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The Role of Synoptic-Scale Flow during Tropical over the North

W. E DWARD BRACKEN AND LANCE F. B OSART Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York

(Manuscript received 13 February 1998, in ®nal form 22 January 1999)

ABSTRACT The synoptic-scale ¯ow during and over the North Atlantic Ocean is inves- tigated using compositing methods. Genesis and lysis are de®ned using the National Hurricane Center (NHC, now known as the Tropical Prediction Center) best-track data. Genesis (lysis) occurs when NHC ®rst (last) identi®es and tracks a tropical depression in the ®nal best track dataset. -centered composites are created with the Analysis of the Tropical Oceanic Lower Level (ATOLL; ϳ900 hPa) and 200-hPa winds for June± November produced by NHC for the years 1975±93. Results show that signi®cant regional differences exist in 200-hPa ¯ow during genesis across the Atlantic basin. Composites of genesis in the western part of the basin show a 200-hPa (ridge) located to the west (east) of the ATOLL disturbance. In the eastern half of the basin composites of genesis show a sprawling 200-hPa ridge centered northeast of the ATOLL disturbance. The major axis of this elliptically shaped 200-hPa extends zonally slightly poleward of the ATOLL level disturbance. Another composite of relatively rare genesis events that are associated with the equatorward end of frontal boundaries show that they generally occur in the equatorward entrance region of a jet streak in conjunction with an ATOLL cyclonic vorticity maximum in a region where vertical shear is minimized. An approximation of the Sutcliffe±Trenberth form of the quasigeostrophic omega equation is used to estimate the forcing for vertical motion in the vicinity of developing tropical . Forcing for ascent is found in all three genesis composites and is accompanied by a nonzero minimum in vertical shear directly above the ATOLL cyclonic vorticity maximum. Vertical shear over developing depressions is found to be near 10 m sϪ1, suggestive that weak shear is necessary during tropical cyclogenesis to help force synoptic-scale ascent. Composites of tropical lysis show much weaker ATOLL cyclonic vorticity when compared to the genesis composites. The magnitude of the vertical shear and the forcing for ascent above the lysis ATOLL disturbance are stronger and weaker, respectively, than in the genesis composites. These differences arise due to the presence of a jet- streak and a longer half-wavelength between the trough and ridge axes in the lysis 200-hPa ¯ow composite. The genesis ¯ow patterns are decomposed by crudely removing the signature of the developing cyclone and its associated . Two separate and very different ¯ow patterns commonly observed during genesis over the eastern and western Atlantic Ocean are found to be very similar once the ¯ows are decomposed. Both ¯ows are characterized by strong deformation at low levels and at 200 hPa with an upper-level jet exit region near the developing depression.

1. Introduction for today's operational tropical is fore- casting which cloud cluster will be the seedling that Animations of satellite imagery illustrate the ubiquity later becomes a . How can one distin- of organized cloud clusters (horizontal scale typically guish between a cloud cluster that will develop and one ϳ400±600 km) in the Tropics. Observations of these cloud clusters (Simpson and Riehl 1981) have shown that will not develop? What processes in¯uence the tran- that only 1%±2% develop into tropical cyclones. Fun- sition from cloud cluster to tropical cyclone? This paper damental atmospheric observations in tropical cyclone will try to address these questions by focusing upon the breeding grounds are available from ships, satellites, role that synoptic-scale systems play in North Atlantic aircraft, and land-based stations, but they are too limited tropical cyclogenesis. in both space and time to adequately sample the cloud The process of tropical cyclogenesis involves a com- clusters. Consequently, one of the most dif®cult tasks plex interaction between different scales of atmospheric motion. In general, the Tropics are characterized by a decrease in entropy of saturated air with height and consequently are conditionally unstable to undilute par- Corresponding author address: Dr. W. Edward Bracken, Depart- cel ascent. Ooyama (1964) and Charney and Eliassen ment of Earth & Atmospheric Sciences, The University at Albany/ SUNY, 1400 Washington Ave., Room ES-234, Albany, NY 12222. (1964) recognized that the circulations of a larger-scale E-mail: [email protected] tropical vortex and embedded cumulus clouds could op-

᭧ 2000 American Meteorological Society

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC 354 MONTHLY WEATHER REVIEW VOLUME 128 erate together in a conditionally unstable atmosphere in tropospheric anticyclone (or ridge) (e.g., Riehl 1948, such a way as to allow the system to amplify in time. 1950; Erickson 1963), 2) an upper-tropospheric cyclone This idea of system ampli®cation caused by a cooper- (or trough) (e.g., Koteswaram and George 1957; Ra- ation between the larger-scale vortex and the cumulative mage 1959; Erikson 1967; Sadler 1967, 1975, 1976, effects of cumulus clouds was termed conditional in- 1978; Yanai 1968; McBride and Keenan 1982; Davidson stability of the second kind (CISK). Emanuel (1986, et al. 1990; Bosart and Bartlo 1991; Reilly 1992; Mont- 1989, 1991) and Rotunno and Emanuel (1987) take a gomery and Farrell 1993; Molinari et al. 1995; Briegel different view. They hypothesize that tropical cyclones and Frank 1997), 3) a lower-tropospheric wind surge are maintained against dissipation by moist convection (e.g., Morgan 1965; Fujita et al. 1969; Love 1985a,b; whose energy source is supplied entirely by self-induced Lee et al. 1989; Zehr 1991; Briegel and Frank 1997), anomalous ¯uxes of moist enthalpy from the sea surface and 4) an upper-tropospheric jet streak (e.g., Bracken with almost no contribution from preexisting condi- and Bosart 1997). tional instability. This type of instability has been The initial cyclonic vorticity maximum that becomes termed wind-induced surface heat exchange (WISHE). the tropical cyclone can be very broadly placed into Theoretically, genesis is a continuous series of events, several categories: 1) a disturbance associated with a or more accurately, a process that culminates in the monsoon trough or the intertropical convergence zone formation of a self-sustaining vortex. Very generally, (Riehl 1954, 1979), 2) a disturbance embedded in east- genesis can be thought of as a process that leads to the erly trade wind ¯ow such as the easterly wave (EW) formation of a tropospheric deep vortex and enables that (Carlson 1969a,b; Burpee 1972, 1974, 1975; Reed et al. vortex to enter a state of self-development or self-in- 1977; Saha et al. 1981; Thorncroft 1995; Thorncroft tensi®cation. This intensi®cation of the vortex can be and Hoskins 1994a,b), 3) stagnant subtropical frontal measured by an increase in interrelated quantities like zones that originate in midlatitudes (Frank 1987), 4) old vorticity and wind speed or a decrease in central pres- midlatitude mesoscale convective systems (MCSs; Bos- sure. Some theories hypothesize that this state of self- art and Sanders 1981), and 5) upper-level cutoff lows development begins when these quantities reach a ®nite that penetrate to lower levels (Avila and Rappaport value (i.e., CISK- or WISHE-like processes begin to 1996). All of the work presented here will focus on become important). In reality, however, the vortex may tropical cyclogenesis in the North Atlantic Ocean basin. never become fully self-sustaining and the onset of this Almost all of the cyclones in the North Atlantic region instability is not instantaneous. In general, the instability have their seeds in the form of African EWs (Riehl 1954; gradually becomes more important as a warm core forms Erickson 1963; Simpson et al. 1968; Frank 1970), old within the vortex. It is this internal instability in con- frontal boundaries that originated in midlatitudes (e.g., junction with external environmental in¯uences such as Bosart and Bartlo 1991), old midlatitude MCSs (e.g., the (SST) distribution and en- Bosart and Sanders 1981), or the low-level re¯ection of vironmental dynamics and thermodynamics that deter- an upper-level cutoff low (Avila and Rappaport 1996). mine the ultimate intensity of the cyclone. Events lead- This paper will examine composite synoptic-scale ing up to the formation of a persistent, tropospheric deep ¯ows using a gridded dataset in the vicinity of devel- vortex will be considered the genesis process. Addi- oping tropical depressions. More speci®cally, this paper tionally, before one can claim an understanding of trop- will identify and examine the synoptic-scale ¯ows most ical cyclogenesis one must also gain an understanding commonly observed during North Atlantic basin trop- of the processes that inhibit genesis and those that cause ical cyclogenesis, and explore the fundamental dynam- tropical cyclone demise (cyclolysis). ics that are implied by those ¯ows. Composites are cre- Previous research on tropical cyclogenesis (i.e., Gray ated for genesis events in very limited areas across the 1968, 1988) has revealed several necessary conditions Atlantic basin and will be storm-centered. This will al- for tropical cyclogenesis. The four most widely agreed low an examination and comparison of genesis events upon are the following: 1) the presence of a synoptic- occurring in climatologically different background scale low-level cyclonic vorticity maximum stronger ¯ows. Section 2 will discuss the dataset used to create than the background planetary vorticity, 2) average oce- the composites. Section 3 will discuss the procedure anic mixed layer temperatures at least 27ЊC, 3) back- used to create the composites. In section 4 genesis and ground planetary vorticity greater than 0.8±1.3 ϫ 10Ϫ5 lysis composite ¯ows and composite diagnostics will be sϪ1 (the value of the Coriolis parameter 3Њ±5Њ off the presented. Section 5 and 6 will present the results of equator), and 4) vertical shear of the horizontal wind the compositing and the conclusions, respectively. over the 850±200-hPa layer generally Ͻ10±15 m sϪ1 [Zehr (1991, 1992) states that shear Ͻ12.5 m sϪ1 is 2. Data conducive for genesis]. In addition, some studies (e.g., Colon and Nightingale 1963; Fett 1966; Gray 1988) Gridded wind analyses for the North Atlantic Ocean have stressed a ®fth condition that involves some ex- created by the National Hurricane Center (NHC, now ternal in¯uence acting upon the disturbance to promote known as the Tropical Prediction Center) and archived genesis. External in¯uences may include 1) an upper- at the Hurricane Research Division of the National Oce-

