(2004) and Yagi (2006) Using Four Methods

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Determining the Extratropical Transition Onset and Completion Times of Typhoons Mindulle (2004) and Yagi (2006) Using Four Methods

QIAN WANG Ocean University of China, Qingdao, and Shanghai Typhoon Institute and Laboratory of Typhoon Forecast Technique, China Meteorological Administration, Shanghai, China

QINGQING LI Shanghai Typhoon Institute and Laboratory of Typhoon Forecast Technique, China Meteorological Administration, Shanghai, China

GANG FU Laboratory of Physical Oceanography, Key Laboratory of Ocean–Atmosphere Interaction and Climate in Universities of Shandong, and Department of Marine Meteorology, Ocean University of China, Qingdao, China

(Manuscript received 5 December 2011, in ﬁnal form 21 July 2012)

ABSTRACT

Four methods for determining the extratropical transition (ET) onset and completion times of Typhoons Mindulle (2004) and Yagi (2006) were compared using four numerically analyzed datasets. The open-wave and scalar frontogenesis parameter methods failed to smoothly and consistently determine the ET completion from the four data sources, because some dependent factors associated with these two methods signiﬁcantly impacted the results. Although the cyclone phase space technique succeeded in determining the ET onset and completion times, the ET onset and completion times of Yagi identiﬁed by this method exhibited a large distinction across the datasets, agreeing with prior studies. The isentropic potential vorticity method was also able to identify the ET onset times of both Mindulle and Yagi using all the datasets, whereas the ET onset time of Yagi determined by such a method differed markedly from that by the cyclone phase space technique, which may create forecast uncertainty.

1. Introduction period due to the complex interaction between the cyclone and the midlatitude systems. Several characteristics The process of extratropical transition (ET) occurs when of this interaction include asymmetries in wind, thermal a tropical cyclone (TC) moves poleward into the mid- structure, moisture field, precipitation, cloud, and con- latitudes from its genesis region in the tropics, accompanied vection (Sekioka 1970; Jones et al. 2003), as well as the by environmental changes including the increasing Coriolis dispersal of the upper-level tropical cyclone warm core force, decreasing sea surface temperatures, and the ambi- (Ritchie and Elsberry 2001). ent atmosphere of increased baroclinicity and vertical wind A TC undergoing ET frequently reintensifies and shear. On average, 46% of Atlantic TCs (Hart and Evans evolves into a fast-moving and rapidly developing extra- 2001), 27% of western North Pacific storms (Klein et al. tropical cyclone (EC), which may produce torrential 2000), 33% in the southwest Pacific basin (Sinclair 2002), rainfall resulting in catastrophic inland flooding (DiMego and about 10% of TCs in the South Indian Ocean (Foley and Bosart 1982a,b), damaging winds (Merrill 1993; Evans and Hanstrum 1994) experience ET. During ET the TC and Hart 2008), and hazardous seas (Sekioka 1956; Sinclair structure is replaced by extratropical features, with a hybrid 2002). A review of the cyclone structure change due to ET, associated disaster threats to land communities and Corresponding author address: Qingqing Li, Shanghai Typhoon maritime activities, and challenges to forecasters can Institute, 166 Puxi Rd., Shanghai 200030, China. be found in Jones et al. (2003). Forecasting when an ET E-mail: [email protected] process starts and finishes thus becomes important for

