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2448 MONTHLY WEATHER REVIEW VOLUME 137

Heavy Precipitation Associated with Southern Appalachian Cold-Air Damming and Carolina Coastal Frontogenesis in Advance of Weak Landfalling Tropical Storm Marco (1990)

ALAN F. SROCK AND LANCE F. BOSART University at Albany, State University of New York, Albany, New York

(Manuscript received 30 September 2008, in ﬁnal form 22 January 2009)

ABSTRACT

An analysis is presented of Tropical Storm Marco (1990), a storm that dropped copious amounts of rain over the southeast United States. Marco was noteworthy because of its role in the formation and evolution of two distinct episodes of cold-air damming and coastal frontogenesis over Georgia and the Carolinas. These mesoscale features led to greater than 300 mm of precipitation in 2 days over the near-coastal southeast United States; much of the rain occurred while Marco was over 400 km away. This case is further complicated by two other nearby tropical cyclones, which affected Marco’s track and the overall rainfall distribution. Synoptic and mesoscale analyses of the development of the coastal front and cold-air damming episodes show that the location of Marco helped to orient low-level winds toward the Appalachians. As rain developed inland, a pocket of relatively cool air, the ‘‘cool pool,’’ formed near the mountain slopes and was partially blocked by the higher terrain. Low-level analyses show that the coastal front on the oceanward edge of the cool pool became a focusing mechanism for ascent and precipitation, as moist, tropical air advected inland by Marco was forced upward at the density gradient. The results indicate that a weak tropical cyclone can directly effectuate intense precipitation distant from the storm center, both by causing moist tropical ﬂow toward land and by inducing mesoscale features that focus the precipitation and lead to heavy rainfall and ﬂooding.

1. Introduction ﬂooding from landfalling TCs are often the most dan- gerous threat because of the loss of life and the de- a. Purpose struction of property. Further study into the effect of Landfalling and near-landfalling tropical cyclones surface winds, wind shifts, and land/ocean airmass (TCs) pose a challenging forecast problem because of contrasts on the precipitation distribution of TCs was the potential devastation to people and property from suggested as a way to improve understanding and storm surges, strong winds, and heavy precipitation (e.g., forecast skill by the Fifth Prospectus Development Sheets 1990; Rappaport 2000). Although storm surges Team of the U.S. Weather Research Program (PDT-5; and high winds tend to be weaker when distant from the Marks et al. 1998). Nearshore surface features can storm, regions of heavy precipitation associated with a modify the ﬁnal TC precipitation distribution through TC can occur hundreds of kilometers away from the storm mesoscale effects such as orographically forced ascent center. In the case of landfalling and near-landfalling TCs, (upslope), coastal frontogenesis, and cold-air damming topography-related mesoscale features can combine (CAD). This paper will examine the formation and with enhanced forcing for ascent and moisture from enhancement of coastal fronts (CFs) and CAD events synoptic features and the TC to cause extremely dam- induced by the presence of a TC in proper position with aging, heavy rainfall over land. Rappaport (2000) states respect to the local terrain. that inland precipitation and associated freshwater The period of 9–13 October 1990 was chosen for study because two distinct episodes of coastal frontogenesis and CAD caused exceptionally heavy precipitation near Corresponding author address: Alan F. Srock, Dept. of Atmo- spheric and Environmental Sciences, University at Albany/SUNY, the southeast U.S. coast; these mesoscale features were Albany, NY 12222. enhanced by Tropical Storm Marco in the eastern Gulf E-mail: [email protected] of Mexico. Marco was the primary TC affecting the

DOI: 10.1175/2009MWR2819.1

Ó 2009 American Meteorological Society Unauthenticated | Downloaded 10/10/21 01:05 PM UTC AUGUST 2009 S R O C K A N D B O S A R T 2449

FIG. 1. Topography (shaded; m), signiﬁcant station location, cross-section line (dashed), and storm positions every 6 h (see legend for symbol reference) from 0000 UTC 9 Oct 1990 to 1200 UTC 13 Oct 1990. The TC position at 1200 UTC on a given date is shown by an open symbol next to the date.

Southeast’s rainfall during the period, but the remnants of the southern Appalachians, each coincident with a of Hurricane Klaus and the rapid approach of Hurri- shallow, intense CF over near-coastal Georgia and cane Lili from the east near the end of the period greatly South Carolina. affected the final rainfall pattern (Fig. 1 shows the tracks of the TCs). Marco and the remnants of Klaus were b. Previous work responsible for over 500 mm of precipitation near the Georgia–South Carolina border, $57 million in damage Topographic effects can significantly enhance pre- (in 1990 dollars), and seven deaths (all due to inland cipitation in any rainfall-producing system. Orographic flooding) from the heavy rainfall (Mayfield and Lawrence precipitation enhancement without a CF or CAD is 1991). The rainfall distribution with this case is uncom- generally related to upslope effects (e.g., Passarelli and mon compared to other landfalling TC events, since much Boehme 1983; Barros and Kuligowski 1998). Intention- of the heaviest rainfall in coastal Georgia and the Caro- ally selecting cases without CFs, Passarelli and Boehme linas occurred while Marco was . 400 km away. Figure 2 (1983) found that regions of upslope precipitation re- shows the National Centers for Environmental Prediction/ ceived 20%–60% more rainfall than nearby flat or Hydrological Prediction Center (NCEP/HPC) archived downslope terrain. Focusing solely on precipitation di- 24-h accumulated precipitation ending at 1200 UTC 11 rectly attributable to a landfalling TC, Haggard et al. October and 1200 UTC 12 October, along with Marco’s (1973) looked at 70 yr of TCs that made landfall in position at the end of each period (Figs. 2a,b, respec- the United States and subsequently traversed the Ap- tively). The heaviest rainfall in both periods is located palachian Mountains (defined as the TC center passing between the Appalachian Mountains and the coast; over elevation greater than 300 m). The authors found however, the highest rainfall totals in the 24-h period TCs that crossed high topography usually had pre- ending at 1200 UTC 11 October appear over near- cipitation maxima in the areas of sharpest elevation coastal South Carolina, 12 h before Marco makes increase. landfall in the Florida Panhandle. In this paper, we will Cold-air damming (e.g., Richwein 1980; Forbes et al. show that the presence and location of Marco were 1987; Bell and Bosart 1988) refers to the process where crucial for the development of two CAD episodes east cold air is slowed by a topographic barrier and moves