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC FEBRUARY 2000 BRACKEN AND BOSART 355 anic and Atmospheric Administration/Atlantic Ocean- ographic and Meteorological Laboratories (NOAA/ AOML/HRD) for the years 1975±93 will be used here to examine the synoptic-scale ¯ow during genesis. Twice-daily analyses of the meridional and zonal wind components were available at two levels: Analysis of the Tropical Oceanic Lower Layer (ATOLL ϳ900 hPa) and 200 hPa (Shapiro 1986; Goldenberg and Shapiro 1996). This ATOLL±200-hPa dataset covers a region from about 124Њ to 5ЊW and 45ЊN to the equator. The grid has 35 and 80 points in the meridional and zonal direction, respectively, giving the dataset a resolution of about 1.5Њϫ1.5Њ. FIG. 1. Formation points of tropical depressions over the North Atlantic Ocean that later went on to become tropical (marked An advantage of the ATOLL±200-hPa dataset is that by triangles) for 1975±93. Points of cyclolysis of tropical depressions the analyses were created using the same methods over (between 30Њ and 70ЊW and equatorward of 25ЊN) that never became a 20-yr period. National Meteorological Center (NMC; tropical storms (marked by squares) for 1975±87. now the National Centers for Environmental Prediction) analyses at 850 and 200 hPa were used as a ®rst guess for the ATOLL±200-hPa analyses. Rawinsondes, pibal that all hurricane forecasters and satellite analysts will winds, ®xed and moving ship surface observations, and wait until persistent deep moist convection is present in manually derived satellite winds were used to improve association with a cyclonic surface circulation before upon the NMC 850-hPa analysis, yielding the ATOLL declaring a system a depression. One might then con- analysis. Aircraft reports, 200-hPa rawinsonde obser- sider the conditions presented in the composites as those vations and manually derived satellite winds were also favorable for persistent deep moist convection in as- used to improve upon the NMC 200-hPa analysis, yield- sociation with a cyclonic surface circulation and weak ing the ®nal 200-hPa analysis. A more detailed discus- shear, conditions highly favorable for tropical depres- sion of the dataset and methods used to create the an- sion formation. alyses can be found in Wise and Simpson (1971). A discussion on the caveats of using this ATOLL±200- 3. Compositing procedure hPa dataset is given in section 5a of this paper. The position of tropical depressions are determined For the purpose of compositing, tropical cyclogenesis by using the NHC best-track data (Jarvinen et al. 1984). (cyclolysis) is de®ned as the time a depression ®rst (last) A second dataset identical in format to the NHC best- appears in the NHC best-track data or the lysis best- track data was used to identify cases of tropical cyclo- track data. Storm-centered composites for an area lysis during the period 1967±87 (hereafter called the ϳ3000 km ϫ 3000 km were created for cyclones during lysis best-track dataset). This dataset was created at 1975±93. Data for each storm is used only once in the NHC during an effort to create a CLIPER-type (cli- composites and only at the time of genesis or lysis. The matology and persistence; Neumann 1972) forecasting genesis of several storms in the composite occurred near model for tropical depressions (C. J. Neumann 1998, the edge of the data domain. For these storms using a personal communication). Unfortunately, the effort to composite area of ϳ3000 km ϫ 3000 km results in create this new forecasting model was halted in 1988 regions of missing data near the edge of the composite when the individual compiling the data passed away. domain. This problem was addressed by not computing Therefore, it might be expected that the data in the lysis a composite at those grid points where data for a par- best-track dataset would be of the same quality as the ticular storm were missing. As a result, some composite original best-tract dataset. This lysis best-track dataset analyses will have regions of missing data along the can be obtained by contacting the ®rst author of this southern and eastern edges. paper. A map of genesis across the basin (Fig. 1) reveals The NHC best-track dataset is not completely objec- two large regions where genesis is favored: 1) east of tive and some caveats on using it will be discussed here. the southeast United States and over the Gulf of Mexico, The dataset was created partially by using the subjective and 2) in the 10Њ±20ЊN band between Africa and South interpretations of hurricane forecasters and satellite an- America. An examination of all genesis events in each alysts. Hence the starting time of a depression given by of these large regions above (158 genesis events in total) NHC might be in error. The foremost reason for this shows that the synoptic-scale ¯ow differs considerably possible error is the lack of observations near devel- between the two regions (not shown). Smaller repre- oping depressions. This may result in an incorrect clas- sentative subregions were selected from the larger re- si®cation of system strength. It is hoped that these types gions to examine the differences in ¯ow during genesis of errors will be random and the compositing procedure (Fig. 1). Choice of genesis subregion locations was pri- will average them out. It should be expected, however, marily based upon the expectation that an increase in

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(١␨avg, (1 ´ frequency and coverage of observations in those regions ␻ ϰϪVt would yield a more reliable analysis. The ®rst genesis where ␻ ϵ vertical velocity (dp/dt), ␨ ϵ relative vor- subregion (24 storms) is the Bahamas region (20Њ±30ЊN, ␨ ϩ ␨ )/2, and)١ ١␨ ϵ , ticity, V ϵ V Ϫ V 60Њ±80ЊW; composite storm position of 26.4ЊN, t 200 900 avg 900 200 ϰϵ``is proportional to.'' The reader is cautioned that 69.6ЊW). The second genesis subregion (31 storms) is this assessment of the sense of vorticity by the Cape Verde region (5Њ±20ЊN, 15Њ±40ЊW; composite the thermal wind given in (1) is highly approximated. storm position of 12.8ЊN, 28.1ЊW). A third composite Using a value of 10Ϫ4 msϪ2 (approximately an accel- (nine storms; composite storm position 18.0ЊN, 85.6ЊW) eration of 10 m sϪ1 over 24 h) for the inertial acceler- of genesis events occurring late in the season near mid- ation and a value of 10 m sϪ1 for the wind velocity, the latitude frontal boundaries in the Gulf of Mexico and magnitude of the Rossby number (Ro) at 15Њ and 10Њ Caribbean Sea will also be presented. A fourth com- lat is ϳ0.25 and 0.40, respectively. These Ro values posite (18 storms; marked by squares in Fig. 1; com- suggest that to ®rst approximation QG theory can be posite storm position 15.8ЊN, 53.8ЊW) consists of over- used qualitatively to help understand the dynamics of water cyclolysis events in the Atlantic basin equator- the composite ¯ows. ward of 25ЊN and between longitudes 30Њ and 70ЊW (total number of overwater lysis events over the entire Atlantic basin was 58). Cyclolysis occurring poleward 4. Compositing results of 25ЊN will be excluded to minimize any biases created a. Bahamas and Cape Verde genesis cases by cold SSTs and strong vertical shear characteristic of midlatitudes and to eliminate the possibility of sampling At the ATOLL level the Bahamas and Cape Verde a tropical cyclone that is becoming extratropical. composite ¯ow streamlines and isotachs (Figs. 2a,c) In a series of papers, McBride (1981a,b) and McBride show ¯ow near the center of the disturbance taking on and Zehr (1981) presented an observational analysis of the shape of an ``inverted V,'' similar to patterns given tropical cyclogenesis in the Atlantic and Paci®c basin. in Frank and Johnson (1969) and Frank (1969). In both Twelve composite datasets were created from rawin- regions the axis of the inverted V is slightly tilted south- sonde data comparing developing and nondeveloping west±northeast with an axis of maximum winds on the poleward side of the inverted V. Based upon the char- cloud clusters. Composites given in McBride and Zehr acteristics in Fig. 2, it seems probable that most of the (1981) showed that developing cloud clusters in the ATOLL disturbances in the Bahamas and Cape Verde western Atlantic were accompanied by a trough±ridge composites are EWs, likely of African origin. The com- couplet in the 200-hPa streamline analyses (their Fig. posite ATOLL relative vorticity shows a maximum in 11). They concluded that relatively strong upward ver- cyclonic vorticity associated with both genesis cases Ϫ1 ( 2cmsϪ1) tical motion on the order of 100 hPa day ϳ (Figs. 3a,c). Although both composite ¯ows at the within a circle 4Њ latitude in radius centered on the clus- ATOLL level are very similar, the composite 200-hPa ter is needed for it to develop. ¯ows are very different (Figs. 2b,d). Contrasting the Synoptic-scale vertical velocities can be qualitatively 200-hPa streamlines in the two genesis composites inferred for the composites shown in this paper using shows that, in general, genesis in the Bahamas region diagnostics based upon quasigeostrophic (QG) theory. is associated with a highly ampli®ed trough±ridge pat- In the QG omega equation the vertical velocity is forced tern and genesis in the Cape Verde region is associated by two competing and overlapping processes: 1) dif- with a zonally oriented elliptically shaped anticyclone. ferential vorticity advection, and 2) the Laplacian of In the Bahamas region, composite 200-hPa isotachs thermal advection. Although an alternative vertical mo- and relative vorticity show several interesting features. tion method based on Q vectors (Hoskins et al. 1978) Strong westerlies are observed along the poleward edge avoids the cancellation problem of the traditional meth- of the domain (Fig. 2b) with weaker ¯ow on each side od, both methods would be impractical here since only of the trough axis. A 200-hPa relative vorticity dipole wind data at ϳ900 and 200 hPa is available. Instead, straddles the ATOLL vorticity maximum with cyclonic the sense of the synoptic-scale forcing will be approx- (anticyclonic) vorticity upstream (downstream) (Fig. imated for each composite by taking advantage of the 3b). At 200-hPa the cyclonic vorticity axis does not Trenberth (1978) QG methodology whereby vertical extend very far poleward and is con®ned to the equa- motion is forced through vorticity advection by the ther- torward side of the strong westerlies, possibly indicating mal wind. Sutcliffe (1947) was the ®rst to demonstrate that this composite trough includes a number of low- the importance of vorticity advection by the thermal latitude trough fracture events (Dean and Bosart 1996). wind for the development of cyclones and . The bulk of the cyclonic vorticity maximum is con®ned A highly simpli®ed version of the Sutcliffe±Trenberth to the southwest quadrant of the composite domain and omega equation will be used here to help locate mid- is located downstream of a positively tilted ridge axis. tropospheric regions where forcing for synoptic-scale Finally, a narrow band of cyclonic vorticity extends east ascent and descent may be present at the level of QG and then northeast from the equatorward edge of the theory. The method is based on vorticity maximum. A larger-scale composite (not

