An Observational Study of Cold Season–Banded Precipitation in Northeast U.S. Cyclones

David R. Novak Scientific Services Division, National Weather Service Eastern Region, Bohemia, New York

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Lance F. Bosart The University at Albany, State University of New York, Albany, New York

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Daniel Keyser The University at Albany, State University of New York, Albany, New York

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Jeff S. Waldstreicher Scientific Services Division, National Weather Service Eastern Region, Bohemia, New York

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Abstract

A climatological and composite study of banded precipitation in the northeast United States during the cold season (October–April) is presented. Precipitation systems in the northeast United States in April 1995 and from October 1996 to April 2001 that exhibited greater than 25.4 mm (1 in.) of rainfall, or 12.7 mm (0.5 in.) liquid equivalent, were identified as cases for study. A total of 111 cases were identified during this period, of which 88 had available radar data. Of these cases, 75 exhibited banded structure whereas 13 did not. A band classification scheme was developed from a subset of study cases. Application of the classification scheme to the 88 cases revealed that banded cases can exhibit a variety of banded events during their evolution. Single-banded events were the most common (48), followed by transitory (40), narrow cold frontal (36), multi (29), and undefined (9). Further investigation of the single-banded events highlighted banded structure in the comma-head portion of storms, with 81% of these events exhibiting a majority of their length in the northwest quadrant of the surface cyclone.

Composites were calculated for cases exhibiting single-banded events in the northwest quadrant of the surface cyclone and for nonbanded cases to distinguish synoptic and mesoscale flow regimes associated with banded events and nonbanded cases. The banded composite was marked by cyclogenesis and the development of a closed midlevel circulation. This flow configuration was associated with deformation and strong midlevel frontogenesis northwest of the surface cyclone center, which coincided with the mean band position. The nonbanded composite exhibited a much weaker cyclone located in the confluent entrance region of an upper-level jet. The absence of a closed midlevel circulation in the nonbanded composite limited deformation and associated frontogenesis northwest of the surface cyclone. Cross-section analysis through the respective composite frontogenesis maxima showed that the banded composite frontal zone exhibited stronger and deeper frontogenesis and weaker conditional stability than the nonbanded composite frontal zone.

Case studies from the northeast United States confirm the composite results, highlighting the importance of deep-layer frontogenesis coincident with weak conditional stability. These results are in qualitative agreement with the Sawyer–Eliassen equation, which predicts that the frontogenetical response will be enhanced (reduced) in the presence of small (large) moist symmetric stability.

Corresponding author address: David Novak, NOAA/NWS Eastern Region Headquarters, 630 Johnson Ave., Bohemia, NY 11716. Email: david.novak@noaa.gov

Abstract

A climatological and composite study of banded precipitation in the northeast United States during the cold season (October–April) is presented. Precipitation systems in the northeast United States in April 1995 and from October 1996 to April 2001 that exhibited greater than 25.4 mm (1 in.) of rainfall, or 12.7 mm (0.5 in.) liquid equivalent, were identified as cases for study. A total of 111 cases were identified during this period, of which 88 had available radar data. Of these cases, 75 exhibited banded structure whereas 13 did not. A band classification scheme was developed from a subset of study cases. Application of the classification scheme to the 88 cases revealed that banded cases can exhibit a variety of banded events during their evolution. Single-banded events were the most common (48), followed by transitory (40), narrow cold frontal (36), multi (29), and undefined (9). Further investigation of the single-banded events highlighted banded structure in the comma-head portion of storms, with 81% of these events exhibiting a majority of their length in the northwest quadrant of the surface cyclone.

Composites were calculated for cases exhibiting single-banded events in the northwest quadrant of the surface cyclone and for nonbanded cases to distinguish synoptic and mesoscale flow regimes associated with banded events and nonbanded cases. The banded composite was marked by cyclogenesis and the development of a closed midlevel circulation. This flow configuration was associated with deformation and strong midlevel frontogenesis northwest of the surface cyclone center, which coincided with the mean band position. The nonbanded composite exhibited a much weaker cyclone located in the confluent entrance region of an upper-level jet. The absence of a closed midlevel circulation in the nonbanded composite limited deformation and associated frontogenesis northwest of the surface cyclone. Cross-section analysis through the respective composite frontogenesis maxima showed that the banded composite frontal zone exhibited stronger and deeper frontogenesis and weaker conditional stability than the nonbanded composite frontal zone.

Case studies from the northeast United States confirm the composite results, highlighting the importance of deep-layer frontogenesis coincident with weak conditional stability. These results are in qualitative agreement with the Sawyer–Eliassen equation, which predicts that the frontogenetical response will be enhanced (reduced) in the presence of small (large) moist symmetric stability.

Corresponding author address: David Novak, NOAA/NWS Eastern Region Headquarters, 630 Johnson Ave., Bohemia, NY 11716. Email: david.novak@noaa.gov

1. Introduction

Mesoscale band formation in extratropical cyclones can dramatically affect the intensity, timing, and subsequent accumulation of precipitation. The effects of mesoscale bands are especially evident during the cold season, when snowfall associated with the bands can produce localized “white out” conditions embedded within relatively lighter snowfall, resulting in extreme snowfall gradients. Such mesoscale characteristics have made the diagnosis and prediction of precipitation bands challenging.

Numerous theoretical and observational studies (e.g., Thorpe and Emanuel 1985; Sanders and Bosart 1985; Moore and Lambert 1993; Nicosia and Grumm 1999) have established that mesoscale bands are primarily forced by frontogenesis in the presence of small moist symmetric stability. Although conditional symmetric instability (CSI; Bennetts and Hoskins 1979) has often been cited as a condition for banded precipitation, the theoretical formulation of CSI and its observed coexistence with frontogenetical forcing and banded elevated upright convection (e.g., Reuter and Yau 1990; Martin 1998a,b) often make operational diagnosis difficult (Schultz and Schumacher 1999). Whether or not pure CSI is found within cases becomes secondary to the fact that frontogenesis in the presence of small moist symmetric stability can result in a focused band of ascent, as shown in numerical simulations by Thorpe and Emanuel (1985) and Emanuel (1985).

Several case studies in the northeast United States have documented the occurrence of mesoscale banding events associated with frontogenesis and small moist symmetric stability. Sanders and Bosart (1985), in a case study of an intense snowband along the northeast U.S. coast, found that the region of banding underwent frontogenetical forcing in the presence of small moist symmetric stability, resulting in a narrow, intense updraft located on the equatorward flank of the frontogenesis maximum. Sanders (1986), in a case study of another major northeast U.S. snowstorm, found that banding observed in the comma head portion of the storm coincided with strong frontogenetical forcing and small moist symmetric stability. In a study of three northeast U.S. banded snowstorms, Nicosia and Grumm (1999) found the snowbands observed in their cases were coincident with frontogenesis within a deep layer of small moist symmetric stability.

Many of the bands in the above case studies were observed in the comma head portion of cyclones. Martin (1998a,b), in a case study of a central U.S. cyclone, demonstrated that deformation and warm air advection within the trough of warm air aloft [trowal; defined as the axis of highest potential temperature (θ) ahead of the upper cold front (Godson 1951; Martin 1998b)] in a developing cyclone may contribute to frontogenesis in the comma head portion of cyclones. Nicosia and Grumm (1999) showed that deformation found north of the developing midlevel cyclone contributed to strong midlevel frontogenesis, which coincided with the snowbands observed in their cases. Nicosia and Grumm (1999) also showed that the juxtaposition of moist and dry airstreams just northwest of the surface cyclone center could reduce the moist symmetric stability. These results suggest that the evolution of the synoptic and mesoscale flow environment may influence mesoscale band formation.