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FIG. 2. The 24-h accumulated precipitation (contoured every 25 mm starting at 50 mm; contours end at 150 mm) ending at (a) 1200 UTC 11 Oct 1990 and (b) 1200 UTC 12 Oct 1990. The location of Marco at the end of the time period is denoted by the M. Adapted from archived image at NCEP/HPC (courtesy of W. Junker). preferentially in the direction of the pressure gradient Bell and Bosart (1988) found that CAD east of the force. This occurs most often on the eastern side of Appalachians most frequently occurs in late fall and approximately north–south-oriented mountains with a early winter, when the temperature difference is great- high pressure system to the north, as cold air moving est between the warm ocean and cold land. Their study toward the eastern slopes has insufficient kinetic energy also found a low-level wind maximum oriented parallel to go over the barrier and is then forced to decelerate. to the mountains in the cold air, which helped to push The deceleration weakens the effect of the Coriolis the cold dome equatorward. Fritsch et al. (1992) sug- force, so the cold air will preferentially move away from gested that the cold pool could be maintained by the the higher pressure to the north, creating an equator- blocking of incoming solar radiation due to cloud cover ward bulge of high pressure and low temperature (e.g., over the cold dome and evaporative cooling due to Bailey et al. 2003, their Fig. 1). Forbes et al. (1987) precipitation falling through the cold dome; however, showed that the presence of the cold pool was important Brennan et al. (2003) found evaporative cooling in the for the location and type of precipitation. As warmer, cold air only helped to maintain the temperature deficit moister air from the Atlantic Ocean was advected to- while the cold pool was unsaturated. To further cate- ward the mountains, it was lifted up and over the density gorize CAD events, Bailey et al. (2003) objectively gradient at the edge of the cold pool, instead of at the classified members of a CAD climatology. Their pri- mountainside topography gradient as would be ex- mary division between CAD types depended on for- pected with terrain-driven ascent alone. mation and maintenance method—whether the polar

Unauthenticated | Downloaded 10/10/21 01:05 PM UTC AUGUST 2009 S R O C K A N D B O S A R T 2451 cold air was advected into the region by equatorward ence of a CF along with the poleward moisture transport synoptic forcing, cooled in situ due to heavy rain and was shown to focus precipitation at and inland of the CF. evaporative cooling, or a combination of both. Most of Precipitation ahead of a landfalling and recurving TC their cases showed a clear signal of high pressure to the track can also be greatly modified by extratropical north or northeast of the mountains, but, in the few transition (ET; e.g., Bosart and Carr 1978; Bosart and cases contained in their last category (unclassifiable), a Lackmann 1995; Klein et al. 2000; Jones et al. 2003; primary feature leading to CAD development was often Evans and Prater-Mayes 2004). The ET process occurs a low pressure to the south. as a TC morphs from a warm-core to cold-core cyclone Coastal fronts (e.g., Bosart et al. 1972; Bosart 1975; and gains extratropical cyclone characteristics (e.g., Nielsen 1989) are thermal and density airmass bound- wind field asymmetries and fronts). An upstream trough aries that form primarily as a result of land–sea tem- and downstream ridge are necessary components for perature contrasts. Usually forming approximately 12 h ET, although the relative strength of the TC vorticity before the passage of a coastal low pressure system, CF and the upstream vorticity in the trough can vary. locations and intensities are affected by ‘‘orography, During ET, precipitation tends to shift toward left of coastal configuration, land–sea temperature contrast and track, as enhanced ascent ahead of the upstream trough friction’’ (Bosart et al. 1972). Coastal fronts are usually shifts the region of strongest precipitation toward the shallow features (e.g., Nielsen and Neilley 1990), and approaching trough (Atallah et al. 2007). A good syn- CAD can assist in the strengthening of the CF gradient if opsis of the current understanding of ET is presented in the cold pool extends close enough to the coastline. Jones et al. (2003, see especially their Fig. 11). However, Nielsen and Neilley (1990) point out that Binary interaction (e.g., Fujiwhara 1921, 1923; Brand CAD and CF events can and often do occur without a 1970) also plays a significant role in Marco’s evolution, significant presence of the other. Riordan (1990) exam- given the proximity of Hurricanes Klaus and Lili. ined a CF that formed off the coast of the Carolinas over Dritschel and Waugh (1992) and Prieto et al. (2003) the sea surface temperature gradient at the edge of the studied the effect of distance, storm radii, and relative Gulf Stream. As this CF moved inland, the temperature vorticity strength on different forms of binary interac- gradient intensified as the CF interacted with an already- tion. These studies have shown that cyclonic vorticity existing CAD event against the Appalachians, which led centers tend to orbit each other cyclonically; however, to especially heavy precipitation in the region. the type of interaction can vary greatly (see especially Coastal fronts are also often found directly ahead of a Prieto et al. 2003, their Fig. 2). TCs track. Bosart and Carr (1978) and Bosart and Dean The paper will be organized as follows. Section 2 will (1991) studied the heavy precipitation ahead of Hurri- discuss the data used for the study. Section 3 will present cane Agnes (1972), which occurred up to one day ahead a synoptic overview of Marco and the surrounding en- of Agnes on a line stretching from Virginia to New vironment, while section 4 will look at the mesoscale York. The authors noted a surface convergence line co- effects (i.e., CAD and CF) that led to the enhanced incident with a strong temperature and dewpoint gradi- rainfall. Section 5 will discuss these results and offer ent, driven primarily by land–sea thermal contrasts. This conclusions and suggestions for future work. CF was directly attributable to the tropical cyclone, forming near the shore and continuing ahead of the TC 2. Data sources as Agnes moved north. Atallah and Bosart (2003) and Colle (2003) examined aspects of the precipitation dis- The primary sources for gridded data are the 40-yr tribution of Hurricane Floyd (1999) through synoptic and European Centre for Medium-Range Weather Fore- modeling analyses; these studies found that precipitation casts (ECMWF) Re-Analysis (ERA-40; Uppala et al. ahead of Floyd’s track was generally enhanced along the 2005), with 1.1258 horizontal grid spacing, 22 vertical CF from approximately 12 h before through the time of levels (pressure coordinates), and 6-h temporal resolu- storm passage. tion, and the NCEP North American Regional Rean- Cote (2007) discussed predecessor rain events (PREs), alysis (NARR; Mesinger et al. 2006), which has ;32-km defined as rainfall . 100 mm in 24 h ahead of a TC. A horizontal grid spacing and 29 vertical levels, with 3-h PRE’s primary moisture source must be advected pole- temporal resolution. Both reanalyses were compared ward by the TC, but the lifting mechanism and precipi- with observations to ensure the validity of any conclu- tation are not directly attributable to the TC. His work sions drawn using the data. Considering the relatively showed that the average distance from TC to PRE is 935 coarse grid spacing of the ERA-40, the exact positioning km, with a ;36-h time lag before TC passage. Only some and values of near-surface fields and derived quantities of the cases in Cote (2007) involved a CF, but the pres- like vertical motion should be treated with some