DOI: 10.1175/WAF-D-11-00148.1

Ó 2012 American Meteorological Society Unauthenticated | Downloaded 09/28/21 06:20 AM UTC DECEMBER 2012 W A N G E T A L . 1395 forecasters and is still one of many challenging issues Operational forecasters commonly employ a variety related to ET due to our incomplete understanding of of numerically analyzed datasets of differing resolutions the interaction between TCs and midlatitude systems. with which to determine the ET onset and completion As pointed out by Jones et al. (2003), to specify a pre- times of a TC. To provide more detailed guidance to cise time at which a TC has become extratropical and forecasters, this study will compare the utility of these propose a universal definition that is based on tools different methods for determining ET onset and com- available to a forecaster would be desirable from an pletion times on four numerically analyzed datasets of operational viewpoint. different horizontal resolutions. Evans and Hart (2003) Klein et al. (2000) proposed a two-stage definition for used the CPS diagnostic to examine the ET onset and ET in the western North Pacific from an observational completion times of several storms with the operational perspective. With the loss of symmetric features of the National Centers for Environmental Prediction (NCEP) cloud field, a TC is in the transformation stage, which Aviation Model and NOGAPS numerical analyses, which completes when the storm becomes embedded in the have comparable resolutions, and found that the diagnosis baroclinic zone. The second stage of ET is called the of ET onset or completion of some cases differed between reintensification stage, in which the transformed TC ei- the two data sources. Nevertheless, intercomparisons of ther reintensifies as a baroclinic system or else dissi- operational feasibility of the aforementioned ET onset pates. In the present study, the beginning (ending) of the and completion diagnostic techniques across datasets of transformation stage of ET is defined as the ET onset differing horizontal resolution are still lacking. Therefore, (completion) time. At present, forecasters mostly rely as the first step, we will evaluate in this study the existing on subjective assessments of the asymmetry of the methods of defining ET onset and completion times with storm’s cloud shield, its travel over colder waters, and two typical ET cases in the northwestern Pacific using the intrusion of cold air into the central storm circulation distinct reanalysis data and numerical simulation results. to discern the onset and completion of ET (Evans and The two TCs chosen in this study are Typhoons Mindulle Hart 2003). However, those TCs that weaken or recurve (2004) and Yagi (2006), with the former reintensifying may also have these traits. Several diagnostic criteria for slightly and the latter undergoing rapid reintensification ET onset and completion times have been suggested in after ET in terms of the reports of the best-track data. The previous studies from different perspectives. Through data and methodology employed in this study are described investigating the frontal characteristics of the ET pro- in section 2, followed by the analysis results in section 3. cess (Keyser et al. 1988; Schultz and Doswell 1999), Harr Finally, a summary and discussion are represented in and Elsberry (2000) deemed that values of the scalar section 4. frontogenesis function could be used for describing the ET of TCs. Another tool that can be used to determine ET onset and completion times is the cyclone phase 2. Data and methodology space (CPS), which is a combination of parameters of a. Typhoons Mindulle (2004) and Yagi (2006) the storm-motion-relative thickness symmetry and thermal wind (Hart 2003; Evans and Hart 2003; Hart et al. The best-track data from the Shanghai Typhoon In- 2006). Demirci et al. (2007) proposed the ET completion stitute of China Meteorology Administration (STI/CMA) time when an open-wave (OW) pattern at 500-hPa geo- shows Typhoon Mindulle formed near Guam at 1800 UTC potential height occurs. More recently, Kofron et al. 21 June 2004 as a tropical depression and then moved west- (2010a) analyzed the applicability of the above-mentioned northwestward (Fig. 1a). It developed into a tropical storm criteria for the ET time using the Navy Operational Global after 36 h and was updated to a typhoon at 0600 UTC Assimilation and Prediction System (NOGAPS) gridded 27 June 2004 (Fig. 1b). This typhoon peaked at 1200 UTC analyses and found that the OW and scalar frontogenesis 28 June 2004 with a minimum sea level pressure (MSLP) of diagnosis both failed to discriminate between transition- 950 hPa (Fig. 1b). Mindulle made its first landfall at the ing ET and recurving non-ET cases, and all of the above- eastern coast of Taiwan (about 20 km south to Hualien) mentioned methods were found to have disadvantages that as a severe tropical storm around 1500 UTC 1 July 2004 preclude them from providing consistent guidance for the (Fig. 1a). After 15 h, it made its second landfall in the city of time when extratropical transition of a poleward-recurving Yueqing in China’s Zhejiang Province with tropical storm TC is occurring. They also suggested that isentropic po- intensity. Henceforth, Mindulle accelerated north- tential vorticity (IPV) can be utilized to define ET onset northeastward toward the East China Sea (Fig. 1a). and completion times and, more importantly, further During this period it underwent an ET process. The STI/ determine whether a cyclone will reintensify after ET CMA best-track data reported the ET completion time of (Kofron et al. 2010b). Mindulle as 0000 UTC 4 July 2004. The MSLP of Mindulle