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FIG. 2. Composite ¯ow streamlines (thin solid), isotachs (every 1 m sϪ1; thick solid), and divergence (every 0.2 ϫ 10Ϫ5 sϪ1 in all panels except every 0.1 ϫ 10Ϫ5 sϪ1 in e; greater than 0.2 and less than Ϫ0.2 shaded with positive values surrounded by thin solid lines and negative values surrounded by thin dashed lines) at the ATOLL level for (a) Bahamas subregion, (c) Cape Verde subregion, and (e) lysis subregionp; and the 200-hPa level for (b) Bahamas subregion, (d) Cape Verde subregion, and (f) lysis subregion. Thin dotted lines cross at the center of the composite depression. Composite domain approximately 3000 km ϫ 3000 km. Blank areas in analyses are regions where data from a storm were missing (see section 3 for further explanation). shown) illustrates that this narrow band of cyclonic vor- is seen over the equatorward half of the domain (Fig. ticity is actually the tail end of the time-mean tropical 2d). As a result of this ¯ow con®guration the ridge axis upper-tropospheric trough (Sadler 1976) over the At- at 200 hPa extends almost zonally northeast of the lantic Ocean. ATOLL vorticity maximum and then arcs southwest- Composite 200-hPa relative vorticity and isotachs in ward ahead of the low-level disturbance over the west- the Cape Verde composite also show several key fea- ern half of the domain. Therefore, for genesis cases in tures. Relatively strong westerly ¯ow is observed on the the Cape Verde region the ATOLL vorticity maxima is poleward edge of the domain and strong easterly ¯ow observed to be squeezed between the poleward edge of

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FIG. 3. Composite ¯ow streamlines (thin solid) and relative vorticity (every 0.3 ϫ 10Ϫ5 sϪ1; with cyclonic vorticity thick solid and anticyclonic vorticity thick dashed) at the ATOLL level for (a) Bahamas subregion, (c) Cape Verde subregion, and (e) lysis subregion; and the 200-hPa level for (b) Bahamas subregion, (d) Cape Verde subregion, and (f) lysis subregion. Thin dotted lines cross at the center of the composite depression. Composite domain approximately 3000 km ϫ 3000 km. a relatively strong easterly jet and the ridge axis at 200 1970). It will be assumed here that vertical velocity at hPa. In addition, relatively strong westerlies are ob- the ground and 100 hPa are zero and divergence varies served at 200 hPa poleward of the ridge axis, especially linearly between levels. Calculations of the kinematic northeast of the ATOLL vorticity maxima. vertical velocity for both genesis composites (not Vertical motions on the synoptic scale are too small shown) indicate the upward vertical velocity over the to be measured directly, but since divergence is avail- developing cyclone center averages 1±2 cm sϪ1 or 70± able at the ATOLL and 200-hPa levels (Fig. 2), the 120 hPa dayϪ1, numbers quantitatively very similar to average vertical velocity in the ATOLL±200-hPa layer the results given in McBride and Zehr (1981). It should may be computed using the kinematic method (O'Brien be noted, however, that small uncertainties in the hor-

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FIG. 4. Composite Sutcliffe±Trenberth forcing (contours every 5 ϫ 10Ϫ11 sϪ2; with positive forcing shaded and surrounded by thin solid lines and negative forcing by thin dashed lines) for (a) Bahamas subregion, (c) Cape Verde subregion, and (e) lysis subregion; and the ATOLL±200- hPa vertical shear of the horizontal wind (alternating solid and dashed lines at 1 msϪ1 intervals, and shaded greater than 16 m sϪ1) for (b) Bahamas subregion, (d) Cape Verde subregion, and (f) lysis subregion. Thin dotted lines cross at the center of the composite depression. Composite domain approximately 3000 km ϫ 3000 km. izontal wind ®eld can create large errors in vertical ve- ing vertical velocities can be developed using QG the- locities computed using the kinematic method. There- ory. As discussed earlier, the Sutcliffe±Trenberth meth- fore, the quantitative estimates of vertical velocities giv- od of inferring vertical motion is used here to locate en here should be viewed with caution. However, the regions of forcing for upward vertical motion. Regions divergence patterns displayed in both genesis compos- of inferred upward (downward) vertical motion will co- ites (ATOLL convergence and 200-hPa divergence) cer- incide with regions of positive (negative) forcing or tainly are consistent with upward vertical motion above cyclonic vorticity advection (anticyclonic vorticity ad- the ATOLL vorticity maximum. vection) by the thermal wind. The Bahamas composite Other perhaps more desirable techniques of comput- (Fig. 4a) shows a very well-de®ned maximum (mini-

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC 360 MONTHLY WEATHER REVIEW VOLUME 128 mum) in forcing, and therefore a maximum in inferred hPa the Bahamas composite shows cyclonic vorticity upward (downward) motion, directly over and slightly advection over and east of the depression position; northeast (west) of the ATOLL vorticity maximum. A therefore, in the Bahamas composite, vorticity advection second maximum in forcing for ascent appears in the increases with height over and to the east of the de- extreme northwest corner of the domain. Two minima pression center. Similarly, in the Cape Verde composite, straddle the ®rst maximum in forcing, indicating in- vorticity advection at the ATOLL level is cyclonic (an- ferred downward motion in those areas. ticyclonic) west and southwest (south and southeast) and The use of QG theory permits a diagnosis of what is nearly neutral north of the depression center. At 200 hPa physically causing (or forcing) upward vertical motion cyclonic vorticity advection is concentrated in a band over the developing disturbance; the kinematic method oriented northeast±southwest located over and slightly does not allow such a physical interpretation. The Sut- northwest of the depression center. Therefore, in the cliffe±Trenberth forcing shows that it is the high-am- Cape Verde composite, vorticity advection increases plitude 200-hPa trough±ridge couplet in the Bahamas with height over and north of the depression center. This genesis composite that creates a situation favorable for qualitative visual inspection of vorticity advection pat- cyclonic vorticity advection by the thermal wind and terns in the Bahamas and Cape Verde region support implied upward vertical motion over the ATOLL vor- the existence of upward motion over the developing ticity maximum. That upward vertical motion in turn depression. likely creates an environment conducive for persistent, Upward vertical motion over a low-level vorticity organized, deep moist convection near the center of the maximum, by itself, does not bring about tropical cy- ATOLL vorticity maximum. clogenesis. Weak vertical is also a necessary, Sutcliffe±Trenberth forcing computed for the Cape but still insuf®cient, condition for tropical cyclogenesis. Verde composite also shows inferred upward vertical Theoretically, shear will in¯uence how mass is circu- motion (Fig. 4c), although the forcing here is weaker lated or ventilated through a vertical column in the cloud than in the Bahamas composite. Note that a given cluster (Gray 1968). If cloud-cluster ventilation is small, amount of synoptic-scale forcing will yield a stronger then enthalpy and moisture in a vertical column can response (stronger upward or downward vertical veloc- increase and hydrostatically result in reduced surface ities) in a warm air mass with small static stability (as pressure. If the ventilation is large, then mass circulation is found in the Tropics) when compared to a cool air through the column is too fast to allow enthalpy and mass with larger static stability [as is found in the mid- moisture to concentrate and sustained surface pressure latitudes (e.g. Doswell 1987)]. In the Cape Verde com- falls cannot occur. An alternate explanation for why posite, the area of inferred upward motion is found near strong vertical shear decreases the intensity of tropical the ATOLL vorticity maximum. The upward vertical cyclones was presented by DeMaria (1996). That paper motion over the developing disturbance is being forced hypothesized that midlevel warming, associated with a primarily through the advection of smaller values of shear-induced tilted maximum in potential vorticity, sta- anticyclonic vorticity toward regions of larger values of bilizes the lower troposphere and reduces convective anticyclonic vorticity, not through the advection of cy- activity near cyclone centers. clonic values of vorticity toward regions of anticyclonic The vertical shear in these composites is computed vorticity as was found in the Bahamas case. What is over the ATOLL±200-hPa layer. In the Bahamas com- important dynamically for forcing upward vertical mo- posite (Fig. 4b) the vertical shear is relatively large over tion is that vorticity advection over the disturbance is the poleward and equatorward edges of the domain. An in the cyclonic sense, the sign of the vorticity does not oblong region of weak shear is observed near the center matter dynamically. Therefore, in the Cape Verde case and immediately south and west of the developing dis- it is the strong anticyclone and the cyclonic vorticity turbance. The Cape Verde composite (Fig. 4d) also advection along its equatorward ¯ank at 200 hPa that shows stronger shear both poleward and equatorward creates an environment favorable for upward vertical with a region of weak shear oriented east-northeast to motion. west-southwest across the center of the developing dis- The results from the Sutcliffe±Trenberth method are turbance. Therefore, both genesis composites do show consistent with qualitative expectations from the QG weak shear, a situation favorable for enthalpy and mois- omega equation. In that equation vertical motion is ture concentration near the center of the developing dis- forced by differential vorticity advection and the La- turbance. placian of temperature advection. If one assumes that Vertical shear in the ATOLL±200-hPa layer at the temperature advection is negligible in the Tropics, then time of genesis was calculated over the grid point near- upward vertical motion occurs where vorticity advection est the depression center and the eight grid points that increases with height. A diagnosis of vorticity advection surround it. The values from those nine grid points were (not shown, but may be qualitatively determined with then averaged to come up with a value of shear directly Figs. 3a±d) indicates that at the ATOLL level in the over the depression. Those calculations show that shear Bahamas composite vorticity advection is cyclonic (an- is weak but nonzero for both the Bahamas and Cape ticyclonic) west (east) of the depression center. At 200 Verde composites during genesis. In the Bahamas com-

