Reexamining the Cold Conveyor Belt

SEPTEMBER 2001 SCHULTZ 2205

DAVID M. SCHULTZ NOAA/National Severe Storms Laboratory, Norman, Oklahoma

(Manuscript received 18 September 2000, in ®nal form 9 January 2001)

ABSTRACT Despite the popularity of the conveyor-belt model for portraying the air¯ow through midlatitude cyclones, questions arise as to the path of the cold conveyor belt, the lower-tropospheric air¯ow 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, air¯ow 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 clari®es the structure of and terminology associated with the cold conveyor belt. It is speculated that cyclones with well-de®ned 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 strophic reasoning and the conversion of available potential energy to kinetic energy within the cyclone). One approach to understanding midlatitude cyclones Although models of air¯ow through midlatitude cy- is to consider the four-dimensional air¯ow through such storms. Conceptual models of such air¯ow represent clones have existed since the nineteenth century (e.g., kinematic descriptions of the paths of air parcels through references within Kutzbach 1979; Cohen 1993; Wernli cyclones. Dynamics, however, cannot be entirely di- 1995), the essence of our modern interpretation was vorced from such models, as proposed air¯ow models perhaps best distilled by Carlson (1980) in his study of should be consistent with the current understanding of the central U.S. cyclone of 5 December 1977. His con- the dynamics of midlatitude cyclones (e.g., poleward- ceptual model based on that storm, the conveyor-belt moving, rising warm air is consistent with quasigeo- model, describes three airstreams that affect the structure of clouds and precipitation: the dry airstream, the warm conveyor belt, and the cold conveyor belt. Sub- Corresponding author address: Dr. David M. Schultz, NOAA/Na- sequently, the conveyor-belt model was applied to other tional Severe Storms Laboratory, 1313 Halley Circle, Norman, OK 73069. cyclones by other investigators, and the model became E-mail: [email protected] quite in¯uential and enjoyed popularity. Occasionally,

᭧ 2001 American Meteorological Society

Unauthenticated | Downloaded 09/23/21 07:12 PM UTC 2206 MONTHLY WEATHER REVIEW VOLUME 129 criticisms that challenge aspects of the conveyor-belt and Wernli (1995, section 1.3). Surface air trajectories model have arisen. Sometimes these criticisms have were ®rst computed by Shaw (1903) for the 26±27 Feb- strengthened or expanded the scope of the conveyor- ruary 1903 storm that devastated the British Isles. Shaw belt model; other times these concerns have been un- laid basic principles for the construction and interpre- heeded. As will be shown later in this paper, some trou- tation of air-parcel trajectories (Shaw 1903; Shaw and bling issues regarding the path of one of those airstreams Lempfert 1906; Shaw 1911, chapter 7). His work also through the cyclone, the cold conveyor belt, remain illustrated convergence of cold easterly ¯ow encircling largely unappreciated. the low center and poleward-moving warm air in the The purpose of this paper is to explore the path of northeastern quadrant of the storm [e.g., Shaw and the cold conveyor belt. As will be shown later in this Lempfert (1906; ®gure reproduced in Kutzbach 1979, paper, studies prior to 1980 (e.g., Shaw and Lempfert p. 182)], questioning a previous hypothesis that mid- 1906; Eliassen and Kleinschmidt 1957) found that the latitude cyclones had symmetric in¯ow. Such surface cold conveyor belt turns cyclonically around the low convergence implied ascent of the poleward-moving center, remaining in the lower troposphere. In contrast, warm air over the cold easterly ¯ow, leading to precip- Carlson (1980) found that the cold conveyor belt turns itation. Bjerknes (1919) presented a schematic illus- anticyclonically and rises to the mid- and upper tro- trating similar air¯ows (Fig. 1a). posphere. Perhaps as a consequence of the popularity A drawback to these early models is that they did not of Carlson's conceptual model, studies subsequent to depict air¯ow very far above the surface. The devel- 1980 continued to identify the anticyclonic path of the opment of upper-air networks and isentropic analysis cold conveyor belt, even in the face of contradictory allowed quantitative means to assess vertical motion evidence. Since the concept of the anticyclonically turn- (e.g., Rossby et al. 1937; Danielsen 1961; Green et al. ing cold conveyor belt originated with the cyclone stud- 1966), as reviewed in Gall and Shapiro (2000). Bjerknes ied by Carlson (1980), a reinvestigation of this case from (1932, Fig. 13) computed storm-relative moist-isentro- a modern perspective should help to clarify this dis- pic ¯ow rising anticyclonically into the base of an al- crepancy. tostratus deck above a warm-frontal surface. He also In section 2, the literature on air¯ow through cyclones noted the apparent inconsistency of this air¯ow turning is reviewed, including an extended discussion of the anticyclonically despite being part of a larger cyclonic conveyor-belt model. Issues related to the cold conveyor circulation. Namias (1939) showed streamlines along belt are discussed in section 3. In section 4, an obser- isentropic surfaces, showing the ascending warm air vational overview of the 5 December 1977 case, in- over the warm front and the dry descending air behind cluding isentropic analysis, is presented. Section 5 fol- the cold front. The dry, descending air was further ex- lows with a mesoscale model simulation of the event, plored by PalmeÂn and Newton (1951) in a cold-air out- with numerically calculated air-parcel trajectories in break over the central United States and by Danielsen section 6. Section 6 also contains a discussion of the (1961, 1964, 1966) in extratropical cyclones over the generality of these results to cyclones other than the 5 United States. Eliassen and Kleinschmidt (1957) pre- December 1977 event. In section 7, the origin of the sented isentropic streamlines relative to the moving sur- cloud and precipitation poleward of the warm front in face low center (hereafter, storm-relative isentropic the 5 December 1977 case is examined and the use of analysis) at different levels through a column of points conveyor-belt concepts in forecasting precipitation is within a warm-frontal zone: the air underneath the discussed. This paper closes with a summary of prin- warm-frontal surface (S1 and S3 in Fig. 1b) gently rose cipal conclusions in section 8. as it encircled the low center, while air above the warm-

frontal surface (S5 and S7 in Fig. 1b) rose abruptly, as described in earlier studies (e.g., Namias 1939). 2. Previous literature Browning and Harrold (1969), Browning et al. In this section, the development of the conveyor-belt (1973), and Harrold (1973) focused on the warm as- model is placed in historical perspective. First, airstream cending airstream. Speci®cally, they found that a drier models that preceded Carlson's (1980) conceptual mod- midtropospheric ¯ow capped the warm ascending air, el are reviewed brie¯y in section 2a, followed by a more generating potential instability as these two airstreams thorough discussion of the elements of the conveyor- ascended over the warm front. They also found that cool belt model portrayed by Carlson (1980) in section 2b. dry air descended from the downstream anticyclone and Finally, the in¯uence of the conveyor-belt model on later ¯owed underneath the warm-frontal zone. PalmeÂn and research is discussed in section 2c, along with some Newton (1969, Fig. 10.20) illustrated three airstreams criticisms that have arisen in section 2d. through midlatitude cyclones. They featured the warm ascending air over the warm front, the dry descending air behind the cold front, and midtropospheric air en- a. Early airstream models tering the cyclone from the west (reminiscent of the Air¯ow models have a long history as reviewed by midtropospheric cyclical trajectories of Whitaker et al. Kutzbach (1979, chapter 6), Cohen (1993, section 2.8), 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 De- cember 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 Gold- ing (1995), and Browning (1997).

