A universal, broad-environment energy conversion signature of explosive cyclones

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 40, , doi: /grl.50114, 2013 A universal, broad-environment energy conversion signature of explosive cyclones Mitchell Timothy Black 1,2 and Alexandre Bernardes Pezza 1 Received 26 November 2012; revised 21 December 2012; accepted 23 December 2012; published 31 January [1] This study presents the first analysis of the Lorenz energetics associated with a global climatology of explosive cyclones. Energy budgets of the large-scale environment are calculated for 32 year climatologies ( ) of explosive cyclones within four of the most active regions in the world: the Northwest Pacific, the North Atlantic, the Southwest Pacific, and the South Atlantic. A robust signature in the Lorenz energy cycle is observed; anomalous energy conversions commence 48 h before explosive cyclone development and remain strong (i.e., significantly above background noise) for 120 h. Remarkably, the calculated signature of energy conversion is virtually identical for all four geographical regions. While the conversions imply a classic baroclinic growth cycle, they are not seen in regular cyclones that undergo a deepening of less than half that exhibited by explosive cyclones. This finding opens a new avenue of exploration of explosive storm behavior based on the large-scale environment. Citation: Black, M. T., and A. B. Pezza (2013), A universal, broad-environment energy conversion signature of explosive cyclones, Geophys. Res. Lett., 40, , doi: /grl Introduction [2] Therapidintensification of extratropical cyclones presents a serious threat to human life and property, via the associated destructive winds, flooding rainfall, and dangerous oceanic conditions. Explosive cyclone development has been traditionally defined by a reduction of central sea level pressure exceeding 24 hpa in 24 h, relative to 60 latitude [Sanders and Gyakum, 1980; Allen et al., 2010]. Despite their seriousness, explosive cyclones remain an important forecasting challenge [Sanders et al., 2000; Allen et al., 2010] with their developing mechanisms both various and complex [Kuwano-Yoshida and Asuma, 2008, and references therein]. [3] Over the past three decades, explosive cyclones have received some attention; however, the focus has been on Northern Hemisphere (NH) events. There, systems are predominantly maritime cold season events that preferentially occur to the east of continental seaboards in regions of enhanced baroclinicity [Sanders and Gyakum,1980;Roebber, 1984; Gyakum et al., 1989; Chen et al., 1992; Lim and 1 School of Earth Sciences, The University of Melbourne, Melbourne, Victoria, Australia. 2 ARC Centre of Excellence for Climate System Science, The University of Melbourne, Melbourne, Victoria, Australia. Corresponding author: M. T. Black, School of Earth Sciences, The University of Melbourne, Melbourne, Vic 3010, Australia. (mtblack@student.unimelb.edu.au) American Geophysical Union. All Rights Reserved /13/ /grl Simmonds, 2002]. While advances in numerical weather prediction in recent decades have improved our ability to predict explosive cyclones, much remains to be done to better understand their physics and underlying mechanisms of formation. In particular, very little is known of how the large-scale circulation may or may not be conducive to explosive cyclone formation, as the classic baroclinic front theory of the Bergen School [Bjerknes and Solberg, 1922] does not distinguish between the conceptual model associated with regular deepening cyclogenesis and explosive cyclones. [4] There has been an increasing awareness in recent times that invaluable information pertaining to the formation and intensification of extratropical cyclones may be achieved through analyses of the Lorenz energetics [Lorenz, 1955, 1967] applied to the local vortex environment [Michaelides, 1987, 1992; Orlanski and Sheldon, 1995; McLay and Martin, 2002; Decker and Martin, 2005]. In this sense the various studies show how the baroclinic cycle unfolds as the vortex dominates the energy transfers, and vice versa. A different approach is to use the broad-scale environment as a large-scale reservoir of energy available for the vortex to evolve, as more recently proposed by Veiga et al. [2008]and Pezza et al. [2010]. These later developments suggest that the broad-scale energetics could be used as a climatological proxy to infer the evolution of a composite of storms targeted over a large area of interest. However, no attempts to address this issue had been made until the present study. [5] This study is the first analysis of the broad environment energetics for a climatology of wintertime explosive cyclones over four of the most active world regions, providing the most thorough global analysis to date. Data and methodology are discussed in section 2 and the main results are presented in section 3. A follow-up discussion is presented in section 4, commenting on new insights and perspectives brought by the results. 2. Data and Methodology [6] A traditional and compact way to study energetics is adopted in this study, whereby the kinetic and available potential energies are partitioned into zonal and eddy components, KZ, KE and AZ, AE, respectively. As in Veiga et al. [2008], the energy budget equations for an open atmospheric system can be ¼ CK þ CZ þ BKZ þ BΦZ DZ (1) ¼ CE CK þ BKE þ BΦE DE ¼ CZ CA þ BAZ þ GZ (3)