Although there have been numerous case studies concerning mesoscale precipitation bands, there have been relatively few observational climatologies (Schultz and Schumacher 1999). Houze et al. (1976), in a radar survey of 11 maritime cyclones affecting western Washington state, identified and defined six types of rainbands (warm frontal, warm sector, cold frontal wide, cold frontal narrow, wave, and postfrontal), providing a contemporary band classification scheme. Byrd (1989), in a composite study of precipitation bands in the southern plains utilizing eight Weather Service Radar-1957 (WSR-57) sites and sounding observations, found that the nonbanded composite sounding exhibited a colder and more stable boundary layer, with less midlevel shear than the strongly banded composite sounding. Reuter and Yau (1990), as part of the Canadian Atlantic Storms Program (CASP; Stewart 1991), consistently found a frontal environment marked by slantwise neutrality in the region of seven precipitation bands observed in Atlantic Canada.

Although the foregoing investigations have provided insight into the observed frequency, variety, and characteristic frontal environments of mesoscale bands, they have been limited by the number of cases studied and a lack of observational data on the scale of banded precipitation structures. With the implementation of the National Weather Service (NWS) Weather Surveillance Radar-1988 Doppler (WSR-88D) national radar network (Klazura and Imy 1993) the coverage and sensitivity of radar observations in the United States have markedly improved, providing an opportunity to build large databases of banded cases for study. Such databases, coupled with mesoscale gridded datasets, facilitate investigation of common band environments and associated synoptic flow evolutions.

This study uses WSR-88D observations during five cold seasons to establish a climatology of banded events in the northeast United States. The resulting case dataset is used to compare the characteristic frontal environments associated with banded and nonbanded cases through composite analysis. The composites are further used to illustrate how the synoptic flow evolution influences cyclone substructure. This component of the study builds on the results of Nicosia and Grumm (1999) by placing mesoscale banding into the context of the synoptic flow evolution.

This paper is organized as follows. Section 2 describes the datasets and methodologies employed to develop the band climatology and composite study. Section 3 presents the climatology results, documenting the frequency and variety of banded features observed in the northeast United States. Section 4 examines the common synoptic and mesoscale dynamics responsible for band formation through composite analysis. Case study analyses are provided as illustrations of the composite results in section 5, and section 6 provides a discussion of the climatology, composite, and case study results. Summary remarks are presented in section 7.

2. Data and methodology

a. Data sources

The Unified Precipitation Dataset (UPD; Higgins et al. 1996) was utilized to identify cases exhibiting heavy precipitation in the northeast United States. This national dataset incorporates National Oceanic and Atmospheric Administration (NOAA) first-order station precipitation measurements, daily cooperative observer measurements, and data from NWS River Forecast Centers (RFCs), representing over 13 000 stations in the contiguous United States. Precipitation amounts represent 24-h accumulation ending at 1200 UTC, interpolated to a 0.25° latitude–longitude grid.

The NOAA Daily Weather Maps (DWMs) weekly series (NOAA 1995–2001) were used to corroborate the UPD measurements and determine the predominant precipitation type (liquid/frozen). This publication provides daily North American surface frontal analyses, surface snow cover, and 500-hPa geopotential height maps valid at 1200 UTC each day. In addition, temperature maxima and minima, and precipitation for the 24-h period ending 1200 UTC are plotted for over 120 stations in the United States.

Mosaic radar data used in this study were composed of reflectivity returns from the 0.5° elevation scan from each radar site, with 2-km spatial and 5-min temporal resolution. Archived data were obtained from the Cooperative Program for Operational Meteorology, Education and Training (COMET) and were available in April 1995 and then from October 1996 to April 2001, although a large portion of the 1996/97 winter season was missing.

The National Centers for Environmental Prediction– National Center for Atmospheric Research (NCEP– NCAR) reanalysis (Kalnay et al. 1996; Kistler et al. 2001) was utilized for the initial composite investigation. This global dataset has a 2.5° latitude–longitude spatial resolution and 6-h temporal resolution. Locally archived NCEP Eta Model (Black 1994) analyses and 6-h forecast fields served as a higher-resolution composite and case study dataset. Although the Eta Model resolution and physics changed during the course of the study period (see http://www.emc.ncep.noaa.gov/mmb/ research/eta.log.html for a list of changes), the Eta Model was chosen since it had a relatively fine resolution, was available for the entire study period, and is a current operational forecast model. Model grids were remapped to the CONUS 211 grid (Lambert conformal projection) with 80-km horizontal resolution and 50-hPa vertical resolution, providing gridded fields that yield smooth diagnostic calculations.

b. Methodology

1) Climatology

The area of study was defined as the latitude–longitude box covering 36.5°–50°N and 65°–85°W. Although a portion of southeast Canada was included in the domain, the study was restricted to data from the U.S. radar network coverage (Maddox et al. 2002). Only cold season (October–April) precipitation systems exhibiting amounts greater than 25.4 mm (1 in.) rain or 12.7 mm (0.5 in.) liquid equivalent in the case of frozen precipitation, during a 24-h period at a point in the study domain were examined. Precipitation systems meeting these criteria were defined as “cases.” On three occasions, two cases were observed within the study domain at the same time. On these occasions both cases were assessed independently.

The UPD was used to identify cases in April 1995 and then from October 1996 to December 1998 (when the dataset ended). A comparison between the UPD and the DWM precipitation reports showed that all cases identified by using the DWM were captured in the UPD, while only a few cases identified in the UPD were not captured in the DWM. This result supported use of the DWM for identifying cases for the remaining study period (January 1999 to April 2001). A total of 111 cases met the precipitation criteria during the study period, of which 88 cases had available mosaic radar data (termed “study cases”; Fig. 1).

Radar data from study cases during October 1996– April 1998 (45 study cases) were reviewed to sample the variety of banded structures observed in the northeast United States. Although some meso-α scale (200– 2000 km, >6 h; Orlanski 1975) elongated precipitation areas may be considered bands, this study focuses on bands on the meso-β scale (20–200 km, <6 h). Based on this radar review, previous classification schemes in the literature (e.g., Browning and Harrold 1969; Houze et al. 1976; Byrd 1989) and consultation with operational forecasters, a subjective classification scheme was developed.

The resulting classification scheme (Table 1) defines single, multi-, narrow cold-frontal, and transitory banded structures. A single-banded structure is defined as a linear reflectivity feature 20–100 km in width and greater than 250 km in length. The banded structure must maintain a minimum intensity of 30 dBZ along a majority of its length for at least 2 h. These criteria are similar to the 3:1 aspect ratios used by Houze et al. (1976) and the 0.5–3-h lifetime criterion used by Byrd (1989). Figure 2a provides an example of a single band shown on mosaic radar imagery.

The multibanded structure is defined as a region exhibiting more than three finescale bands with similar spatial orientation and periodic spacing. The finescale bands are defined as having widths between 5 and 20 km, spaced no greater than 40 km apart, and having intensities greater than 10 dBZ over the background reflectivity. This structure must be maintained for at least 2 h.