2) WARM CONVEYOR BELT As noted by Harrold (1973), the term conveyor belt was ®rst used in 1969 in a discussion at the Conference on the Global Circulation of the Atmosphere speci®cally to describe the warm, moist air from the warm sector of a midlatitude cyclone that ¯ows poleward and ascends over the warm-frontal zone, forming a cloud mass called the polar-front cloud band. Carlson (1980) termed this ¯ow the warm conveyor belt and found that it start- ed 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 ¯ow is an open wave (no closed ¯ow), 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 ¯ow is characterized by closed ¯ow, some of the warm conveyor belt air may turn cyclonically around the low FIG. 1. Early models of air¯ow through midlatitude cyclones: (a) center [the so-called trowal airstream of Martin (1999)]. schematic air¯ow of warm air (open arrows) overriding cyclonically Thus, the deep cloud mass composing the head of the turning cold air (solid arrows) (Bjerknes 1919, Fig. 6) and (b) storm- comma-shaped cloud pattern, at least in some cases, is relative isentropic streamlines at different levels through a point northwest of the warm front of a cyclone (Eliassen and Kleinschmidt due to the cyclonically turning portion of the warm con- 1957, Fig. 35). Notation along the streamlines is found in the legend veyor belt [see also Reed et al. (1994)]. It is interesting on the bottom right of the ®gure. to note that Bjerknes (1932) suggested that, if the rising warm sector air was unstable, cyclonic ¯ow would be favored over anticyclonic ¯ow, perhaps indicating some

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FIG. 2. The conveyor-belt model of air¯ow 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 ing in the cold conveyor belt due to melting of snow- ¯ow). ¯akes 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 hydro- 3) COLD CONVEYOR BELT meteors will reduce the amount and intensity of pre- In contrast to the dry airstream and the warm con- cipitation reaching the ground; in some cases, only virga veyor belt, relatively little research has explored the cold may be observed. On the other hand, if the cold con- conveyor belt, its properties, and its path through the veyor belt is nearly saturated, precipitation reaching the cyclone. Air in the cold conveyor belt originates in the surface could be substantial. lower troposphere of the downstream anticyclone and The literature suggests that the cold conveyor belt passes underneath the warm-frontal zone (Fig. 2). Thus, takes one or both of two paths after passing underneath the warm front separates the warm and cold conveyor the warm-frontal zone. Studies prior to 1980 show the belts. Potential vorticity is created in the cold conveyor cold conveyor belt turning cyclonically around the low belt beneath the area of latent heat release in the as- center, remaining in the lower troposphere behind the cending warm conveyor belt. The increasing potential cold front (e.g., Fig. 1), hereafter the cyclonic path. vorticity within the cold conveyor belt is transported Carlson's (1980) analysis departed from earlier research westward toward the lower-tropospheric cyclone center. in that the cold conveyor belt turns sharply anticyclon- As demonstrated by Stoelinga (1996) and Rossa et al. ically and rises to the upper troposphere, hereafter the (2000), the diabatically generated potential vorticity can anticyclonic path (Figs. 2 and 3). This rising air pro- enhance the cyclonic circulation about the surface cy- duces the structure termed the cloud head, which, in clone without appreciably affecting the location and some cyclones, emerges from underneath the polar-front overall structure of the cyclone. cloud band (e.g., BoÈttger et al. 1975; Monk and Bader Since any precipitation generated within the warm 1988; Bader et al. 1995; Dixon 2000). Since 1980, con- conveyor belt must fall through the cold conveyor belt, ceptual models of the air¯ow through cyclones tend to the temperature and humidity of the cold conveyor belt incorporate both interpretations (e.g., Carlson 1987; can play an important role in controlling the type and Browning 1990, 135±136; Carlson 1991, 318±319). amount of precipitation reaching the surface. Three ex- Thus, the popularity of Carlson's (1980) conceptual amples follow. First, if the cold conveyor belt is char- model seems to have led to general acceptance of the acterized by a shallow layer of subfreezing air, sleet, anticyclonic path of the cold conveyor belt, a concept and/or freezing rain may occur. Second, low-level cool- that was hitherto undescribed in the literature.

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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 ␪ ϭ 297 K surface

and ␪w ϭ 12ЊC surface (when saturated). Stippling indicates region of sustained precipitation associated with the cloud shield. LSC identi®es the limiting streamline of the cold conveyor belt. Other notation as in legend. c. In¯uence of the conveyor-belt model on later through cyclones, thus giving the conveyor-belt model research a modern perspective. A testament to the substantial value and in¯uence of Carlson's (1980) conveyor-belt model, it has appeared d. Criticisms and ambiguity of the conveyor-belt in review articles (e.g., Browning 1986, 1990, 1999) model and textbooks (e.g., Kocin and Uccellini 1990, 67±74, 78; Carlson 1991, section 12.4; Bluestein 1993, section Despite enjoying popularity, the simple picture of the 1.6.2; Bader et al. 1995, section 3.1). The conveyor-belt conveyor-belt model has been criticized in the past for model also has served as a starting point for exploring the following reasons. Schultz (1990, p. 219), Kuo et cyclone structures and evolutions that do not ®t into the al. (1992), and Reed et al. (1994) argued that airstreams Norwegian cyclone model. For example, such airstream in cyclones do not resemble ¯at conveyors or three- analyses have been performed for cyclones conforming dimensional tubes with well-de®ned boundaries. In- to the evolution described by Shapiro and Keyser (1990) stead, Wernli (1997) described airstreams as coherent (e.g., Kuo et al. 1992; Reed et al. 1994; Browning and ¯exible tubes that evolve over time rather than being Roberts 1994), split fronts (e.g., Browning and Monk steady-state entities. Mass and Schultz (1993), Reed et 1982; Young et al. 1987; Kuo et al. 1992; Mass and al. (1994), and Bader et al. (1995, p. 213) showed that Schultz 1993), cyclones affected by topography (e.g., the three-conveyor-belt model may be an oversimpli- Hobbs et al. 1990, 1996; Steenburgh and Mass 1994; ®cation of the air¯ow through some cyclones because Bierly and Winkler 2001), and cyclones in different more than three airstreams may be needed to describe large-scale ¯ow regimes (e.g., Evans et al. 1994; Bader the variety of individual air-parcel trajectories. Grotjahn et al. 1995, section 5.2). In addition, with the increasing and Wang (1989) showed that surface ¯uxes of heat and availability of mesoscale models, calculating air-parcel moisture can modify the air¯ow through marine cy- trajectories has become a more accurate method than clones; speci®cally, the cold lower-tropospheric air from storm-relative isentropic analysis to envision the air¯ow the downstream anticyclone (i.e., cold conveyor belt air)