2 @AE ¼ CA CE þ BAE þ GE (4) [7] The energy budget equations are expressed in terms of the generation of AZ and AE (GZ and GE, respectively), the dissipation of KZ and KE (DZ and DE, respectively), and the conversion between the various energy forms given by CA (from AZ to AE), CE (from AE to KE), CK (from KE to KZ), and CZ (from AZ to KZ). In addition, the respective boundary transports (BAZ, BAE, BKZ, and BKE) and the zonal and eddy kinetic energies associated with the boundary pressure work (BΦZ and BΦE) are also included. CA, CE, andcz are often referred to as baroclinic conversion terms due to their association with horizontal temperature gradients in the atmosphere, whereas CK is a barotropic conversion term reflecting momentum transport [e.g., Michaelides, 1992]. The generation, dissipation, and pressure work terms are calculated as residuals. Detailed mathematical expressions for all the components of the energy budget can be found in Michaelides [1987], Veiga et al. [2008], and references therein. [8] Six-hourly data from the ERA-Interim reanalysis ( resolution) [Dee et al., 2011] were used for both the cyclone identification and gridded energy calculations. Climatologies of NH wintertime (December to February) and Southern Hemisphere (SH) wintertime (June to August) explosive cyclones were compiled for the 32 year period , using the Melbourne University automatic tracking scheme [Simmonds et al., 1999] and following the recently improved criterion for identifying explosive cyclones proposed by Allen et al. [2010]. [9] Gridded energy calculations were performed over four equally sized domains: W, N (North Atlantic (NA)); 120 E 165 W, N (Northwest Pacific (NWP)); 65 W 10 E, S (South Atlantic (SA)); and 145 E 140 W, S (Southwest Pacific (SWP)). These domains were chosen to capture the entire explosive period for each of the systems identified within each region (Figure 1), although sensitivity tests showed that our results hold if slightly smaller domains and sample sizes are used (not shown). Explosive cyclones were identified over the 32 year period by considering the 24 h pressure deepening from each six-hourly time step, making this the most thorough compilation of explosive cyclones to date. [10] Key to our methodology is also to investigate the energetics of regular deepening cyclones, hereby defined to be cyclonic systems whose pressure fields experience a deepening of no more than 12 hpa in 24 h (relative to 60 latitude) (i.e., half of the pressure change defined for explosive cyclones). Hence the signal for regular deepening cyclones is robustly compared to explosive cyclones as they all undergo a deepening period. All cyclone systems considered in this study are identifiable on synoptic charts as closed cyclones throughout their period of pressure deepening. 3. Results 3.1. Global Explosive Cyclone Distribution [11] Figure 1 shows the complete tracks (blue) and the explosive tracks (red) associated with all explosive cyclones identified throughout the 32 winter seasons, accompanied by the corresponding densities of explosive occurrence and composite synoptic conditions. A total of 314 and 585 explosive cyclones were identified in SH and NH, respectively. [12] The cyclone trajectories (Figures 1a and 1b) show a global pattern of occurrence concentrated in mid and high latitudes, over the ocean. The red trajectories also show that explosive development tends to occur on the equatorward side of the spread particularly on the eastern seaboard of the continents, threatening several inhabited areas and many shipping and aviation routes. [13] Figures 1c and 1d show that while the track density is relatively smooth in the SH, two distinct local maxima can be distinguished in the NH: one in the North Atlantic and one in the North Pacific. Those areas have been identified inside red boxes (coincident with the boundaries of the energetics domains) that are used hereafter to help characterize the key features associated with this class of cyclone. In the SH, two boxes were also chosen based on the adverse impacts on the eastern