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The Role of Synoptic-Scale Flow During Tropical Cyclogenesis Over the North Atlantic Ocean

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The Role of Synoptic-Scale Flow During Tropical Cyclogenesis Over the North Atlantic Ocean FEBRUARY 2000 BRACKEN AND BOSART 353 The Role of Synoptic-Scale Flow during Tropical Cyclogenesis over the North Atlantic Ocean 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 tropical cyclogenesis and cyclolysis 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. Storm-centered composites are created with the Analysis of the Tropical Oceanic Lower Level (ATOLL; ;900 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 trough (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 anticyclone 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 cyclones. 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 s21, suggestive that weak shear is necessary during tropical cyclogenesis to help force synoptic-scale ascent. Composites of tropical cyclone 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 convection. 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 meteorologist is fore- casting which cloud cluster will be the seedling that Animations of satellite imagery illustrate the ubiquity later becomes a tropical cyclone. How can one distin- of organized cloud clusters (horizontal scale typically guish between a cloud cluster that will develop and one ;400±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- q 2000 American Meteorological Society 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 sea surface temperature (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 278C, 3) back- used to create the composites.
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