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC FEBRUARY 2000 BRACKEN AND BOSART 361 posite the mean shear magnitude is 11.9 m sϪ1 and the in a very strong gradient of shear across the ATOLL mean shear direction is southwesterly (from 215Њ). In vorticity maximum. In addition, the areal coverage of the Cape Verde composite the mean shear magnitude is strong shear is much larger and closer to the ATOLL 9.1msϪ1 and the mean shear direction is south-south- vorticity maximum, especially to the north and west. It easterly (from 160Њ). These results will be discussed in is hypothesized that strong shear near the disturbance more detail in section 5b of this paper (see also Fig. center associated with the jet streak on the east side of 11). the trough axis and weak forcing for ascent over and near the center of a very weak vortex all act to destroy b. Lysis cases the developing warm core and cause cyclolysis. It is hypothesized that the quintessential difference between The disturbances in the lysis composite can be viewed the Cape Verde genesis composite and the lysis com- as a special set of lysis cases. These disturbances are posite is the presence of strong shear in the lysis com- tropical depressions that obviously must have formed posite near the center of the ATOLL disturbance. in an environment that was once conducive to genesis but has become unfavorable for the continued existence of the depression (or conducive for cyclolysis). These c. Anomalies of composites from climatology depressions in the lysis best-track dataset never went on to become tropical storms. It must be remembered that Anomalies from climatology for the Bahamas, Cape the disturbances in the lysis composite are undergoing Verde, and lysis composites were created to show how cyclolysis and therefore the composite ¯ow pattern is the genesis and lysis composites differ from the mean an environment that will not allow for the continued conditions over the basin. Anomaly ®elds are de®ned existence of the tropical cyclone. Hence, rather than as departures of the composite ®elds from a weighted considering the lysis cases as representative of nondev- climatology created using the daily ATOLL±200-hPa elopers it is more accurate to interpret them to be de- analyses from 1975 to 1994. Shown in Fig. 5 are the velopers that were cut off in the ``prime of their life.'' composite analyses minus the climatological analyses. Ideally one would want to contrast a composite of non- At the ATOLL level all three composites show an anom- developers against the genesis composite, but that is not alous cyclonic vortex of 2.7 ϫ 10Ϫ5 sϪ1, 2.1 ϫ 10Ϫ5 possible with these datasets. sϪ1, and 0.9 ϫ 10Ϫ5 sϪ1 in the Bahamas, Cape Verde, At the ATOLL level the disturbance for the lysis com- and lysis composites, respectively (Figs. 5a±c). In the posite weakly resembles an EW de®ned by a feeble Bahamas composite (Fig. 5a) the subtropical ridge is inverted trough and vorticity maximum (Figs. 2e, 3e). anomalously strong on the poleward side of the anom- At 200 hPa (Figs. 2f, 3f) a trough axis is located west alous cyclonic vortex. In the Cape Verde composite (Fig. of the ATOLL vorticity maximum with a very promi- 5b) anomalous southerly ¯ow is observed on the south- nent jet streak in the southwest ¯ow east of the trough eastern side of the cyclonic vortex, possibly indicative axis that extends from near the center of the disturbance of a cross-equatorial ¯ow into the disturbance from the northeastward. A vorticity maximum is observed in the . The lysis anomalies (Fig. 5c) base of the trough at 200 hPa. An area of strong anti- show only an anomalous cyclonic vortex with an anom- cyclonic vorticity dominates the east and northeast part alously strong anticyclone east-northeast of that vortex of the domain and is part of a dipole of vorticity that and a very weakly anomalous cyclonic vortex north of straddles the jet streak axis. It is interesting that the ¯ow the main vortex. in the extreme eastern half of the domain is qualitatively The 200-hPa anomalies in the Bahamas region (Fig. very similar to the Cape Verde genesis composite at this 5d) show that during genesis the westerly component level. Since these westward-moving lysis cases were at of the wind is anomalously weak and the ¯ow is anom- some time in their past genesis cases (note that these alously cyclonic (anticyclonic) west (east) of the cases are not included in the Cape Verde composite) it ATOLL vortex. This anomalous vorticity dipole is the is perhaps not surprising that the ¯ow to the east re- source of the observed 200-hPa trough±ridge couplet sembles the genesis composite. seen in Fig. 3b. In the Cape Verde composite (Fig. 5e) Sutcliffe±Trenberth forcing of inferred vertical mo- the 200-hPa ridge axis on the poleward and westward tion does show weak upward vertical velocities over side of the developing vortex is seen as anomalously and near the ATOLL vorticity maximum in the lysis strong. In addition, the ¯ow poleward and westward of composite. Composites of divergence (Figs. 2e,f) in- the vortex has an anomalously strong southerly com- dicate ATOLL convergence and 200-hPa divergence ponent. Therefore, during Cape Verde genesis events and therefore upward motion over the disturbance cen- the 200-hPa ridge poleward and west of the lower-tro- ter. Cyclonic vorticity advection at 200 hPa (not shown) pospheric disturbance is anomalously strong. In the lysis and Sutcliffe±Trenberth diagnostics (Fig. 4e) both in- composite (Fig. 5f) the 200-hPa ¯ow pattern shows an dicate the presence of weak forcing for upward vertical anomalously strong trough west of the lower tropo- motion. ATOLL±200-hPa shear (Fig. 4f) shows weak spheric vortex and a stronger ridge east of the vortex. shear equatorward and strong shear poleward, resulting Therefore, during these lysis events the ¯ow at 200 hPa

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FIG. 5. Composite anomalies (composite minus weighted climatology) for the Bahamas and Cape Verde genesis cases and the lysis cases. (a), (b), (c) ATOLL anomalous streamlines (thick lines) and relative vorticity (every 0.3 ϫ 10Ϫ5 sϪ1; with cyclonic vorticity thin solid and anticyclonic vorticity thin dashed) for the Bahamas, Cape Verde, and lysis composites, respectively. (d), (e), (f) Same as in (a), (b), and (c) except at 200 hPa. (g), (h), (i) Anomalous Sutcliffe±Trenberth forcing (thick solid contours every 5 ϫ 10Ϫ11 sϪ2 with only positive values contoured) and 200- hPa±ATOLL shear anomalies (contoured every1msϪ1; less than 5 m sϪ1 of shear shown by thin lines with positive values solid and negative values dashed; shear greater than 6 m sϪ1 shaded every1msϪ1 according to the color ). Composite domain approximately 3000 km ϫ 3000 km. was anomalously strong from the south and southwest 5h) is anomalously strong over almost the entire analysis over and northwest of the low-level vortex. domain with a relative minimum east and south of the Anomalies of the vertical shear of the horizontal wind center and a relative maximum poleward of the low- and the Sutcliffe±Trenberth forcing appear in Figs. 5g±i. level vortex. In the lysis composite (Fig. 5i) Sutcliffe± In the Bahamas region (Fig. 5g) the Sutcliffe±Trenberth Trenberth forcing is again anomalously strong over and forcing is anomalously strong over and northeast of the northeast of the low-level vortex. Shear in that com- low-level vortex. Shear in the Bahamas composite is posite (Fig. 5i) is anomalously strong over the entire anomalously weak well poleward and southeast of the domain and is anomalously highest northeast of the cen- low-level vortex center and anomalously strong both ter. This observed tendency for genesis to occur away east and west of the vortex (Fig. 5g). The Sutcliffe± from those regions where shear is anomalously low and Trenberth forcing in the Cape Verde composite (Fig. 5h) lysis to occur where shear is anomalously high possibly is anomalously strong over and poleward of the low- indicates that shear must be carefully balanced between level vortex. Shear in the Cape Verde composite (Fig. values too low to force upward motion and values too