coasts of South America, Australia, and throughout New Zealand, where the density is generally higher than in other areas. Over 60% of all explosive cyclones occurred within these study domains: 47 in SWP, 78 in SA, 210 in NA, and 224 in NWP. [14] Figures 1e and 1f show the atmospheric circulation associated with the onset of explosive development for cyclones within the respective study domains; composites of the mean sea level pressure anomaly and 300 hpa wind magnitude are drawn in and around each domain. A blocking structure is seen in all cases, with an anticyclonic anomaly poleward of the upper level jet stream blocking the cyclonic anomalies within the jet. The Icelandic Low is particularly developed in the NA composite, depicting the well-known relationship of the North Atlantic storms with the North Atlantic Oscillation [Hurrell, 1995; Serreze et al., 1997]. The jet is slightly weaker in the SA when compared to the other areas and is more vigorous in the NWP where it is also climatologically stronger Universal Signature in the Energy Conversions [15] Figure 2 shows the evolution of the average energy conversion term anomalies associated with each domain prior to, during and after the explosive development (Figures 2a 2d) as well as the equivalent evolution of the anomalous energetics for 200 regular deepening cyclones for the NWP (Figure 2e) and the NA (Figure 2f). It becomes immediately obvious from this figure that there is a robust global signature in the energy field associated with explosive cyclones which is not present for regular deepening cyclones. This signature is modulated by a positive anomalous peak in CZ and a negative anomalous peak in CE and CA that start to develop between 48 and 24 h before the explosive phase begins, followed by a stronger opposite pulse that re-emerges between 12 and 48 h after the onset of explosive development. The invoked terms are regarded as baroclinic as they involve horizontal temperature gradients. The barotropic term CK does not present a coherent fluctuation pattern distinguishable from random fluctuations, suggesting that the barotropic instability is not relevant in explosive cyclones [Orlanski and Sheldon, 1995].Remarkably, the results show that regular deepening cyclones are related to an average energy signature which cannot be distinguished from noise. The fact that explosive cyclones do associate with above-noise, robust environmental energy conversions is relevant because it signifies that this class of dangerous cyclones can be monitored via large-scale energy metrics that do not rely on high-resolution analyses of individual storms. 453

3 BLACK AND PEZZA: ENERGY SIGNATURE OF EXPLOSIVE CYCLONES (a) (b) (c) (d) (e) (f) Figure 1. Complete trajectories (blue) and explosive segments (red) of all wintertime explosive cyclones identified from the ERA-Interim data set ( ) for (a) the SH and (b) NH. The corresponding frequencies of explosive events are shown in Figures 1c and 1d, expressed as multiples of 10-5 explosive cyclones per ( lat)2 per 6 h. A total of 47, 78, 210, and 224 explosive cyclones occurred within the SWP, SA, NA, and NWP study domains, respectively (domain boundaries indicated in red). Composite MSLP anomalies (2 hpa interval; negative contours dashed; zero contour black) and 300 hpa wind (magnitude in m/s, shaded) at the onset of explosive development are shown for the four study domains in Figures 1e and 1f. H and L identify regions of anomalous high and low pressure, respectively. [16] The global environmental signature associated with explosive cyclones can be best interpreted with the help of the Lorenz diagram presented in Figure 3a. In a regular baroclinic cycle, the zonal available potential energy is converted into both zonal kinetic energy (via CZ) and eddy available potential energy (via CA), which can be further converted into eddy kinetic energy (via CE). The preexplosive signature suggests that the environment is first accumulating both zonal available potential energy and zonal kinetic energy relative to the climatology (i.e., either absolutely accumulating or decreasing the rate of loss). In this stage, the jet stream is intensifying, as well as the horizontal temperature gradients associated with the reservoir of potential energy. [17] During and after the explosive phase, the anomalies reverse (as seen in Figure 2), and the jet becomes an additional source of energy to feed the baroclinic cycle, where a vigorous generation of eddy kinetic energy is first observed. Hence the 454