The narrow cold-frontal band (Houze et al. 1976) for this study is defined as a narrow (10–50 km wide), long (>300 km in length) band found along the surface cold front or in the warm sector of a cyclone, which maintains an intensity of at least 40 dBZ along its length for at least 2 h. The definition used here is deliberately more general than that used by Houze et al. (1976) in order to incorporate warm sector bands (Browning and Harrold 1969; Houze et al. 1976) into the category. The narrow cold-frontal structure is differentiated from the single-banded structure based on its location relative to the cyclone (equatorward versus poleward), intensity (40 versus 30 dBZ), and minimum width (10 versus 20 km).

Transitory bands are defined as a structure that meets all respective criteria in a given category, except one. In most cases either the lifetime or intensity criterion is not met. Transitory bands are analogous to the weakly banded class defined by Byrd (1989), allowing for a full continuum of banding. Banded structures that were ambiguous due to bright banding or incomplete radar data were classified as undefined.

The occurrence of a banded structure as defined above was termed an “event” (Fig. 1). Note that several cases exhibited more than one type of banded event during their evolution. Cases that did not exhibit a banded event during their evolution were termed “nonbanded cases.” Figure 2b provides an example of a mosaic radar image from a nonbanded case.

2) Composites

Composite analyses were created for single- and multibanded events and nonbanded cases (Fig. 1). In order to develop composite analysis stratifications, a surface cyclone-relative composite of the single band distribution was developed. Surface cyclone positions at the analysis time (0000, 0600, 1200, and 1800 UTC) most representative of the banded structure were determined by utilizing the NCEP–NCAR reanalysis dataset 1000-hPa height fields (henceforth used as a surrogate for the sea level pressure field), and the observed band location at this time was determined from radar. Note that all events exhibited an identifiable surface low at their respective analysis time. For occurrences of secondary cyclogenesis, the surface low closest to the banded structure was used as the surface cyclone position.

The single-banded events were stratified into two composite classes according to the surface cyclone-relative distribution. Events in which the single band exhibited a majority of its length in the northwest quadrant relative to the surface cyclone were classified as the northwest class (Fig. 1). This class was subdivided into events where the bands exhibited a majority of their length within (northwest near) and beyond (northwest far) 500 km of the surface cyclone. Events in which the single band exhibited a majority of its length in the northeast or southeast quadrant relative to the surface cyclone were classified as the east class. One event could not be clearly identified as a northwest or east class event, and was included in both composite classes. Also, a few study cases exhibited bands in more than one quadrant of the cyclone, and therefore were included in more than one composite class. Study cases that did not exhibit any type of banding were defined as nonbanded cases, and were included in the nonbanded composite class (Fig. 1).

The initial analysis time (t = 0 h) for each banded event was defined as the closest NCEP–NCAR reanalysis time (0000, 0600, 1200, and 1800 UTC) to the onset of the event. The initial analysis time for the nonbanded cases was defined as the analysis time closest to the middle of the time period when the case was within the study domain. A program was developed to composite cases in each composite class. This program takes the reanalysis data within a 60° by 120° latitude–longitude box centered on the cyclone position at t = 0 h, and averages fields from all banded events and nonbanded cases included in each composite class. If more than one event occurred at the same time in the same composite class, only one analysis time was included in the composite. The composites were calculated at 6-h intervals from 12 h before (t = −12 h) to 12 h after (t = +12 h) the initial analysis time (t = 0 h).

The composite program was modified to incorporate the resolution and domain of the Eta Model grids. Because the archived CONUS 211 grid has an eastern boundary roughly along 55°W longitude, a new cyclone-centered data box approximately 2500 km by 2500 km in size was defined to accommodate the grid boundaries. If any portion of a particular event data box went off the Eta Model domain, the specific grid points included in that portion were not included in the composite calculation. This contingency was only an issue on the eastern edge of the composite, where the lack of data resulted in noisy composite fields. The NCEP– NCAR reanalysis composite data were used to guide subjective smoothing near the eastern edge of the model composite figures shown in this paper. The centroid position of each composite class at t = 0 h was used as geographical reference for displaying the resulting composite fields.

The General Meteorology Package (GEMPAK; desJardins et al. 1991) version 5.6 was used to calculate kinematic parameters and visualize the various gridded datasets. The simplified 2D form of the Miller (1948) frontogenesis equation,
i1520-0434-19-6-993-e1
was used to examine the frontogenetical effect of the horizontal wind. This simplification was made since the frontal zones included in this study were nearly saturated, so that the tilting and diabatic terms oppose one another in the updraft region of a thermally direct frontogenetical circulation.

3. Climatology

a. Application of classification scheme

The band classification scheme (Table 1) was applied to the 88 study cases to document the observed mesoscale band type and frequency in the northeast United States, and to build a database of banded events to study. The results of this application are summarized in Table 2. Note that several cases exhibited a particular band type more than once, and/or more than one band type during the time period when the case was within the study domain. Since a given case can feature multiple events, the total number of events (162) is nearly double the total number of cases (88).

The most common event type observed in the northeast United States was the single (48 events), followed by the transitory (40), narrow cold frontal (36), and multi- (29). Nine events were classified as undefined. The large number of transitory events reveals that banded structure is often observed that does not maintain its identity for more than 2 h. Additionally, the predominance of single-banded events over narrow cold-frontal events reflects a relative lack of narrow cold-frontal events observed in the northeast United States during the cold season as compared with the Pacific Northwest (Houze et al. 1976) and the United Kingdom (Browning 1985). This finding also agrees with the relative proportion of single bands compared to narrow cold-frontal bands found in adjacent Atlantic Canada (Stewart 1991).

Not all cases exhibiting heavy precipitation in the northeast United States exhibit mesoscale banding. A total of 13 study cases were classified as nonbanded, representing ∼15% of the study cases. This subset of cases serves as an important dataset to be used to discriminate between banded and nonbanded cases.

b. Single-band characteristics

The geographical distribution of the 48 single-banded events is shown in Fig. 3a. Although only five cold seasons were studied, single-banded events were distributed relatively evenly over the northeast United States. The surface low positions corresponding to the analysis time of the 48 single-banded events are shown in Fig. 3b. Although the low centers represent a single time, the clustering of centers off the East Coast highlights the dominant coastal storm track, with evidence of a secondary storm track apparent in the Midwest.

The surface cyclone-relative single-band distribution is shown in Fig. 4. The predominance of bands in the northwest quadrant is readily evident. Thirty-nine of the 48 single-banded events (81%) exhibited a majority of their length in this quadrant. Also note that bands to the northeast of the surface cyclone exhibited a northwest– southeast mean orientation, while bands to the northwest exhibited a southwest–northeast mean orientation. These orientations are consistent with the idealized orientation of the thermal wind in a developing baroclinic system (an “S”-shaped thermal structure).

Analysis of the lifetimes of single-banded events (Table 3) shows that the events are generally short lived, as the frequency of events rapidly decreases with increasing event duration. Several events just barely met the 2-h minimum, consistent with the large number of transitory bands observed (Table 2). This outcome suggests that the conditions favoring single-band formation occur on limited time scales. However, a number of events persisted beyond 12 h, the longest of which lasted 22.5 h (4–5 February 1998).