Unauthenticated | Downloaded 09/23/21 07:12 PM UTC 2210 MONTHLY WEATHER REVIEW VOLUME 129 can gain heat and moisture by traveling over warmer synoptic-scale cyclones. For example, an air¯ow model water, thus becoming ``warm conveyor belt air,'' a pro- by Lemon and Doswell (1979), a revision of an earlier cess recognized by Shaw (1911, p. 213). Some studies such model developed by Browning (1964), was used of marine cyclones were unable to identify a well-de- to represent air¯ow in supercell thunderstorms. Al- ®ned cold conveyor belt (e.g., Reed et al. 1994; Brown- though not described as such originally, the air¯ow ing et al. 1995) or found no sharp boundary between models used by Browning (1964) and Lemon and Do- the warm and cold conveyor belts (e.g., Kuo et al. 1992; swell (1979) for supercells are similar in spirit to the Browning and Roberts 1994). Tung (1990) and Mass conveyor-belt model for midlatitude cyclones (e.g., and Schultz (1993) found no evidence for the anticy- Carlson 1991, section 12.5; Lemon 1998). clonic path of the cold conveyor belt. A ®nal concern Despite the criticisms summarized in this section, the is ambiguity about what the conveyor belts actually rep- conveyor-belt model survives. This paper will focus on resent: streamlines, trajectories, streak lines, or some just a small number of these issues, those related to the combination of the above. cold conveyor belt. In particular, what is the path of the Classically, air¯ow is represented by one of three cold conveyor belt after it passes underneath the warm- methods: streamlines, trajectories, or streak lines. A frontal surface? streamline is a line tangent to the instantaneous velocity of the ¯uid at every point, a trajectory is the path of a 3. The path of the cold conveyor belt material parcel of ¯uid through some time period, and a streak line is a line along which the ¯uid elements Although it is not unreasonable to postulate the cold constituting the line have passed through a particular conveyor belt splitting and taking two different paths, point in space over some time period (e.g., Huschke in much the same manner as the dry airstream fans out 1959). These three are equivalent only when the ¯ow behind the cold front or the warm conveyor belt splits is steady state. when rising over the warm front, evidence described In schematic air¯ow models, it may not be clear that below questions the existence of the cold conveyor belt what is being depicted is uniquely associated with splitting and rising anticyclonically. Additionally, evi- streamlines or trajectories. A steady-state assumption is dence suggests that the bulk of the cold conveyor belt typically invoked implicitly in conveyor-belt studies to remains in the lower troposphere and travels cycloni- allow for the simpli®cation that isentropic streamlines cally around the low center. Four arguments describing can be considered trajectories. The steady-state as- the evidence for these two claims are advanced in this sumption, however, requires that the cyclone is neither section. changing its intensity nor its structure, generally an unlikely prospect. In addition, storm-relative isentropic a. Lack of evidence for the anticyclonic path analysis assumes that the local vertical velocity of the isentropic surface is zero, which may not be the case, Table 1 lists 19 studies in which only the cyclonic particularly in a rapidly evolving situation. Although it path of the cold conveyor belt was identi®ed. Previous is easier to calculate streamlines than trajectories, strict- literature with quanti®able airstreams or calculated tra- ly, streamlines can represent trajectories only approxi- jectories that demonstrate the anticyclonic path, in fact, mately. are rare. Many schematic diagrams of storm-relative is- Whereas numerically calculated trajectories appear to entropic ¯ow exist, but these do not show the actual resolve the dilemma of objectively determining the con- wind observations used to draw such anticyclonically veyor belts, spatial and temporal resolution limits the curving streamlines (e.g., Fig. 5 in Carlson 1980; Fig. accuracy of the calculations. Moreover, not all of the 4 in Kurz 1988; Fig. 12.18a in Carlson 1991; Fig. 8c air¯ow in a cyclone will coincide with these aggrega- in Browning and Roberts 1994). tions of trajectories; it may be possible to ®nd trajec- In all the literature surveyed, only three studies pos- tories that do not belong to any ensemble of trajectories. sess numerically calculated air-parcel trajectories pur- Nevertheless, it is plausible to assume that it is possible ported to follow the anticyclonic path of the cold con- to determine collections of suf®ciently similar trajec- veyor belt. First, trajectory 5 in Whitaker et al. (1988, tories that their aggregation into conveyor belts will both Fig. 18b) undergoes anticyclonic turning as the parcel represent the air¯ow properly and encompass the ma- approaches the low center and rapidly ascends to jet jority of a cyclone's air¯ow. Thus, conveyor belts can level. Hibbard et al. (1989) also present trajectories cal- be considered equivalent to the coherent ensembles of culated from a model simulation of the same storm. In trajectories de®ned by Wernli and Davies (1997) and their Figs. 3 and 4, yellow-colored trajectories (repre- Wernli (1997), separated by the airstream boundaries of senting those that originate in the ``ocean-in¯uenced Cohen and Kreitzberg (1997). For the present study, I planetary boundary layer east of the cyclone,'' which am assuming that the conveyor belts can be represented suggests that they belong to the cold conveyor belt) are by collections of trajectories that follow similar paths. shown, some of which originate in the warm sector and The ambiguity in what air¯ow models represent is rise over the warm front, taking a path similar to that not limited to schematic depictions of air¯ow through 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) Fig. 40 in Kutzbach (1979, p. 182) Surface trajectories Bjerknes (1919) Fig. 6 Schematic Eliassen and Kleinschmidt (1957) Pp. 134±137 and Fig. 35 Storm-relative isentropic streamlines Browning and Harrold (1969) Fig. 2a Storm-relative isentropic streamlines Golding (1984) Fig. 2 Schematic Figs. 5, 6, 9, 10, 13, 14 Calculated trajectories Carr and Millard (1985) Figs. 7, 8 Storm-relative isentropic streamlines Iskenderian (1988) Fig. 5a Storm-relative isentropic streamlines Fig. 5b Isentropic trajectories Fig. 7 Schematic Hibbard et al. (1989) Figs. 3d, 4d Calculated trajectories Stewart and Macpherson (1989) Fig. 4 Storm-relative isentropic streamlines Fig. 9 Schematic Tung (1990) Figs. 4.5, 4.11a, 5.6 Calculated trajectories Kuo et al. (1992) Fig. 8 Calculated trajectories Reed et al. (1992) Pp. 911±912 and Fig. 16 Schematic Fig. 6b Calculated trajectories Cohen (1993) Fig. 2.1 Schematic Mass and Schultz (1993) P. 910 and Figs. 16, 18a, 22 Calculated trajectories Schultz and Mass (1993) Figs. 10, 11a,c, 12a Calculated trajectories Reed et al. (1994) P. 2699 and Fig. 10 Schematic Pp. 2699±2701 and Fig. 11c Calculated trajectories Browning and Roberts (1994) Fig. 8d Storm-relative isentropic streamlines Browning et al. (1995) Figs. 5, 9a Storm-relative isentropic streamlines Browning and Roberts (1996) Fig. 11 Schematic such air parcels originate in the warm sector indicates zone would ascend in such a rapid manner. As remarked that they might best be labeled as belonging to the warm by Shaw (1911, p. 213), ``the cold air of the easterly conveyor belt, not the cold conveyor belt. The second current is very unpromising material out of which to study showing anticyclonic cold conveyor belt trajec- make a rising current.'' If the depiction of the anticy- tories examined model simulations of two oceanic ex- clonic path of the cold conveyor belt is to be taken plosively developing cyclones (Liu 1997). Liu found literally, strong ascentÐperhaps the strongest associated that trajectories from a narrow range of initial altitudes with the cycloneÐmust be present in this region. rose rapidly and turned anticyclonically, carrying much Browning and Mason (1981, p. 579) attribute the ascent of the moisture in the storm. Because these air parcels to frictional convergence in the sheared part of the entrained moisture and sensible heat ¯uxes, they did not boundary layer just ahead of the surface warm front, remain on a moist-isentropic surface, making Liu (1997) but this hypothesis has not been tested rigorously. reluctant about equating them to the cold conveyor belt. Therefore, the process forcing this supposed rapid as- Finally, Kuo et al. (1992, their Figs. 9 and 12) examine cent in the cold conveyor belt has not been elucidated. an oceanic cyclone in which several trajectories begin Golding (1984), Carr and Millard (1985, p. 386), in the lower troposphere, poleward of the warm front, Browning et al. (1995, p. 1245), and Wernli and Davies and rise to the mid and upper troposphere while turning (1997, Fig. 9), among others, show no large ascent as- anticyclonically. In this case, however, strong fronto- sociated with trajectories in the cold conveyor belt. Con- genesis in a region of strong baroclinicity occurs pole- sequently, there appears to be a question about the mag- ward of the original warm front. Thus, it may be argued nitude of the vertical motion in this region of the cold that this air did not originate within the cold air un- conveyor belt. derneath the warm-frontal zone either. These three studies indicate that de®ning what exactly is the cold conveyor belt in certain instances may be problematic (e.g., c. Ambiguity of streamline analyses Grotjahn and Wang 1989). Carlson's (1980) isentropic analysis, reproduced in Fig. 3, is based on a storm-relative streamline analysis b. Unknown mechanism to force ascent of cold stable of the available radiosonde data on the ␪ ϭ 297 K air surface when unsaturated and on the ␪w ϭ 12ЊC surface For the air within the cold conveyor belt to rise to when saturated. The sharp anticyclonic ascent of the the upper troposphere would imply that air would have cold conveyor belt as depicted occurs over Illinois, In- to pass through the stable layer that characterizes the diana, and Michigan, in a region of strong shear of the warm-frontal zone. It seems unlikely that such cold low- horizontal wind on the isentropic surface. Isentropic sur- er-tropospheric air moving underneath the warm-frontal faces within the cold conveyor belt from other storms

