1. Introduction The National Hurricane Center (nhc) online glossary defines a subtropical cyclone (stc) as a “non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones…
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- 4. Discussion and conclusions
3. North Atlantic STC climatology The objective identification technique for detecting STC formation applied to STC Sean (2011) was subsequently applied to the subset of baroclinically influenced tropical cyclogenesis cases specified in section 2a using the 0.5° CFSR dataset. Of the 222 candidate cyclones, 67 were identified as STCs (~2 STCs a year). Figure 5 illustrates the intraseasonal variability associated with the location of STC formation in the North Atlantic basin during 1979–2010. STC formation primarily occurs over the southern Gulf Stream and western Caribbean Sea during April–July, coinciding with the highest mean SSTs in the North Atlantic basin during that period (Fig. 6). An intrusion of relatively cold upper-tropospheric air accompanying an upper-tropospheric disturbance moving over the southern Gulf Stream/western Caribbean Sea during April–July would steepen local lapse rates and facilitate the development of deep convection that serves as a catalyst for STC formation. STC formation becomes more frequent over the central and eastern North Atlantic during August–December (Fig. 3) as mean SSTs increase throughout the basin (Fig. 6). The observed increase in mean SSTs during August–December causes the central and eastern North Atlantic to become favorable for the development of deep convection following an intrusion of relatively cold upper-tropospheric air accompanying an upper-tropospheric disturbance. Intraseasonal variability is also associated with the frequency of STC formation in the North Atlantic basin. Figure 7 separates the 67 STCs identified in this study by the month during which they formed. STC formation occurs most frequently in September and October, with a secondary peak in June. A seasonal minimum in the frequency of STC formation is observed in July, likely associated with the lack of relatively cold upper-tropospheric air impinging upon the subtropics during that month (McTaggart-Cowan et al. 2008). Figure 7 also indicates that STC formation can occur from April through December in the North Atlantic basin, outside the range of the official North Atlantic TC season (June–November). Figure 7 reveals that STCs forming during the spring and fall are typically classified as Strong TT events by McTaggart-Cowan et al. (2013), indicating that STCs forming during these periods develop in the presence of strong lower-tropospheric thermal gradients (Table 1). STCs forming during boreal summer are typically classified as Weak TT events by McTaggart-Cowan et al. (2013). STCs forming during this period develop in the presence of moderate lower-tropospheric thermal gradients (Table 1); an unsurprising result as the Northern Hemisphere warms and lower-tropospheric baroclinicity is reduced in the subtropics. Only four STCs are classified as Trough Induced events in this study. This result (and the results stated above) suggests that STC formation is inherently associated with some degree of lower-tropospheric baroclinicity, and that the magnitude of lower-tropospheric baroclinicity associated with STC formation may be a function of season and the location of formation within the North Atlantic basin. The intraseasonal variability associated with the location and frequency of North Atlantic STC formation presented in this study are similar to those found in the previous North Atlantic STC climatology constructed by Guishard et al. (2009). STCs identified in Guishard et al. (2009) form more frequently over the western North Atlantic than over the eastern North Atlantic, a result that is replicated in the present study (Fig. 5). STC formation occurs most frequently in September and October in both studies, with a secondary peak in June and seasonal minimum in July (Fig. 7). The similar intraseasonal variability associated with the location and frequency of North Atlantic STC formation observed in both studies, despite the use of inherently different STC identification techniques, is reassuring and suggests that the results of the present study are robust. It is important to note that ~4 STCs were identified a year in the 1957–2002 North Atlantic STC climatology constructed by Guishard et al. (2009), while ~2 STCs were identified a year in the 1979–2010 North Atlantic STC climatology presented in this study. The difference in the number of STCs identified per year may be explained by considering the 1) higher resolution reanalysis dataset, 2) smaller number of candidate cyclones, and 3) the more restrictive STC identification technique used to identify STCs in this study. The intraseasonal variability associated with the location and frequency of North Atlantic STC formation is similar, but not identical, to the intraseasonal variability associated with the location and frequency of North Atlantic TCs. Figure 8 compares the intraseasonal variability associated with the location of North Atlantic STC and North Atlantic TC formation during the a) spring and early summer and b) late summer and fall of 1979 through 2010. Like TC formation, STC formation primarily occurs over the southern Gulf Stream and western Caribbean Sea