Radar animations of the single-banded structure show that the evolution may locally exhibit short-lived (0.5 h) finescale bands (widths < 20 km, intensities > 10 dBZ over the background reflectivity) that reach maturity in place, dissipate, but then are readily replaced by another finescale band in the same location. This observation is consistent with the theoretical work of Xu (1992), who showed through idealized numerical modeling that large-scale ascent can evolve into multiple smaller-scale updrafts when the geostrophic saturation equivalent potential vorticity (EPV*g) is reduced to negative values. In this environment, bands can exhibit finescale multibanded structures that stretch and merge into the dominant single band of moist ascent. In this case the single-banded structure may be considered a dominant band axis, or a band accumulation zone, rather than one single band. In fact, on 10 occasions there was interaction between single- and multibanded events in which individual bands in the region of multibanding merged with the single band for at least 2 h.

Radar animations of northwest class bands also revealed that many of the bands pivoted as they translated with the system. In fact, 24 of the 39 (62%) single-banded events in the northwest class were associated with an identifiable pivot point, with the band typically pivoting cyclonically. Radar animations of the east class bands showed a common motion approximating the velocity of the low- to midtropospheric wind (often toward the northeast).

4. Composite results

a. Composite evolutions

The composite dataset allows examination of the cyclone evolution in each composite class through a 24-h period. For the purposes of this paper, only the northwest and nonbanded composite results will be shown at 12-h intervals. Results of the east, multibanded, northwest-near, and northwest-far composites are shown in Novak (2002, his section 3.2).

1) Northwest composite

The results of the northwest composite at t = −12 h are summarized in Fig. 5. A surface cyclone is found in the Carolinas (Fig. 5a) associated with a 500-hPa trough (Fig. 5b). There is evidence that the cyclone is deepening since the disturbance tilts toward the west with height, and progressively exhibits a more open structure with height (Figs. 5a–c). A strong 300-hPa jet is found at the base of the trough, and another 300-hPa jet is found in an area of confluence off the northeast U.S. coast (Fig. 5b). This pattern resembles the common jet configuration that Kocin and Uccellini (1990, 58– 62) cited for northeast U.S. snowstorms. The 700-hPa flow features a broad closed low over the Ohio River Valley (Fig. 5c). At this time only a small east–west-oriented band of 750–650-hPa layer-averaged (midlevel) frontogenesis is found across southern New York (Fig. 5c). This band of frontogenesis is located within a broad and weak deformation zone (Fig. 5c; note deformation values are just under the 3 × 10−5 s−1 threshold) on the northern fringe of the midlevel warm air advection (Fig. 5d).

Twelve hours later at t = 0 h (Fig. 6) the composite surface cyclone has deepened approximately 50 m and moved northeast (Fig. 6a) as the 500-hPa trough becomes negatively tilted (Fig. 6b). The presence of warm air advection (Fig. 6a), the implied upward increase in cyclonic vorticity advection (Figs. 6a–c), and the double jet structure (Fig. 6b) provide support for large-scale ascent over the northeast United States. Consistent with cyclogenesis, the closed low at 700 hPa has deepened 60 m (Fig. 6c). The development of a deep midlevel low establishes a well-defined midlevel deformation zone (outlined by the 3 × 10−5 s−1 dotted contour) that is associated with a confluent asymptote over Pennsylvania and western New York. This deformation acting on the thermal gradient contributes to midlevel frontogenesis found north and northwest of the surface low (Fig. 6c). A complementary interpretation is that the midlevel frontogenesis has developed northwest of the cyclone in response to increasing warm air advection (which has doubled in value over the previous 12-h period; Figs. 5d and 6d) and associated warm air advection gradient, with the maximum midlevel frontogenesis located on the poleward side of the warm air advection maximum. Note that the mean band position is located on the equatorward flank of the midlevel frontogenesis maximum centered over Pennsylvania (Fig. 6d).

During the next 12 h the composite surface cyclone continued to deepen as it moved northeast along the northeast U.S. coast. At t = +12 h a closed circulation extended nearly to 500 hPa (Figs. 7a–c) as the cyclone became nearly vertically stacked. Large-scale forcing for ascent has shifted eastward, as implied by the location of the thermal and vorticity advections (Figs. 7a,b) and the position of the double jet structure (Fig. 7b). The 700-hPa low has also deepened and broadened, displacing the primary deformation zone to the northeast quadrant of the cyclone (Fig. 7c). Similar to the composite fields 12 h earlier, midlevel frontogenesis was found within this deformation zone (Fig. 7c) along the poleward side of midlevel warm air advection (Fig. 7d); however, in accordance with the northeastward shift of the dynamics, frontogenesis is nearly absent in the northwest quadrant of the cyclone.

2) Nonbanded composite

Not all cyclones associated with heavy precipitation in the northeast United States exhibit mesoscale banding. These cases serve as a type of null case, highlighting the synoptic and meso-α-scale features important in the formation of mesoscale bands. The composite of the 13 nonbanded cases at t = −12 h is shown in Fig. 8. A weak surface low is found over Tennessee (Fig. 8a). This surface low is associated with a weak (∼12 × 10−5 s−1) 500-hPa vorticity maximum (Fig. 8b) embedded in the confluent entrance region of a 40 m s−1 300-hPa jet (Fig. 8b). Midlevel confluence ahead of the surface cyclone contributes to weak deformation (Fig. 8c) and subsequent frontogenesis, located in an elongated band across southern New York along the poleward side of a broad area of warm air advection (Fig. 8d).

Unlike the northwest (Figs. 5–7) and east (see Novak 2002, his section 3.2) composites, little development occurs over the next 12-h period (Fig. 9a). This observation can be understood by noting that a confluent entrance region of the 300-hPa jet is anchored over the northeast United States (Fig. 9b) as southwesterlies ahead of the short-wave disturbance merge with westerlies across southern Canada. Downstream ridging ahead of the short-wave disturbance is limited by the position of the jet (Fig. 9b), reducing vorticity advection and subsequent surface development [1000-hPa height barely below 90 m (Fig. 9a)]. Despite this lack of development, midlevel confluence contributes to frontogenesis northeast of the surface cyclone (Fig. 9c) that is comparable to the magnitude calculated in the northwest composite at t = 0 h (Fig. 6c). The placement of the frontogenesis maximum is consistent with the expected location in the equatorward entrance region of the jet, and is found on the poleward side of a broad region of warm air advection (Fig. 9d). Note that a 700-hPa low has not formed, reducing the likelihood of significant deformation and frontogenesis northwest of the surface cyclone (Figs. 9c,d).

Twelve hours later (t = +12 h) an elongated surface cyclone is found off the mid-Atlantic coast (Fig. 10a) as the 500-hPa trough is sheared while it moves into the confluent entrance region of the upper-level jet (Fig. 10b). Midlevel confluence (Fig. 10c) and warm air advection (Fig. 10d) still contribute to midlevel frontogenesis well ahead of the surface cyclone.

b. Composite cross sections

A comparison of the composite frontal environments can be made by cross-sectional analysis. Note that the lengths (∼2000 km) and frontal baroclinicity were nearly equal among the northwest and nonbanded class composite cross sections, facilitating comparisons of key features and frontal slope.

