveyor belt, a mesoscale model
simulation was performed of this case and was diagnosed using air-parcel trajectories, which do not require
these assumptions.
5. Case study: Mesoscale model simulation
To examine further the airflow through this cyclone
using high spatial and temporal resolution, a mesoscale
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FIG. 5. Storm-relative isentropic analysis on the 297-K isentropic surface from radiosonde
observations at 1200 UTC 5 Dec 1977. Thin solid lines represent pressure every 100 hPa. Gray
dashed–dotted line encloses the area where relative humidity is greater than 80%. Storm-relative
winds (one pennant, full barb, and half barb denote 25, 5, and 2.5 m s 21 , respectively). Arrows
represent my analysis of streamlines. The legend for the station model is found in the lower
right of the figure.
model simulation was performed. Section 5a describes
the mesoscale model used to produce the simulation of
Carlson’s case, which is discussed in section 5b.
a. Model description
The Pennsylvania State University–National Center
for Atmospheric Research fifth-generation Mesoscale
Model (MM5), a nonhydrostatic, primitive-equation
model (Dudhia 1993; Grell et al. 1994), was employed
to simulate the cyclone. The simulation was initialized
at 0000 UTC 5 December 1977, was ended at 0000 UTC
7 December 1977, and featured 23 variably spaced terrain-following-coordinate (s) levels in the vertical. Here,
s 5 (p 2 ptop )/(psfc 2 ptop ), p is pressure, ptop is the
pressure at model top (100 hPa), and psfc is the surface
pressure. The horizontal grid spacing was 50 km over
a domain of 140 by 90 points that was roughly bounded
by 208 and 608N and by 1308 and 608W. A grid spacing
of 50 km captures the essence of the cyclone structure
and evolution, without introducing small mesoscale
structures that would result in unnecessary detail. Precipitation processes were parameterized using an explicit-moisture scheme that includes prognostic equations for water vapor, cloud water, and rainwater; cloud
water and rainwater are assumed to be cloud ice and
snow if the temperature is below 08C (Dudhia 1989;
Grell et al. 1994, section 5.3.1.1). The Kain–Fritsch
cumulus parameterization (Kain and Fritsch 1993) was
used to represent subgrid-scale convective precipitation.
Other parameterizations included a multilevel planetary
boundary layer (Zhang and Anthes 1982) and a radiative
upper boundary condition (Klemp and Durran 1983).
To create the initial conditions for the model simulation,
National Centers for Environmental Prediction (NCEP)
reanalyses (Kalnay et al. 1996) were interpolated to the
model grid and the integrated mean divergence was removed to avoid the production of spurious gravity
waves. Lateral boundary conditions were generated by
linear interpolation of the reanalyses at 12-h intervals.
b. Evolution
At 12 h into the simulation (5/12), the model placed
the surface cyclone center over western Kentucky (Fig.
6a), in agreement with the surface analysis from the
observations at this time (Fig. 4a), although the model’s
central pressure was about 5 hPa too high. The modeled
500-hPa flow captured the associated vorticity maxima
over Arkansas and Minnesota (cf. Figs. 4b and 6c). At
36 h into the simulation (6/12), the surface cyclone
deepened to 982 hPa with a secondary development
north of Cape Cod of 987 hPa (Fig. 6b). The observed
centers at this time were 990 and 986 hPa, respectively
(not shown). At 500 hPa, the shortwave troughs merged
to form a broad closed low (Fig. 6d). Despite the errors
in the central pressures of the primary cyclone (forecast
too shallow early, forecast too deep later), the locations
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FIG. 6. Sea level pressure (thick solid lines every 4 hPa) and temperature on the s 5 0.995 (approximately 40 m
above the ground) surface (thin solid lines every 48C). Large Ls and Hs indicate position of surface cyclone and
anticyclone centers: (a) 1200 UTC 5 Dec and (b) 1200 UTC 6 Dec 1977. The 500-hPa geopotential height (solid
lines every 60 dam) and absolute vorticity of the total wind (10 25 s 21 , shaded according to scale). The Xs represent
local maxima of absolute vorticity: (c) 1200 UTC 5 Dec and (d) 1200 UTC 6 Dec 1977.
of the low centers and fronts are well forecast for a
model with 50-km grid spacing. Therefore, this model
simulation should be adequate for addressing the issues
of airflow within the cyclone.