Unauthenticated | Downloaded 09/23/21 07:12 PM UTC 2212 MONTHLY WEATHER REVIEW VOLUME 129 also show this region of strong shear (e.g., Fig. 12.18a 310 K) surface, when saturated. Because the mixing in Carlson 1991; Fig. 8 in Browning and Roberts 1994). ratio (and hence ␪w and ␪e) varies horizontally along As discussed by Schaefer and Doswell (1979) and the ␪ ϭ 297 K surface (e.g., mixing ratios vary from Doswell and Caracena (1988), analysis of a vector ®eld 2.0gkgϪ1 at Cape Hatteras, NC, to 15.7 g kgϪ1 at from sparse data can be ambiguous, especially in regions Tallahassee, FL, in Fig. 5), a single value of ␪w/␪e cannot of strong shear. This statement is also con®rmed by be assigned to a given value of ␪. Thus, Fig. 5 avoids Cohen and Kreitzberg (1997, their Fig. 5) who show this ambiguity and is consistent with the more traditional that their integrated contraction rate, a measure of the method of constructing isentropic maps that follow a rate at which nearby air-parcel trajectories approach single dry-adiabatic surface. Nevertheless, the differ- each other, will be large in a region of shear. Conse- ence between these two approaches is reconsidered in quently, interpretating the path of the cold conveyor belt section 6a of this paper. Another difference is that Carl- based solely on isentropic streamline analysis can be son (1980, p. 1502) uses the phase speed of the long risky. wave to determine the storm motion (6.7 m sϪ1 to the east), whereas the present analysis uses the motion of the surface cyclone (13.2 m sϪ1 to the northeast). Since d. Complexity and confusion this paper concerns the cold conveyor belt, a lower- The discrepancy over the path of the cold conveyor tropospheric airstream, it is more appropriate to consider belt leads to potential complexity and confusion. In their the ¯ow relative to a lower-tropospheric feature like the discussion of the variety of cyclogenesis events, Bader center of the surface cyclone rather than a midtropos- et al. (1995, p. 213) state: ``In the cyclogenesis types pheric feature like the long wave. described in this section, it has been found necessary Figure 5 shows the 297-K isentropic surface sloping to introduce a second warm conveyor belt and to reserve upward toward the north: the pressure of the surface the term `cold conveyor belt' for a third conveyor belt, was greater than 900 hPa over the southern United States con®ned to low levels, which appears later in the cy- and less than 400 hPa over much of Minnesota and the clogenesis.'' This complexity leads to potential confu- Dakotas. Relative humidities were greater than 80% sion over airstream classi®cation (i.e., cold conveyor throughout Alabama, Tennessee, and the Ohio River belt vs warm conveyor belt) and under what conditions valley. Storm-relative air¯ow on this isentropic surface the cold conveyor belt clearly exists and is well de®ned. consisted of southeasterlies and easterlies undergoing In hopes of clarifying this situation, the 5 December gentle ascent over the depth of 1000±740 hPa over the 1977 cyclone studied by Carlson (1980) is revisited southeast United States, mid-Atlantic, and upper Mid- from a modern perspective. west. Farther west, northwesterly ¯ow descended down the isentropic surface from 500 hPa in Wyoming, Ne- braska, and western Iowa to 980 hPa over Texas, Lou- 4. Case study: Analysis of observations isiana, and Mississippi. Another branch of the north- To reexamine the storm studied by Carlson (1980), westerly ¯ow curved cyclonically toward the east over an observational analysis is performed ®rst. At 1200 Minnesota, and then spread anticyclonically toward the UTC 5 December 1977 (hereafter, 5/12), a surface cy- south over the Great Lakes, descending and meeting the clone with a central pressure around 986 hPa was cen- southeasterly ¯ow along a con¯uent asymptote. Finally, tered over western Kentucky (Fig. 4a). At 500 hPa, a an approximately closed cyclonic circulation was cen- broad planetary-scale trough was situated over the cen- tered over southern Indiana. tral United States, with embedded shortwave troughs Other than schematic streamlines, Carlson's (1980) located over Minnesota and slightly to the west of the analysis (his Fig. 5 or Fig. 3 in this paper) displays no surface center (Fig. 4b). data and was constructed differently from Fig. 5, so a To illustrate the cold conveyor belt, a storm-relative direct comparison is not possible. In Carlson's analysis, isentropic chart was constructed from the available the low-level easterly ¯ow turns into upper-level south- soundings at 5/12. An average velocity for the surface westerly ¯ow, making the abrupt anticyclonic ascent cyclone center of 13.2 m sϪ1 in the direction of 47Њ from 700 hPa to above 400 hPa over Illinois. In Fig. 5, (northeast) was determined from 5/03 to 5/18. This vec- my analysis, albeit on a different isentropic surface, tor storm motion was subtracted from the total wind shows two adjacent airstreams. None of the upper-air speed at each level for each sounding to determine the data indicate any appreciable ascending motion over storm-relative wind. A dry-isentropic chart along the Illinois. Thus, there is considerable ambiguity in the ␪ ϭ 297 K surface, the same dry-isentropic surface path of the cold conveyor belt that cannot be resolved selected by Carlson (1980, Fig. 5) for the cold conveyor without further information (i.e., section 3c). belt, then was constructed (Fig. 5). Although the storm-relative isentropic method pro- Note that this methodology differs slightly from that vides an easy way to assess streamlines through cy- of Carlson (1980). His storm-relative isentropic chart clones, the usefulness of this method is limited by the was constructed from the ␪ ϭ 297 K surface, when the availability of upper-air data, the steady-state assump-