The northwest class composite cross section (orientation is shown in Fig. 6d) and nonbanded composite cross section (orientation is shown in Fig. 9d) at t = 0 h are presented in Figs. 11a,b and Figs. 11c,d, respectively. Moist symmetric stability is assessed in Figs. 11a,c by evaluation of the full wind saturation equivalent potential vorticity (EPV*). The full wind was used as opposed to the geostrophic wind in the calculation of EPV* since the full wind is likely more representative of the curved flow noted in the composites, although the applicability of the full wind to the assessment of moist symmetric stability in three-dimensional flow configurations continues to be debated (e.g., Schultz and Schumacher 1999, their section 3; Gray and Thorpe 2001; Clark et al. 2002). Note that the mean band position is centered in the cross section in Figs. 11a,b, whereas the center of the nonbanded cross section corresponds to the 750–650-hPa frontogenesis maximum.

The northwest class composite cross section shows that the mean band position is embedded within a sloping frontal zone (Fig. 11a). Examination of the saturation equivalent potential temperature (θes) and EPV* fields reveals that this front separates a conditionally unstable atmosphere on the equatorward side from a conditionally stable atmosphere on the poleward side. However, it is important to note the existence of a layer of small EPV* [i.e., values < 0.25 PVU (potential vorticity unit)] between 550 and 350 hPa in the center of the cross section, which is coincident with the mean band position. Although a large portion of this area is unsaturated (Fig. 11b), the portion directly above the band position exhibits relative humidity values near 80%, indicating the presence of small moist symmetric stability in this location. Frontogenesis is readily apparent in the cross section (Fig. 11a), with the maximum vertically aligned between 800 and 600 hPa and collocated with the mean band position under the area of small EPV*. As shown by Emanuel (1985), the frontogenetical response in the presence of small moist symmetric stability takes the form of a concentrated updraft on the equatorward flank of the frontal zone. Examination of the composite vertical velocity field (Fig. 11b) reveals a narrow ascent maximum with a slight tilt toward the cold air, consistent with Emanuel (1985). The band position is clearly evident in the relative humidity field (Fig. 11b), with a narrow relative humidity maximum correlated with the ascent maximum. The tilt of these two fields toward the cold air is suggestive of slantwise ascent.

The nonbanded composite cross section (Figs. 11c,d) exhibits similar features to the northwest composite cross section, but differences are apparent. Although a region of midlevel conditional instability is found equatorward and above the frontal zone (Fig. 11c), the frontal zone has a shallower slope in the 800–500-hPa layer than in the northwest composite cross section [1:160 versus 1:90; based on considering the slope of the maximum θes gradient (not shown)]. For a given baroclinicity, the shallower sloped frontal zone would imply greater conditional stability within the frontal zone. Although small EPV* values are found above the frontal zone, relative humidities through the entire cross section are near or below 70% (Fig. 11d). Note also that the frontogenesis maximum is not as vertically aligned as in the northwest composite cross section, especially below 600 hPa. The maximum frontogenesis magnitude in the nonbanded cross section [0.4°–0.8°C (100 km)−1 (3 h)−1] is also weaker than the northwest composite [0.8°–1.6°C (100 km)−1 (3 h)−1]. Consistent with weaker frontogenesis and relatively larger conditional stability, the ascent accompanying this frontogenesis is relatively broad and weak, with magnitudes less than −4 × 10−3 hPa s−1 (Fig. 11d).

5. Case studies

A representative single-banded and nonbanded study case are each presented to highlight the primary dynamical differences evident in each of the composites. Synoptic analyses and cross sections are shown at the key analysis times for each study case.

a. 5–6 February 2001

On 5–6 February 2001, a major winter storm produced widespread snow accumulations greater than 30 cm throughout most of New England, with over 75 cm locally in New Hampshire. The large snowfall accumulations were primarily attributed to an intense single band that developed during the storm. Snowfall rates in excess of 10 cm h−1 were observed in the extensive band, producing near white out conditions. Mosaic radar imagery shows the band at 0000 UTC 6 February 2001 (Fig. 2a; hereafter times will be abbreviated as “UTC/ day”).

A synoptic analysis of the event at 0000/6 is shown in Fig. 12. Rapid cyclogenesis occurred during the previous 6 h (700- and 1000-hPa height falls of 120 m) as the 500-hPa trough became negatively tilted (Fig. 12b). A closed circulation is noted from the surface to nearly 500 hPa (Figs. 12a–c), which results in a deep-layer deformation zone associated with a confluent asymptote northwest of the surface cyclone (Fig. 12c), just as in the t = 0 h northwest composite (Fig. 6c). This deformation acting on the thermal gradients results in midlevel frontogenesis, with maxima exceeding 2°C (100 km)−1 (3 h)−1 (Fig. 12c), found on the poleward side of a focused band of midlevel warm air advection (Fig. 12d). The observed band location (refer to Fig. 2a) closely corresponds to the axis of maximum midlevel frontogenesis (Fig. 12c).

b. 14–15 February 2000

On 14–15 February 2000 a weak cyclone moved from the Ohio Valley to the Gulf of St. Lawrence. Although not intense, the cyclone was still responsible for a large swath of over 25 mm of precipitation that fell as a mix of rain, sleet, and freezing rain over coastal locations, with over 20 cm of snowfall accumulation in interior New England. In contrast to the 5–6 February 2001 storm, no mesoscale banding was evident during the evolution of the 14–15 February 2000 storm (Fig. 2b).

Many of the same synoptic features noted in the nonbanded composite (Fig. 9) are present in the 14–15 February 2000 case. At 1200/14 the surface cyclone centered in Pennsylvania (Fig. 13a) was associated with a short-wave trough over the Maryland panhandle (Fig. 13b). This short wave was embedded within the confluent entrance region of a 70 m s−1 jet (Fig. 13b). The 700-hPa flow featured a short-wave trough over the mid-Atlantic states, but in the absence of a closed circulation, the flow continues northeastward (Fig. 13c) instead of a portion of the flow turning cyclonically as in the 5–6 February 2001 storm. This flow pattern limits the development of deformation and warm air advection northwest of the surface cyclone, reducing the potential for frontogenesis in this sector of the cyclone (Fig. 13c). However, ahead (east) of the surface cyclone, midlevel warm air advection was found over a large portion of the northeast United States (Fig. 13d) and once again frontogenesis was found on the poleward side of this warm air advection (Figs. 13c,d).

c. Cross-section comparison

Cross sections through the midlevel frontogenesis maximum in each case show similar features to the respective composite cross sections. The 0000/6 cross section (Figs. 14a,b) exhibits intense frontogenesis within the frontal zone (Fig. 14a). Although the frontogenesis maximum and θes contours slope toward the colder air, the frontal zone is relatively upright, suggesting weak conditional stability. Conditional instability is noted south of the frontal boundary, but an area of small EPV* is found in the 600–400-hPa layer above the band position and frontogenesis maximum. The frontogenetical response is readily evident in Fig. 14b, with a narrow, sloping ascent maximum on the equatorward flank of the frontal zone.

In contrast, the 1200/14 cross section (Figs. 14c,d) exhibits a shallow sloped frontal zone (Figs. 14c), with two disconnected and weak frontogenesis maxima at 1000 and 700 hPa (Fig. 14c). Although a region of conditional instability is found equatorward and above the frontal zone, the shallow slope of the front and weak disconnected frontogenetical forcing limits the frontal circulation as is evident in Fig. 14d. Note that the ascent is less than half the magnitude at the corresponding time in the 5–6 February 2001 storm (Fig. 14b) and occurs over a broader region, consistent with the comparison between the banded and nonbanded composite cross sections (Figs. 11b,d).