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FIG. 1. Observed (dots) and simulated (squares) tracks of Typhoons (a) Mindulle and (c) Yagi. The dually nested grid configurations used for the simulations are also shown in (a) and (c), with the domain numbers in the bottom-left corners of the domains. Time series of 21 best-track central pressure (SLP; solid lines with dots; hPa), maximum sustained wind speed (Vmax; solid lines with circles; m s ), and simulated central pressure (solid lines; hPa) of (b) Mindulle and (d) Yagi. dropped 2 hPa from 1200 UTC 3 July to 1200 UTC 4 July next 30 h beginning at 0000 UTC 25 September 2006 2004 (Fig. 1b). (Fig. 1d). Typhoon Yagi originated from a tropical depression As noted above, Mindulle (2004) and Yagi (2006) both about 1200 km east-northeast of Guam and had a clock- underwent ET processes. The former only slightly reinte- wise loop in its track afterward (Fig. 1c). Yagi reached its nsified while the later exhibited a striking reintensification peak intensity on 21 September 2006 with an MSLP of after ET. These two cases, which experienced distinct in- 920 hPa (Fig. 1d); the storm then recurved northeastward tensity change after the ET, are chosen to investigate the (Fig. 1c). With the increasing wind shear in midlatitudes, ET processes in the present study. Yagi began to weaken and underwent ET. This storm b. Data was identified as an EC at 0600 UTC 25 September 2006 by the CMA. The STI/CMA best-track data indicated a To examine the possible influences of different data reintensification, with a 22-hPa decrease in MSLP during sources on determining the ET onset and completion

Unauthenticated | Downloaded 09/28/21 06:20 AM UTC DECEMBER 2012 W A N G E T A L . 1397 times, three reanalysis datasets with different horizontal surface model (Chen and Dudhia 2001), a simple cloud- resolutions ranging from 20 km to 1.258 in longitude and interactive shortwave radiation scheme (Dudhia 1989), the simulation results from the mesoscale Weather Research Rapid Radiative Transfer Model (RRTM) longwave radi- and Forecasting Model (WRF) are examined. Two global ation (Mlawer et al. 1997) scheme, the Betts–Miller–Janjic reanalysis datasets, the Japan Meteorological Agency’s cumulus scheme (Betts 1986; Betts and Miller 1986; Janjic (JMA) 25-yr Reanalysis Project (JRA-25) data (Hatsushika 1994) on the 50-km grids, and the Lin microphysics scheme et al. 2006; Onogi et al. 2007; Song et al. 2011) with 1.2583 (Lin et al. 1983) on all grids. The NCEP FNL gridded 1.258 resolution and the National Center for Atmospheric analyses are used as the initial and lateral boundary con- Research (NCAR)-archived National Centers for Envi- ditions for the simulations. The starting times of the simu- ronmental Prediction (NCEP) Global Final (FNL; Wang lations are 0000 UTC 3 July 2004 with an integration length et al. 2009) analysis data with 1.0831.08 horizontal reso- of 48 h for Mindulle and 0000 UTC 23 September 2006 with lution at 6-h intervals, are used in this study. In addition to an integration length of 72 h for Yagi, respectively. The these two relatively coarser datasets, JMA Regional simulation periods covered the respective ET processes of Spectral Model 20-km reanalysis data (RSM-20km; NPD/ Typhoons Mindulle and Yagi as discussed below. JMA 1997) are employed to investigate the ET evolution Figure 1 shows that the simulated tracks of Mindulle of the two typhoons. It should be noted that assimilated and Yagi approximate well the observations, and the data, assimilation techniques, model dynamic codes, phys- simulated storm intensities are fairly reasonable, particu- ics packages, and reanalysis methods also evidently differ larly with the slight (rapid) reintensification of Mindulle among the above datasets, and these differences must (Yagi) being reproduced well. Because model verification cause uncertainty and discrepancies in determining ET is not the focus of this study, only preliminary model onset and completion times. However, an evaluation of verification was conducted. For instance, in comparison the influences of these differences is not possible. Nev- with the National Aeronautics and Space Administration ertheless, the possible impact of data resolution on the Quick Scatterometer (QuikSCAT) surface winds (Fig. 2), identification of ET onset and completion times can be at the asymmetric surface wind distributions associated with least qualitatively examined, as is discussed below. both Mindulle and Yagi were captured well in the simu- Because it is often difficult for authors to gain access to lations. Around 1000 UTC 4 July 2004, Typhoon Mindulle 2 higher-resolution (e.g., grid spacing of ;10 km or less) triggered higher surface winds (.25 m s 1) near the reanalysis data, an alternative is to investigate results of northeastern coasts of the Korean Peninsula (Fig. 2a), a high-resolution mesoscale numerical simulations. Also, trend that was found in the modeling results as well high-resolution mesoscale regional numerical models have (Fig. 2b). For Yagi, a wavenumber-2 asymmetry of surface been widely employed for operational TC forecasting, and winds was noted in the core region with elevated wind forecasters may diagnose ET processes using mesoscale velocities in the northwest and southeast at 1855 UTC numerical forecast results. It is expected that the evalua- 24 September 2006 (Fig. 2c). The WRF model successfully tion of the determination techniques of ET times based on simulated this asymmetric feature, with the peak surface 2 high-resolution mesoscale numerical model results enables wind velocity greater than 30 m s 1 (Fig.2d).Thecloud us to provide more operational guidance for forecasters. features (e.g., cloud shapes and occurrence of active con- Therefore, the addition of high-resolution numerical model vection) indicated by the model-derived topcloud temper- results is employed in this study. Simulations of Typhoons atures of the two storms (not shown) appear similar to the Mindulle and Yagi were conducted with WRF version 3.1 observations from the infrared satellite imagery as well. (Skamarock et al. 2008). Here, two grids nesting down to Therefore, the simulations are believed to have reproduced 10-km horizontal resolution are employed in the simula- reasonably well the structures of these two typhoons. tions of these two storms. The outer domains both have c. Methodology in determining ET onset and a grid length of 50 km and contain 150 3 150 grid points in completion times the x and y directions. A 10-km nested mesh is used with grid dimensions of 320 3 320 and 400 3 400 in the simu- Four methods employed to determine the ET onset lations of Mindulle and Yagi, respectively, and is large and completion times will be discussed in this section. enough to include the regions where the typhoons moved Harr et al. (2000), as well as Ritchie and Elsberry (2003), during their ET processes. Corresponding domains used in indicated that ET was dependent primarily on mid- the simulations are illustrated in Figs. 1a and 1c, respec- latitude structures. Demirci et al. (2007) proposed that tively. All meshes have 57 vertical levels with the top level at the occurrence when the TC became an OW on the 10 hPa. The two simulations employ the same physics op- 500-hPa map (contoured every 20 m) denoted the com- tions, including the Yonsei University boundary layer pletion of ET. At that time, the cyclone was properly scheme (Noh et al. 2003; Hong et al. 2006), the Noah land embedded in the baroclinic zone. In addition, the OW

