JUNE 2007 ATALLAH ET AL. 2185
Precipitation Distribution Associated with Landfalling Tropical Cyclones over the Eastern United States
EYAD ATALLAH
Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
LANCE F. BOSART AND ANANTHA R. AIYYER*
Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York
(Manuscript received 28 November 2005, in final form 25 August 2006)
ABSTRACT
Tropical cyclones (TCs) making landfall over the United States are examined by separating those asso- ciated with precipitation predominantly left of their tracks from those with the same to the right of their tracks. Composites of atmospheric variables for these two TC categories are performed and analyzed using potential vorticity (PV) and quasigeostrophic (QG) frameworks. Dynamical signatures are retrieved from these composites to help understand the evolution of precipitation in these storms. Results indicate that a left of track precipitation distribution (e.g., Floyd 1999) is characteristic of TCs undergoing extratropical transition (ET). In these cases, a positively tilted midlatitude trough approaches the TC from the northwest, shifting precipitation to the north-northwest of the TC. Potential vorticity redistribution through diabatic heating leads to enhanced ridging over and downstream of the TC, resulting in an increase in the cyclonic advection of vorticity by the thermal wind over the transitioning TC. A right of track precipitation distri- bution is characteristic of TCs interacting with a downstream ridge (e.g., David 1979). When the down- stream ridge amplifies in response to TC-induced diabatic heating ahead of a weak midlatitude trough, the PV gradient between the TC and the downstream ridge is accentuated, producing a region of enhanced positive PV advection (and cyclonic vorticity advection by the thermal wind) over the TC. The diabatic enhancement of the downstream ridge is instrumental in the redistribution of precipitation about the transitioning TCs in both cases and poses a significant forecast challenge.
1. Introduction 2002. Floyd (1999) proved to be a particularly difficult forecast challenge. While most of the hazardous a. Purpose weather warnings as the storm approached the coast Several recent storms have highlighted the impor- focused on the potential for wind damage, flooding due tance of understanding the effect of the interaction of to heavy precipitation proved to be a graver source of landfalling tropical systems with midlatitude features. destruction. More than 50 cm of precipitation fell over Notable recent events impacting the United States in- sections of North Carolina, and amounts exceeding 20 clude Floyd and Irene in 1999, and Isidore and Lili in cm occurred over a wide area stretching from the Pied- mont region of North Carolina to southeastern New York. Atallah and Bosart (2003) provided a detailed case study of Floyd and indicated that the precipitation * Current affiliation: Department of Marine, Earth, and Atmo- spheric Sciences, North Carolina State University at Raleigh, Ra- distribution and intensity associated with the storm re- leigh, North Carolina. sulted from its interaction with a potent midlatitude trough and subsequent extratropical transition (ET). This study will attempt to elucidate a general paradigm Corresponding author address: Anantha Aiyyer, Dept. of Ma- rine, Earth, and Atmospheric Sciences, North Carolina State Uni- for understanding the dynamics responsible for modu- versity at Raleigh, Raleigh, NC 27695. lating the distribution, spatial extent, location, and in- E-mail: [email protected] tensity of precipitation associated with tropical cyclones
DOI: 10.1175/MWR3382.1
© 2007 American Meteorological Society
Unauthenticated | Downloaded 09/27/21 02:06 PM UTC
MWR3382 2186 MONTHLY WEATHER REVIEW VOLUME 135 subsequent to landfall, regardless of whether or not the wind and precipitation distribution concentrated near tropical cyclone (TC) undergoes a successful ET. the center of the tropical cyclone often evolves into a broad, asymmetric configuration. Klein et al. (2000) b. ET climatologies showed that the inner core of the tropical cyclone tends Past studies suggest that ET is a relatively common to lose its symmetric appearance as strong frontogen- phenomenon in many subtropical ocean basins (e.g., esis develops poleward of the tropical cyclone. Al- Foley and Hanstrum 1994; Klein et al. 2000; Hart and though the expanding cloud field associated with the Evans 2001; Sinclair 2002; Jones et al. 2003; Dare and TC can include large amounts of high clouds due to Davidson 2004). Foley and Hanstrum (1994) found that outflow into the midlatitude westerlies, regions of sig- a key precursor to ET includes a highly