For comparison to Fig. 5, the 297-K storm-relative
isentropic surface from the model simulation is presented in Fig. 7. Although the magnitude of the stormrelative wind differs somewhat from the observations
in some locations (e.g., North Carolina, West Virginia,
Illinois, Michigan), the essence of the pattern is the
same, illustrating tropospheric-deep descending air over
the central United States, mid- and upper-tropospheric
westerly flow across the northern United States and
southern Canada, and ascending east/southeast flow
over the southeast United States and Ohio River valley.
A potentially significant difference occurs along the
Great Lakes where observations at Green Bay, Wisconsin; Flint, Michigan; and Buffalo, New York, indicate
northeasterly flow (Fig. 5), whereas the model simulation has weak flow from the west or east (Fig. 7). A
cross section through the warm-frontal zone shows that
such northeasterlies may be found at lower isentropic
levels (,290 K) over the same area in a region of strong
shear within the warm-frontal zone (Fig. 8). Neverthe-
less, this otherwise favorable comparison between Figs.
5 and 7 supports proceeding with the analysis.
6. Trajectory analysis
To explore the actual airflow within the storm, threedimensional air-parcel trajectories were computed. The
accuracy of air-parcel trajectories is limited by factors
that include the spatial resolution of the model simulation, temporal resolution of model output, time step
of trajectory calculation, specification of and degree of
mixing (both in the model and in the real atmosphere),
and strength of wind gradients. Using the approach described by Seaman (1987), trajectory calculations were
performed from 60-min model output files with a 10min time step, values consistent with the results of Doty
and Perkey (1993).
a. 1200 UTC 5 December 1977
To examine flow in the vicinity of the warm front, a
roughly north–south cross section across the warm front
is constructed (Fig. 8). The 297-K isentropic surface
(thick dashed line) is selected for comparison to Carl-
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FIG. 7. Storm-relative isentropic analysis on the 297-K isentropic surface from model simulation at 1200 UTC 5 Dec 1977. Thin solid lines represent pressure every 100 hPa. Dashed line
encloses area where relative humidities are greater than 80%. Storm-relative winds (one pennant,
full barb, and half barb denote 25, 5, and 2.5 m s 21 , respectively). Thick solid lines represent
locations of cross sections AB (Fig. 8) and Des Moines, IA (DSM), to Atlantic City, NJ (ACY)
(Fig. 14).
son’s choice of the dry-isentropic surface for his stormrelative isentropic analysis. Consistent with Figs. 5 and
7, the 297-K isentropic surface lies within southerlies
equatorward of the surface warm-frontal position, within easterlies near the leading edge of the warm front
around 700 hPa, and within westerlies above 600 hPa
(Figs. 5, 7, and 8). When the air is saturated, however,
ascending flow would not remain on a dry-isentropic
surface, but, as indicated by Carlson (1980), would ascend more steeply moist adiabatically. Issues about
which moist-adiabatic surface aside (i.e., section 4),
Carlson (1980) chose the u w 5 128C surface (approximately the u e 5 310 K surface, the thick solid line in
Fig. 8). The steepness of the u e 5 310 K surface means
that parcels following this surface are far removed from
the cold air underneath the warm-frontal zone. Thus,
the u 5 297 K and u e 5 310 K surfaces are too high
to be unequivocally within the cold air underneath the
warm front (Fig. 8). This fact alone leads to potential
confusion in interpreting the airflow.