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21 FIG. 2. Comparison of (a) the surface wind vectors (arrows) and velocity (shading; m s ) derived from QuikSCAT at 0954 UTC with (b) WRF-simulated surface winds at 1000 UTC 4 July 2004 for Typhoon Mindulle (2004). (c),(d) As in (a),(b), respectively, but for Typhoon Yagi (2006) observed at 1855 UTC 24 Sep 2006 in (c) and simulated at 1900 UTC 24 Sep 2006 (d). method provided an objective way to define the ET time scalar frontogenesis parameter [SFP; see Eq. (A1) in the that can be viewed using gridded datasets by nonexperts appendix] calculated in this study follows Schultz and (Kofron et al. 2010a). Doswell (1999) and Harr and Elsberry (2000). More spe- A number of previous studies found that the ET process cifically, Harr and Elsberry (2000) suggested that the time could be considered to be an interaction between a vortex when the sum of the scalar frontogenesis in the northeast and a baroclinic zone (Doswell 1984, 1985; Davies-Jones and southwest quadrants of the cyclone within the 500-km 1985; Keyser et al. 1988), accompanied by the develop- radius shows a distinct increase could potentially indicate ment of midlatitude frontal boundaries. Harr and Elsberry the ET completion time. (2000) demonstrated that the development of frontal Another method for analyzing the process of ET is the characteristics manifested various complex physical pro- use of phase-space diagrams (Hart 2003; Evans and Hart cesses associated with ET, and proposed that the rate of the 2003; Hart et al. 2006; Kitabatake 2011). The CPS (Hart increase in frontogenesis peaked at a time that may be 2003) is a continuum describing the broad evolution of defined as the ET completion time. The three-dimensional all synoptic-scale cyclones. Three parameters (i.e., B,