meridional nificant precipitation are typically embedded in the trough–ridge pair upstream of the TC and that ET oc- large cloud shield (Jones et al. 2003). curs most frequently when a large-amplitude cold front Several recent papers (e.g., Klein et al. 2002; Ritchie approaches within 1700 km of the tropical cyclone. and Elsberry 2003; McTaggart-Cowan et al. 2003; Sin- Harr and Elsberry (2000) and Harr et al. (2000) noted clair 2004) have attempted to quantify the effect of mid- that, during ET, significant precipitation to the north- latitude troughs on ET. Klein et al. (2002) showed that east of the TC favors diabatic ridging poleward of the reintensification is favored when the upper-level TC TC, increasing the amplitude of the trough–ridge cou- outflow enhances the equatorward jet-entrance region plet, resulting in an enhanced jet streak poleward of of a downstream jet streak. This occurs as the diabatic the TC. heating associated with the TC redistributes potential Hart and Evans (2001) found that, relative to the vorticity (PV) such that the anticyclonic circulation on number of TCs in each basin, ET is more frequent in the equatorward side of the jet streak is strengthened. the North Atlantic as compared with the North Pacific. Similarly, Ritchie and Elsberry (2003) found that the They found that 46% of TCs during 1950–96 underwent proper phasing of the tropical cyclone and the midlati- some form of ET, with the east coast of the United tude trough (i.e., the TC becomes collocated with a States and the Canadian Maritime Provinces most region of differential cyclonic vorticity advection in- likely to experience transitioning storms. Jones et al. creasing with height associated with the midlatitude (2003) showed that ET occurs in nearly every ocean trough) results in substantial enhancement of the up- basin that experiences tropical cyclone activity. An ob- per-level divergence, a feature observed in many case jective definition and quantification of ET has recently studies of ET. Sinclair (2004) showed that cyclonic vor- been proposed in Evans and Hart (2003) and Hart ticity advection (CVA) at jet level is the strongest in- (2003) that uses changes in the low-level thickness field dicator of significant redevelopment during ET. and midtropospheric thermal wind to signal ET onset Dare and Davidson (2004) found that the maximum and completion. Arnott et al. (2004) used an objective intensity of TCs near Australia is generally followed by cluster analysis methodology to quantify the ET pro- recurvature, which suggests the importance of trough cess and showed that distinct frontal and thermal struc- interactions. The importance of the tropical cyclone on tures can be identified as TCs undergo ET. These struc- the eventual strength of ET was addressed by Agustí- tural changes are evident in the Hart (2003) cyclone Panareda et al. (2004) who found that the impact of the phase space analysis and are consistent with the empiri- TC was maximized when the positive PV anomaly was cal formulation of ET as provided by Evans and Hart situated equatorward of the jet in a region of anticy- (2003). clonic equivalent barotropic shear. Hart et al. (2006) examined the factors that deter- mine whether a TC undergoing ET will reintensify as d. ET case studies an extratropical cyclone. Their results show that the presence of a negatively tilted baroclinic trough up- Palmén (1958) and Anthes (1990) examined the stream of a TC undergoing ET is an important indicator large-scale dynamics associated with the ET of Hurri- of post-ET reintensification, whereas the presence of a cane Hazel (1954) along the east coast of the United positively tilted upstream baroclinic trough precludes States, and concluded that the transport of tropical air significant extratropical reintensification once ET has masses into the midlatitudes resulted in a large increase occurred. in the available potential energy (APE) of the atmo- sphere. Bosart and Carr (1978), DiMego and Bosart c. Storm structures and intensity (1982a,b), and Bosart and Dean (1991) discussed the Palmén (1958), Muramatsu (1985), and Foley and precipitation distribution and dynamics associated with Hanstrum (1994) have found that the nearly symmetric the transition of Hurricane Agnes (1972). DiMego and
Unauthenticated | Downloaded 09/27/21 02:06 PM UTC JUNE 2007 ATALLAH ET AL. 2187