To demonstrate this point, storm-relative forward trajectories starting along the 297-K isentropic surface
within cross section AB at 5/12 and ending at 6/12 are
computed (Fig. 9). Trajectories numbered 1 and 2 start
in the lower troposphere below 750 hPa and rise anticyclonically during this 24-h period to above 300 hPa
(Fig. 9a). The initial locations of these trajectories in
cross section AB (Fig. 8), however, indicate that they
began in the warm air equatorward of the warm-frontal
zone, not in the cold air behind the warm front. In addition, these two trajectories resemble the storm-relative
isentropic streamlines depicted in Carlson’s (1980, his
Fig. 4) warm conveyor belt. Thus, in the conveyor-belt
conceptual model, this strongly rising air would best be
classified as belonging to the warm conveyor belt.
Trajectories 3–7 represent storm-relative, westwardmoving air that originates in the lower to midtroposphere (750–600 hPa) and rises anticyclonically to about
350 hPa, a height somewhat less than that obtained by
trajectories 1 and 2 (Fig. 9b). These trajectories resemble the anticyclonic path of the cold conveyor belt as
depicted in Carlson’s analysis (Fig. 3 in this paper). This
airstream lies within much of the warm-frontal zone
along the 297-K isentrope. Finally, trajectories 8–10
represent air that originates at 550–600 hPa, sinks anticyclonically around the downstream anticyclone over
New England to 600–750 hPa, and rises as it approaches
the cyclone to 650–400 hPa (Fig. 9c). If these trajectories were computed forward from these locations, they
would cyclonically circle the low center (not shown).
Thus, these trajectories belong to the cyclonic path of
the cold conveyor belt.
If some air parcels beginning on the 297-K isentropic
surface belong to the warm conveyor belt, then air belonging to the true cold conveyor belt is likely to be
found at a lower level (colder potential temperature).
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FIG. 8. Cross section AB at 1200 UTC 5 Dec 1977 (location in Fig. 7). Equivalent potential
temperature (thin solid lines every 5 K), u e 5 310 K (thick solid line), u 5 297 K (dashed line),
u 5 290 K (dashed–dotted line), relative humidity equal to 90% (dotted line), storm-relative
winds (one pennant, full barb, and half barb denote 25, 5, and 2.5 m s 21 , respectively), and
vertical velocity (cm s 21 shaded according to scale at top right of figure). Beginning locations
of trajectories displayed in Figs. 9, 10, and 11 numbered 1–30. Gray box represents inset cross
section in Fig. 12.
Indeed, trajectories beginning on the 290-K isentropic
surface (Fig. 10) show drastically different paths from
those on the 297-K isentropic surface. All trajectories
show a cyclonic turning around the low center, characteristic of the cyclonic path of the cold conveyor belt.
To compare trajectories on the u 5 297 K surface to
those on the u e 5 310 K surface, 24-h forward trajectories beginning at 5/12 are constructed along the u e 5
310 K surface (Fig. 11a). These trajectories break into
some of the same groups previously seen on the u 5
297 K surface (Fig. 9). Trajectories 21–23 resemble the
cyclonic path of the cold conveyor belt, whereas trajectories 24 and 25 resemble the anticyclonic path of
the cold conveyor belt (Fig. 11a). Trajectories 26–30,
on the other hand, seem to be unrelated to the ascending
anticyclonic path of the cold conveyor belt trajectories.
If these trajectories are taken backward from 5/12 to
5/00, this distinction is made more clear (Fig. 11b).
Trajectories 25–27, ascending through the midtroposphere, transition to upper-tropospheric trajectories
28–30, which ascend about 100 hPa, likely similar to
the cyclical airstream noted by Whitaker et al. (1988).
Thus, two noninteracting airstreams coexist on the same
moist-isentropic surface in the mid- and upper troposphere.