6. Discussion

a. Climatology

Despite the rather strict criteria used to define mesoscale precipitation bands, 162 banded events were documented in the northeast U.S. study region during the study period. This number testifies to the frequency with which operational forecasters are challenged by mesoscale band formation during the cold season in the northeast United States. However, the occurrence of 13 nonbanded cases during the study serves as a reminder that not all heavy precipitation systems exhibit mesoscale banding as defined in this paper.

The predominance of single-banded events in the northwest quadrant relative to the surface cyclone is consistent with case studies conducted in the eastern half of the United States (e.g., Sanders and Bosart 1985; Nicosia and Grumm 1999; Martin 1998a,b); however, it differs from the predominant single-band location (ahead of the warm front) documented in the Pacific Northwest (Houze et al. 1976) and in the United Kingdom (Browning 1985). Note that these latter two studies were located in the exit region of the Pacific and Atlantic storm tracks (Pacific Northwest, United Kingdom), which may influence the storm-relative location of single-band formation. This hypothesis is supported by the results of this study and Nicosia and Grumm (1999), which suggest that single-banded events in the northeast United States are often associated with rapid cyclogenesis, which is more frequent in the western ocean basins (Whittaker and Horn 1984).

The tendency for single bands to be short lived and the large number of transitory events suggest that the conditions favoring single-band development occur on limited time scales, although in rare cases single bands persisted for more than 12 h. Also of interest is the common pivoting motion of the observed single bands, since the pivot point experiences heavy precipitation for a sustained period. Operational quantitative precipitation forecasts (QPF) could benefit from understanding the dynamics associated with this pivoting motion.

b. Composites and case examples

Composites calculated from the Eta Model analyses and 6-h forecast fields for banded and nonbanded cases highlight the role deformation, frontogenesis, and moist symmetric stability play in mesoscale band formation, and places these elements into the context of the synoptic and mesoscale flow evolution.

In the northwest composite the midlevel deformation zone forms northwest of the surface cyclone when a closed midlevel circulation develops and strengthens as the cyclone deepens. Consistent with the case studies of Nicosia and Grumm (1999), the deformation is found in diffluent flow north of the 700-hPa low center. This deformation acting on the temperature gradient produces differential temperature advections and associated frontogenesis. The midlevel frontogenesis develops in response to increasing warm air advection and associated warm air advection gradient, and is focused along the confluent asymptote of the deformation zone through a deep layer, providing forcing for the single-banded events.

The northwest composite frontal zone was marked by deep-layer frontogenesis coincident with small moist symmetric stability. Application of the semigeostrophic form of the Sawyer–Eliassen equation (Sawyer 1956; Eliassen 1962) would predict that the direct circulation induced by the frontogenesis in the presence of small moist symmetric stability would lead to an intense, narrow sloping updraft on the equatorward flank of the frontal zone. The northwest composite vertical velocity fields exhibited this very feature. In fact, all single-banded composite frontal zones (northwest near, northwest far, and east) exhibited the above frontal features (see Novak 2002, his section 3.2), suggesting that deep-layer frontogenesis in the presence of small moist symmetric stability is the primary forcing mechanism for single bands in both the northwest and east quadrants.

In contrast, the nonbanded composite cyclone evolution was dominated by its position in the confluent entrance region of an upper-level jet, limiting the development of a closed midlevel circulation and an associated deformation zone northwest of the surface cyclone. Although the cases in the nonbanded composite did not exhibit mesoscale banding, it is important to note that midlevel deformation and frontogenesis were evident, associated with confluence ahead of the cyclone. In this regard, the existence of midlevel frontogenesis alone does not ensure mesoscale band formation. Composite and case study cross sections through respective frontogenesis maxima showed that compared to their banded counterparts, the frontogenesis was weaker (especially near the surface in the nonbanded composite) and lacked vertical continuity (as in the 14– 15 February 2000 case). Additionally the nonbanded composite and the 14–15 February 2000 case exhibited a relatively shallow slope (see Figs. 11c and Fig. 14c), suggesting greater conditional stability. The combination of weaker frontogenesis and greater conditional stability likely limited the frontal circulation and subsequent band development.

The stark contrast in the degree of development between the northwest and nonbanded composite and case study evolutions supports the assertion made by Nicosia and Grumm (1999) that banded structure is favored during cyclogenesis, when temperature advections are enhanced (contributing to frontogenesis) and differential moisture advection can reduce moist symmetric stability. Also, the results from the present study suggest that single-band formation in the comma-head portion of cyclones occurs during favored periods when midlevel deformation and associated frontogenesis are maximized as the midlevel circulation forms, and when the moist symmetric stability is minimized. The northwest composite also shows that as the midlevel center deepens and broadens during the occlusion process (see Fig. 7), the primary deformation zone and associated frontogenesis shift northeastward, leading to band dissipation in the northwest quadrant. This evolution may explain the limited lifetime of single-banded events noted in Table 3, at least with respect to single-banded events observed in the northwest quadrant.

c. Conceptual models

Conceptual models of single-banded events and nonbanded cases are presented in Fig. 15. The single-banded event (Fig. 15a) exhibits a double-jet structure, placing the surface cyclone under upper-level divergence, which promotes cyclogenesis. The existence of a closed midlevel circulation creates a deformation zone associated with a confluent asymptote northwest of the surface cyclone. This deformation zone supports frontogenesis, which provides forcing for band development northwest of the surface cyclone. Diffluent flow ahead of the midlevel disturbance also contributes to deformation and associated frontogenesis, supporting band development ahead of the warm front. Note that this conceptual model is consistent with the findings of Nicosia and Grumm (1999, see their Fig. 17). The nonbanded case (Fig. 15b) highlights the dominance of the upper-level jet. A weak surface low associated with a weak midlevel trough is found in the equatorward entrance region of the jet. Confluent flow ahead of this midlevel trough contributes to deformation and associated frontogenesis. The conceptual models of single-banded events and nonbanded cases are also consistent with the combined observational and idealized modeling study of Schultz et al. (1998), who found that cyclones in diffluent flow tend to exhibit frontogenesis in the northwest quadrant of the cyclone (as seen in the single-banded conceptual model; Fig. 15a), whereas cyclones in confluent flow tend to feature frontogenesis in the northeast quadrant (as seen in the nonbanded conceptual model; Fig. 15b).

A schematic of the characteristic single-banded and nonbanded cross-sectional environments is shown in Fig. 16. The banded frontal zone is marked by a sloping region of frontogenesis extending through the midtroposphere, coincident with weak conditional stability. The direct circulation induced by the deep-layer frontogenesis in the presence of small moist symmetric stability results in a narrow, intense sloping updraft on the equatorward flank of the frontal zone (Fig. 16a). The nonbanded frontal zone (Fig. 16b) has similar features as the banded cross section, except that the frontogenesis is weaker and the associated ascent is weaker and broader. In addition to the weaker frontogenetical forcing, the slope of the frontal zone is shallower than in the banded cross section. In this situation, the Sawyer–Eliassen equation would predict that the ascent forced by the frontogenesis would be weaker and broader, as depicted in the vertical velocity field (Fig. 16b).

7. Summary

A climatology of banded precipitation events, composites of banded and nonbanded cases, and selected case studies in the northeast United States during the cold season were presented. It was established that mesoscale banding is a frequent phenomenon in the northeast United States, with over 85% (75 of 88) of the study cases exhibiting some type of mesoscale banding. While a number of cases exhibited more than one type of banded event during their duration, single-banded events were found to be the most common, predominantly occurring in the comma-head portion of developing cyclones.