Bosart (1982a,b) found that Agnes reintensified and diagnosis, via its invertibility property, of the role underwent ET as large-scale ascent ahead of the ap- played by the TC and key features of the environment proaching trough overspread the periphery of the rem- (e.g., Agustí-Panareda et al. 2005). nant low-level circulation. However, a transitioned cyclone does not have to be f. Goals intense in order to have significant impact, as flooding can accompany relatively weak remnant cyclones. Hur- The main focus of this paper is to understand the ricane Camille (1969) produced catastrophic flooding in synoptic-scale dynamics modulating the precipitation the Virginia Piedmont as a relatively weak transitioned distribution associated with landfalling TCs. While pre- cyclone (Chien and Smith 1977). Bosart and Lackmann vious studies have generally concentrated on ET and (1995) documented the case of David (1979), which the associated dynamics, this paper will examine all reintensified as a tropical system over land prior to un- landfalling systems exclusively from the point of view of dergoing ET while it interacted with a relatively weak the precipitation distribution, regardless of whether or midlatitude trough. Sinclair (1993) undertook a study not the system successfully undergoes ET. As such, the of the ET of Tropical Cyclone Patsy (1986) and found retrieval of characteristic signatures associated with ET that the greatest ascent in the redevelopment of Patsy is illustrative, but essentially coincidental to the study. also occurred in a region of an upward increase in CVA No assumptions are made or imposed regarding the in the equatorward entrance region of a subtropical jet. initial strength of the TC at landfall or the eventual Evans and Prater-Mayes (2004) also showed that the strength of any transitioned cyclone. Therefore, this ET and reintensification of Irene (1999) was aided by study hopes to provide a more generalized framework an upper-level trough and jet streak. Colle (2003) illus- than previous studies for understanding the dynamics trated the importance of latent heating on frontogen- modulating the precipitation distribution as opposed to esis in the mid- and lower troposphere, and quantified the dynamics modulating the strength or intensity of the impact of the latent heating on the strength and ET. A PV and quasigeostrophic (QG) framework is precipitation distribution associated with Floyd (1999). adopted to help illustrate the essential dynamics. The Abraham et al. (2004) documented observations from paper is organized as follows. Section 2 contains the the ET of Hurricane Michael (2000) and showed that data and methodology, section 3 presents the results of the entrainment of relatively cool and dry air into the the diagnostics, and section 4 summarizes and discusses western quadrant of the storm effectively diminished the significance of these results. the precipitation in the southwestern quadrant of the cyclone. 2. Data and methodology e. PV thinking a. Gridded fields Studies of tropical systems have also been facilitated All gridded fields for use in the composites are taken by the use of PV (Rossby 1940; Ertel 1942; Hoskins et from the National Centers for Environmental Predic- al. 1985). The “PV thinking” perspective (Morgan and tion–National Center for Atmospheric Research Nielsen-Gammon 1998) has been utilized in numerous (NCEP–NCAR) reanalysis at 0000 and 1200 UTC (Kal- studies of hurricane–trough interactions (e.g., Molinari nay et al. 1996; Kistler et al. 2001). The NCEP–NCAR et al. 1995, 1997; Bosart and Lackmann 1995; Bosart et reanalysis provides a relatively uniform dataset over a al. 2000) and transitioning TCs (e.g., Browning et al. long period, and is useful for the production of com- 1998; Henderson et al. 1998; Thorncroft and Jones posites. The disadvantage of the reanalysis is the rela- 2000; McTaggart-Cowan et al. 2001). tively coarse grid resolution of 2.5° 2.5°. However, Potential vorticity is particularly useful in the study this drawback is mitigated because the focus of this of TC trough interactions. The TC corresponds to a study is synoptic scale in nature. It is understood that positive PV anomaly concentrated in the lower and the representations of the core of any tropical systems midtroposphere at small radii and a broad and shallow in the reanalysis are inadequate. Moreover, no attempt negative PV anomaly at larger radii in the upper tro- is made to diagnose the change of structure in the core posphere. On the other hand, extratropical cyclones are of the systems in this study. Rather, general signatures often associated with maxima of PV in the mid- and and changes in the structure of the mass field will be upper levels of the troposphere at large radii. The util- used to diagnose the sense of the QG forcing for ascent ity of PV lies not only in the visualization of the evo- as a first-order dynamical explanation of the precipita- lution of the interaction between the two, but also in tion distribution around the storms in question.