To explore further the trajectories within the vicinity
of the warm front, an inset from Fig. 8 is constructed
and 24-h forward trajectories are calculated from locations starting within that inset (Fig. 12). Three different types of trajectories are classified within this inset: trajectories belonging to the warm conveyor belt,
the anticyclonic path of the cold conveyor belt, and the
cyclonic path of the cold conveyor belt (trajectory beginning locations labeled W, A, and C, respectively).
Like the warm-frontal surface, the boundary between
the warm conveyor belt and the cyclonic path of the
cold conveyor belt slopes rearward with height and is
approximately coincident with the u e 5 310 K surface
(thick solid line in Fig. 12). Above 850 hPa, there are
one or more trajectories at the boundary between the
warm conveyor belt and the cyclonic path of the cold
conveyor belt, which ascend anticyclonically. Between
700 and 750 hPa, a sharp boundary exists between trajectories belonging to the cyclonic and anticyclonic
paths of the cold conveyor belts (Fig. 12). This boundary
is caused by a change in the flow field with height: The
flow around the cyclone is closed below 700 hPa, whereas the flow is characterized by an open wave above 700
hPa (not shown). The closed flow below 700 hPa is
more likely to feature cold conveyor belt trajectories in
the cyclonic circulation around the low, whereas trajectories turning anticyclonically, not in the closed circulation, dominate above 700 hPa.
Thus, in this inset, the anticyclonic-turning trajecto-
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FIG. 9. Twenty-four-hour storm-relative forward trajectories starting on the 297-K isentropic
surface in cross section AB at 1200 UTC 5 Dec 1977 (Fig. 8). Thickness of trajectory corresponds
to pressure, as in legend. Trajectories numbered 1–10 at the starting and ending locations. Arrows
along trajectory occur every 3 h. Sea level pressure (thin solid lines every 4 hPa) and pressure
on the u 5 297 K surface (dashed lines every 100 hPa). (a) Warm-conveyor-belt trajectories 1
and 2, (b) anticyclonic path of the cold-conveyor-belt trajectories 3–7, and (c) cyclonic path of
the cold-conveyor-belt trajectories 8–10.
ries are shown to be associated with a transition between
the warm conveyor belt and the cyclonic path of the
cold conveyor belt. As the gradient in wind direction
across the frontal zone becomes less well defined with
height, the boundary between the airstreams (i.e., the
zone of anticyclonic-turning trajectories) becomes
broader also. It is interesting to note that the u 5 297
K and ue 5 310 K surfaces lie nearly along the zone
in which the anticylonic path of the cold conveyor belt
trajectories dominate. Thus, Carlson’s choice of the
u 5 297 K and ue 5 310 K surfaces was fortuitous in
that it lay at the boundary between the warm and cold
conveyor belts and his analysis of the cold conveyor
belt reflected only the anticylonic path. Other choices
of isentropic surfaces would have produced analyses
entirely within either the warm conveyor belt or the
cyclonic path of the cold conveyor belt. Therefore, in
this section, calculation of storm-relative trajectories
along isentropic surfaces clarifies the results from Carlson’s (1980) analysis of the same storm.
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FIG. 10. Twenty-four-hour storm-relative forward trajectories starting on the 290-K isentropic
surface in cross section AB at 1200 UTC 5 Dec 1977 (Fig. 8). Thickness of trajectory corresponds to pressure, as in legend. Trajectories numbered 11–20 at the starting and ending
locations. Arrows along trajectory occur every 3 h. Sea level pressure (thin solid lines every
4 hPa) and pressure on the u 5 290 K surface (dashed lines every 100 hPa).
b. 1200 UTC 6 December 1977
Twenty-four hours later (6/12), storm-relative trajectories belonging to the anticyclonic path of the cold
conveyor belt do not exist on the u 5 297 K surface
anymore (Fig. 13). The absence of the anticyclonic path
of the cold conveyor belt at this level is due to the
evolving midlevel flow: whereas the 500-hPa flow was
an open trough at 5/12 (Fig. 6c), by 6/12, the flow
became increasingly closed, with substantial flow
around the storm at midlevels (Fig. 6d). The closed
circulation favors the prominence of the cyclonic path
of the cold conveyor belt.