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2 L 2 U VT and VT ) are calculated in the CPS method as when the storm exceeds an IPV threshold value of 2 2 2 indicated in the appendix [Eqs. (A2)–(A4)]. Parameter 1.6 PVU (1 PVU 5 10 6 Km2 kg 1 s 1) at the 330-K B reflects the 900–600-hPa thickness asymmetry relative potential temperature isentropic level. 1 2 L to the moving direction of the storm. Parameters VT 2 U and VT indicate the 900–600- and 600–300-hPa thermal winds, respectively. These CPS parameters are computed 3. Results within a 500-km radius from the storm center based on a. Open-wave features in the 500-hPa geopotential the geopotential height field. Positive (negative) values of 2 L 2 U height fields VT or VT indicate a warm- (cold-) core structure according to the thermal wind relationship. Large positive Demirci et al. (2007) found that the time when the last values of B reveal a highly asymmetric (frontal) structure closed contour of the 500-hPa geopotential height fields while near-zero values of B indicate the symmetry. The related to the cyclone becomes open (namely OW struc- start of an ET event is decided when jBj becomes greater ture) seemingly coincides with the end of the trans- 2 L than 10 m, and the ET completion is determined as VT formation stage defined in Klein et al. (2000), when the 2 U and VT both change from positive to negative (Hart and storm has the characteristics of a baroclinic cyclone in Evans 2001; Hart 2003; Evans and Hart 2003; Hart et al. both satellite imagery and numerical weather prediction 2006; Song et al. 2011). analyses, and the center of the storm is embedded in Potential vorticity (PV) has been used to describe the cold, descending air. Demirci et al. (2007) proposed that structure of the cyclone and the surrounded balanced such time is thus potentially taken as the ET completion atmospheric flow related to ET events (Bosart and time. The ET completion times of Typhoon Mindulle Lackmann 1995; Thorncroft and Jones 2000; McTaggart- and Yagi are examined in this section using the three Cowan et al. 2001), and has recently been considered to reanalysis data and WRF simulation results, based on the be a potentially useful tool for determining ET onset and OW criterion. completion times (Kofron et al. 2010b). Because TCs Figure 3 depicts the 500-hPa geopotential height fields have higher PV values at lower levels with high PV gra- associated with Typhoon Mindulle. Mindulle was located dients confined to the inner core (Jones et al. 2003; to the southeast of a weak trough at 0600 UTC 4 July Shapiro and Franklin 1995) and the preexisting trough 2004 (Figs. 3a, 3d, 3g, and 3j). The coarser-resolution data interactingwiththeTChashigherPVvaluesinupper (JRA-25 and FNL) show the occurrence of OW structures levels associated with the upper-level jet, the interaction both at 1200 UTC 4 July 2004 (Figs. 3b and 3e), which is of the TC and the trough may be described by the evolving 12 h later than the completion time in the STI/CMA best- midlevel isentropic PV [IPV; Eq. (A5) in the appendix; track data. Note that, although the cyclone tended to Kofron et al. (2010b)]. Using the Navy Operational Global slightly intensify, a closed-contour feature only recurred in Atmospheric Prediction System analyses on a 18 latitude– the FNL analyses at 1800 UTC 4 July 2004. The 500-hPa longitude grid, Kofron et al. (2010b) found that the time geopotential height fields from RSM-20km do not exhibit when the TC-centered circular average IPV on the 330-K the open-wave feature during the period shown in Figs. potential temperature isentropic surface shows a minimum 3g–i, although a typical EC feature was seen with the is a good indicator of ET onset. Moreover, the comple- lower-level center of the cyclone significantly separating tion of ET for reintensifying cases is defined as occurring from the midlevel one (e.g., Fig. 3i). Additionally, the absence of open-wave features was also found throughout the whole simulation of Mindulle (seen in Figs. 3j–l), 1 Sinclair (2004) deemed that calculations based on the method suggesting that it is unable to ascertain the ET completion of Evans and Hart (2003) yielded fluctuating B values for the TCs time of Mindulle based on the simulated 500-hPa geo- whose moving directions were varying, and proposed referencing B potential height fields. to the area-averaged thermal wind vector. The method used in For the rapidly reintensifying case of Yagi, all datasets Sinclair (2004) produced smoothly varying values of B for the show the presence of open-wave features (Fig. 4), but southwest Pacific TCs. We have also employed the method of Sinclair (2004) to calculate the B values of Typhoons Mindulle and the corresponding ET completion times are distinct. At Yagi, and found that the B values (not shown) often seemed to be 0000 UTC 24 September 2006, a pattern with a closed larger than those calculated with the method of Evans and Hart 500-hPa geopotential height contour associated with (2003), with the exception of the WRF results for Mindulle. In Yagi lays in the south of a deep trough in the JRA-25 addition, whether using 10 m as the B threshold is reasonable for data (Fig. 4a). An open-wave feature was present 6 h western North Pacific TCs, still needs to be considered further if B is calculated as in Sinclair (2004). Therefore, considering the later (Fig. 4b), indicating the ET completion. This OW smooth motion of Mindulle and Yagi, we still used the method of figure still existed at 1200 UTC 24 September 2006 Evans and Hart (2003) for the calculation of B in the current study. (Fig. 4c), with pronounced cyclone tilting. The ET