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC FEBRUARY 2000 BRACKEN AND BOSART 363 high to allow moisture and enthalpy accumulation in a of a well-de®ned 200-hPa trough and its associated deep vertical column near the center of the depression. southwesterly jet streak in the vicinity of the lysis de- pression. This trough and its associated jet streak may act to produce strong vertical shear of the horizontal d. Composite ¯ow decompositions wind in the vicinity of the depression, precluding de- In an effort to determine commonalties and differ- velopment. ences between genesis and lysis ¯ows the composites will be decomposed and examined here. Flows will be decomposed by removing the approximate signature of e. Late season genesis in the western Caribbean and the cyclone/easterly wave and its associated convection. southern Gulf of Mexico This is done by computing and subtracting out a sym- metric, convergent cyclonic vortex at the ATOLL level As was mentioned previously, a small portion (3%± and a symmetric, divergent anticyclonic vortex at the 5%) of tropical cyclones form in subtropical regions 200-hPa level centered on the ATOLL level disturbance. along stagnant frontal zones, which originate in mid- The result of that subtraction (hereafter called the back- latitudes associated with more baroclinic environments. ground ¯ow) reveals similarities in the background ¯ow This section will brie¯y describe a composite of nine between the two genesis composites and differences be- tropical cyclogenesis events in the western Caribbean tween the genesis and lysis composite background ¯ows and southern Gulf of Mexico (15Њ±25ЊN, 80Њ±95ЊW) at the ATOLL and 200-hPa level. subjectively picked from the September±November pe- An initial comparison of the undecomposed Bahamas riod of 1977±91. Satellite images and synoptic surface and Cape Verde genesis composites reveals similarities maps were inspected. Those storms that appeared to be only at the ATOLL level and large differences at the associated with old midlatitude frontal boundaries were 200-hPa level. However, many similarities at both the included in the composite (storms included in the com- ATOLL and 200-hPa level are revealed once the ¯ow posite can be determined by using Fig. 10). Composites is decomposed. At the ATOLL level the background were created using methods exactly the same as those ¯ow is characterized by strong deformation with the used to create the Bahamas and Cape Verde composites. axis of dilatation in the Bahamas region oriented south- An examination of satellite imagery during the nine west±northeast northwest of the center of the composite genesis events (not shown) indicated that all events oc- (Fig. 6a) and east±west over the center of the composite curred in close proximity to the equatorward end of an in the Cape Verde region (Fig. 6c). This ¯ow con®g- old midlatitude frontal boundary. The possible presence uration favors a concentration of moisture and thermal of a frontal boundary indicates that jet streak dynamics gradients near the depression in the genesis composites. may be playing an important role during these tropical At the 200-hPa level the background ¯ows are nearly cyclogenesis events. Therefore, these events will be pre- identical and again characterized by strong deformation. sented and discussed separately from the Bahamas and Both 200-hPa background ¯ows (Figs. 6b,d) show two Cape Verde composites. regions of cyclonic ¯ow north and south of the center Composite ®elds for these cases of ``frontal'' tropical connected to each other by an axis of cyclonic vorticity cyclogenesis are given in Fig. 7. The composite ATOLL that passes just to the west of the center and two regions streamlines (Fig. 7a) show a pattern different from the of anticyclonic vorticity east and west of the center. This Bahamas and Cape Verde composites (Figs. 2a,c). The results in the placement of the depression in a general inverted V pattern is not observed in the streamlines of southeasterly ¯ow on the equatorward side of a zonally the frontal composite, possibly indicating that the de- oriented ridge axis, which then allows for the existence veloping disturbances in these cases may not originate of vorticity advection in the cyclonic sense and forces from EWs. Instead, the streamlines indicate a more sym- upward motion over the developing depression. There- metric vortex at the ATOLL level with stronger ¯ow on fore, the genesis composites exhibit some fundamental its poleward side. At 200 hPa (Fig. 7b) an anticyclone similarities once the signature of the developing cy- is centered just east of the ATOLL-level cyclonic vor- clone/easterly wave and its associated convection are ticity maximum (Fig. 7c). Parcels that ¯ow poleward removed. on the west side of the 200-hPa anticyclone should ex- A comparison of the decomposed genesis and lysis perience strong acceleration into a con¯uent jet-entrance ¯ows reveal differences, the most striking of which is region located between a weak trough and the poleward seen at the 200-hPa level (Figs. 6b,d,f). At 200 hPa the side of the anticyclone (Fig. 7b). Since the ageostrophic lysis depression is located directly beneath a zonally wind blows perpendicular and to the left of the parcel oriented ridge axis and is not equatorward of a ridge acceleration (in the ) the acceler- axis as was seen in both genesis composites (Fig. 6f). ation of parcels into the jet should result in a thermally This may indicate that cyclonic vorticity advection at indirect transverse circulation across the jet axis at 200 200 hPa over the depression is precluded, resulting in hPa. Divergence (convergence) on the right (left) side an absence of forced upward motion. Another signi®cant of the 200-hPa jet-entrance region (Fig. 7b) supports difference between the two composites is the presence this conjecture. This divergence above ATOLL level

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FIG. 6. Background ¯ow computed by decomposing the total composite ¯ow as described in text (full barbs 5 m sϪ1 and half-barbs 2.5 m sϪ1) for (a) Bahamas subregion, (c) Cape Verde subregion, and (e) lysis subregion at ATOLL level; and for (b) Bahamas subregion, (d) Cape Verde subregion, and (f) lysis subregion at 200 hPa. Composite domain approximately 3000 km ϫ 3000 km. convergence (Fig. 7a) kinematically indicates upward At 200 hPa (Fig. 7d) a vorticity dipole straddles the jet- vertical motion over the low-level vorticity maximum. entrance region poleward of the anticyclone. This 200- The ATOLL and 200-hPa vorticity ®elds (Figs. 7c,d) hPa anticyclone is characterized by two major ridge for the frontal composite do show signi®cant differences axes. As in the Cape Verde composite a zonal ridge axis from the Bahamas and Cape Verde composites. At the extends westward from the anticyclone center. The pres- ATOLL level (Fig. 7c) the cyclonic vorticity maximum ence of a jet poleward of the anticyclone helps create is stronger than in the Bahamas and Cape Verde com- a second ridge axis that extends poleward almost me- posites. A band of cyclonic vorticity extends east-north- ridionally from the anticyclone center. east from the ATOLL vorticity maximum, possibly in- Sutcliffe±Trenberth forcing diagnostics (Fig. 7e) dicating the presence of a composite frontal boundary. show three regions of inferred upward motion over the

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FIG. 7. Frontal genesis composite; ¯ow streamlines (thin solid), isotachs [every 1 m sϪ1 in (a) and every2msϪ1 in (b) thick solid], and divergence [every 0.2 ϫ 10Ϫ5 sϪ1 in (a) and every 0.3 ϫ 10Ϫ5 sϪ1 in (b); greater than 0.2 and less than Ϫ0.2 shaded in (a) and greater than Ϫ0.3 and less than Ϫ0.3 shaded in (b) with positive values surrounded by thin solid lines and negative values surrounded by thin dashed lines] for (a) the ATOLL level and (b) the 200-hPa level. Composite ¯ow streamlines (thin solid) and relative vorticity (every 0.5 ϫ 10Ϫ5 sϪ1; with cyclonic vorticity thick solid and anticyclonic vorticity thick dashed) for (c) the ATOLL level and (d) the 200-hPa level. Composite Sutcliffe±Trenberth forcing (every 5 ϫ 10Ϫ11 sϪ2; with positive forcing shaded and surrounded by thin solid lines and negative forcing in thin dashed lines) in (e). ATOLL±200- hPa vertical shear of the horizontal wind (thick solid every 2 m sϪ1 and shaded greater than 16 m sϪ1) in (f). Thin dotted lines cross at the center of the composite depression. Composite domain approximately 3000 km ϫ 3000 km. composite domain. The most prominent area of inferred equatorward edge of the domain. As was noted in the upward motion extends northeast±southwest over the other genesis composites the area of inferred upward ATOLL cyclonic vorticity maximum. Two other regions motion over and near the ATOLL cyclonic vorticity of implied ascent are observed over the northwest and maximum is forced by cyclonic vorticity advection by