Unauthenticated | Downloaded 09/27/21 02:06 PM UTC 2188 MONTHLY WEATHER REVIEW VOLUME 135 b. Storms and compositing TABLE 1. A list of TCs included in the study. The dates and times listed represent the initial time that a storm was included in All the storms used in the composites are listed in each of the composites. The numbers in parentheses show the Table 1. (Full details on the life cycle and track of each maximum sustained wind (kt) of the cyclone at the time listed as storm and the best track data can be found online at taken from the best-track data. http://www.nhc.noaa.gov/pastall.shtml.) The first time Storm LOC ROC period used for each storm in each composite is listed. This time (t 0) is defined as the first time that the Hazel (1954) 1200 UTC 15 Oct (110) storm exhibits a left of track (LOC) or right of track Audrey (1957) 1200 UTC 28 Jun 0000 UTC 27 Jun (ROC) precipitation distribution. The storm is then (45) (90) composited relative to a reference storm track. The Ethel (1960) 0000 UTC 16 Sep evolution in each of the composites is taken as the 24-h (45) period following the time listed in Table 1. To be in- Carla (1961) 1200 UTC 13 Sep 1200 UTC 12 Sep (30) (100) cluded in any of the composites, each storm had to meet Candy (1968) 0000 UTC 24 Jun the following criteria: (60) • Landfall was along either the Gulf or Atlantic coasts Camille (1969) 1200 UTC 18 Aug (65) of the United States. Doria (1971) 1200 UTC 27 Aug • The storm had to display a poleward component in its (50) track. Agnes (1972) 1200 UTC 21 Jun • The storm was far enough inland to permit rain mea- (30) surements in all quadrants of the storm. Eloise (1975) 1200 UTC 23 Sep (110) The storms are composited relative to a specified lati- David (1979) 0000 UTC 5 Sep tude–longitude point based on a reference storm track. (65) All the grids in the LOC composite at time t 0 are Frederic (1979) 0000 UTC 14 Sep (40) moved so that the center of each storm occupies the Alicia (1983) 1200 UTC 19 Aug same latitude–longitude coordinate. This process is re- (25) peated for each of the following time steps in each com- Bob (1985) 0000 UTC 25 Jul posite. The apparent motion of the storm in the com- (65) posites is therefore an artifact of the reference storm Danny (1985) 1200 UTC 18 Aug 1200 UTC15 Aug (25) (80) track. Elena (1985) 1200 UTC 3 Sep Applying the above criteria resulted in 32 storms be- (25) ing chosen from the period of 1950–98. Figure 1 dis- Gloria (1985) 1200 UTC 27 Sep plays the best-track data of a few representative storms (85) and the associated precipitation. Note that if storm po- Bonnie (1986) 1200 UTC 26 Jun (65) sitions were needed after the best-track data was no Chris (1988) 1200 UTC 29 Aug 0000 UTC 28 Aug longer available (i.e., the storm underwent ET), the (20) (30) position of the associated 850-hPa vorticity maximum Bob (1991) 1200 UTC 19 Aug in the reanalysis was used for the purposes of compos- (95) iting. All storms were composited with reference to a Andrew (1992) 0000 UTC 28 Aug 1200 UTC 26 Aug (20) (80) storm track with the same precipitation characteristics Beryl (1994) 1200 UTC 15 Aug and evolution. The dates along the tracks represent the (35) 1200 UTC time and the dots are in 6-h increments. Erin (1995) 1200 UTC 4 Aug Figures 1a,b show the tracks of Audrey (1957) and (20) Carla (1961), respectively, and are cases where the Bertha (1996) 1200 UTC 13 Jul (60) storms can be binned at different times in both the left Danny (1997) 1200 UTC 24 Jul of track and right of track composites. Both storms (30) make landfall with the bulk of the precipitation falling to the right of the storm. After recurvature, precipita- tion shifts to the left of the storm track. For example, in Agnes (1972) and David (1979) are shown as examples Fig. 1b, Carla (1961) would be binned as an ROC storm of storms with a consistent LOC and ROC precipitation from 1200 UTC 11 September to 0000 UTC 13 Septem- distribution, respectively. In general, each storm is used ber 1961and as an LOC storm from 1200 UTC 13 Sep- only when the precipitation distribution about the tember to 1200 UTC 14 September 1961. In Figs. 1c,d, storm is unambiguous. The 32 cases represent about 1/3