Figure 13 also shows that trajectories originating closer to the low center are more likely to curve cyclonically
around the low center than trajectories originating farther east of the low center on the same isentropic surface. This observation is consistent with results from
Market (1996, Fig. 4.65) and Liu (1997, 114–115).
c. Generality of conclusions
In this paper, the anticyclonic path of the cold conveyor belt is shown to be a transition zone between the
more substantial warm conveyor belt and cyclonic path
of the cold conveyor belt. Because this study represents
only a single case, however, the generality of this conclusion can be questioned. In this section, we explore
two aspects of the generality of this result.
1) THE
CYCLONIC PATH OF THE COLD CONVEYOR
BELT AS A TRANSITION ZONE
Table 1 presents 19 studies that did not find evidence
of the anticyclonic path of the cold conveyor belt, outweighing the three previous studies described in section
3a that found anticyclonically turning trajectories (i.e.,
Whitaker et al. 1988; Kuo et al. 1992; Liu 1997). One
explanation for why studies illustrating the cyclonic path
outnumber those illustrating the anticyclonic path is that
the anticyclonic path may be a relatively narrow transition zone and may be found within a small range of
isentropic surfaces in the lower to midtroposphere.
Thus, sampling the anticyclonic path with trajectories
or isentropic analyses may be more difficult. For example, Browning and Roberts (1994, their Fig. 8) show
storm-relative moist-isentropic streamlines along three
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FIG. 11. Storm-relative trajectories starting on the u e 5 310 K surface in cross section AB
at 1200 UTC 5 Dec 1977 (Fig. 8). Thickness of trajectory corresponds to pressure, as in legend.
Trajectories numbered 21–30 at the starting and ending locations. Arrows along trajectory occur
every 3 h. Sea level pressure (thin solid lines every 4 hPa) and pressure on the u e 5 310 K
surface (dashed lines every 100 hPa). (a) Twenty-four-hour forward trajectories; (b) 12-h backward trajectories.
surfaces within the transition zone between the anticyclonic and cyclonic paths of the cold conveyor belt,
showing that this transition occurs over a u w depth of
3 K.
It may be that some cyclones possess a weak anticyclonic path of the cold conveyor belt or do not possess
one at all. Tung (1990), Kuo et al. (1992), and Browning
and Roberts (1994, p. 1552) found that there is no sharp
demarcation between the cold conveyor belt and the
overlying warm conveyor belt in the cases they examined. This is likely related to the sharpness of the
warm front, which, for marine cyclones at the end of
storm tracks (e.g., over the eastern ocean basins), has
been characterized as weak, ‘‘stubby,’’ or nonexistent
by western U.S. and European meteorologists (e.g., Wallace and Hobbs 1977, p. 127; Friedman 1989, p. 217;
Schultz et al. 1998, 1787–1788). In the extreme case
where no sharp boundary exists, trajectories may
smoothly transition from one regime to another, prohibiting categorization into discrete conveyor belts. As
discussed by Cohen (1993, section 2.8.5), the so-called
polar trough conveyor belt of Browning and Hill (1985)
and Browning (1990) may be similar to such cases,
where no sharp boundary between cold air and warm
air exists, merely a zone of weak warm advection. These
transition trajectories likely become more numerous
when the gradient in wind direction associated with the
warm front is less intense than that during the 5 December 1977 storm.
That the anticyclonic path of the cold conveyor belt
is a transitional airstream between the warm conveyor
belt and the cyclonic path of the cold conveyor belt
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SCHULTZ
especially during the incipient cyclogenesis. Therefore,
this present study helps to clarify the role of the different
conveyor-belt terminologies found in the literature.