¯ow was unsaturated, but along the ␪w ϭ 12ЊC(␪e ഠ tion, 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 ЊF. (b) The 500-hPa chart at 1200 UTC 5 Dec 1977. Notation is standard. Temperature and dewpoint depressions are reported in ЊC. fortunately, the sparsity of the sounding stations limits simulation was performed of this case and was diag- unambiguous determination of the path of the cold con- nosed using air-parcel trajectories, which do not require veyor belt in the crucial region of interest northwest of these assumptions. the surface low center. Also, in this case, the storm deepens and evolves from 5/12 to 6/12 and precipitation 5. Case study: Mesoscale model simulation occurs, violating the steady-state and dry-isentropic assumptions. To make a more accurate determination of To examine further the air¯ow through this cyclone the path of the cold conveyor belt, a mesoscale model 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 Ϫ1, respectively). Arrows represent my analysis of streamlines. The legend for the station model is found in the lower right of the ®gure. model simulation was performed. Section 5a describes used to represent subgrid-scale convective precipitation. the mesoscale model used to produce the simulation of Other parameterizations included a multilevel planetary Carlson's case, which is discussed in section 5b. boundary layer (Zhang and Anthes 1982) and a radiative upper boundary condition (Klemp and Durran 1983). a. Model description To create the initial conditions for the model simulation, National Centers for Environmental Prediction (NCEP) The Pennsylvania State University±National Center reanalyses (Kalnay et al. 1996) were interpolated to the for Atmospheric Research ®fth-generation Mesoscale model grid and the integrated mean divergence was re- Model (MM5), a nonhydrostatic, primitive-equation moved to avoid the production of spurious gravity model (Dudhia 1993; Grell et al. 1994), was employed waves. Lateral boundary conditions were generated by to simulate the cyclone. The simulation was initialized linear interpolation of the reanalyses at 12-h intervals. at 0000 UTC 5 December 1977, was ended at 0000 UTC 7 December 1977, and featured 23 variably spaced ter- b. Evolution rain-following-coordinate (␴) levels in the vertical. Here,

␴ ϭ (p Ϫ ptop)/(psfc Ϫ ptop), p is pressure, ptop is the At 12 h into the simulation (5/12), the model placed pressure at model top (100 hPa), and psfc is the surface the surface cyclone center over western Kentucky (Fig. pressure. The horizontal grid spacing was 50 km over 6a), in agreement with the surface analysis from the a domain of 140 by 90 points that was roughly bounded observations at this time (Fig. 4a), although the model's by 20Њ and 60ЊN and by 130Њ and 60ЊW. A grid spacing central pressure was about 5 hPa too high. The modeled of 50 km captures the essence of the cyclone structure 500-hPa ¯ow captured the associated vorticity maxima and evolution, without introducing small mesoscale over Arkansas and Minnesota (cf. Figs. 4b and 6c). At structures that would result in unnecessary detail. Pre- 36 h into the simulation (6/12), the surface cyclone cipitation processes were parameterized using an ex- deepened to 982 hPa with a secondary development plicit-moisture scheme that includes prognostic equa- north of Cape Cod of 987 hPa (Fig. 6b). The observed tions for water vapor, cloud water, and rainwater; cloud centers at this time were 990 and 986 hPa, respectively water and rainwater are assumed to be cloud ice and (not shown). At 500 hPa, the shortwave troughs merged snow if the temperature is below 0ЊC (Dudhia 1989; to form a broad closed low (Fig. 6d). Despite the errors Grell et al. 1994, section 5.3.1.1). The Kain±Fritsch in the central pressures of the primary cyclone (forecast cumulus parameterization (Kain and Fritsch 1993) was 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 ␴ ϭ 0.995 (approximately 40 m above the ground) surface (thin solid lines every 4ЊC). 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 Ϫ5 sϪ1, 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 less, this otherwise favorable comparison between Figs. model with 50-km grid spacing. Therefore, this model 5 and 7 supports proceeding with the analysis. simulation should be adequate for addressing the issues of air¯ow within the cyclone. 6. Trajectory analysis For comparison to Fig. 5, the 297-K storm-relative isentropic surface from the model simulation is pre- To explore the actual air¯ow within the storm, three- sented in Fig. 7. Although the magnitude of the storm- dimensional air-parcel trajectories were computed. The relative wind differs somewhat from the observations accuracy of air-parcel trajectories is limited by factors in some locations (e.g., North Carolina, West Virginia, that include the spatial resolution of the model simu- Illinois, Michigan), the essence of the pattern is the lation, temporal resolution of model output, time step same, illustrating tropospheric-deep descending air over of trajectory calculation, speci®cation of and degree of the central United States, mid- and upper-tropospheric mixing (both in the model and in the real atmosphere), westerly ¯ow across the northern United States and and strength of wind gradients. Using the approach de- southern Canada, and ascending east/southeast ¯ow scribed by Seaman (1987), trajectory calculations were over the southeast United States and Ohio River valley. performed from 60-min model output ®les with a 10- A potentially signi®cant difference occurs along the min time step, values consistent with the results of Doty Great Lakes where observations at Green Bay, Wiscon- and Perkey (1993). sin; Flint, Michigan; and Buffalo, New York, indicate northeasterly ¯ow (Fig. 5), whereas the model simu- a. 1200 UTC 5 December 1977 lation has weak ¯ow from the west or east (Fig. 7). A cross section through the warm-frontal zone shows that To examine ¯ow in the vicinity of the warm front, a such northeasterlies may be found at lower isentropic roughly north±south cross section across the warm front levels (Ͻ290 K) over the same area in a region of strong is constructed (Fig. 8). The 297-K isentropic surface shear within the warm-frontal zone (Fig. 8). Neverthe- (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 sϪ1, 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 storm- began in the warm air equatorward of the warm-frontal relative isentropic analysis. Consistent with Figs. 5 and zone, not in the cold air behind the warm front. In ad- 7, the 297-K isentropic surface lies within southerlies dition, these two trajectories resemble the storm-relative equatorward of the surface warm-frontal position, with- isentropic streamlines depicted in Carlson's (1980, his in easterlies near the leading edge of the warm front Fig. 4) warm conveyor belt. Thus, in the conveyor-belt around 700 hPa, and within westerlies above 600 hPa conceptual model, this strongly rising air would best be (Figs. 5, 7, and 8). When the air is saturated, however, classi®ed as belonging to the warm conveyor belt. ascending ¯ow would not remain on a dry-isentropic Trajectories 3±7 represent storm-relative, westward- surface, but, as indicated by Carlson (1980), would as- moving air that originates in the lower to midtropos- cend more steeply moist adiabatically. Issues about phere (750±600 hPa) and rises anticyclonically to about which moist-adiabatic surface aside (i.e., section 4), 350 hPa, a height somewhat less than that obtained by