4 Figure 2. Time series of the averaged anomalies of the energy conversion terms vertically integrated between 925 and 100 hpa for explosive cyclones in (a) SWP (47 cyclones), (b) SA (78 cyclones), (c) NWP (224 cyclones), (d) NA (210 cyclones), and regular deepening cyclones in (e) NWP and (f) NA (200 cyclones each). Shading indicates 95% confidence intervals about the mean while hatching highlights the period of explosive development. Units are W/m 2. signature is a reflection of an environment marked by baroclinic instability. While the jet stream is one of the elements that give dynamic support to the baroclinic growth, a range of other synoptic processes may be present within the domain over the composite. From an energetics viewpoint, the reversion of CZ in the Lorenz cycle should not be interpreted as a direct energy transfer from the jet in a strict sense. Rather, the environment suggests that the energy available to be converted into zonal kinetic energy is reduced. [18] Figures 3b 3d show the conversion cycles for the NWP box before, during, and after explosive development, which is here taken as an example with the strongest global density. Here we show the actual values of energy and conversion, as opposed to the anomalies shown in Figure 2. The generation of zonal available potential energy (RGZ) is of similar order of magnitude to the largest conversion terms in the cycle; however, this input is approximately constant throughout and should not affect the comparisons. All other boundary terms are at least one order of magnitude less than the most significant conversion terms. [19] The boxes show that while the absolute values of potential and kinetic energy retain the same order of magnitude throughout the life cycle, their temporal derivatives present expressive changes that help understand the global signature of explosive cyclones. While the zonal available potential and kinetic energies increase and the eddy terms slightly decrease before the explosive phase (Figure 3b), the opposite holds during the explosive period yet with increased intensity (Figure 3c), losing strength after the explosive phase has ended (Figure 3d). [20] The conversion terms maintain a baroclinic cycle, as denoted by the direction of the arrows from zonal available potential energy toward eddy kinetic energy. This is also observed in the other geographical regions (figure not shown). The direction of CZ is negative in the NH and variable in the SH (figure not shown), but the anomalies preceding the explosive deepening are always positive (as per Figure 2). 455

5 Figure 3. Volume-integrated energy terms for the 224 Northwest Pacific wintertime explosive cyclones. Time 0 h refers to the onset of explosive development. Figure 3a shows the energy cycle here adopted as a reference guide. Figures 3b 3d show the average energy cycles for the 48 h period before, the 24 h period during, and the 48 h period after explosive development, respectively. Units are in 10 5 J/m 2 and W/m 2, respectively, for energy and conversion terms. The barotropic conversion is comparatively weak during the whole process. 4. Concluding Remarks [21] In this paper, we presented a global climatology of explosively developing cyclones using a state-of-the-art automatic tracking scheme. This class of cyclone falls in the most extreme end of the intensity spectrum, with very rapid pressure falls of at least 24 hpa in 24 h relative to 60 latitude. They are often associated with hurricane force winds accompanied by heavy rainfall, posing significant threats to human activities. [22] Our results document the most detailed global climatology to date, with all individual trajectories and their densities of occurrence derived by six-hourly temporal resolution for the explosive characterization. The global hot spots were identified, with sharper density peaks in the NH particularly on the eastern coast of the U.S. and Canada, and on the eastern coast of Japan. In the SH, there is a relatively greater spread toward lower latitudes, with maximum frequency on the eastern coast of South America, Australia, and New Zealand. [23] We reveal that this class of severe storms is associated with a global signature in the large-scale energetics calculated over very large domains. The signature is robust and not region nor time dependent, having the same magnitude and energy conversion profile for all four domains representing the global hot spots. Physically they imply a vigorous baroclinic cycle in the very broad scale environment, with overall conversion of zonal available potential energy into eddy kinetic energy. The jet stream is noted as having an important participation in the domains, acting as a source of available potential energy particularly after the explosive phase, while the barotropic conversion remained small. No doubt this process agrees with our overall knowledge of the role of a baroclinic cycle behind extratropical cyclone formation, but for the first time we show that the broad