2) THE
CYCLONIC PATH OF THE COLD CONVEYOR
BELT IN DIFFERENT FLOW REGIMES
FIG. 12. Inset from cross section AB (Fig. 8) at 1200 UTC 5 Dec
1977. Equivalent potential temperature (thin solid lines every 5 K),
u e 5 310 K (thick solid line), u 5 297 K (dashed line), u 5 290 K
(dashed–dotted line), and storm-relative winds (one pennant, full
barb, and half barb denote 25, 5, and 2.5 m s 21 , respectively). Beginning locations of trajectories labeled by type of trajectory: W 5
warm conveyor belt, A 5 anticyclonic path of the cold conveyor
belt, and C 5 cyclonic path of the cold conveyor belt.
suggests that the airstream identified as W2, the second
warm conveyor belt (e.g., Bader et al. 1995, section
5.2), may also be the same feature. Airstream W2 is
described as ‘‘composed of air having a u w intermediate
between that of the main warm conveyor belt and the
cold conveyor belt. In some cases the origin of W2 is
obvious. However, in other cases, W2 probably originates near or ahead of the warm front or is the lower
part of W1 that curves around the developing low’’
(Bader et al. 1995, p. 213). Airstream W2 also bears a
striking resemblance to the cold conveyor belt in the
case presented by Carlson (1987) and the cyclonically
turning moist airstream of Bierly and Winkler (2001),
A second hypothesis to explain the apparent lack of
anticyclonic-path trajectories in the previous literature
is that, in cyclones with a deep closed storm-relative
circulation extending to the mid- and upper troposphere,
cold conveyor belt trajectories would be more likely to
follow the cyclonic path than the anticyclonic path. This
hypothesis is consistent with the majority of the cyclones in Table 1, which are rapidly developing and
well-developed marine cyclones. These results are consistent with those for the warm conveyor belt, too. For
example, for open wave cyclones, more of the warm
conveyor belt flow ascends anticyclonically than cyclonically. The events described by Whitaker et al.
(1988) and Kuo et al. (1992) are both associated with
mobile shortwave troughs, rather than closed lows in
the mid- and upper troposphere, suggesting that the
storm-relative flow is open. The open flow in these
events explains the preference for the ascending anticyclonic path of the warm conveyor belt (e.g., Fig. 18b
in Whitaker et al. 1988 and Fig. 16 in Kuo et al. 1992).
Extending this thought, the large-scale flow in which
the cyclone is embedded is important in controlling the
path and strength of the cold conveyor belt, and conveyor belts in general, as discussed by Evans et al.
(1994) and Bader et al. (1995, section 5.2).
Additional insight can be gained by considering the
cyclone from a storm-relative framework. Because more
lower-tropospheric flow would be ‘‘closed’’ in a storm-
FIG. 13. Twelve-hour storm-relative forward trajectories starting on the 297-K isentropic surface
at 1200 UTC 6 Dec 1977. Thickness of trajectory corresponds to pressure, as in legend. Arrows
along trajectory occur every 3 h. Sea level pressure (thin solid lines every 4 hPa) and pressure
on the u 5 297 K surface (dashed lines every 100 hPa).
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relative perspective when the mean storm motion is removed (e.g., Sinclair 1994, his Fig. 1), the cyclonic path
of the cold conveyor belt may become even more apparent in such analyses, compared to the anticyclonic
path of the cold conveyor belt.
7. Origin of cloud head
The purpose of Carlson’s (1980) paper was to explain
how the cloud pattern evolved into the comma-shaped
cloud head. Carlson concluded that the air in the cloud
head originated in the anticyclonic path of the cold conveyor belt. Later studies of other cyclones (e.g., Mass
and Schultz 1993; Martin 1999) have shown that air in
the cloud head originated from the cyclonic path of the
warm conveyor belt. Given the mesoscale model simulation from Carlson’s cyclone event, the origin of the
cloud and precipitation for this system can be determined.
A cross section parallel to the warm-frontal zone (Fig.
