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6B.3 Synoptic Composites of the Extratropical Transition Lifecycle of North Atlantic Tropical Cyclones: Factors Determining Post-Transition Evolution

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6B.3 Synoptic Composites of the Extratropical Transition Lifecycle of North Atlantic Tropical Cyclones: Factors Determining Post-Transition Evolution 6B.3 SYNOPTIC COMPOSITES OF THE EXTRATROPICAL TRANSITION LIFECYCLE OF NORTH ATLANTIC TROPICAL CYCLONES: FACTORS DETERMINING POST-TRANSITION EVOLUTION Robert E. Hart1, Jenni L. Evans2, and Clark Evans1 1Department of Meteorology, Florida State University 2Department of Meteorology, Pennsylvania State University 1. INTRODUCTION 2. DATA AND METHODOLOGY Over the past five years, the extratropical a. Datasets transition (ET) lifecycle has been defined based upon Storm composites performed here were satellite signatures (Klein et al. 2000) and objective based upon 1°×1° resolution Navy Operational measures of dynamic and thermal structure (Evans Global Atmospheric Prediction System (NOGAPS; and Hart 2003). Following ET, tropical cyclones Hogan et al. 1991) operational analyses for the period (TCs) can become explosive cold-core cyclones, 1998-2003. During these six years, 34 Atlantic TCs explosive warm-seclusion cyclones, or simply decay underwent resolvable ET as diagnosed within the as a cold-core cyclone (Hart 2003; Evans and Hart cyclone phase space (CPS) 2003). Each of these three evolution paths has (http://moe.met.fsu.edu/cyclonephase). dramatically different wind and precipitation field b. Methodology (Bosart et al. 2001), and ocean wave (Bowyer 2000; i. Normalizing the ET timeline Bowyer and Fogarty 2003) responses associated with The ET lifecycle defined in Evans and Hart it. Furthermore, the envelope of realized intensity is (2003) is used here, with the beginning of ET fundamentally related to the structure obtained after (labeled as TB) as the time when the storm becomes ET (Hart 2003), so incorrect forecasts of storm significantly asymmetric (B>10) and end of ET structure have the effect of also degrading the (labeled as TE) the time when the lower-tropospheric L intensity forecasts. Yet, numerical models currently thermal wind indicates a cold core (i.e. –VT <0). struggle in forecasting the basic nature of TC These threshold for beginning and end of transition evolution during and after ET (McTaggart-Cowan et were confirmed by the objective clustering of al. 2001; Ma et al. 2003). Indeed, the predictability transitioning storms in the CPS by Arnott et al. of the long-wave pattern of the northern hemisphere (2004). often becomes unusually low when an ET event is To examine the precursor and subsequent upcoming (Jones et al. 2003a,b). synoptic evolution of the cyclones, seven additional The goal of the research presented here is to times were defined with respect to these two points: define and distinguish the precursor environments TB-72h, TB-48h, TB-24h, TMID [= ½ (TB+ TE)], that lead to cases of post-ET weakening, TE+24h, TE+48h, and TE+72h. This normalizing of strengthening, and/or cold-core evolution or warm- the ET time line of each TC enables direct seclusion. The approach is to normalize the TC comparison of the evolution of one system to another. lifecycles of each of these 34 cases using the cyclone While all nine milestones are used to produce a phase space (CPS; Hart 2003) characterization of composite mean cyclone phase lifecycle, for purposes cyclone structure. This framework not only permits of brevity only four of the nine milestones are objective diagnosis of the post-transition structure examined here and illustrated with the synoptic (cold-core or warm-seclusion), but also enables composites: TB-24h, TB, TE, and TE+24h. compositing of precursor and subsequent storm ii. Compositing environments for similar phase evolutions. This For each cyclone, a storm-relative 1°×1° facilitates a detailed examination of the differing resolution grid, spanning 91° of longitude and 91° of nature of underlying interaction between the TC and latitude, was centered over the cyclone center. The trough. Further, composites of the precursor NOGAPS-based storm-relative grid was then environments will provide forecasters with produced for each of the four ET milestones defined schematics for future cyclone evolution, even in above. After this process was repeated for each of the situations of low predictability using the numerical 34 cyclones, the composite mean storm-relative grid model guidance alone. was produced. *Corresponding author address: Robert Hart, 404 Love Building, Florida State University, Tallahassee, FL 32306- 4520. Email: [email protected] a) intensity generally takes several days yet a TC only has to survive for 24h after weakening from peak intensity to reach a baroclinic environment supporting the commencement of ET. Within 24h of transition completing (TE+24hr), the North Atlantic cyclone has achieved its largest value of B and also has significantly decreased in pressure (Figure 1). The former suggests that upon completion of ET the mean cyclone