Unauthenticated | Downloaded 09/27/21 02:06 PM UTC JUNE 2007 ATALLAH ET AL. 2189
FIG. 1. Storm track (black lines) and precipitation (shaded starting at 10 mm every 40 mm) for Hurricanes (a) Audrey (1957), (b) Carla (1961), (c) Agnes (1962), and (d) David (1979). The dots show the storm positions and the numbers mark the 1200 UTC position for each date. of all storms making landfall in the eastern United It should be noted that he UPD underestimates the States and approximately 1/2 of all storms that exhibit rainfall maximum by about 50% on approximately 13% some northward component in their tracks over land. of the storm days (not shown). However, the spatial or The precipitation distribution was taken from the qualitative distribution of the precipitation for most NCEP Unified Precipitation Dataset (UPD), which storms is well replicated. Figures 2a,b show subjective provides gridded daily precipitation amounts (1200– hand analysis of the storm totals for Bertha (1996; Fig. 1200 UTC) on a 0.25° 0.25° latitude–longitude grid 2c) and Fran (1996; Fig. 2d) as taken from the National covering the region from 20°–60°Nto140°–60°W. The Weather Service forecast offices in Sterling, Virginia, data were analyzed using a Cressman (1959) objective and Raleigh, North Carolina, respectively. It is clear analysis scheme. (Computational details can be found when comparing Figs. 2a,b with Figs. 2c,d that the UPD online at http://www.cpc.ncep.noaa.gov/research_ qualitatively represents the distribution of the precipi- papers/ncep_cpc_atlas/7/toc.html.) tation well, albeit the maxima are somewhat reduced.
Unauthenticated | Downloaded 09/27/21 02:06 PM UTC 2190 MONTHLY WEATHER REVIEW VOLUME 135
FIG. 2. (a) Subjective analysis of rainfall associated with Hurricane Bertha (1996) from the Climate Prediction and National Hurricane Centers; (b) subjective analysis of rainfall associated with Hurricane Fran (1996) from the National Weather Service Forecast Office in Sterling, VA; storm total precipitation (mm) from the UPD for (c) Bertha (1996) and (d) Fran (1996).
3. Results the storm track was still on the west side of the Appa- lachian Mountain chain. This suggests that while orog- The tracks of the storms used to produce the LOC raphy and cold-air damming may be significant in many and ROC precipitation composites are shown in Fig. 3. cases, it is not a necessary condition to achieve the LOC The precipitation associated with some of the storms precipitation distribution. The LOC composites are switches sides during their evolution, and consequently created in storm relative coordinates with respect to the some tracks appear in both Figs. 3a,b. The reader is track of Hurricane Floyd (1999) as shown in Fig. 4. referred back to Table 1 for the starting date for each Clearly, not all storms in the composite moved at the storm for the respective composites. Of the 14 storms same speed. Consequently, the 12-h position of each used in the LOC composite, six made landfall along the storm was relocated to the 12-h position of Floyd. It east coast of the United States while eight of the storms should be noted that Floyd was chosen as a reference made landfall along the Gulf Coast. A total of six of the track since it is roughly representative of the average 14 storms had a LOC precipitation distribution while LOC track vector.