environment can be used as a proxy for explosive cyclogenesis. We hypothesize that for regular deepening cyclones, the baroclinic cycle is not vigorous enough to leave a distinguishable trail in the environmental energetics. Our discovery opens an important door for future studies, in that predicted changes in the energetics of the broad environment [e.g., Hernández-Deckers and von Storch, 2010] may be associated with changes in extreme storminess worldwide. [24] Acknowledgment. A. B. Pezza would like to acknowledge the Australian Research Council for funding parts of this work. References Allen, J. T., A. B. Pezza, and M. T. Black (2010), Explosive cyclogenesis: A global climatology comparing multiple reanalyses, J. Climate, 23(24), , doi: /2010jcli Bjerknes, J., and H. Solberg (1922), Life cycle of cyclones and the polar front theory of atmospheric circulation, Geofysiske Publikationer, 3, Chen, S.-J., Y.-H. Kuo, P.-Z. Zhang, and Q.-F. Bai (1992), Climatology of explosive cyclones off the east Asian coast, Mon. Weather Rev., 120(12), , doi: / (1992)120<3029:coecot>2.0.co;2. Decker, S. G., and J. E. Martin (2005), A local energetics analysis of the life cycle differences between consecutive, explosively deepening, continental cyclones, Mon. Weather Rev., 133(1), , doi: /mwr Dee, D. P., S. M. Uppala, A. J. Simmons, P. Berrisford, P. Poli, S. Kobayashi, U. Andrae, M. A. Balmaseda, G. Balsamo, P. Bauer, et al (2011), The ERA-Interim reanalysis: Configuration and performance of the data assimilation system, Q. J. Roy. Meteor. Soc., 137(656), , doi: /qj.828. Gyakum, J. R., J. R. Anderson, R. H. Grumm, and E. L. Gruner (1989), North Pacific cold-season surface cyclone activity: , Mon. Weather Rev., 117(6), , doi: / (1989) 117<1141:NPCSSC>2.0.CO;2. Hernández-Deckers, D., and J.-S. von Storch (2010), Energetics responses to increases in greenhouse gas concentration, J. Climate, 23(14), , doi: /2010jcli Hurrell, J. W. (1995), Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation, Science, 269(5224), , doi: /science Kuwano-Yoshida, A., and Y. Asuma (2008), Numerical study of explosively developing extratropical cyclones in the northwestern Pacific region,mon. Weather Rev., 136(2), , doi: /2007mwr Lim, E.-P., and I. Simmonds (2002), Explosive cyclone development in the Southern Hemisphere and a comparison with Northern Hemisphere events, Mon. Weather Rev., 130(9), , doi: / (2002)130<2188:ECDITS>2.0.CO;2. 456

6 Lorenz, E. N. (1955), Available potential energy and the maintenance of the general circulation, Tellus, 7(2), , doi: /j tb01148.x. Lorenz, E. N. (1967), The Nature and Theory of the General Circulation Of The Atmosphere, Report, 161 pp., World Meteorol. Org., Geneva. McLay, J. G., and J. E. Martin (2002), Surface cyclolysis in the North Pacific Ocean. Part III: Composite local energetics of tropospheric-deep cyclone decay associated with rapid surface cyclolysis, Mon. Weather Rev., 130(10), , doi: / (2002)130<2507: SCITNP>2.0.CO;2. Michaelides, S. C. (1987), Limited area energetics of Genoa cyclogenesis, Mon. Weather Rev., 115(1), 13 26, doi: / (1987) 115<0013:LAEOGC>2.0.CO;2. Michaelides, S. C. (1992), A spatial and temporal energetics analysis of a baroclinic disturbance in the Mediterranean, Mon. Weather Rev., 120(7), , doi: / (1992)120<1224:asatea>2.0.co;2. Orlanski, I., and J. P. Sheldon (1995), Stages in the energetics of baroclinic systems, Tellus A, 47(5), , doi: /j x. Pezza, A. B., J. A. P. Veiga, I. Simmonds, K. Keay, and M. D. Mesquita (2010), Environmental energetics of an exceptional high-latitude storm, Atmos. Sci. Lett., 11(1), 39 45, doi: /asl.253. Roebber, P. J. (1984), Statistical analysis and updated climatology of explosive cyclones, Mon. Weather Rev., 112(8), , doi: / (1984)112<1577:SAAUCO>2.0.CO;2. Sanders, F., and J. R. Gyakum (1980), Synoptic-dynamic climatology of the Bomb, Mon. Weather Rev., 108(10), , doi: / (1980)108<1589:sdcot>2.0.co;2. Sanders, F., S. L. Mullen, and D. P. Baumhefner (2000), Ensemble simulations of explosive cyclogenesis at ranges of 2 5days, Mon. Weather Rev., 128(8), , doi: / (2000)128<2920:esoeca>2.0.co;2. Serreze, M. C., F. Carse, R. G. Barry, and J. C. Rogers (1997), Icelandic low cyclone activity: Climatological features, linkages with the NAO, and relationships with recent changes in the Northern Hemisphere circulation, J. Climate, 10(3), , doi: / (1997)010<0453: ILCACF>2.0.CO;2. Simmonds, I., R. J. Murray, and R. M. Leighton (1999), A refinement of cyclone tracking methods with data from FROST, Aust. Meteorol. Mag., 14, Veiga, J. A. P., A. B. Pezza, I. Simmonds, and P. L. Silva Dias (2008), An analysis of the environmental energetics associated with the transition of the first South Atlantic hurricane, Geophys. Res. Lett., 35, L15806, doi: /2008gl