14a), along the cold conveyor belt as it travels underneath the warm-frontal zone, shows that the bulk of the
cloud lies above the warm-frontal zone. Two maxima
of cloud liquid water are shown (Fig. 14a): the maximum over Ohio (C1) is associated with a rainwater maximum (R1), which mostly evaporates before reaching
the ground (Fig. 14b). This precipitation falls through
the subsaturated portion of the cold conveyor belt where
relative humidities are less than 90% (not shown), allowing significant evaporation. Farther west along the
path of the cold conveyor belt, a second maximum in
cloud liquid water above the warm-frontal surface over
Illinois (C2) is associated with a maximum in rainwater
(R2) (cf. Figs. 14a,b). Backward air-parcel trajectories
from these two maxima of cloud water (not shown)
demonstrate that these trajectories belonged to the warm
conveyor belt. Whereas Carlson’s results showed that
the clouds and precipitation originated within the anticyclonic path of the cold conveyor belt, the results
from the model simulation show that the clouds and
precipitation originated within the ascending warm conveyor belt.
Given the popularity of the Carlson (1980) characterization of the cold conveyor belt, it is likely that some
forecasters use this concept to forecast precipitation
since, according to Carlson (1980), the so-called wraparound precipitation in the cloud head is attributed to
the rising anticyclonic cold conveyor belt. Using this
methodology, forecasting bursts of heavier precipitation
would rely on watching for fluctuations in the intensity
of the cold conveyor belt. The results of this paper call
that approach into question because the largest ascent
and generation of heavy precipitation occurs primarily
in the warm conveyor belt, not in the cold conveyor
belt (e.g., Figs. 9 and 14). This result is consistent with
that offered by Martin (1999), who expounds on the
importance of the cyclonic path of the warm conveyor
belt (i.e., trowal airstream) to heavy precipitation in the
FIG. 14. Cross section from Des Moines (DSM) to Atlantic City
(ACY) at 1200 UTC 5 Dec 1977 (location in Fig. 7). Equivalent
potential temperature (thin solid lines every 5 K), u e 5 310 K surface
(thick solid line), u 5 297 K surface (thick dashed line), u 5 290
K surface (thick dashed-dotted line), and storm-relative winds (one
pennant, full barb, and half barb denote 25, 5, and 2.5 m s 21 , respectively). (a) Cloud liquid water mixing ratio (0.01 g kg 21 , shaded
according to legend), C1 and C2 represent maxima in cloud liquid
water mixing ratio described in text. (b) Rainwater mixing ratio (0.01
g kg 21 , shaded according to legend), R1 and R2 represent maxima
in cloud liquid water mixing ratio described in text.
northwest quadrant of cyclones, but contradicts that of
Liu (1997) who showed that as much as 70% of the
moisture transport in cyclones could be associated with
the cold conveyor belt. Thus, there appears to be some
controversy in the literature whether the air that feeds
the cloud head is from the warm or cold conveyor belt
(e.g., Browning and Roberts 1994; Browning et al.
1995; Liu 1997; Dixon 2000, p. 77). Instead, the usefulness of the cold conveyor belt seems to be in recognizing the potential for evaporation of falling hydrometeors from the warm conveyor belt, melting of frozen
hydrometeors (e.g., Kain et al. 2000), or precipitationtype forecasting (e.g., rain, snow, sleet, or freezing rain).
This is not to say that individual cyclones that form
relatively shallow cloud heads may not be caused by
SEPTEMBER 2001
2223
SCHULTZ
the cyclonic or anticyclonic paths of the cold conveyor
belt in the head of the comma cloud, but heavy precipitation rates associated with strong lifting are less likely
in such a stable environment. Heavy precipitation rates
are more likely to be caused by deep clouds, perhaps
associated with midlevel conditional or moist symmetric
instabilities formed by wraparound precipitation generated at midlevels by the cyclonic path of the warm
conveyor belt. It may be that the tendency to identify
the cloud head with the cold conveyor belt in oceanic
cyclones is due to the tendency for the cold conveyor
belt, which has a potentially large fetch over the oceans,
to be nearly saturated and emerge from underneath the
polar front cloud band.