The composite and case study results largely confirm previous work on mesoscale band formation, reiterating that deep-layer frontogenesis in the presence of small moist symmetric stability is the primary forcing mechanism for single-band formation. Additionally, the composites and case studies demonstrate how cyclogenesis and the attendant synoptic flow evolution influence the likelihood, timing, and location of mesoscale band formation via the location and magnitude of deformation, which directly affects the location, magnitude, and depth of frontogenesis. Viewing single-band formation in the context of favored synoptic and mesoscale flow evolutions affords operational forecasters a heightened awareness of the potential for single-banded events. Future work will demonstrate how this perspective can be utilized in the operational forecast environment to anticipate mesoscale band formation.

Acknowledgments

The authors wish to thank Anantha Aiyyer (University at Albany, SUNY), who contributed to the development of the composite program; David Schultz (NSSL/CIMMS) and Ron Horwood (NOAA/NWS Northeast RFC), who provided insightful perspectives on this research; Elizabeth Page and Dolores Kiessling (COMET), who helped acquire model and radar data; Celeste Iovinella (University at Albany, SUNY), who provided critical support in manuscript preparation; and the three anonymous reviewers, who provided constructive comments leading to improvements in the presentation of this research. This research was supported by NOAA Grant NA07WA0458, awarded to the University at Albany, SUNY as part of the CSTAR program.

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Fig. 1.
Fig. 1.

Chart of the study method and terminology. The number of each element is denoted in the parentheses

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Fig. 2.
Fig. 2.

WSR-88D radar mosaic of (a) a single-banded event valid at 0000 UTC 6 Feb 2001 and (b) a nonbanded case valid at 1200 UTC 14 Feb 2000. Color scale along left side is partitioned every 5 dBZ

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Fig. 3.
Fig. 3.

(a) Geographical distribution of the 48 single-banded events studied. The axis of each band at the most representative analysis time (0000, 0600, 1200, or 1800 UTC) is identified by a solid black line. (b) Geographical distribution of surface lows observed at each respective single-banded event analysis time. Each surface low position is identified by an “x.”

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Fig. 4.
Fig. 4.

Distribution of single-banded events relative to surface cyclone position (origin). Each black line represents the axis of a single band at the most representative analysis time. Geographic-relative north is denoted by the “N” at the top of the figure. The radial distance scale is in km

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Fig. 5.
Fig. 5.

Northwest composite at t = −12 h: (a) 1000-hPa heights (solid) every 30 m and 1000– 500-hPa thickness (dashed) every 6 dam; (b) 500-hPa heights (solid) every 6 dam, absolute vorticity (dashed) every 2 × 10−5 s−1 starting at 12 × 10−5 s−1, and 300-hPa wind speed (shaded) starting at 35 m s−1; (c) 700-hPa heights (solid) every 3 dam, 750–650-hPa layer-averaged resultant deformation equal to 3 × 10−5 s−1 (dotted), and 750–650-hPa layer-averaged Miller 2D frontogenesis (shaded) in °C (100 km)−1 (3 h)−1; (d) 700-hPa heights (solid gray) every 3 dam, 750–650-hPa Miller 2D frontogenesis [as in (c)], and 750–650-hPa layer-averaged temperature advection (solid black) contoured for positive values every 5°C (day)−1

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Fig. 6.
Fig. 6.

As in Fig. 5, except for northwest composite at t = 0 h with cross-section orientation and composite band position for Figs. 11a,b shown in (d)

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Fig. 7.
Fig. 7.

As in Fig. 5, except for northwest composite at t = +12 h

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Fig. 8.
Fig. 8.

As in Fig. 5, except for nonbanded composite at t = −12 h

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Fig. 9.
Fig. 9.

As in Fig. 5, except for nonbanded composite at t = 0 h with cross-section orientation for Figs. 11c,d shown in (d)

Citation: Weather and Forecasting 19, 6; 10.1175/815.1

Fig. 10.
Fig. 10.

As in Fig. 5, except for nonbanded composite at t = +12 h

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Fig. 11.
Fig. 11.

(a) Cross section through the composite band position (black bar along x axis) in the northwest composite at t = 0 h showing Miller 2D frontogenesis [shaded for positive values according to the grayscale in units of °C (100 km)−1 (3 h)−1]. Saturation equivalent potential temperature (thin solid) every 3 K, saturation equivalent potential vorticity computed using the full wind (heavy solid) every 0.25 PVU (1 PVU = 10−6 m2 s−1 K kg−1) contoured at and below 0.25 PVU. (b) Relative humidity beginning at 70%, shaded according to the grayscale, and vertical motion contoured every 2 × 10−3 hPa s−1 with dashed (solid) contours denoting ascent (descent); zero contour is drawn solid. Cross-section orientation in (a) and (b) is shown in Fig. 6d. (c) As in (a) except for the nonbanded composite. (d) As in (b) except for the nonbanded composite. Cross-section orientation in (c) and (d) is shown in Fig. 9d

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Fig. 12.
Fig. 12.

Synoptic analysis valid at 0000 UTC 6 Feb 2001: (a) 1000-hPa heights (solid) every 30 m and 1000–500-hPa thickness (dashed) every 6 dam; (b) 500-hPa heights (solid) every 6 dam, absolute vorticity (dashed) every 4 × 10−5 s−1 starting at 16 × 10−5 s−1, and 300-hPa wind speed (shaded) starting at 40 m s−1; (c) 700-hPa heights (solid) every 3 dam, 750–650-hPa layer-averaged resultant deformation equal to 6 × 10−5 s−1 (dotted), and 750–650-hPa layer-averaged Miller 2D frontogenesis (shaded) in °C (100 km)−1 (3 h)−1; (d) 700-hPa heights (solid gray) every 3 dam, 750–650-hPa Miller 2D frontogenesis [as in (c)], and 750–650-hPa layer-averaged temperature advection (solid black) contoured for positive values every 20°C (day)−1. Cross-section orientation for Figs. 14a,b shown in (d)

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Fig. 13.
Fig. 13.

As in Fig. 12, except for 1200 UTC 14 Feb 2000. Cross-section orientation for Figs. 14c,d shown in (d)

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Fig. 14.
Fig. 14.

(a) Cross section through the observed band position (black bar along x axis) at 0000 UTC 6 Feb 2001 showing Miller 2D frontogenesis [shaded for positive values according to the grayscale in units of °C (100 km)−1 (3 h)−1]. Saturation equivalent potential temperature (thin solid) every 3 K, saturation equivalent potential vorticity computed using the full wind (heavy solid) every 0.25 PVU contoured at and below 0.25 PVU. (b) Relative humidity beginning at 70%, shaded according to the grayscale, and vertical velocity contoured every 3 × 10−3 hPa s−1 with dashed (solid) contours denoting ascent (descent); zero contour is drawn solid. Cross-section orientation in (a) and (b) is shown in Fig. 12d. (c) As in (a) except for cross section through midlevel frontogenesis maximum at 1200 UTC 14 Feb 2000. (d) As in (b) except for 1200 UTC 14 Feb 2000. Cross-section orientation in (c) and (d) is shown in Fig. 13d

Citation: Weather and Forecasting 19, 6; 10.1175/815.1

Fig. 15.
Fig. 15.