during the spring and early summer (Fig. 8a), coinciding with the highest mean SSTs in the North Atlantic basin (Fig. 6). TC formation becomes more frequent over the main development region and Cape Verde Islands during the late summer and fall as African easterly wave activity increases (Fig. 8b). In contrast to the location of TC formation, STC formation becomes more frequent over the central and eastern North Atlantic during the late summer and fall (Fig. 8b) as mean SSTs increase (Figs. 6) and more of the basin becomes favorable for the development of deep convection following an intrusion of relatively cold upper-tropospheric air accompanying an upper-tropospheric disturbance. Figure 9 compares the intraseasonal variability associated with the frequency of North Atlantic STC and North Atlantic TC formation during 1979–2010. Like TC formation, STC formation in the North Atlantic basin occurs most frequently in September and October. However, Fig. 9 also indicates that STC formation occurs relatively frequently in June, a result inconsistent with the frequency of TC formation. This secondary peak in the frequency of STC formation is likely due to the presence of sufficiently warm ocean waters in the North Atlantic basin (Fig. 6) and sufficiently cold upper-tropospheric air in the midlatitudes that, when injected into the subtropics, can facilitate the development of deep convection in the early summer. 4. Discussion and conclusions The NHC STC definition, specified in section 1, suggests that both baroclinic and diabatic processes contribute to STC formation. The 1979–2010 North Atlantic STC climatology presented in this study is the first STC climatology to consider baroclinic and diabatic processes in conjunction with the NHC STC definition, using the adapted Davis (2010) methodology for STC identification to quantify the relative contributions of these processes during the evolution of individual cyclones. The results of this study suggest that considerable intraseasonal variability is associated with the location and frequency of North Atlantic STC formation, supporting the earlier results of Guishard et al. (2009) and substantiating the results of both studies. STC formation primarily occurs over the southern Gulf Stream and western Caribbean Sea during the spring and early summer, becoming more frequent over the central and eastern North Atlantic in the late summer and fall (Fig. 5). STC formation occurs most frequently in September and October, with a secondary peak in June (Fig. 7). The intraseasonal variability associated with the location and frequency of North Atlantic STC formation is similar, but not identical, to the intraseasonal variability associated with the location and frequency of North Atlantic TCs (Figs. 8,9). The dynamically based 1979–2010 North Atlantic STC climatology presented in this study provides the foundation from which to investigate the upper-tropospheric features linked to North Atlantic STC formation. A cyclone-relative composite analysis performed on subjectively constructed clusters of STCs identified in this climatology will be presented in a subsequent study to document the structure, motion, and evolution of the upper-tropospheric features linked to North Atlantic STC formation. The authors hypothesize that the upper-tropospheric feature linked to STC formation can influence the life cycle of an STC and result in similar contributions of baroclinic and diabatic processes during the evolution of STCs. The adapted Davis (2010) methodology for STC identification presented in this study has the potential to be applied to oceanic cyclones in real time. The real-time application of this methodology would benefit operational forecasters and research scientists by providing further insight into the relative contributions of baroclinic and diabatic processes occurring during the evolution of individual cyclones, as well as providing an additional tool for forecasting the TT of STCs. The adapted Davis (2010) methodology for STC identification, depicted graphically for STC Sean (2011) in Fig. 3, is well-designed for implementation in an ensemble-forecasting framework due to its ability to illustrate the changing contributions of baroclinic and diabatic processes during the evolution of individual cyclones. Utilizing the adapted Davis (2010) methodology for STC identification in an ensemble-forecasting framework would allow operational forecasters to better assess the timing of STC formation and the likelihood of TT. Directory: student -> abentley -> research images -> mthesis -> paper -> paper1 -> outline sample student -> Comparing Hurricanes and Hurricane Seasons student -> Storm Surge: a rising Danger student -> Dr. Jeff Masters' WunderBlog student -> April 9, 2008 Hurricanes (Continued) student -> Instructions for Writing Hurricane Term Paper research images -> Tropical Transition of an Unnamed, High-Latitude, Tropical Cyclone over the Eastern North Pacific research images -> References List (Updated: 8 January 2015) Bosart, L. F., and J. A. Bartlo, 1991: Tropical storm formation in a baroclinic environment. Mon. Wea. Rev., 191 research images -> Tropical Transition in the Eastern North Pacific: Sensitivity to Microphysics outline sample -> AL192011 Sean pdf.] Download 104.76 Kb. Share with your friends:
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