Unauthenticated | Downloaded 10/10/21 01:05 PM UTC 2452 MONTHLY WEATHER REVIEW VOLUME 137 skepticism; however, relative maxima and minima still previous times (not shown), had moved off to the provide useful information. The ERA-40 proved supe- northeast as the precursor to Lili began to travel west- rior for locating the tropical cyclones and for large-scale ward under the ridge. By 1200 UTC 10 October, the flow features, but comparison of the reanalyses with primary midlatitude surface cyclone was over Indiana observed soundings and surface data suggests that the and Ohio, associated with the thermal trough over the NARR better represented the state of the atmosphere Midwest, whereas the sharpest 1000–500-hPa thickness on the mesoscale in the CF and CAD regions. Surface gradient was located west of the Appalachians, stretch- data were collected from three sources at NCAR: the ing southwestward to the Mississippi Delta (Fig. 3b). A NCEP Automated Data Processing Global Surface weak CAD signature was present in the central Caro- Observations (ADP), the U.S. Air Force Data Surface linas, as shown by a MSLP maximum, which bulged Airways, version 3 (DATSAV3), and the International southwestward parallel to the mountain axis. At the same Comprehensive Ocean–Atmosphere Dataset (ICOADS); time, an area of 700-hPa ascent ,24.0 3 1023 hPa s21 duplicate stations were removed from the file. The was over near-coastal Florida, Georgia, and South National Hurricane Center (NHC) best-track dataset Carolina. Marco had moved over the Florida Keys, and [i.e., the Atlantic basin hurricane database (HURDAT)] a tightening of the pressure gradient to its east and was used for tropical cyclone positions while the TCs northeast over the previous 24 h suggested an even were contained in the dataset; note that Lili was clas- stronger feed of tropical air. Precipitation was heavy sified as a subtropical storm until 0000 UTC 11 October. in parts of central Georgia and South Carolina during Also, Klaus was not tracked in HURDAT after 1200 this period, as shown by the accumulated rainfall at UTC 9 October, so its position for the next 24 h was Columbia (CAE), South Carolina, Augusta (AGS), subjectively determined using low-level vorticity from Georgia, and Athens (AHN), Georgia (Fig. 4). the NARR and ERA-40 and nearby surface observa- By 1200 UTC 11 October, the dominant 700-hPa as- tions. After 1200 UTC 10 October, Klaus’s location was cent maximum was located ahead of the synoptic-scale indeterminable using available data. Other data sources thermal trough stretching from North Carolina to include archived radiosonde data from the National Ontario (Fig. 3c). The offshore shift of the 700-hPa as- Climatic Data Center (NCDC) and visible satellite im- cent maximum near South Carolina suggests that pre- ages from the University at Albany archive. cipitation in the most heavily flooded areas (over near- The General Meteorological Package (GEMPAK; coastal Georgia and South Carolina) waned during this Koch et al. 1983) was used for all data archival and most period. Radar summaries (not shown) and accumulated of the plotting routines. To facilitate calculation of dif- precipitation (Fig. 4) at the time confirm that the stron- ferences and gradients using surface data, the irregu- gest echoes and rainfall had moved toward the coast and larly positioned surface observations were interpolated offshore, and would remain there over the next 12 h. At using a Barnes analysis (e.g., Barnes 1964) in GEMPAK 1200 UTC 11 October, Marco was near its peak inten- to a 0.3830.38 horizontal grid. The interpolation was sity just off the western coast of Florida, barely grazing only performed on the land- and buoy-based observa- the coast for its first landfall (Fig. 1; e.g., Mayfield and tions (ADP and DATSAV3), since the ship data from Lawrence 1991). Marco made a second landfall at TD the ICOADS were too sparse and erratic to include at strength near 0000 UTC 12 October, and had reached the grid spacing desired over land for objective analysis. central Georgia by 1200 UTC 12 October (Fig. 1). Thus, surface data figures will plot station observations Rainfall had returned to central Georgia and South and subjectively analyzed contours using both land and Carolina by 1200 UTC 12 October (see Fig. 4), as shown marine observations, but objectively analyzed features by two distinct 700-hPa ascent maxima within 300 km of will be shaded using only the ADP and DATSAV3 data. Marco: one just off the northern Georgia coast and the other over higher terrain in northern Georgia (Fig. 3d). Marco weakened further after this time, and the asso- 3. Synoptic (large scale) setup ciated ascent and precipitation induced by the TC decreased rapidly as well. a. Case overview b. Large-scale environment and key features Marco officially reached tropical storm strength over northern Cuba at 1200 UTC 9 October. At that time, the At 1200 UTC 9 October, a southwesterly jet .75 tightest mean sea level pressure (MSLP) and 10002500- ms21 on the dynamic tropopause (DT; e.g., Hoskins et al. hPa thickness gradients were located just ahead of the 1985; Nielsen-Gammon 2001) stretched from Texas to midlatitude trough over the western Great Plains (Fig. 3a). Hudson Bay on the downstream side of the associated A surface high, which dominated the central Atlantic at DT potentially cold (cyclonic vorticity) anomaly (Fig. 5a).