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC 366 MONTHLY WEATHER REVIEW VOLUME 128 the thermal wind. But in this composite (and in the Cape North Atlantic basin show how often genesis is accom- Verde composite), the area of inferred upward motion panied by upper-tropospheric troughs and ridges. Er- is located upstream of a 200-hPa ridge axis. This frontal ickson (1963) gives a description of observations during composite is different from the Cape Verde composite the development of Tropical Cyclone Debbie (1961) in that there are two prominent 200-hPa ridge axes that near Cape Verde and notes the presence of an upper- play a role in creating inferred upward motion. One axis level anticyclone during the genesis process. Colon and extends westward from the anticyclone and the other Nightingale (1963) noted that out of 40 cases of cyclone extends poleward almost meridionally from the anti- development over the North Atlantic, 28 occurred west cyclone center. Cyclonic vorticity advection upstream of an upper-level anticyclone or east of a trough in pole- of the two axes combine to create one coherent area of ward ¯ow. The development of Tropical Cyclone Alma inferred upward motion with the strongest forcing over (1962) is discussed in Yanai (1968). Alma is shown to and poleward of the ATOLL cyclonic vorticity maxi- form as a result of the interaction between an African mum. easterly wave (AEW) and an upper-tropospheric trough As might be expected, shear is very strong over the over the Caribbean Sea region. In a case study by Bosart poleward half of the composite domain in the vicinity and Bartlo (1991), the genesis of Tropical Cyclone Di- of the jet (Fig. 7f). But over the equatorward half of ana is shown to occur as the result of the interaction of the domain shear is small, thereby creating a strong an upper-tropospheric trough and an old frontal bound- gradient of shear across the developing ATOLL cyclonic ary off the southeast United States. In a case study of vorticity maximum. This results in shear values near the the high-latitude development of Tropical Cyclone Do- developing disturbance larger than those observed in rothy (1966), Erikson (1967) emphasizes the important the lysis composite. However, in this genesis composite role of upper-tropospheric vorticity advection associated the ATOLL cyclonic vorticity and forcing for ascent are with an upper-tropospheric trough above a well-de®ned much stronger than in the lysis composite. This result low-level disturbance. suggests that a particular shear value alone does not determine if tropical cyclogenesis occurs. a. Validity of composites An obvious question that must be answered is, Do 5. Discussion the individual genesis cases that make up the composite The Bahamas and Cape Verde genesis composites resemble the composite analysis? This question can be presented here possess characteristics that are very sim- approached quantitatively using the two-sided Student's ilar to those seen in published papers on tropical cy- t-test or by a qualitative subjective inspection of the clogenesis. A case study in the Australian±South Paci®c individual cases in the composites. Both methods will region of cyclones Irma and Jason by Davidson et al. be used here. The two-sided Student's t-test is used to (1990) and work by McBride and Keenan (1982) sug- inquire whether the mean of a sample (the composite) gests that tropical cyclogenesis can occur in association is signi®cantly different than the mean of the population with an equatorward-moving upper-tropospheric trough. of all genesis events (Panofsky and Brier 1968). Cal- In the western North Paci®c region upper-tropospheric culations of the statistical signi®cance of composite vor- troughs and ridges in close proximity to developing ticity, wind speed, vertical shear, and Sutcliffe±Tren- tropical cyclones have been associated with genesis berth forcing indicate that all features in the Bahamas, (Sadler 1976, 1978). Figures 1b,c in Sadler (1976) pos- Cape Verde, and lysis composites are at least signi®cant sess features that are very similar to those seen in the to the 95% level (not shown). The only exception to Bahamas and Cape Verde genesis composites. Both Sad- the last statement is the 200-hPa cyclonic vorticity max- ler (1976) ®gures show essentially the same dynamical imum in the lysis composite (note that the 200-hPa jet signature. In his ®gures the developing system is located streak in that composite is signi®cant to the 95% level). southwest of an upper-level anticyclone and equator- Before a subjective investigation of the individual ward of a zonally oriented ridge axis, a ¯ow pattern composite members begins, several issues concerning very similar to the Cape Verde genesis composite. It is the characteristics of the datasets should be noted. The also interesting to note the presence of an upper-level ATOLL±200-hPa analyses are created objectively and trough in the Sadler (1976) ®gures, as was seen in the as a result will contain errors. Analysis errors may arise Bahamas composite. Sadler (1976) suggests that the for several reasons: 1) measuring error, 2) analysis ¯ow pattern given in those ®gures leads to enhanced scheme smoothing, 3) a lack of observations, and 4) the out¯ow and divergence in the cyclone out¯ow layer. As tendency for the analyses to resemble the 12-h forecast was shown in this paper, those ¯ow patterns create re- in data void regions. Errors in the best-track dataset may gions of upper-tropospheric cyclonic vorticity advection arise due to the highly subjective nature of the decision that are favorable for ascent and therefore create an process at NHC and the lack of observations near de- environment that will support persistent, deep moist veloping cyclones. NHC's methods of identifying the convection. beginning or end of a tropical cyclone's life cycle are Published observations of cyclogenesis in the western not objective. An exact determination of the time and

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC FEBRUARY 2000 BRACKEN AND BOSART 367 location of tropical cyclogenesis is extremely dif®cult, of a westerly jet streak; instead Henri develops in a if not impossible. Undoubtedly there will be some dif- poleward jet-entrance region of an easterly jet. ferences in opinion among the individual forecasters at A ®nal check on the representativeness of the com- NHC as to exactly where and when a tropical cyclone posites can be made by using composites for other in- has developed or decayed. It is hoped that the criteria dependent regions. Since the area covered by the Ba- to declare a cloud cluster a tropical depression will be hamas and Cape Verde subregions are relatively small, made primarily based upon observations and be con- additional composites can be created for regions adja- sistent among NHC forecasters. It is also expected that cent to the Bahamas and Cape Verde subregions with the errors in both datasets will be random and that by an independent set of depressions. If the composite an- creating composites these random errors will be re- alyses in the Bahamas and Cape Verde subregions match moved from the analyses and only features that play a the composite analyses in adjacent regions, then one signi®cant role in the tropical cyclogenesis process will might conclude that the composites are representative remain. These errors may occasionally be observed in of conditions during genesis. Or, in statistical terms, if the individual members that make up the composite. two independent subsamples of a given population both Therefore, one should not expect each member to look yield the same mean (or composite), then the means are exactly like the composite. One should expect most likely representative of the mean of the total population. composite members to be similar to the composite with Composites analyses in regions adjacent to the Bahamas variations in the position and intensity of signi®cant and Cape Verde regions were calculated using a total features due to the errors noted above. An additional of 60 separate genesis events (not shown). A comparison source of difference is natural variability. No two cases of the Bahamas and Cape Verde composites, with com- of genesis look exactly alike. If there is a common dy- posite analyses in those adjacent regions, shows that namical signature it should be observed in the com- they do very closely match each other. This indicates posite. that the Bahamas and Cape Verde composite ¯ows are A subjective examination of the 200-hPa ¯ow for representative of conditions commonly observed in each member in the Cape Verde composite (Fig. 8) those regions during tropical cyclogenesis. shows that 87% (27 of 31; note that storms with missing data were not tabulated) of the members are similar to b. Role of synoptic-scale ¯ows the composite analysis (Figs. 2, 3). These 27 cases are A primary requirement for tropical cyclogenesis is marked by the presence of an anticyclone or ridge axis the existence of persistent, organized, deep moist con- to the poleward or east and easterly ¯ow over or equa- vection. Sustained convection is the essential ingredient; torward of the ATOLL-level vorticity maxima. A small without a mechanism to produce it, other quantities like shift in the location/time of genesis or the strength of vertical shear of the horizontal wind, low-level vorticity, the synoptic-scale features would result in excellent and SST are irrelevant. Existence of organized deep agreement between these 27 cases and the composite. moist convection in association with a lower-tropo- The four remaining cases (Frances 1976; Dennis 1981; spheric cyclonic vorticity maximum, small vertical Irene 1981; Dennis 1987) are accompanied by ¯ows that shear, and SST Ͼ27ЊC should be favorable for tropical are not very similar to the composite but are not com- cyclogenesis. The role of the synoptic-scale ¯ow in cre- pletely contrary to the composites either. Subjective in- ating this favorable genesis environment is now dis- spection of the 200-hPa ¯ow for each member in the cussed. Bahamas composite (Fig. 9) show close similarities be- who forecast weather for midlatitude tween 83% (20 of 24) of the cases and the composite locations quickly realize that precipitation is commonly analysis. All 20 cases possess a trough±ridge couplet in observed on the downstream side of an upper-tropo- the ¯ow streamlines that would very closely match the spheric trough axis. The physical reasons for why this overall composite analysis if the strength of some of the occurs are found within QG theory. If the upper-level synoptic-scale features were altered or if a small shift ¯ow is assumed to be in geostrophic balance and tem- in the location/time of genesis occurred. The four cases perature advection is neglected, then for an idealized that do not match the composite (Blanche 1975; Car- sinusoidal 200-hPa ¯ow pattern, upward vertical motion oline 1975; Barry 1983; Ana 1991) do not possess a on the downstream side of a 200-hPa trough axis and signi®cant trough±ridge couplet at 200 hPa. Even the the upstream side of a 200-hPa ridge axis must be pri- nine frontal cases (Fig. 10) generally match the com- marily forced through an upward increase in cyclonic posite analyses. All frontal cases are characterized by vorticity advection. A fundamental difference between the presence of a 200-hPa trough and ATOLL distur- the Cape Verde and Bahamas composites is the existence bance (not shown) located near the equatorward jet en- and orientation of the trough and ridge axes. In the trance region of a 200-hPa westerly jet streak located Bahamas composite both the trough and ridge axis are poleward of the developing storm on the downstream meridionally oriented so that forcing for upward motion side of the trough. Henri (1979) is the only storm that exists between the trough and ridge axes. In the Cape does not develop in the equatorward jet-entrance region Verde composite the trough axis is absent so that upward

