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

Bracken, W. Edward; Bosart, Lance F.

doi:10.1175/1520-0493(2000)128<0353:trossf>2.0.co;2

Cited by 107 publications

(88 citation statements)

References 43 publications

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“…For example, Riehl (1948) studied TC formation in the North Pacific and found the upper-level disturbances in the vicinity of the low-level trades tended to enhance low-level instability, which can lead to TC formation. The global observational study of Gray (1968) noted that TCs tend to form in regions of weak vertical shear of the horizontal winds, which has been confirmed by others (e.g., Gray 1984;McBride and Zehr 1981;Bracken and Bosart 2000). Palmé n (1948) discussed the link between TC formation and sea surface temperature (SST) and concluded that TCs typically form in regions where the SST exceeds 268C.…”

Section: Introductionmentioning

confidence: 77%

“…In addition, several studies have suggested that some weak amount of vertical shear is necessary for TC development (e.g., McBride and Zehr 1981;Bracken and Bosart 2000). This relationship between vertical shear and TC formation was recognized by Gray (1968) and DeMaria et al (2001), and was included as dynamical parameters in their respective TC genesis parameters.…”

Section: E Vertical Shearmentioning

confidence: 99%

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Objective Estimation of the 24-h Probability of Tropical Cyclone Formation

Schumacher

DeMaria

Knaff

2009

Weather and Forecasting

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A new product for estimating the 24-h probability of TC formation in individual 58 3 58 subregions of the North Atlantic, eastern North Pacific, and western North Pacific tropical basins is developed. This product uses environmental and convective parameters computed from best-track tropical cyclone (TC) positions, National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) analysis fields, and water vapor (;6.7 mm wavelength) imagery from multiple geostationary satellite platforms. The parameters are used in a two-step algorithm applied to the developmental dataset. First, a screening step removes all data points with environmental conditions highly unfavorable to TC formation. Then, a linear discriminant analysis (LDA) is applied to the screened dataset. A probabilistic prediction scheme for TC formation is developed from the results of the LDA.Coefficients computed by the LDA show that the largest contributors to TC formation probability are climatology, 850-hPa circulation, and distance to an existing TC. The product was evaluated by its Brier and relative operating characteristic skill scores and reliability diagrams. These measures show that the algorithmgenerated probabilistic forecasts are skillful with respect to climatology, and that there is relatively good agreement between forecast probabilities and observed frequencies. As such, this prediction scheme has been implemented as an operational product called the National Environmental Satellite, Data, and Information Services (NESDIS) Tropical Cyclone Formation Probability (TCFP) product. The TCFP product updates every 6 h and displays plots of TC formation probability and input parameter values on its Web site. At present, the TCFP provides real-time, objective TC formation guidance used by tropical cyclone forecast offices in the Atlantic, eastern Pacific, and western Pacific basins.

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Section: Introductionmentioning

confidence: 77%

Section: E Vertical Shearmentioning

confidence: 99%

Objective Estimation of the 24-h Probability of Tropical Cyclone Formation

Schumacher

DeMaria

Knaff

2009

Weather and Forecasting

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“…Subsequent workers (e.g. Ferreira and Schubert, 1997;Bracken and Bosart, 2000) have referred to inverted-V systems in the Tropics but mostly concentrated over ocean regions. This is the first detailed documentation of such a structure over the West Africa region.…”

Section: Introductionmentioning

confidence: 99%

Anatomy of an observed African easterly wave in July 2006

Bain

Parker

Dixon

et al. 2011

Quart J Royal Meteoro Soc

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The detailed structure of an African easterly wave (AEW) observed during the AMMA field campaign is analysed. The wave was present from 25 to 29 July 2006. A complex circulation pattern was observed: the overall structure of convection and the positive vorticity of the trough region had an elongated inverted-V appearance, wrapped around an area of low winds and clear skies. Satellite imagery showed that the AEW was a significant influence on the modulation of convection on the large scale.The wave was identified initially through its strong signature on soil moisture and convection. The AEW structure observed was not anticipated and has not been discussed in previous literature. In addition, wave tracking using a Hovmöller diagram of meridional winds did not detect the wave, and a Hovmöller of vorticity showed the wave moved at a slower speed than other AEWs in July.New schematics explaining the structure are presented, describing the case as observed by satellites and analysed by a limited-area version of the Met Office Unified Model. It is proposed that the positive vorticity branches of the inverted-V can be regarded as analogous to atmospheric fronts, with characteristic gradients in winds and thermodynamic properties, acting as locations for enhanced convection. The implications of the new case are discussed in relation to previous theory and it is suggested that the accepted model of an idealised AEW is incomplete and should be extended to include more complex structures.

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“…While the previous conditions are arguably informative indicators of Atlantic tropical cyclone formation in deep easterlies equatorward of about 20ЊN, there is evidence that many tropical cyclones forming further north are influenced by extratropical weather systems and their attendant large vertical wind shear, both from a theoretical (Montgomery and Farrell 1993) and observational perspective (Vega and Binkley 1994;Bracken and Bosart 2000). Bracken and Bosart (2000) composited the synoptic-scale flow associated with developing and decaying tropical disturbances over the northern Caribbean Sea and found that the incipient low-level cyclone was typically located beneath the inflection point between an upshear trough and downshear ridge.…”

Section: Introductionmentioning

confidence: 99%

“…Bracken and Bosart (2000) composited the synoptic-scale flow associated with developing and decaying tropical disturbances over the northern Caribbean Sea and found that the incipient low-level cyclone was typically located beneath the inflection point between an upshear trough and downshear ridge. Quasigeostrophic theory was used to characterize this location as consisting of gentle, subsynoptic-scale upward motion, for which a nontrivial vertical shear was crucial.…”

Section: Introductionmentioning

confidence: 99%

Baroclinically Induced Tropical Cyclogenesis

2003

Self Cite

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The authors investigate the transition of numerous subtropical cyclones into late season tropical storms and hurricanes during the 2000 and 2001 Atlantic tropical cyclone seasons. In all transitioning cases (10), the 900-200-hPa wind shear was initially near or in excess of the upper limit of vertical shear deemed suitable for tropical cyclogenesis. In many of these cases, the vertical shear decreased markedly near or prior to the time of tropical cyclone formation. In cases that did not become named tropical cyclones, either the tropospheric vertical shear remained in excess of 15-20 m s Ϫ1 or the underlying sea surface temperature (SST) dropped below about 26ЊC prior to or during the weakening of the shear. Cases in which the shear remained large featured multiple, shortwave upper-tropospheric troughs interacting with the developing lower-tropospheric disturbance such that classical occlusion did not occur.Through detailed analysis and simulation of the development of Hurricane Michael in 2000, it was found that the rapid reduction of vertical shear occurred within the precursor baroclinic development. This shear reduction is explained by the diabatic redistribution of potential vorticity, involving both the nonconservative redistribution of potential vorticity (PV) along the three-dimensional vorticity vector and divergent, diabatically induced outflow in the upper troposphere. While some shear reduction occurred in an adiabatic simulation, the diabatic processes were found crucial to reducing the shear on a short (12 h) timescale. Such a rapid reduction in shear may be particularly important for the formation of late season hurricanes because of the greater poleward steering influence of disturbances in the westerlies that tends to move such storms over cool water.

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

Cited by 107 publications

References 43 publications

Objective Estimation of the 24-h Probability of Tropical Cyclone Formation

Objective Estimation of the 24-h Probability of Tropical Cyclone Formation

Anatomy of an observed African easterly wave in July 2006

Baroclinically Induced Tropical Cyclogenesis

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