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23 21 FIG. 3. ERA-40 MSLP (contoured, hPa), 1000–500-hPa thickness (dashed; dam), and 700-hPa vertical motion (shaded; 310 hPa s ) at 1200 UTC for (a) 9 Oct, (b) 10 Oct, (c) 11 Oct, and (d) 12 Oct 1990. The positions of Klaus, Lili, and Marco are indicated by using the symbol convention in Fig. 1.

The equatorward entrance region of this jet was lo- Marco, or a combination of both. Marco and the rem- cated over a cold front with the same orientation (not nants of Klaus moved primarily northward over the next shown), stretching southwestward from the low-level 24 h (Fig. 1), although Klaus shows a much more pro- cyclone over Lake Erie (Fig. 5a). Farther downstream, nounced westward path. Klaus’s westward shift was potential temperatures on the DT near Marco and likely partially due to a binary interaction; Marco was Klaus were . 360 K, and nearby wind shear between far stronger than Klaus at this time, and the TCs were the DT and 850-hPa levels was anticyclonic (not within 1000 km of each other, suggesting Klaus was shown), suggesting a warm-core structure through the undergoing some form of straining out while moving depth of the troposphere, consistent with 1000–500-hPa cyclonically around Marco (e.g., Prieto et al. 2003). thickness values .576 dam (Fig. 3a). The poleward A visible satellite image from 1831 UTC 10 October advection of this warm air would have likely aided in shows the strong tropical moisture feed east of Florida building the ridge that was developing over the western that curved cyclonically around Marco’s northern and Atlantic. The ERA-40 analyzed stronger low-level eastern ﬂank and brought warm, moist air to the coastal vorticity to the west of Klaus’s estimated position at Carolina and Georgia region (Fig. 6a). Backward tra- 1200 UTC 10 October (Fig. 5b); this may be related to jectories calculated from the ERA-40 at approximately Klaus advecting too far west in the model representa- the same time show the source region of the air at 850 tion, vorticity elongation stretching northward from hPa over coastal South Carolina and Georgia was located

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FIG. 4. The 3-h accumulated precipitation (mm) ending at the period listed for CAE (black bars), AGS (gray bars), and AHN (white bars) for periods ending 1200 UTC 10 Oct–1800 UTC 12 Oct 1990. Data are from the NCDC hourly precipitation dataset. over the tropical Atlantic between 950 and 1000 hPa 48 h Carolina to Pennsylvania formed from a combination earlier (Fig. 7a). These trajectories approximately follow of vorticity advection over the Appalachians (not the track of Klaus (Fig. 1), and are consistent with a stream shown) and lee troughing on the eastern slopes. Visible of air with precipitable water (PW) values .55 mm that satellite from 1331 UTC 11 October shows the cloud had reached Georgia and South Carolina at 1800 UTC shield above Marco and coastal Georgia and South 10 October. For comparison, the mean precipitable Carolina, as well as the continued moisture feed from the water derived from soundings at Charleston, South tropics (Fig. 6b). Backward trajectories from 1200 UTC Carolina for the month of October (from 1948 to 2005) 11 October conﬁrm the near-surface, tropical source is approximately 33 mm, whereas the plus-two standard with PW values .45 mm advecting around the east side deviation-level for the same period is ;50 mm (see of Marco toward the Georgia coast (Fig. 7c). However, http://www.crh.noaa.gov/unr/include/pw.php?sid5chs); PW values at the Georgia coast are nearly 10 mm less at this shows the anomalously high moisture available for 1200 UTC 11 October than at 1800 UTC 10 October; precipitation, especially during the early stages of this since both low-level trajectories show a south to south- event. Extrapolation of Klaus’s track indicates that the east (tropical) source region, the difference in PW near moist air from Klaus would have reached the southeast the coast was most likely due to the added moisture U.S. coast around 1800 UTC 10 October, suggesting that associated with Klaus at the former time. the especially high PW values near the coast are partially At 1200 UTC 12 October, Marco was located south- due to the remnant moist air of the decaying TC. Back- east of the upper-level jet associated with the synoptic ward trajectories from 0000 UTC 11 October show a trough to the west and the associated positive PV continuation of the tropical feed, although the ﬂow at the maximum (Fig. 5d). Jones et al. (2003) stated that a TC time was more southerly (Fig. 7b). Precipitation between just downstream of an upper-level jet and positive PV 1200 UTC 10 October and 1200 UTC 11 October was maximum is in a favorable position for ET, but Marco exceptionally heavy (greater than 150 mm) over coastal did not redevelop or accelerate poleward over the next Georgia and South Carolina (Fig. 2a); thus, much of the 24 h. Instead, Marco slowed its northward motion and decrease in precipitable water between 1800 UTC 10 turned to the east. Marco’s northward deceleration was October and 1200 UTC 11 October would likely be ex- partially a result of the upper-level trough lifting out plained by heavy rainfall over the southeast United States. and not picking up the TC, whereas its eastward shift By 1200 UTC 11 October, the primary low-level vor- was likely related to another binary interaction with ticity maximum associated with the midlatitude trough Lili, which was moving rapidly westward (Fig. 1). Both had moved north of the Great Lakes, but the DT trough TCs had a northward component of motion after this still stretched equatorward to the Gulf of Mexico (Fig. 5c). time, but the weakening Marco moved southeastward The low-level vorticity maximum stretching from North relative to (or cyclonically around) Lili. Therefore,