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC 368 MONTHLY WEATHER REVIEW VOLUME 128 shaded) for all cases included in Cape Verde subregion composite. 1 Ϫ s 5 Ϫ 10 ϫ . 8. 200-hPa ¯ow streamlines (thick solid) and ATOLL-level cyclonic vorticity (greater than 1.0 IG F

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC FEBRUARY 2000 BRACKEN AND BOSART 369 shaded) for all cases included in Bahamas subregion composite. 1 Ϫ s 5 Ϫ 10 ϫ . 9. 200-hPa ¯ow streamlines (thick solid) and ATOLL-level cyclonic vorticity (greater than 1.0 IG F

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC 370 MONTHLY WEATHER REVIEW VOLUME 128 ) for all cases included in frontal tropical cyclogenesis composite. 1 Ϫ shaded every 5 m s 1 Ϫ . 10. 200-hPa ¯ow streamlines (thick solid) and isotachs (greater than 15 m s IG F

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC FEBRUARY 2000 BRACKEN AND BOSART 371 motion is forced only equatorward of a zonally oriented ridge axis. However, both composites are accompanied by forced upward motion created by the same mecha- nism, an upward increase in cyclonic vorticity advec- tion. Likewise, it can be shown that in the frontal genesis composites that upward QG vertical motion over the ATOLL cyclonic vorticity maximum lies beneath the 200-hPa equatorward jet-entrance region and that this ascent is forced by the same mechanism as in the Ba- hamas and Cape Verde composites, that is, an upward increase in cyclonic vorticity advection. This inferred ascent pattern is consistent with that found in the vi- cinity of jet streaks (e.g., Bluestein 1993, 397±407). The coexistence of SST Ͼ27ЊC, a synoptic-scale ¯ow pattern favorable for convection, and a lower-tropo- spheric cyclonic vorticity maximum (i.e., an EW or an old midlatitude frontal boundary) is not enough for trop- ical cyclogenesis. It is believed that weak vertical wind shear is required so that enthalpy and moisture can ac- cumulate and increase in magnitude in a vertical column near the depression center (Gray 1968). If ventilation is minimized then enthalpy and moisture in the vertical FIG. 11. Histogram of ATOLL±200 hPa (ϳ900 hPa) layer-average column can increase and hydrostatically result in lower shear magnitude (m sϪ1) at the time of genesis for all depressions during the time period 1975±93 (139 storms). Shear is computed by surface pressure. Alternatively, DeMaria (1996) hy- subtracting the ATOLL wind from the 200-hPa wind. The calculation pothesized that strong shear inhibits convection near the is made for the grid point nearest the depression and the eight grid cyclone center by stabilizing the lower troposphere. points that surround it. The shear value used is the average of those Nevertheless, shear will not directly play a role in the nine grid points. initiation of deep moist convection; it primarily deter- mines the structure of convection after it is initiated. As the vertical shear increases, the structure of convection the longer half-wavelength between the trough and ridge will move from ordinary cells (or single cells) to mul- axis and the existence of a strong jet streak in the south- ticells (e.g., Bluestein 1993, 487±492). Ordinary-cell westerly ¯ow between the trough and ridge in the lysis type are characterized by small ventila- composite at 200 hPa. The consequences of these dif- tion and therefore are conducive to the accumulation of ferences are weaker cyclonic vorticity advection and enthalpy and moisture in a vertical column. forcing for ascent and stronger shear over the ATOLL The Bahamas and Cape Verde genesis composite ¯ow vorticity maximum in the lysis composite. analyses do show weaker shear over the ATOLL-level Most of the previous discussion has concentrated on disturbance. Weak shear in the Cape Verde composite the detrimental effects of shear. Therefore, it may seem is perhaps not surprising given the existence of a strong paradoxical to hypothesize that some weak shear is nec- anticyclone aloft, but weak shear in the Bahamas com- essary for tropical cyclogenesis, but that idea will be posite associated with an upper-level trough might be investigated here. The magnitude of vertical shear dur- unexpected. Weak shear in the Bahamas composite is ing genesis has been the subject of controversy. In the the result of the superposition of generally weak south- past it has been suggested that zero or near-zero vertical erly ¯ow at both 200 hPa and the ATOLL level. In shear must be present during genesis (e.g., McBride and general, upper-level troughs (especially those at mid- Zehr 1981). An examination of vertical shear for all latitudes) are associated with jet streaks and relatively genesis events available in the ATOLL±200-hPa dataset stronger ¯ow. The Bahamas composite trough is not from 1975 to 1993 over the Atlantic basin (a total of accompanied by very strong ¯ow. This trough is not 139 storms) is shown in Fig. 11. Shear is computed by typical of troughs commonly seen embedded in the subtracting the ATOLL wind from the 200-hPa wind. westerlies at midlatitudes. Wind speeds at 200 hPa are The calculation is made for the grid point nearest the strongest well poleward of the trough and weakest along depression and the eight grid points that surround it. the trough axis and at the center of the anticyclone. No The average value at those nine grid points for all 139 strong jet streaks are observed upstream or downstream storms are shown in Fig. 11. The same calculations were of the trough axis. made using the grid point nearest the depression and The favorable trough±ridge pattern in the Bahamas the grid point northwest, northeast, southwest, and composite should be contrasted with the unfavorable southeast of it (5 points in total). The results from the trough±ridge pattern in the lysis composite. The most 5-point calculation were nearly identical to the 9-point signi®cant differences between the two composites are calculation. The results show that during genesis the

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200-hPa±ATOLL layer vertical shear magnitude is near cyclogenesis on any given day is not simple. Locating 10msϪ1 (the mean is 10.5 m sϪ1, the median is 10.0 regions of weak shear by identifying upper-level anti- msϪ1, and the standard deviation is 4.8 m sϪ1). cyclones or regions of easterly ¯ow aloft is not adequate The results presented here do not show a tendency enough to determine if conditions are favorable for gen- for the vertical shear to be near zero during genesis. It esis. One must locate regions where the synoptic-scale is interesting to note that while it is commonly believed ¯ow will favor persistent, organized deep moist con- that shear inhibits the genesis process, the existence of vection. An examination of satellite images shows that some weak shear is required to force synoptic-scale as- over the tropical Atlantic Ocean MCSs commonly do cent. This suggests that shear magnitude during genesis not last more than one or two diurnal heating cycles must be carefully balanced between values small enough (Laing and Fritsch 1997). The initiation of an MCS to prevent strong ventilation and values large enough depends upon the existence of three things: 1) low-level to be dynamically signi®cant (i.e., allow for advective moisture, 2) instability, and 3) lift (Doswell 1987). The processes, e.g., forcing for ascent, to exist). Figure 5 supply of the ®rst ingredient, low-level moisture, is cer- can also be used to give some credibility to this hy- tainly adequate for deep moist convection in the Tropics. pothesis. For the second ingredient, instability, an assumption In the Bahamas genesis composite the shear is anom- commonly made is that the air mass above the tropical alously weak north and southeast of the low-level vor- oceans is almost always conditionally unstable and sup- tex; the rest of the analysis domain is covered by anom- plies an abundant and widespread energy source for alously strong shear (Fig. 5g). East and west of the deep moist convection. This assumption is valid only center the shear attains its maximum anomalous value. for undilute vertical displacements of air parcels that do Directly over and near the center, shear is only anom- not include the effects of water loading. The idea that alously high by 0±5 m sϪ1. The area of shear that is convective available potential energy (CAPE) in the weakly anomalously positive in the Bahamas composite Tropics is large has recently been challenged by Betts is collocated with an area of anomalously high Sut- (1982) and Xu and Emanuel (1989). They showed that cliffe±Trenberth forcing for ascent (Fig. 5g). The same soundings from the deep Tropics are nearly neutral to general characteristics found in the Bahamas composite reversible adiabatic ascent when adiabatic condensate can be seen in the Cape Verde composite (Fig. 5h). Shear is included in the de®nition of buoyancy. Therefore, the over and near the developing low-level vortex is only assumption of widespread regions of large CAPE over anomalously high by 0±5 m sϪ1 in a region where the the tropical oceans may need to be replaced by wide- Sutcliffe±Trenberth forcing for ascent is anomalously spread near neutrality with only small regions where high. In the lysis composite the Sutcliffe±Trenberth CAPE is large. The ®nal ingredient in the initiation of forcing for ascent is again anomalously high (Fig. 5i), an MCS, lift, is a mechanism that is forced by processes but in that composite the shear over and near the de- acting on many different scales of motion (Doswell pression is greater than 5 m sϪ1 above normal. 1987). Lift is required to bring a parcel to its level of To summarize, the composites show that shear during free convection. Lift can be generated on several dif- genesis is a relative minimum over and near the de- ferent scales: storm scale, mesoscale, synoptic scale, and veloping depression. However, genesis does not tend to planetary scale. Planetary-scale upward motion occurs occur in regions where shear is nonexistent (zero) or only very slowly over areas much larger than an MCS even anomalously weak. Genesis tends to occur where and will likely not play the primary role in the initiation the shear is weak (when compared to the environment of convection and tropical cyclogenesis. Planetary-scale surrounding the storm) but just slightly (0±5 m sϪ1) features such as the Madden±Julian Oscillation (Madden anomalously positive. One possible explanation of this and Julian 1994) may act to enhance convection over is that in order for there to be forced synoptic-scale large areas of the Tropics by creating a larger-scale en- ascent there must be some shear. On the other hand, vironment that is favorable for ascent forced by pro- there cannot be so much shear that mass circulation cesses on smaller scales. In contrast, storm-scale lift is through the column is too fast to allow enthalpy and very localized and cannot initiate upward motion over moisture concentration. The case of too much shear an area large enough to have an impact on the scale of seems to be what is observed in the lysis composite. an MCS. Therefore, it may be concluded that shear magnitude The initiation of deep moist convection, assuming during genesis must be carefully balanced between val- instability and moisture exist, is then mostly controlled ues small enough to prevent strong ventilation and val- by lift on the mesoscale and synoptic scale. Both scales ues large enough to create forcing for ascent. However, interact to produce the lift that may initiate an MCS. from these results the exact nature of the most favorable Lift and subsidence on different scales can oppose one shear pro®le for genesis is unclear. Perhaps a concen- another and result in weak vertical velocities. But some- tration of shear over a given layer of the troposphere times lift on different scales can cooperate to produce might be more favorable than shear of similar magnitude strong upward motion. It is theorized that during tropical over other layers in the troposphere. cyclogenesis the mesoscale and synoptic scale both co- The identi®cation of regions favorable for tropical operate to create an environment favorable for persis-