Unauthenticated | Downloaded 09/27/21 02:06 PM UTC JUNE 2007 ATALLAH ET AL. 2191
FIG. 3. The tracks of the tropical cyclones (based on the National Hurricane Center best-track data) for (a) storms with precipitation to the left of track and (b) storms with precipitation to the right of track.
The tracks of the storms used to produce the ROC a. Synoptic overview composites are shown in Fig. 3b. The storms in the The 1000–500-hPa thickness, the 1000-hPa heights, ROC bin were composited with respect to Audrey at and 850–200-hPa wind shear for both composites are the time of landfall (1957; Fig. 1a) as it is roughly rep- shown in Fig. 5. At time t 0 (Fig. 5a), the LOC com- resentative of the average ROC track vector. As in the posite TC is still offshore with a minimum central LOC composites, the first time in the composite is the height value of about 45 m at 1000 hPa. The cyclone first time the storm exhibits a clear ROC precipitation circulation center is located near the thickness ridge distribution over land. Of the 16 storms used in the (576 dam), indicative of its warm-core structure. A ROC composite, 11 made landfall along the Gulf Coast, while only five storms made landfall along the East large synoptic-scale thermal trough is located in the Coast. Interestingly, all of the storms that made landfall Midwest, with the trough axis lying approximately along the East Coast made landfall near the Georgia– along the Mississippi River. Warm-air advection is al- South Carolina border, and then proceeded to track ready evident along the Virginia and North Carolina along or just east of the spine of the Appalachians. Piedmont, with cold-air advection becoming prevalent While the location of landfall may be incidental, along and to the west of the Appalachians. A weak storms making landfall farther north are inherently trough axis in the 1000-hPa height field lies north of the more likely be associated with cold-air damming and/or TC, presumably a surface reflection of the midlatitude a significant midlatitude trough, and consequently ex- thermal trough. hibit a LOC precipitation distribution (as will be dis- Twelve hours later (t 12), the cyclone has moved cussed later). The track along the spine of the Appala- onshore and the cyclonic curvature of the thermal chians, on the other hand, does seem intuitive for the trough has increased concomitantly with an increase in ROC composite, as the wind field around the tropical the anticyclonic curvature in the thickness field imme- cyclone center would produce upslope (downslope) diately along the East Coast (Fig. 5b). This is indicative conditions in the eastern (western) quadrant of the of an increase in the vorticity gradient across the storm storm. The western extent and strong curvature of the and represents an increase in the differential cyclonic tracks over Gulf Coast indicates that these systems of- vorticity advection with height and consequently the ten encounter a strong east–west ridge axis with west- quasigeostrophic forcing for ascent. The S-shaped con- erlies off to the west and north. Also, 13 of the 16 figuration now evident in the thickness field, is a pat- storms exhibited an ROC precipitation distribution tern indicative of midlatitude cyclogenesis. The implied while still south of 38°N and at least eight of the storms maximum in warm-air advection is now located near in the composite later evolved into LOC storms. populated areas of the Northeast, stretching from
Unauthenticated | Downloaded 09/27/21 02:06 PM UTC 2192 MONTHLY WEATHER REVIEW VOLUME 135