Carlson (1980) chose the ␪w ϭ 12ЊC surface (approx- trajectories 1 and 2 (Fig. 9b). These trajectories resem- imately the ␪e ϭ 310 K surface, the thick solid line in ble the anticyclonic path of the cold conveyor belt as Fig. 8). The steepness of the ␪e ϭ 310 K surface means depicted in Carlson's analysis (Fig. 3 in this paper). This that parcels following this surface are far removed from airstream lies within much of the warm-frontal zone the cold air underneath the warm-frontal zone. Thus, along the 297-K isentrope. Finally, trajectories 8±10 the ␪ ϭ 297 K and ␪e ϭ 310 K surfaces are too high represent air that originates at 550±600 hPa, sinks an- to be unequivocally within the cold air underneath the ticyclonically around the downstream anticyclone over warm front (Fig. 8). This fact alone leads to potential New England to 600±750 hPa, and rises as it approaches confusion in interpreting the air¯ow. the cyclone to 650±400 hPa (Fig. 9c). If these trajec- To demonstrate this point, storm-relative forward tra- tories were computed forward from these locations, they jectories starting along the 297-K isentropic surface would cyclonically circle the low center (not shown). within cross section AB at 5/12 and ending at 6/12 are Thus, these trajectories belong to the cyclonic path of computed (Fig. 9). Trajectories numbered 1 and 2 start the cold conveyor belt. in the lower troposphere below 750 hPa and rise anti- If some air parcels beginning on the 297-K isentropic cyclonically during this 24-h period to above 300 hPa surface belong to the warm conveyor belt, then air be- (Fig. 9a). The initial locations of these trajectories in longing to the true cold conveyor belt is likely to be cross section AB (Fig. 8), however, indicate that they 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), ␪e ϭ 310 K (thick solid line), ␪ ϭ 297 K (dashed line), ␪ ϭ 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 Ϫ1, respectively), and vertical velocity (cm sϪ1 shaded according to scale at top right of ®gure). 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 and 24-h forward trajectories are calculated from lo- surface (Fig. 10) show drastically different paths from cations starting within that inset (Fig. 12). Three dif- those on the 297-K isentropic surface. All trajectories ferent types of trajectories are classi®ed within this in- show a cyclonic turning around the low center, char- set: trajectories belonging to the warm conveyor belt, acteristic of the cyclonic path of the cold conveyor belt. the anticyclonic path of the cold conveyor belt, and the To compare trajectories on the ␪ ϭ 297 K surface to cyclonic path of the cold conveyor belt (trajectory be- those on the ␪e ϭ 310 K surface, 24-h forward trajec- ginning locations labeled W, A, and C, respectively). tories beginning at 5/12 are constructed along the ␪e ϭ Like the warm-frontal surface, the boundary between 310 K surface (Fig. 11a). These trajectories break into the warm conveyor belt and the cyclonic path of the some of the same groups previously seen on the ␪ ϭ cold conveyor belt slopes rearward with height and is

297 K surface (Fig. 9). Trajectories 21±23 resemble the approximately coincident with the ␪e ϭ 310 K surface cyclonic path of the cold conveyor belt, whereas tra- (thick solid line in Fig. 12). Above 850 hPa, there are jectories 24 and 25 resemble the anticyclonic path of one or more trajectories at the boundary between the the cold conveyor belt (Fig. 11a). Trajectories 26±30, warm conveyor belt and the cyclonic path of the cold on the other hand, seem to be unrelated to the ascending conveyor belt, which ascend anticyclonically. Between anticyclonic path of the cold conveyor belt trajectories. 700 and 750 hPa, a sharp boundary exists between tra- If these trajectories are taken backward from 5/12 to jectories belonging to the cyclonic and anticyclonic 5/00, this distinction is made more clear (Fig. 11b). paths of the cold conveyor belts (Fig. 12). This boundary Trajectories 25±27, ascending through the midtropo- is caused by a change in the ¯ow ®eld with height: The sphere, transition to upper-tropospheric trajectories ¯ow around the cyclone is closed below 700 hPa, where- 28±30, which ascend about 100 hPa, likely similar to as the ¯ow is characterized by an open wave above 700 the cyclical airstream noted by Whitaker et al. (1988). hPa (not shown). The closed ¯ow below 700 hPa is Thus, two noninteracting airstreams coexist on the same more likely to feature cold conveyor belt trajectories in moist-isentropic surface in the mid- and upper tropo- the cyclonic circulation around the low, whereas tra- sphere. jectories turning anticyclonically, not in the closed cir- To explore further the trajectories within the vicinity culation, dominate above 700 hPa. of the warm front, an inset from Fig. 8 is constructed 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 ␪ ϭ 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 ␪ ϭ 297 K and ␪e ϭ 310 K surfaces was fortuitous in the warm conveyor belt and the cyclonic path of the that it lay at the boundary between the warm and cold cold conveyor belt. As the gradient in wind direction conveyor belts and his analysis of the cold conveyor across the frontal zone becomes less well de®ned with belt re¯ected only the anticylonic path. Other choices height, the boundary between the airstreams (i.e., the of isentropic surfaces would have produced analyses zone of anticyclonic-turning trajectories) becomes entirely within either the warm conveyor belt or the broader also. It is interesting to note that the ␪ ϭ 297 cyclonic path of the cold conveyor belt. Therefore, in