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21 21 21 FIG. 5. ERA-40 dynamic tropopause potential temperature (shaded; K), winds (m s ; full barb is 5 m s and pennant is 25 m s ), and 925–850-hPa mean relative vorticity (contoured every 0.5 3 1024 s21 starting at 0.5 3 1024 s21) at 1200 UTC for (a) 9 Oct, (b) 10 Oct, (c) 11 Oct, and (d) 12 Oct 1990. The positions of the TCs are as in Fig. 3. instead of potentially undergoing ET and rapidly lifting Most of the synoptic features discussed in this section out of the region with the trough to the north, Marco’s suggest a favorable environment for precipitation over poleward progression slowed, which kept the storm Georgia and the Carolinas, but these features alone over South Carolina. cannot fully account for the extremely heavy and lo- Visible satellite imagery at 1331 UTC 12 October calized rainfall with this case. The next section will focus showed good vertical extent in the cloudiness just ahead on the low-level mesoscale features that led to the local of Marco (Fig. 6c). The southwestern sharp back edge extreme enhancement of precipitation. of convection near the Georgia–South Carolina border was primarily due to the advection of low-level cold air 4. Mesoscale analysis around Marco from west of the Appalachians (discussed a. Surface observations further in section 4). Backward trajectories from 1200 UTC 12 October again show a tropical, near-surface To locate the CAD/CF events, surface observations source region for the air over coastal South Carolina and and meteograms will be used to show the formation and Georgia, with PW .45 mm (Fig. 7d). Again, Marco’s development of the integral mesoscale features. Al- circulation sustained the tropical moisture feed along though a front should be technically based solely on a the storm’s eastern side, which led to the continued thermal and/or density gradient (e.g., Sanders 1999), a heavy rainfall. dynamical deﬁnition may also assist in locating the

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FIG. 6. Visible satellite images from University at Albany archive for (a) 1831 UTC 10 Oct, (b) 1331 UTC 11 Oct, and (c) 1331 UTC 12 Oct 1990. The position of Marco is given by M at 1800 UTC 10 Oct, 1200 UTC 11 Oct, and 1200 UTC 12 Oct 1990, respectively.

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21 21 21 FIG. 7. ERA-40 PW (shaded above 25 mm), 850-hPa winds (m s , where full barb is 5 m s and pennant is 25 m s ), and 48-h back trajectories from 850-hPa starting at (a) 1800 UTC 10 Oct, (b) 0000 UTC 11 Oct, (c) 1200 UTC 11 Oct, and (d) 1200 UTC 12 Oct 1990. Trajectory shadings (blue, purple, gray, and black) represent the 850–900-, 900–950-, 950–1000-, and .1000-hPa layers, respectively. The TC positions are as in Fig. 3.

boundary. For the thermal definition of a front, surface uy began to bulge southwestward to the east of the virtual potential temperature (uy) is used, since it elimi- Appalachians, with a strong uy gradient at the ocean- nates effects from varying elevation and is directly re- ward edge of the potentially cold pool (Figs. 8a,b). lated to density. For the dynamical definition, we will use Winds blew almost directly onshore at Georgia and South the horizontal velocity gradient tensor, which accounts Carolina coastal stations, whereas inland winds were for convergence, deformation, and vorticity (VTEN; e.g., oriented roughly parallel to the mountains. A .10 K 21 Stonitsch and Markowski 2007). The velocity gradient (100 km) uy gradient was located to the west of the tensor will be large when horizontal convergence, con- Appalachians, associated with a cold front trailing fluence, or vorticity is large in the area; since all of these southward from the synoptic cyclone to the north (see can be significant in frontal zones, regions of large VTEN Figs. 3b and 5b), but remained west of the mountains suggest that the dynamical aspects of fronts may be throughout both CF/CAD events. A cyclonic wind shift 21 present in the area. A region with a tight uy gradient and was collocated with the ;8 K (100 km) coastal uy gra- large VTEN collocated would suggest the potential for a dient in coastal North and South Carolina, while another convectively active frontal area. cyclonic wind shift was located near the ;4 K (100 km)21

As the ﬁrst signiﬁcant CF/CAD event began around uy gradient in southeast Georgia and northeast Florida. 1200 UTC 10 October, a region of high pressure and low Although the intensities of both wind shifts had not

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FIG. 8. Observed surface analysis of (a) MSLP (contoured; hPa), temperature (dashed; 8C), and standard station observations at 1200 UTC 10 Oct; and (b) virtual potential temperature (plotted and contoured; K), velocity gradient tensor (objectively shaded every 2 3 1025 s21 starting at 4 3 1025 s21), and winds (m s21, where full barb is 5 m s21 and pennant is 25 m s21) at 1200 UTC 10 Oct 1990. The TC positions are as in Fig. 3.

Unauthenticated | Downloaded 10/10/21 01:05 PM UTC AUGUST 2009 S R O C K A N D B O S A R T 2459 reached the minimum threshold for VTEN shading, the CAD/CF event was shorter lived and more transitory, as combination of the uy gradient and wind shift suggest a the both the tightest uy gradient and highest VTEN CF-type boundary was forming in both locations. values stayed just ahead of Marco and remained oriented By 1800 UTC 10 October, the temperature at nearly parallel to the mountains. Previous CF research Charleston (CHS), South Carolina has increased 28Cin has emphasized a strong CF event and intense precipi- 6 h, whereas the temperature at CAE remained nearly tation 12–24 h ahead of the storm passage along the constant over the same period (Fig. 9a; see Fig. 1 for storm track (e.g., Bosart and Dean 1991; Atallah and station locations). Sea level isobars from central North Bosart 2003; Colle 2003); the development of the second Carolina to northeastern Georgia bulged to the south- CF event just ahead of Marco shows similar character- west, highlighting the relatively high pressure air par- istics. Cooler and drier continental air was beginning to tially blocked by the mountains; winds in this region advect toward Marco’s southern ﬂank from west of the were oriented nearly parallel to the mountain axis, as Appalachians around 1200 UTC 12 October (Fig. 12), expected with CAD. Along the coast, the uy gradient in limiting the primary convection to the eastern and southeastern North Carolina had weakened over the northern sides of the weakening storm as Marco con- previous 6 h (Fig. 9b), suggesting that differential dia- tinued to move northeast (Fig. 6c). Cooling from clearing batic heating due to the lack of cloud cover over land had over eastern North Carolina helped to tighten the uy weakened the northern end of the boundary (Fig. 6a). gradient at the coast, but daytime heating weakened this