has reached the strongest storm-motion-relative temperature gradient it will attain. One to two days after this, the cyclone has continued to intensify and expand considerably in size, with B decreasing as the baroclinic instability is removed. Soon after TE+72h, the occlusion process has begun (not shown). b. Composite mean evolution during ET b) Prior to the start of transition (TB-24h), the TC has commenced recurvature, but remains distinct from the midlatitude environment (Figures 2a, 3a). The storm’s tropical connection is also evident in the equivalent potential temperature field (not shown). While this connection has weakened by the start of transition, the local minimum of static stability associated with the TC (Figure 4a) serves to enhance the Eady baroclinic growth rate, σ, (Eady 1949; -1 σ=fUZN ) of the environment between the trough and TC. Due to the decreased static stability, the trough is effectively made a more dynamically deep baroclinic system through the introduction of the TC and its environment (Harr et al. 2000). Although not shown, the lifting of the tropopause ahead of the Figure 1: 1998-2003 34-cyclone composite mean cyclone trough by the TC’s anticyclone increases the L trajectory using NOGAPS 1° analysis. a) B vs. –VT and b) – L U baroclinicity of the upper troposphere, leading to VT vs. –VT . TB and TE represent the start and end of ET, enhancement of the jet streak that is typically found respectively, as defined in Evans and Hart (2003), and TMID represents the mid-point of ET. The size of the circular marker with the trough. indicates the average radius of 925hPa gale force winds, from By the end of transition (Figure 5c) the 225km at TB-72h to 570km at TE+72h. The gray shading of the moist, unstable core of the remnant TC has weakened marker indicates minimum mean sea level pressure, from 1001hPa considerably; however, a local maximum of 850hPa at TB-72h to 982hPa at TE+72h. The numbers of available cases for each of the nine times above are 33, 34, 34, 34, 31, 31, 26, 17, equivalent potential temperature at the end of and 11. For reasons of brevity, only four of the milestones will be transition (Figure 5c) is consistent with Thorncroft examined here: TB-24h, TB, TE, and TE+24h. and Jones (2000). The remnant maximum of moisture 3. COMPOSITE MEAN EVOLUTION in the presence of a pressure minimum without a AND ET VARIABILITY larger maximum of temperature illustrates why a. Composite mean cyclone phase evolution equivalent potential temperature alone cannot be used The 34-storm composite mean cyclone as an indicator of warm vs. cold-core structure in phase evolution is depicted in Figure 1. The mean cyclones. evolution of cyclone structure from symmetric warm- A potential vorticity perspective on the ET core, to hybrid, to asymmetric cold-core to eventual lifecycle (Figures 6 and 7) illustrates the approach of occlusion is well illustrated. The average the trough while the low-level PV maximum of the transitioning TC peaks in warm-core intensity around TC is maintained. Since the 300hPa static stability of L the atmosphere is approximately 20% less 24h prior to the start of ET (TB-24h with –VT ≈ 100). This result is consistent with Hart and Evans (2001) immediately surrounding the cyclone than between and illustrates why a significant percentage of the cyclone and the approaching trough (dotted Atlantic TCs are capable of undergoing ET in the contours in Figure 7a,b), the TC introduced an first place. Complete dissipation of a TC from peak environment at upper levels that enhanced the Eady baroclinic growth rate of the wave (Figure 7). By the end of transition (Figure 2c), the TC has interacted with the trough (the nature of which will be discussed next), with the 0.75PVU contour extending below 500hPa by the completion of ET (Figure 4c). By 24h after transition, there is no distinction between the former TC remnant and the trough that interacted with it (Figure 3d). To illustrate more clearly the trough’s forcing upon the TC, the Eliassen-Palm flux analysis in cylindrical-isentropic coordinates of Molinari et al. (1995) and Harr et al. (2000) was performed (Figure 5a-d) using the mean atmospheric state of the 34- cyclone composite for each of the four key ET milestones highlighted here: TB-24hr, TB, TE, and TE+24hr. From these analyses, we can determine the Figure 2: Composite mean 500hPa height (contour, dm) and relative contribution of the trough in the eddy angular anomaly from monthly mean (shaded, m) for T -24h, T , T , and B B E momentum and eddy heat flux forcing (and, TE+24h. The transitioning storm is located at (0,0) in each panel. implicitly, the eddy potential vorticity flux) within the transitioning TC. Comparisons to the Elena (1985) trough interaction analyses described in Molinari et al. (1995) and the case studies of Harr et al. (2000) can also be drawn. One day prior to the start of transition (Figure 5a), there is evidence of weak inward eddy angular momentum flux beyond 1000km radius in the middle troposphere, resulting from the approaching trough (Figure 3a).
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