K and ␪e ϭ 310 K surfaces lie nearly along the zone this section, calculation of storm-relative trajectories in which the anticylonic path of the cold conveyor belt along isentropic surfaces clari®es the results from Carl- trajectories dominate. Thus, Carlson's choice of the son'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 ␪ ϭ 290 K surface (dashed lines every 100 hPa). b. 1200 UTC 6 December 1977 more substantial warm conveyor belt and cyclonic path Twenty-four hours later (6/12), storm-relative trajec- of the cold conveyor belt. Because this study represents tories belonging to the anticyclonic path of the cold only a single case, however, the generality of this con- conveyor belt do not exist on the ␪ ϭ 297 K surface clusion can be questioned. In this section, we explore anymore (Fig. 13). The absence of the anticyclonic path two aspects of the generality of this result. of the cold conveyor belt at this level is due to the evolving midlevel ¯ow: whereas the 500-hPa ¯ow was 1) THE CYCLONIC PATH OF THE COLD CONVEYOR an open trough at 5/12 (Fig. 6c), by 6/12, the ¯ow BELT AS A TRANSITION ZONE became increasingly closed, with substantial ¯ow around the storm at midlevels (Fig. 6d). The closed Table 1 presents 19 studies that did not ®nd evidence circulation favors the prominence of the cyclonic path of the anticyclonic path of the cold conveyor belt, out- of the cold conveyor belt. weighing the three previous studies described in section Figure 13 also shows that trajectories originating clos- 3a that found anticyclonically turning trajectories (i.e., er to the low center are more likely to curve cyclonically Whitaker et al. 1988; Kuo et al. 1992; Liu 1997). One around the low center than trajectories originating far- explanation for why studies illustrating the cyclonic path ther east of the low center on the same isentropic sur- outnumber those illustrating the anticyclonic path is that face. This observation is consistent with results from the anticyclonic path may be a relatively narrow tran- Market (1996, Fig. 4.65) and Liu (1997, 114±115). sition zone and may be found within a small range of isentropic surfaces in the lower to midtroposphere. Thus, sampling the anticyclonic path with trajectories c. Generality of conclusions or isentropic analyses may be more dif®cult. For ex- In this paper, the anticyclonic path of the cold con- ample, Browning and Roberts (1994, their Fig. 8) show veyor belt is shown to be a transition zone between the storm-relative moist-isentropic streamlines along three

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FIG. 11. Storm-relative trajectories starting on the ␪e ϭ 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 ␪e ϭ 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 anti- Schultz et al. 1998, 1787±1788). In the extreme case cyclonic and cyclonic paths of the cold conveyor belt, where no sharp boundary exists, trajectories may showing that this transition occurs over a ␪w depth of smoothly transition from one regime to another, pro- 3K. hibiting categorization into discrete conveyor belts. As It may be that some cyclones possess a weak anti- discussed by Cohen (1993, section 2.8.5), the so-called cyclonic path of the cold conveyor belt or do not possess polar trough conveyor belt of Browning and Hill (1985) one at all. Tung (1990), Kuo et al. (1992), and Browning and Browning (1990) may be similar to such cases, and Roberts (1994, p. 1552) found that there is no sharp where no sharp boundary between cold air and warm demarcation between the cold conveyor belt and the air exists, merely a zone of weak warm advection. These overlying warm conveyor belt in the cases they ex- transition trajectories likely become more numerous amined. This is likely related to the sharpness of the when the gradient in wind direction associated with the warm front, which, for marine cyclones at the end of warm front is less intense than that during the 5 De- storm tracks (e.g., over the eastern ocean basins), has cember 1977 storm. been characterized as weak, ``stubby,'' or nonexistent That the anticyclonic path of the cold conveyor belt by western U.S. and European meteorologists (e.g., Wal- is a transitional airstream between the warm conveyor lace and Hobbs 1977, p. 127; Friedman 1989, p. 217; belt and the cyclonic path of the cold conveyor belt

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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 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 cy- FIG. 12. Inset from cross section AB (Fig. 8) at 1200 UTC 5 Dec clones in Table 1, which are rapidly developing and 1977. Equivalent potential temperature (thin solid lines every 5 K), well-developed marine cyclones. These results are con- ␪e ϭ 310 K (thick solid line), ␪ ϭ 297 K (dashed line), ␪ ϭ 290 K (dashed±dotted line), and storm-relative winds (one pennant, full sistent with those for the warm conveyor belt, too. For barb, and half barb denote 25, 5, and 2.5 m sϪ1, respectively). Be- example, for open wave cyclones, more of the warm ginning locations of trajectories labeled by type of trajectory: W ϭ conveyor belt ¯ow ascends anticyclonically than cy- warm conveyor belt, A ϭ anticyclonic path of the cold conveyor belt, and C ϭ cyclonic path of the cold conveyor belt. clonically. 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 suggests that the airstream identi®ed as W2, the second the mid- and upper troposphere, suggesting that the warm conveyor belt (e.g., Bader et al. 1995, section storm-relative ¯ow is open. The open ¯ow in these 5.2), may also be the same feature. Airstream W2 is events explains the preference for the ascending anti- described as ``composed of air having a ␪w intermediate cyclonic path of the warm conveyor belt (e.g., Fig. 18b between that of the main warm conveyor belt and the in Whitaker et al. 1988 and Fig. 16 in Kuo et al. 1992). cold conveyor belt. In some cases the origin of W2 is Extending this thought, the large-scale ¯ow in which obvious. However, in other cases, W2 probably origi- the cyclone is embedded is important in controlling the nates near or ahead of the warm front or is the lower path and strength of the cold conveyor belt, and con- part of W1 that curves around the developing low'' veyor belts in general, as discussed by Evans et al. (Bader et al. 1995, p. 213). Airstream W2 also bears a (1994) and Bader et al. (1995, section 5.2). striking resemblance to the cold conveyor belt in the Additional insight can be gained by considering the case presented by Carlson (1987) and the cyclonically cyclone from a storm-relative framework. Because more turning moist airstream of Bierly and Winkler (2001), lower-tropospheric ¯ow 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 ␪ ϭ 297 K surface (dashed lines every 100 hPa).