However, over Georgia and South Carolina, cloudiness gradient over the next 12 h (not shown). The uy gradient and precipitation limited the inland heating and main- and VTEN ahead of Marco also weakened as Marco tained a stronger uy gradient (e.g., Fritsch et al. 1992). In dissipated after this time. southern Georgia and northeastern Florida, rainfall and To further examine the temporal evolution of the two cloudiness over land helped to maintain the cold pool intense CF/CAD periods, meteograms were constructed and tighten the uy gradient, while the collocated VTEN for CHS, CAE, AHN, and AGS (Fig. 13). At CHS (Fig. increased to .6 3 1025 s21 over the same 6-h period. 13a), winds were generally from the southeast through- At 0000 UTC 11 October, the temperature and out both CF events (approximately 1200 UTC 10 Octo- dewpoint at CHS continued to rise during the day, ber–0600 UTC 11 October and 0000–1200 UTC 12 Oc- whereas both decreased slightly at CAE (Fig. 10a). The tober). Winds at CHS had a northerly component from combination of higher pressure to the northeast and 1800 UTC 11 October to 0000 UTC 12 October; during Marco to the south-southwest helped to maintain geo- this period, radar summaries position the primary con- strophic flow toward the Appalachians, and heavy rain vection offshore, and only CHS (near the coast) re- continued inland in the Carolinas. The overlap of the uy ported significant rainfall accumulation (not shown). gradient and the VTEN maximum was especially ap- Inland at AHN, rainfall was generally light through the parent at this time over near-coastal Georgia and South first event and only strong for a few hours of the second Carolina, attesting to the strong thermal and dynamical CF event (Figs. 4 and 13b). During both intense CF structure of this CF. Since extrapolation of Klaus’s track events, winds at AHN had a northeasterly component, placed the decayed TC inland over the Carolinas by suggesting that AHN was in the cold dome for both 0000 UTC 11 October, the exceptionally moist air as- CAD events. The temperature and dewpoint remained sociated with the former TC (PW . 55 mm, 6 h earlier; nearly constant at AHN and stayed colder than CHS Fig. 7a) was forced up and over the CF, leading to en- throughout the period, suggestive of a persistent surface hanced rainfall. Soon after this period, both the uy gra- boundary between these two stations. dient and VTEN weakened in Georgia and the Caro- Two stations in the center of the cold pool highlight linas, and neither reinvigorated significantly until ;0000 the two intense periods of heavy precipitation. At CAE UTC 12 October (not shown). (Fig. 13c), moderate and heavy rain began just after At the peak of the second CF/CAD event (0600 UTC 1200 UTC 10 October and continued though 0700 UTC 12 October), dynamical forcing took on a larger role, as 11 October. Light, intermittent precipitation followed shown by the large swath of VTEN . 4 3 1025 s21 ahead until 0000 UTC 12 October; then, after a few hours of of Marco (Fig. 11). During this second CF/CAD event, moderate rain, died off again until Marco moved nearly the CF boundary was located farther inland, likely be- overhead around 1500 UTC 12 October. Observations cause it was advected closer to the mountains by the were similar at AGS (Fig. 13d), where precipitation star- cyclonic flow around Marco. Winds inland paralleled the ted just after 1200 UTC 10 October, weakened by 0600 mountains and were oriented nearly perpendicular to UTC 11 October and was intermittent until 0000 UTC isobars, suggesting that ageostrophic flow was dominant 12 October, when moderate rain and thunderstorms were and that CAD was an important feature. This second reported (cf. the heavy precipitation recorded between

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FIG. 9. As in Fig. 8, but for 1800 UTC 10 Oct 1990.

0600 and 1200 UTC 12 October in Fig. 4). During both ing that both CAE and AGS were within the cold pool. CF/CAD events, winds at CAE and AGS were from the The two distinct periods of signiﬁcant precipitation were northeast, whereas temperatures averaged ;38C colder reported at both stations in the center of the cold pool than CHS but only slightly warmer than AHN, illustrat- while the CF boundaries were most intense over land.

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FIG. 10. As in Fig. 8, but for 0000 UTC 11 Oct 1990. b. Vertical structure needed to measure the depth of the cold air. The NARR was chosen to supplement sounding and surface obser- To determine the vertical structure of the CAD/CF vations for examination of this case, as it had the best events, a high-resolution, three-dimensional dataset was resolution and the most accurate low-level winds of

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FIG. 11. As in Fig. 8, but for 0600 UTC 12 Oct 1990. the available datasets. Comparison of a plan view of mary regions of frontogenesis roughly collocated (the

975-hPa frontogenesis from the NARR (Fig. 14a) with synoptic cold front and CF). The NARR uy gradient just frontogenesis calculated from surface observations southeast of the Appalachians was nearly 8 K (100 km)21 (Fig. 14b) at 0000 UTC 11 October shows the two pri- stronger than observed, primarily because of the

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FIG. 12. As in Fig. 8, but for 1200 UTC 12 Oct 1990.

especially low uy pool of air in central Georgia. This general structure of the thermal and height (pressure) thermal analysis discrepancy explains the higher front- fields are sufficiently similar to suggest that qualitative ogenesis values at the coast in the NARR representa- analyses can be performed using NARR low-level fields tion. Except for the temperature in the cold pool, the at 0000 UTC 11 October. Comparisons of other times

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FIG. 13. Meteograms of surface observations from 0000 UTC 10 Oct to 1800 UTC 12 Oct 1990 of temperature (8C), dewpoint (8C), MSLP (hPa), winds (m s21, where full barb is 5 m s21 and pennant is 25 m s21), and present weather for (a) CHS, (b) AHN, (c) CAE, and (d) AGS.

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FIG. 14. (a) NARR 975-hPa geopotential heights (contoured; m), virtual potential temperature (dashed; K), horizontal frontogenesis [shaded beginning at 2 K (3 h)21 (100 km)21], and winds (m s21, where full barb is 5 m s21 and pennant is 25 m s21) at 0000 UTC 11 Oct 1990. (b) Surface MSLP (contoured; hPa), virtual potential temperature (dashed; K), horizontal frontogenesis [shaded beginning at 2 K (3 h)21 (100 km)21] and winds (m s21, where full barb is 5 m s21 and pennant is 25 m s21) at 0000 UTC 11 Oct 1990.