FIG. 4. Schematic of the track and storm total precipitation (in.; 1 in. 2.54 cm) associated with Hurricane Floyd as reproduced from the Weekly Weather and Crop Bulletin (Vol. 86, No. 38). southeastern Pennsylvania to southern Massachusetts. hPa resides over southeast Texas, west of a relatively The decrease in minimum 1000-hPa height of 30 m may strong high pressure system with a maximum central be indicative of storm strengthening and/or an artifact height of about 180 m located northeast of Bermuda of the expanding scale of the transitioning storm. (Fig. 5d). The relative weakness of the ROC cyclone is Twenty-four hours after first exhibiting a LOC pre- more an artifact of the relatively small scale of the com- cipitation distribution (t 24), the cyclone center is posite cyclones than an actual indication of intensity as located over Long Island, New York (Fig. 5c). A local indicated in Table 1. Although strong geostrophic maximum in the thickness field has propagated pole- southerlies are evident in the eastern quadrant of the ward and is situated almost due east of Virginia, ap- composite TC, the TC remains isolated from any sig- proximately 300–400 km southeast of the cyclone cen- nificant baroclinic zone, precluding any notable tem- ter. Meanwhile, the cyclone continues to move toward perature advections. The only appreciable region of colder thickness values (576 dam). The axis of the ther- warm-air advection is located near the main band of mal trough has become negative as indicated by the westerlies to the northeast of the cyclone, on the north- solid line (an indication of the increasing magnitude of west periphery of the Bermuda high. differential cyclonic vorticity advection) with the The cyclone weakens (true for almost every member trough axis stretching from southeastern Michigan to of the composite) as it moves north toward an east– coastal Georgia. The minimum central height of the west-oriented baroclinic zone with a central minimum storm continues to decrease to 15 m at 1000 hPa. The height now of 90 m at 1000 hPa (t 12; Fig. 5e). The S-shaped configuration of the thickness field has be- strong high pressure system east of the cyclone center come more pronounced as the cyclone entrains colder remains the dominant feature on the map. As the ridge air into the southwestern quadrant of the storm. The axis retreats poleward of the cyclone, the geostrophic implied warm-air advection increases in magnitude as winds turn southerly on the west side of the anticy- the gradients in both the thickness field and height field clone, and the warm-air advection becomes more pro- intensify. It is now essentially impossible to distinguish nounced to the north-northeast of the TC center. As the structure of this system from that of an extratropical the TC continues to track to the northeast (t 24), it cyclone, suggesting that ET is occurring or has taken comes into closer contact with the baroclinic zone on place. the east side of a weak trough in the 1000–500-hPa At t 0 in the ROC composite, a relatively weak thickness field that has moved out of the Rockies and cyclone with a minimum central height of 75 m at 1000 into the western Plains (Fig. 5f). The orientation and
Unauthenticated | Downloaded 09/27/21 02:06 PM UTC JUNE 2007 ATALLAH ET AL. 2193
FIG. 5. The 1000–500-hPa thickness (dashed black lines, contoured every 60 m), 1000-hPa geopotential height (solid black lines, contoured every 30 m), and the 850–200-hPa wind shear (shaded, contoured every 5 m s 1, starting at 20 m s 1) for (a) LOC at t 0, (b) LOC at t 12, (c) LOC at t 24, (d) ROC at t 0, (e) ROC at t 12, and (f) ROC at t 24. Thick solid lines denote thermal trough axes. The thick arrows in (a) and (d) depict the composite storm tracks.