Unauthenticated | Downloaded 09/23/21 07:12 PM UTC 2222 MONTHLY WEATHER REVIEW VOLUME 129 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), al- lowing signi®cant 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 FIG. 14. Cross section from Des Moines (DSM) to Atlantic City (R2) (cf. Figs. 14a,b). Backward air-parcel trajectories (ACY) at 1200 UTC 5 Dec 1977 (location in Fig. 7). Equivalent from these two maxima of cloud water (not shown) potential temperature (thin solid lines every 5 K), ␪e ϭ 310 K surface demonstrate that these trajectories belonged to the warm (thick solid line), ␪ ϭ 297 K surface (thick dashed line), ␪ ϭ 290 K surface (thick dashed-dotted line), and storm-relative winds (one conveyor belt. Whereas Carlson's results showed that pennant, full barb, and half barb denote 25, 5, and 2.5 m s Ϫ1, re- the clouds and precipitation originated within the an- spectively). (a) Cloud liquid water mixing ratio (0.01 g kg Ϫ1, shaded ticyclonic path of the cold conveyor belt, the results according to legend), C1 and C2 represent maxima in cloud liquid from the model simulation show that the clouds and water mixing ratio described in text. (b) Rainwater mixing ratio (0.01 precipitation originated within the ascending warm con- gkgϪ1, shaded according to legend), R1 and R2 represent maxima in cloud liquid water mixing ratio described in text. veyor belt. Given the popularity of the Carlson (1980) charac- terization of the cold conveyor belt, it is likely that some northwest quadrant of cyclones, but contradicts that of forecasters use this concept to forecast precipitation Liu (1997) who showed that as much as 70% of the since, according to Carlson (1980), the so-called wrap- moisture transport in cyclones could be associated with around precipitation in the cloud head is attributed to the cold conveyor belt. Thus, there appears to be some the rising anticyclonic cold conveyor belt. Using this controversy in the literature whether the air that feeds methodology, forecasting bursts of heavier precipitation the cloud head is from the warm or cold conveyor belt would rely on watching for ¯uctuations in the intensity (e.g., Browning and Roberts 1994; Browning et al. of the cold conveyor belt. The results of this paper call 1995; Liu 1997; Dixon 2000, p. 77). Instead, the use- that approach into question because the largest ascent fulness of the cold conveyor belt seems to be in rec- and generation of heavy precipitation occurs primarily ognizing the potential for evaporation of falling hydro- in the warm conveyor belt, not in the cold conveyor meteors from the warm conveyor belt, melting of frozen belt (e.g., Figs. 9 and 14). This result is consistent with hydrometeors (e.g., Kain et al. 2000), or precipitation- that offered by Martin (1999), who expounds on the type forecasting (e.g., rain, snow, sleet, or freezing rain). importance of the cyclonic path of the warm conveyor This is not to say that individual cyclones that form belt (i.e., trowal airstream) to heavy precipitation in the relatively shallow cloud heads may not be caused by

Unauthenticated | Downloaded 09/23/21 07:12 PM UTC SEPTEMBER 2001 SCHULTZ 2223 the cyclonic or anticyclonic paths of the cold conveyor belt in previous literature. Perhaps, in the future, re- belt in the head of the comma cloud, but heavy precip- naming the anticyclonic path of the cold conveyor belt itation rates associated with strong lifting are less likely a transition airstream may be more appropriate and less in such a stable environment. Heavy precipitation rates confusing. are more likely to be caused by deep clouds, perhaps The variability of observed cyclones in different associated with midlevel conditional or moist symmetric large-scale ¯ow patterns (e.g., con¯uence, dif¯uence) instabilities formed by wraparound precipitation gen- has been explored previously by Evans et al. (1994), erated at midlevels by the cyclonic path of the warm Bader et al. (1995), and Schultz et al. (1998). Whereas conveyor belt. It may be that the tendency to identify this study focused on the cold conveyor belt through a the cloud head with the cold conveyor belt in oceanic single cyclone, implications for the air¯ow in other cy- cyclones is due to the tendency for the cold conveyor clones of different strength and shape, and embedded belt, which has a potentially large fetch over the oceans, in different large-scale ¯ows, have been proposed and to be nearly saturated and emerge from underneath the remain to be tested rigorously. polar front cloud band. In fact, this property of the cold conveyor belt raises Acknowledgments. The following individuals are the following forecasting issue. While the warm con- gratefully acknowledged for their contributions to this veyor belt air that arrives at a given point comes from work: Jack Kain and David Stensrud for their assistance the equatorward direction, the cold conveyor belt air using MM5; Mark Stoelinga for providing software to comes from the east. Therefore, an indication of the calculate and display trajectories; Charles Doswell for potential for evaporation or melting of falling precipi- stimulating discussions on streamline analysis and air- tation into the cold air can be obtained from examination stream modelsÐsection 2d is largely derived from those of the properties of the cold air from upstream (i.e., discussions; Toby Carlson for his comments on his orig- more east). For example, if such air is more moist farther inal analysis of the cold conveyor belt; Robert Rabin east, then surface precipitation rates are more likely to for translating portions of Bjerknes (1932); Keith increase with time (assuming a given precipitation rate Browning, James Moore, and Carl Kreitzberg for pro- generated in the warm conveyor belt). On the other viding relevant references; and Richard Grotjahn for hand, drier air being advected from farther east may providing his thoughts on Grotjahn and Wang (1989). indicate a lessening of the precipitation rate at the sur- Toby Carlson, Robert Cohen, Charles Doswell, Jack face. This is in contrast to the air in the warm conveyor Kain, John Locatelli, Anthony Lupo, Patrick Market, belt, for which an understanding of its properties before James Steenburgh, David Stensrud, Mark Stoelinga, ascent comes from farther equatorward in the warm Heini Wernli, and two anonymous reviewers provided sector. valuable comments on earlier versions of this manuscript. 8. Summary I am grateful to the Data Support Section of the Sci- enti®c Computing Division of National Center for At- This study explored the path of the cold conveyor mospheric Research and the National Climatic Data belt through the midlatitude cyclone of 5 December Center for providing data used in this study, the Storm 1977, previously studied by Carlson (1980). Storm-rel- Prediction Center for access to their archives of NMC ative isentropic analysis and air-parcel trajectories cal- maps on micro®lm, and the Mesoscale and Microscale culated from a mesoscale model simulation of Carlson's Meteorology Division of NCAR for their support of (1980) storm showed that both the cyclonic and anti- MM5. cyclonic paths of the cold conveyor belt existed. The anticyclonic path represented a transition between the two more substantial airstreams in the lower tropo- REFERENCES sphere: the warm conveyor belt and the cyclonic path of the cold conveyor belt. This transition zone began in Bader, M. 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