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FIG. 15. Comparison of NARR (solid blue) and observed (dashed red) soundings for (a) CHS and (b) AHN at 0000 UTC 11 Oct 1990. Winds are plotted using same convention as in Fig. 14. and locations shows that the NARR was too cool in the represent the vertical structure of the atmosphere suf- cold dome in both events, which led to slightly stronger ficiently well throughout this case. winds parallel to the mountains as the along-mountain Cross sections from the NARR showing the low-level pressure gradient intensified (not shown). structure of the two CAD/CF events are shown in NARR-derived soundings are compared with ob- Fig. 16 (see Fig. 1 for cross-section line). At 1200 UTC served soundings at 0000 UTC 11 October to show the 10 October, when the first CF/CAD episode began, the validity of NARR vertical profiles of temperature, near-surface wind shift boundary was coincident with dewpoint, and wind. At CHS (Fig. 15a), the thermo- the eastern edge of the cold pool and an area of front- dynamic and wind fields show good qualitative and ogenesis .5 K (100 km)21 (3 h)21 (Fig. 16a; cf. Fig. 8b). quantitative agreement, though the near-surface air in This near-surface frontogenesis caused enhanced forc- the NARR is slightly cooler and drier than in the ob- ing for ascent at the boundary, which combined with servations. The NARR and observations at AHN show background isentropic lift to induce heavy precipitation good qualitative agreement above 550 hPa but have at and just inland of the leading edge of the cold dome. important differences at low levels (Fig. 15b). The pri- This temperature and wind structure continued through mary discrepancy is the temperature and dewpoint be- 1800 UTC 10 October and 0000 UTC 11 October (Figs. low 900 hPa; the NARR temperature in the lowest 16b,c). Though the NARR cold pool became too cold 100 hPa is ;58C colder than observations. Above the cold during this event (as noted earlier), the area above the dome, the NARR and observed winds generally agree cold dome between the mountains and the coast was within ;5ms21 and ;208 in orientation throughout the still dominated by ascent, with an ascent maximum di- depth of the troposphere, excepting a significant westerly rectly above the surface horizontal uy gradient and shift between 550 and 750 hPa in the observed data, frontogenesis maximum. Thus, one would have ex- suggestive of warm-air advection in that layer. Com- pected the strongest precipitation at and just inland of parison of other soundings with the NARR at other the CF (e.g., CAE, Figs. 4 and 13c), whereas stations times show that the cold dome is ;58C too cool during farther inland would have accumulated far less rain both CAD events (not shown). However, except for the during this period (e.g., AHN; Figs. 4 and 13b). The top thermal structure of the cold dome, the NARR seems to of the cold pool was approximately the height of the

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23 21 FIG. 16. NARR cross sections of virtual potential temperature (contoured; K), vertical motion (dashed; 310 hPa s , only con- touring ascent), horizontal winds (m s21, where full barb is 5 m s21 and pennant is 25 m s21), and horizontal frontogenesis [shaded beginning at 2.5 K (3 h)21 (100 km)21] at (a) 1200 UTC 10 Oct, (b) 1800 UTC 10 Oct, (c) 0000 UTC 11 Oct, and (d) 0600 UTC 12 Oct 1990. The cross-section line is shown in Fig. 1; the approximate location of the coastline is delineated by the triangle below the plot. higher terrain to the northwest, so flow retardation due boundary and ascent maximum move north-northeastward to CAD was still occurring. The low-level air is likely ahead of Marco (not shown). too stable in the NARR representation, since the lapse rate at the top of the cold pool is not as strong in the 5. Discussion and conclusions observed thermal vertical structure; however, given the thermal structure indicated in the observed soundings, A case study of the development of CAD and associ- some flow blocking would be expected. Winds contin- ated CFs induced and enhanced by nearby Tropical Storm ued to blow parallel to the Appalachians in the cold Marco has been presented. In this case, the presence, lo- dome to the southeast of the ridge axis, whereas cation, and track of Marco, along with Marco’s interac- southerly and southeasterly flow dominated both the tion with both Hurricanes Klaus and Lili, were neces- offshore flow and the flow above the cold dome. sary ingredients for heavy precipitation in near-coastal A cross section through the peak of the second CF Georgia and the Carolinas. By helping to induce and event shows a similar structure at 0600 UTC 12 October enhance two distinct CAD/CF events, relatively weak (Fig. 16d). The core of the frontogenesis was closer to TCs were able to cause significant damage through co- the mountains in this case, as the CF was advected pious amounts of rainfall and resultant flooding. farther inland by the easterly flow around Marco. The Cold-air damming was important because it shifted the cross section at this time sliced through the core of the primary core of ascent toward the CF at the oceanward maximum NARR ascent, which was farther inland and edge of the cold dome. The development and maintenance above the strongest low-level frontogenesis. The second of these CAD events are noteworthy because the pres- CF event is more transitory as the low-level density sure gradient against the southern Appalachians was