Unauthenticated | Downloaded 09/27/21 02:06 PM UTC 2194 MONTHLY WEATHER REVIEW VOLUME 135 strength of the thickness field indicate the storm is be- PVU). The upper-level PV anomaly takes on a negative ginning to experience increased west-southwesterly tilt as it wraps cyclonically around the surface cyclone. shear. The negative tilt indicates an increasingly potent trough owing to an increase in the differential positive PV ad- b. A PV perspective vection. Also at this time, a northeastward extension of the low-level PV anomaly becomes evident in associa- 1) HORIZONTAL MAPS tion with a developing warm front (Fig. 6c). A PV perspective shows that the composite TC ex- In contrast, the ROC composite at time t 0 shows hibits a maximum of approximately 0.75 potential vor- only a weak midlatitude trough located over Montana, ticity units (1 PVU 10 6 Kkg 1 m2 s 1) in the 850– approximately 2000 km to the north-northwest of the 700-hPa layer (lower troposphere; Fig. 6a). The wind composite TC (0.6-PVU anomaly) over eastern Texas field in this layer shows that the circulation center is not (Fig. 6d). A low-level southerly jet is located east of the quite closed, as evidenced by a trough in the wind field TC (not shown), while winds in the upper troposphere extending north of the low center (not shown). The are from the south-southwest across the TC center in storm is already experiencing some wind shear as indi- response to the weak thermal trough to the west (see cated by the 20 m s 1 southwesterly wind barb in the Fig. 5d). The lower-tropospheric PV anomaly dissipates 300–200-hPa layer (upper troposphere) over the cy- slightly as the composite storm moves to the northeast clone center. Meanwhile, a strong upper-level trough ahead of the midlatitude trough approaching the west- with a maximum of 5 PVU approaches the storm from ern Great Lakes (Fig. 6e). The low-level jet provides the northwest. This trough exhibits a positive tilt and is moisture into and ahead of the low-level PV anomaly located approximately 1500 km to the northwest of the lying across the upper Midwest. This implied moisture cyclone center. A band of positive PV advection is oc- transport can lead to enhanced precipitation ahead of curring on the east side of the trough, stretching from the midlatitude trough to the north, similar to the sche- northern Ohio to the Labrador coast of Canada, sug- matic illustration shown in Bosart and Carr (1978, their gestive of synoptic-scale forcing for ascent in this re- Fig. 3). As the storm moves toward the main band of gion. westerlies, the upper-level flow strengthens across the Twelve hours later (t 12), as the region of positive center of the TC from 5 to 10 m s 1 as the flow veers to potential vorticity advection overspreads the composite the southwest. TC, the area covered by the 0.75 PVU contour in the By t 24 h, the low-level PV anomaly is strengthen- 850–700-hPa layer increases considerably, while the dis- ing slightly as a region of weak positive PV advection tance between the cyclone and upper-level trough de- spreads equatorward (Fig. 6f). Brisk low-level south- creases (Fig. 6b). The positive PV advection now over- easterlies of 10 m s 1 (not shown) are now feeding into spreading the cyclone indicates that there is synoptic- a confluent zone beneath an equatorward jet-entrance scale forcing for ascent occurring over the cyclone region as indicated by the upper-level wind field. The resulting in an expansion in the scale of the low-level strength of the upper-level westerlies continues to in- PV anomaly. Shear has increased slightly as indicated crease across the TC as the storm moves poleward to- by the upper-tropospheric 25 m s 1 southwesterly ward the main baroclinic zone. winds over the cyclone center. There is a change in the 2) CROSS SECTIONS structure of the upper-level PV anomaly, which has ex- perienced considerable erosion over southern Ontario In Fig. 7a, the TC is represented by the relatively and eastern New York, presumably in association with isolated PV maximum at 700 hPa ( 0.7 PVU) at time the diabatic redistribution of PV by latent heat release t 0 in the LOC composite. A column of exceptionally (precipitation). Note that values of PV in this region low-PV air lies to the right of the TC in the cross section have decreased, in spite of the positive PV advection in and the TC itself is located underneath a steeply sloped this region evident in Fig. 6a. The front edge of this PV dynamic tropopause (DT; PV wall as discussed by anomaly now exhibits more anticyclonic curvature, re- Hoskins and Berrisford 1988) as measured by the 1.5- sulting in a trough with a more neutral tilt. In response PVU contour. At this time, the storm is still warm core to this rearrangement of the upper-level PV, the winds as indicated by the downward-bulging isentropes ahead of the trough turn more southerly, allowing the stretching from 200 hPa down to near the surface. The storm to track closer to the coast. midlatitude trough is located about 1500 km to the By t 24, the upper-level trough and the cyclone northwest of the storm as defined by the combination center are superposed (Fig. 6c). The lower-level PV of the lowest depression of the DT and the associated anomaly continues to grow and strengthen ( 0.9 upward bulge (cold-core structure) in the isentropes
Unauthenticated | Downloaded 09/27/21 02:06 PM UTC JUNE 2007 ATALLAH ET AL. 2195