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Edward K. Vizy and Kerry H. Cook

thought that tropical development is unlikely when the shear is greater than 15 m s −1 (e.g., Gray 1968 ; DeMaria et al. 2001 ). Other studies (e.g., Zehr 1991 , 1992 ; Gallina and Velden 2002 ) suggest lower threshold values ranging between 7.5 and 15 m s −1 . Over the eastern North Atlantic off the West African coast, strong lower-to-middle tropospheric vertical shear exists in association with the midlevel African easterly jet (AEJ). The AEJ develops over western Africa as the result of

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Zhuo Wang, M. T. Montgomery, and T. J. Dunkerton

between observationally resolvable scales (synoptic, meso- α ) and infrequently sampled cloud scale processes (meso- γ , including VHTs) that operate within the trough region of tropical easterly waves equatorward of the easterly jet. Toward this end, our study (reported in two parts) explores the multiscale interaction and evolution of the parent wave and its pouch during tropical cyclogenesis for the real-case example of pre–Hurricane Felix that formed in the main development region (MDR) of the

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Chanh Q. Kieu and Da-Lin Zhang

circulation in the eastern Pacific and an anticyclonic ridge extending westward along the southern coast of the United States. In a vertical plane, the trade wind convergence zone is characterized by southwesterly flow turning clockwise with height and southeasterly flow turning anticlockwise to easterlies above 850 hPa to the south and north of the ITCZ, respectively ( Figs. 4 and 5a ). Note the development of a midlevel jet of 10–12 m s −1 on each side, that is, along 17° and 10°N. The two jets are

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Jonathan Zawislak and Edward J. Zipser

1962 ). Two circulation tracks are related to this growth ( Carlson 1969b ; Burpee 1972 , 1974 ; Reed et al. 1977 ): one is at the level of the African easterly jet (AEJ) (∼600 hPa) at approximately 9°–11°N, related to the sign reversal of the meridional potential vorticity (PV) gradient, and a second is at low levels north of the AEJ (∼18°–20°N) in the presence of the low-level potential temperature gradient between the hot, dry air of the Sahara to the north of the jet and relatively cooler

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Zhuo Wang, M. T. Montgomery, and T. J. Dunkerton

the lower troposphere plays an essential role in tropical cyclogenesis. At the synoptic scale, the precursor easterly wave has maximum amplitude near the jet level (600–700 hPa over the east Atlantic and about 850 hPa or lower over the west Atlantic). At the meso- α scale, the structure and temporal development of the critical layer varies from case to case, but we infer from the numerous events studied by DMW09 that the critical layer is aligned vertically (if not initially in some cases

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Syed Ismail, Richard A. Ferrare, Edward V. Browell, Gao Chen, Bruce Anderson, Susan A. Kooi, Anthony Notari, Carolyn F. Butler, Sharon Burton, Marta Fenn, Jason P. Dunion, Gerry Heymsfield, T. N. Krishnamurti, and Mrinal K. Biswas

temperature inversion at the base (∼800–900 hPa) and is associated with a midlevel (∼600–800 hPa) easterly jet. Dunion and Velden pointed out that these characteristics of the SAL tend to inhibit convection and TC development. This inference is also supported by other studies. A study of TC activity over a 25-yr period over the Atlantic indicated a strong anticorrelation between TC activity and dust (i.e., SAL) events ( Evan et al. 2006 ). Lau and Kim (2007a , b) suggest that dust within the SAL reduces

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Oreste Reale, William K. Lau, Kyu-Myong Kim, and Eugenia Brin

. To further confirm the legitimacy of our analyses, we also use NAMMA data for validation. In Fig. 3 , the profiles of the zonal and meridional wind components, obtained from the vertical ground-based 1200 UTC soundings from the Cape Verde Islands, at 14.92°61′11″N, 23.49°46′11″W and an altitude of 85 m, are reported (radiosonde data are available online at http://namma.msfc.nasa.gov ). The zonal wind time series clearly shows the African easterly jet (AEJ) located between 800 and 500 hPa with

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Robert Cifelli, Timothy Lang, Steven A. Rutledge, Nick Guy, Edward J. Zipser, Jon Zawislak, and Robert Holzworth

ahead (west) of the 700-mb trough. Moreover, the low-level vorticity centers straddled the African easterly jet (AEJ) (located near 15°N), and OLR data showed that the convection was associated primarily with the southern vortex. The complex vorticity evolution observed in wave 5 is consistent with a number of previous studies examining AEW structure ( Reed et al. 1988 ; Pytharoulis and Thorncroft 1999 ; Fink and Reiner 2003 ; Ross and Krishnamurti 2007 ). The low-level centers merged as wave 5

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Scott A. Braun, Michael T. Montgomery, Kevin J. Mallen, and Paul D. Reasor

of more intense precipitation were present at 0000 UTC 24 July. The first was just west of the trough axis along a convergence zone where the northerly flow associated with the trough met a low-level barrier jet east of the mountains. The second was an area of organizing deep convection embedded within the strong southeasterly flow on the northeastern side of the trough. Convection began to rapidly expand by 0600 UTC 24 July ( Fig. 2f ). In the southwestern Bay of Campeche, the easterly flow to

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Wallace Hogsett and Da-Lin Zhang

does not seem to be associated with the supergradient outflow jet, as discussed in Zhang et al. (2001) . Our analysis reveals that it is associated with evaporatively driven outflows from moist downdrafts that are induced on the upshear side, as precipitation particles advect cyclonically in the eyewall and then encounter a dry intrusion in the 550–800-hPa minimum θ e layer (cf. Figs. 6c and 7 ). As discussed in Kieu and Zhang (2008) , the generation of moist downdrafts in the minimum θ e

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