Unauthenticated | Downloaded 10/10/21 01:05 PM UTC 2468 MONTHLY WEATHER REVIEW VOLUME 137 primarily caused by a low to the south of the mountains. The heaviest precipitation in this case fell primarily Although there was a fairly strong high over the central within 200 km of the coast (Figs. 2a,b). As the cold dome Atlantic at the beginning of the first CAD/CF event developed and the boundary at the coastal edge evolved (around 1200 UTC 10 October), its location alone was into a CF, moist tropical onshore flow was forced up and not favorable for the ideal pressure gradient against the over the cold dome. The CF, just inland from the coast, southern Appalachians. As Marco developed and moved acted as a focusing mechanism for ascent during each into the eastern Gulf of Mexico, the pressure gradient event. Mesoscale analyses of uy gradient and VTEN tightened and became more orthogonal to the spine of highlighted both the thermodynamic and dynamic fea- the Appalachians. Without Marco reorienting and in- tures of the CF. The heaviest precipitation (Fig. 2) and tensifying the pressure gradient, the development of this intense flooding noted over near-coastal Georgia and CAD event would have likely been far weaker. Branick South Carolina (Mayfield and Lawrence 1991) were col- et al. (1988) discussed a mesoscale convective system located with the strongest ascent and most favorable which traveled eastward out of New Mexico and Texas VTEN and uy gradient. The NARR reasonably repli- toward the Georgia and Florida coasts. They noted that cated both CAD/CF episodes, and its analysis showed CAD formed in the Carolinas due to pressure falls induced the expected strong ascent at and just inland of the CF by the passage of a squall line through Georgia. Marco (Figs. 16a–d). Unlike the results from Haggard et al. caused a similar development of CAD (predominantly (1973), which found the heaviest TC rainfall in regions of induced by lower pressures to the south), but was even flow ascending over higher terrain, the heaviest precipi- more damaging to the Carolinas because of the tropical tation with Marco was shifted away from the mountains moisture advection toward and over the cold dome. by the CAD/CF episodes. Thus, the position, longevity, Traditionally, CAD has been considered a cold-season and intensity of the two CAD/CF events were important phenomenon, where a high pressure system to the north for the overall precipitation location and distribution. led to geostrophic winds oriented toward the moun- The moist remnants from Hurricane Klaus also added tains. With a dominant northern high, the cold dome to the heavy rainfall in the first CAD/CF event. The often develops through the advection of cold air as well PW difference between the ERA-40 at 1800 UTC as in situ cooling (e.g., Bailey et al. 2003). However, 10 October (Fig. 7a) and 1200 UTC 11 October (Fig. 7c) especially in the first CAD/CF event in this study, the shows a decrease in PW of ;10 mm in the core of the poleward source of cold air (through advection) is a tropical moisture plume over the southeast United minor factor, since much of the flow directed at the States. As the remnants of Klaus moved over coastal mountains has tropical or near-tropical origins. Cooling Georgia and South Carolina, the especially moist air in central South Carolina and near-coastal Georgia in would have been forced up and over the leading edge of the clear air overnight preceding 1200 UTC 10 October the CF boundary, creating additional rain and helping to likely combined with latent cooling from nearby pre- maintain the cold dome. If Klaus was not present, heavy cipitation to create the initial pocket of cooler air, which rainfall would probably still have been recorded in was then maintained by the continuous rain and lack of Georgia and the Carolinas, but Klaus’s additional mois- solar heating due to cloud cover during the first heavy ture most likely amplified the final precipitation totals. rainfall episode. This conclusion can be verified by ex- Previous research has discussed the heavy precipita- amining the development of the cold dome; the difference tion associated with landfalling TCs; however, many of between uy inside and outside of the cold pool is small those studies have focused on the heaviest rainfall oc- before precipitation begins (not shown), but magnifies curring just ahead and within 12–24 h of storm passage with time because the onshore, oceanic flow warms dur- (e.g., Bosart and Dean 1991; Atallah and Bosart 2003; ing the day (see Figs. 8b, 9b, and 10b). CAD development Colle 2003). The results of these studies show a similar without a dominant poleward high would be expected far evolution and rainfall distribution to the second CAD/ less frequently than with a cold poleward high, since it CF event studied herein. Bosart and Carr (1978) and requires both a strong low equatorward of the mountains Cote (2007) examined cases of enhanced precipitation andmechanismstodiabaticallycoolandthenmaintain well downstream of the TC. In both studies, poleward the cool air in the blocked region (e.g., radiative cooling, moisture transport on the eastern side of the TC was the evaporative cooling from precipitation, cloud cover). For primary source of rain. Cote (2007), looking at storms this reason, a cold dome in a CAD event primarily caused affecting the East Coast, stated that the average PRE by a low to the south would likely be better termed a forms nearly 1000 km away and 36 h ahead of TC pas- ‘‘cool pool,’’ since it is relatively cold in comparison with sage. The first CAD/CF event with Marco could be its surroundings, but significantly warmer than the cold considered a form of PRE because the additional pool in a classic wintertime CAD event. moisture over 400 km and 36 h away from Marco was

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FIG. 17. Conceptual model of a cold-air damming and coastal front event induced by a nearby tropical cyclone. Important features are denoted on the diagram. advected there primarily by the TC. However, this case on the leeward side of the mountains. Although this differs from the traditional PRE because the augmented type of CAD/CF development is uncommon, the po- precipitation was not induced by midlatitude, synoptic- tential for significant precipitation well ahead of a scale features, but was rather focused to a specific region nearby TC should be considered if flow blocking is by CAD and coastal frontogenesis, mesoscale phenom- possible against nearby topography. ena directly induced by the nearby TC. These results highlight the need to consider weak TCs Acknowledgments. Funding for this work was pro- as possible heavy rain producers, even at a distance. vided by NSF Grants ATM-0304254 and ATM-0553017, Marco (and the remnant moisture from Klaus) inun- as well as an AMS 21st Century Campaign Graduate dated near-coastal Georgia and South Carolina with Fellowship. The authors thank Dr. Anantha Aiyyer and precipitation. Figure 17 shows a conceptual model that Jason Cordeira for their assistance in developing the includes some of the key features in this event. A backward trajectory code, Celeste Iovinella for assorted tropical cyclone west of Florida suggests geostrophic technical help with the manuscript preparation, and flow oriented toward the Appalachian Mountains, with Nicholas Metz for many useful discussions throughout the moist tropical air advected around the TC’s eastern side research process. The authors also thank two reviewers toward the southeast U.S. coast. If a thermal boundary for their comments on the manuscript. Much of the data or CF already exists at the coastline (possibly formed by were provided by the National Climatic Data Center, the differential diabatic heating or land/sea temperature ECMWF, and the Data Support Section of the Compu- contrasts), the tropical air would be forced to rise at the tational and Information Systems Laboratory at NCAR. boundary and would lead to heavy precipitation at and NCAR is supported by grants from the National Science just inland of the CF. Precipitation which fell on the Foundation. inland side of the CF would evaporatively cool the air until saturation, and would help to decrease the tem- REFERENCES perature in the cool pool. The more dense, colder air Atallah, E. H., and L. F. Bosart, 2003: The extratropical transition would experience flow blocking, which would in turn and precipitation distribution of Hurricane Floyd (1999). enhance CAD and cause a push of cold air equatorward Mon. Wea. Rev., 131, 1063–1081.

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