In fact, this property of the cold conveyor belt raises
the following forecasting issue. While the warm conveyor belt air that arrives at a given point comes from
the equatorward direction, the cold conveyor belt air
comes from the east. Therefore, an indication of the
potential for evaporation or melting of falling precipitation into the cold air can be obtained from examination
of the properties of the cold air from upstream (i.e.,
more east). For example, if such air is more moist farther
east, then surface precipitation rates are more likely to
increase with time (assuming a given precipitation rate
generated in the warm conveyor belt). On the other
hand, drier air being advected from farther east may
indicate a lessening of the precipitation rate at the surface. This is in contrast to the air in the warm conveyor
belt, for which an understanding of its properties before
ascent comes from farther equatorward in the warm
sector.
8. Summary
This study explored the path of the cold conveyor
belt through the midlatitude cyclone of 5 December
1977, previously studied by Carlson (1980). Storm-relative isentropic analysis and air-parcel trajectories calculated from a mesoscale model simulation of Carlson’s
(1980) storm showed that both the cyclonic and anticyclonic paths of the cold conveyor belt existed. The
anticyclonic path represented a transition between the
two more substantial airstreams in the lower troposphere: the warm conveyor belt and the cyclonic path
of the cold conveyor belt. This transition zone began in
the lower troposphere where the circulation around the
cyclone was closed and widened with height to the point
where the cyclone became an open wave. Above this
point, the anticyclonic path became the dominant airstream. The prevalence of the anticyclonic path of the
cold conveyor belt in Carlson’s analysis is shown to be
due to his selection of isentropic surfaces at the leading
edge of the warm front. Had Carlson picked a colder
isentropic surface, the cyclonic path of the cold conveyor belt would have been more apparent. This serendipity also explains the relative paucity of trajectories
belonging to the anticyclonic path of the cold conveyor
belt in previous literature. Perhaps, in the future, renaming the anticyclonic path of the cold conveyor belt
a transition airstream may be more appropriate and less
confusing.
The variability of observed cyclones in different
large-scale flow patterns (e.g., confluence, diffluence)
has been explored previously by Evans et al. (1994),
Bader et al. (1995), and Schultz et al. (1998). Whereas
this study focused on the cold conveyor belt through a
single cyclone, implications for the airflow in other cyclones of different strength and shape, and embedded
in different large-scale flows, have been proposed and
remain to be tested rigorously.
Acknowledgments. The following individuals are
gratefully acknowledged for their contributions to this
work: Jack Kain and David Stensrud for their assistance
using MM5; Mark Stoelinga for providing software to
calculate and display trajectories; Charles Doswell for
stimulating discussions on streamline analysis and airstream models—section 2d is largely derived from those
discussions; Toby Carlson for his comments on his original analysis of the cold conveyor belt; Robert Rabin
for translating portions of Bjerknes (1932); Keith
Browning, James Moore, and Carl Kreitzberg for providing relevant references; and Richard Grotjahn for
providing his thoughts on Grotjahn and Wang (1989).
Toby Carlson, Robert Cohen, Charles Doswell, Jack
Kain, John Locatelli, Anthony Lupo, Patrick Market,
James Steenburgh, David Stensrud, Mark Stoelinga,
Heini Wernli, and two anonymous reviewers provided
valuable comments on earlier versions of this manuscript.
I am grateful to the Data Support Section of the Scientific Computing Division of National Center for Atmospheric Research and the National Climatic Data
Center for providing data used in this study, the Storm
Prediction Center for access to their archives of NMC
maps on microfilm, and the Mesoscale and Microscale
Meteorology Division of NCAR for their support of
MM5.
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Reexamining the Cold Conveyor Belt

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