Conceptual model of the synoptic and mesoscale flow environment associated with (a) a single-banded event and (b) a nonbanded case highlighting the key features. Features shown include midlevel frontogenesis (red shading), midlevel deformation zone (encompassed by scalloped blue line) and associated primary dilatation axes [dashed lines in (a)], midlevel streamlines (black lines), and upper-level jet cores (wide dashed arrows)

Citation: Weather and Forecasting 19, 6; 10.1175/815.1

Fig. 16.
Fig. 16.

Schematic cross sections through a typical (a) single-banded and (b) nonbanded environment. Fields shown are frontogenesis (red shading), saturation equivalent potential temperature (thin solid), and ascent (dashed) with length of arrow proportional to the magnitude of ascent and orientation representative of air parcel trajectory. Cross-section length is approximately 1000 km

Citation: Weather and Forecasting 19, 6; 10.1175/815.1

Table 1.

Band classification scheme

Table 1.
Table 2.

Band climatology results

Table 2.
Table 3.

Single-banded event duration frequency

Table 3.
Save
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  • Fig. 1.

    Chart of the study method and terminology. The number of each element is denoted in the parentheses

  • Fig. 2.

    WSR-88D radar mosaic of (a) a single-banded event valid at 0000 UTC 6 Feb 2001 and (b) a nonbanded case valid at 1200 UTC 14 Feb 2000. Color scale along left side is partitioned every 5 dBZ

  • Fig. 3.

    (a) Geographical distribution of the 48 single-banded events studied. The axis of each band at the most representative analysis time (0000, 0600, 1200, or 1800 UTC) is identified by a solid black line. (b) Geographical distribution of surface lows observed at each respective single-banded event analysis time. Each surface low position is identified by an “x.”

  • Fig. 4.

    Distribution of single-banded events relative to surface cyclone position (origin). Each black line represents the axis of a single band at the most representative analysis time. Geographic-relative north is denoted by the “N” at the top of the figure. The radial distance scale is in km

  • Fig. 5.

    Northwest composite at t = −12 h: (a) 1000-hPa heights (solid) every 30 m and 1000– 500-hPa thickness (dashed) every 6 dam; (b) 500-hPa heights (solid) every 6 dam, absolute vorticity (dashed) every 2 × 10−5 s−1 starting at 12 × 10−5 s−1, and 300-hPa wind speed (shaded) starting at 35 m s−1; (c) 700-hPa heights (solid) every 3 dam, 750–650-hPa layer-averaged resultant deformation equal to 3 × 10−5 s−1 (dotted), and 750–650-hPa layer-averaged Miller 2D frontogenesis (shaded) in °C (100 km)−1 (3 h)−1; (d) 700-hPa heights (solid gray) every 3 dam, 750–650-hPa Miller 2D frontogenesis [as in (c)], and 750–650-hPa layer-averaged temperature advection (solid black) contoured for positive values every 5°C (day)−1

  • Fig. 6.

    As in Fig. 5, except for northwest composite at t = 0 h with cross-section orientation and composite band position for Figs. 11a,b shown in (d)

  • Fig. 7.

    As in Fig. 5, except for northwest composite at t = +12 h

  • Fig. 8.

    As in Fig. 5, except for nonbanded composite at t = −12 h

  • Fig. 9.

    As in Fig. 5, except for nonbanded composite at t = 0 h with cross-section orientation for Figs. 11c,d shown in (d)

  • Fig. 10.

    As in Fig. 5, except for nonbanded composite at t = +12 h

  • Fig. 11.

    (a) Cross section through the composite band position (black bar along x axis) in the northwest composite at t = 0 h showing Miller 2D frontogenesis [shaded for positive values according to the grayscale in units of °C (100 km)−1 (3 h)−1]. Saturation equivalent potential temperature (thin solid) every 3 K, saturation equivalent potential vorticity computed using the full wind (heavy solid) every 0.25 PVU (1 PVU = 10−6 m2 s−1 K kg−1) contoured at and below 0.25 PVU. (b) Relative humidity beginning at 70%, shaded according to the grayscale, and vertical motion contoured every 2 × 10−3 hPa s−1 with dashed (solid) contours denoting ascent (descent); zero contour is drawn solid. Cross-section orientation in (a) and (b) is shown in Fig. 6d. (c) As in (a) except for the nonbanded composite. (d) As in (b) except for the nonbanded composite. Cross-section orientation in (c) and (d) is shown in Fig. 9d

  • Fig. 12.

    Synoptic analysis valid at 0000 UTC 6 Feb 2001: (a) 1000-hPa heights (solid) every 30 m and 1000–500-hPa thickness (dashed) every 6 dam; (b) 500-hPa heights (solid) every 6 dam, absolute vorticity (dashed) every 4 × 10−5 s−1 starting at 16 × 10−5 s−1, and 300-hPa wind speed (shaded) starting at 40 m s−1; (c) 700-hPa heights (solid) every 3 dam, 750–650-hPa layer-averaged resultant deformation equal to 6 × 10−5 s−1 (dotted), and 750–650-hPa layer-averaged Miller 2D frontogenesis (shaded) in °C (100 km)−1 (3 h)−1; (d) 700-hPa heights (solid gray) every 3 dam, 750–650-hPa Miller 2D frontogenesis [as in (c)], and 750–650-hPa layer-averaged temperature advection (solid black) contoured for positive values every 20°C (day)−1. Cross-section orientation for Figs. 14a,b shown in (d)

  • Fig. 13.

    As in Fig. 12, except for 1200 UTC 14 Feb 2000. Cross-section orientation for Figs. 14c,d shown in (d)

  • Fig. 14.

    (a) Cross section through the observed band position (black bar along x axis) at 0000 UTC 6 Feb 2001 showing Miller 2D frontogenesis [shaded for positive values according to the grayscale in units of °C (100 km)−1 (3 h)−1]. Saturation equivalent potential temperature (thin solid) every 3 K, saturation equivalent potential vorticity computed using the full wind (heavy solid) every 0.25 PVU contoured at and below 0.25 PVU. (b) Relative humidity beginning at 70%, shaded according to the grayscale, and vertical velocity contoured every 3 × 10−3 hPa s−1 with dashed (solid) contours denoting ascent (descent); zero contour is drawn solid. Cross-section orientation in (a) and (b) is shown in Fig. 12d. (c) As in (a) except for cross section through midlevel frontogenesis maximum at 1200 UTC 14 Feb 2000. (d) As in (b) except for 1200 UTC 14 Feb 2000. Cross-section orientation in (c) and (d) is shown in Fig. 13d

  • Fig. 15.

    Conceptual model of the synoptic and mesoscale flow environment associated with (a) a single-banded event and (b) a nonbanded case highlighting the key features. Features shown include midlevel frontogenesis (red shading), midlevel deformation zone (encompassed by scalloped blue line) and associated primary dilatation axes [dashed lines in (a)], midlevel streamlines (black lines), and upper-level jet cores (wide dashed arrows)

  • Fig. 16.

    Schematic cross sections through a typical (a) single-banded and (b) nonbanded environment. Fields shown are frontogenesis (red shading), saturation equivalent potential temperature (thin solid), and ascent (dashed) with length of arrow proportional to the magnitude of ascent and orientation representative of air parcel trajectory. Cross-section length is approximately 1000 km

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