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC FEBRUARY 2000 BRACKEN AND BOSART 373 tent, organized deep moist convection. The role of the storm-centered composites. Composites are created us- synoptic scale in producing ascent in the Tropics and ing only wind data at the ATOLL (ϳ900 hPa) and 200- subtropics is believed to be twofold: 1) produce a large hPa level. The results suggest that two different large- area of tropospheric-deep lift over the low-level cy- scale upper-tropospheric ¯ow patterns are most com- clonic vorticity maximum, thereby creating a favorable monly observed during genesis over the basin. One ¯ow environment for strong upward motion and thunder- pattern is characterized by an upper-tropospheric storm initiation when mesoscale lift is also present; and trough-ridge couplet and is most commonly observed 2) destabilize the air and/or remove any capping trade in the Bahamas region. The low-level cyclonic vorticity wind inversions through that ascent if the air is neutrally maximum in the Bahamas composite is located beneath buoyant or stable over large areas. Given favorable syn- the poleward ¯ow east (west) of the upper-level trough optic-scale conditions for ascent accompanied by a low- (ridge) (Fig. 2). The second ¯ow pattern is commonly er-tropospheric cyclonic vorticity maximum, weak ver- observed in the Cape Verde region and characterized by tical shear, and SST Ͼ27ЊC, then the situation might be an upper-tropospheric ridge axis poleward of the low- considered favorable for tropical cyclogenesis. It has level cyclonic vorticity maximum (Fig. 2). A third less been shown here that the conditions in the frontal, Ba- commonly observed ¯ow pattern is observed in asso- hamas, and Cape Verde genesis composites meet all ciation with the equatorward end of old, stagnant mid- these requirements (SSTs are not shown in composites, latitude frontal boundaries. This third pattern is char- but are assumed Ͼ27ЊC since genesis occurred and the acterized by a 200-hPa jet streak entrance region on the climatological SSTs are Ͼ27ЊC) and therefore illustrate downstream side of a trough (Fig. 10). the most common example of conditions favorable for Although the two most commonly observed genesis tropical cyclogenesis over the North Atlantic basin. patterns are very different at the 200-hPa level, they do One of the primary results presented here appears to have many similarities when the ¯ow ®eld is decom- con¯ict with results presented in Zehr (1992). That pa- posed. Removal of the signature of the developing cy- per concludes that upper-level wind patterns do not take clone/easterly wave and its associated convection from an active role as a forcing function on tropical cyclo- the composite ¯ow ®eld leaves a ¯ow pattern charac- genesis over the western North Paci®c Ocean. In con- terized by strong deformation (Fig. 6). Both composites trast, the upper-level synoptic-scale wind pattern ap- show a jet exit region at 200 hPa in the vicinity of the pears to have a strong control over where and when developing depression with an axis of dilatation oriented tropical cyclogenesis occurs over the Atlantic Ocean in northeast±southwest to the northwest of the developing part through the creation of synoptic-scale ascent and disturbance. At the ATOLL level the decomposed ¯ow persistent organized, deep moist convection. Perhaps ®eld again shows strong deformation with an axis of these contrasting results indicate that tropical cyclogen- dilatation northwest and over the depression in the Ba- esis is fundamentally very different over the different hamas and Cape Verde composites, respectively. ocean basins and/or is a consequence of differences in The con®guration of the undecomposed full ¯ow computational methods. ®elds presented in all three genesis composites are fa- Finally, the most important question is then, How vorable for tropical cyclogenesis for several reasons. does the contemporaneous existence of synoptic-scale First, they provide a low-level cyclonic vorticity max- ascent, weak shear, low-level cyclonic vorticity, and imum. Second, the ¯ow patterns force synoptic-scale SST Ͼ27ЊC ultimately result in a warm core vortex? ascent through cyclonic vorticity advection by the ther- That process is highly dependent upon interactions be- mal wind above the low-level cyclonic vorticity max- tween different scales of motion and will not be ad- imum. It is hypothesized that this inferred synoptic-scale dressed here, but will be addressed in a later paper. The upward motion then creates an environment favorable synoptic scale plays several different roles in tropical for the creation of persistent, deep moist convection cyclogenesis; it can 1) provide a low-level cyclonic vor- through several different processes: 1) destabilization of ticity maximum; 2) create an environment with weak the lower troposphere; 2) removal of capping trade wind vertical shear of the horizontal wind; 3) create an en- inversions that may preclude deep, moist convection; 3) vironment favorable for persistent, organized deep moist generation of low-level cyclonic vorticity by horizontal convection; and 4) perhaps moisten mid- and upper lev- convergence associated with Ekman pumping; and 4) els through that convection thereby diminishing the neg- creation of a favorable environment for mesoscale lift. ative impacts of evaporatively cooled downdrafts. The Third, the ¯ow pattern is con®gured in such a way that details of the interactions between these synoptic-scale the vertical shear of the horizontal wind is minimized processes and the mesoscale ¯ow will ultimately deter- over and near the low-level cyclonic vorticity maximum mine where, when, and how a warm core vortex forms. but yet large enough to force synoptic-scale ascent. Last, the presence of the persistent deep, moist convection might then act to moisten mid- and upper levels of the 6. Summary troposphere and thereby diminish the stabilizing effects The synoptic-scale ¯ow during tropical cyclogenesis of evaporatively cooled downdrafts. in the North Atlantic basin has been examined using Composites of overwater tropical cyclolysis events

Unauthenticated | Downloaded 10/02/21 04:40 PM UTC 374 MONTHLY WEATHER REVIEW VOLUME 128 illustrate that not all 200-hPa troughs are favorable for never have been undertaken. Peter Black, Frank Marks, tropical cyclogenesis. Some 200-hPa troughs are asso- and Hugh Willoughby all made contributions to this ciated with weakening depressions. Tropical cyclolysis paper through conversations on the topic of tropical events are generally associated with 1) small forcing for cyclogenesis. Sam Houston was kind enough to help synoptic-scale ascent resulting from weak cyclonic vor- prepare some ®gures in this paper. Thanks also to Bob ticity advection by the thermal wind, 2) a weak ATOLL Kohler who assisted in the retrieval of the ATOLL±200- cyclonic vorticity maximum, and 3) strong shear as- hPa data from the HRD archives. Howard Friedman also sociated with a jet streak on the downstream side of the deserves a special thanks for all his administrative work 200-hPa trough axis. in arranging WEB's stay at HRD. Celeste Iovinella has Another factor that has not been addressed here but also been a huge help in preparing this paper. Comments could be playing a role in tropical cyclogenesis is the from the three anonymous reviewers greatly improved intrinsic potential of the incipient disturbance. Some this paper also. This research was supported by NSF lower-tropospheric cyclonic vorticity maxima might Grants ATM9612485 and ATM9413012. have a structure that would make them more favorable for genesis. A modeling study by Kwon and Mac (1990) suggests that some AEWs are more favorably structured REFERENCES thermodynamically for growth. Unfortunately, the da- Avila, A. A., and E. N. Rappaport, 1996: season taset used here contains no thermodynamic information of 1994. Mon. Wea. 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