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FIG. 3. The 500-hPa geopotential height associated with Typhoon Mindulle (2004). The corresponding data sources and time are shown in the bottom-left and -right corners of each panel. completion time determined with FNL data was 6 h geopotential height map. As a storm encounters a strong earlier than that in JRA-25, with the OW structure found pressure gradient and accelerates, the OW signal may at 0000 UTC 24 September 2006 (Fig. 4e). For the RSM- occur. These dependent factors complicate the identiﬁ- 20km dataset, the ending time of ET based on the OW cation of the ET completion time using the OW method. method was much later than those with JRA-25 and FNL As a result, we could obtain relatively consistent ET data, with the open-wave feature occurring at 0600 UTC completion times of these two typhoons with the JRA- 25 September 2006 (Fig. 4i). The ET completion time 25 and FNL datasets, which have coarser resolutions, based on WRF simulation results is about the same as whereas the ending time of the ET of Mindulle could not that in RSM-20km. Figure 4k illustrates an OW signal at be determined with the RSM-20km and WRF simulation 0100 UTC 25 September 2006. results and the achieved ET times of Yagi with the RSM- The above-mentioned analyses suggest that the OW 20km and WRF simulation results obviously lag those method seems not to be a feasible operational tool for with JRA-25 and FNL data. determining the ET completion time, and it is at least b. Frontogenesis sensitive to data sources. The presence of OW structure is also dependent on other factors, such as model reso- Harr and Elsberry (2000) examined the frontogenesis lution, storm translation speed, and the latitude where of cyclones interacting with a preexisting trough to the the storm is located. As grid spacing becomes smaller, it northwest and northeast, and they hypothesized that becomes easier to ﬁnd a closed contour on the 500-hPa scalar frontogenesis could be used to describe the ET of

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FIG. 4. As in Fig. 3, but for Typhoon Yagi (2006).

TCs. More specifically, the addition of the SFP in the Figure 5 shows the evolution of scalar frontogenesis in northeastern and southwestern quadrants of the cyclone four quadrants within the 500-km radius from the center at 500 hPa and averaged within the 500-km radius would of Typhoon Mindulle. It seems that frontogenesis domi- show a distinct increase, which could indicate the ET nated at the mid- and upper levels in the case of Mindulle. completion time. The SFPs are calculated from Eq. (A1) However, different data sources represented distinct par- in the appendix by using all of the three reanalysis data ticulars in different quadrants. The low-resolution data and the WRF simulation results, revealing that the tilt- (i.e., JRA-25 and FNL) show similar patterns; however, ing term dominates positive frontogenesis at all times the frontolysis in the northwestern quadrant and fronto- for the two cases during the period of interest, consistent genesis in the northeastern quadrant from FNL are both with the finding of Schultz and Doswell (1999). more striking than those from JRA-25 (Figs. 5a, 5b, 5e, The horizontal distribution of scalar frontogenesis and 5f). RSM-20km also suggests the presence of signifi- and the evolution of potential temperature indicated cant frontogenesis in the mid- and upper layers in its that there existed obvious warm frontogenesis in the northeastern quadrant (Fig. 5j), but the frontogenesis northeastern quadrant of Mindulle, which was related features in other quadrants are different from those found with the frontal cloud feature (not shown). Additionally, with the JRA-25 and FNL data. In particular, pronounced a T-bone feature in the frontogenesis horizontal distri- frontogenesis was observed in the northwestern and bution was found in Mindulle (not shown), which was southwestern quadrants (Figs. 5i and 5k). WRF simu- generally observed in warm seclusion ET cases (Shapiro lation results of Mindulle reflected relatively noisy and Keyser 1990; Kitabatake 2008). signals of the SFP (Figs. 5m–p), resulting from the

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21 21 FIG. 5. The temporal evolution of SFPs [K (100 km) (3 h) ] associated with Typhoon Mindulle (2004) averaged with the 500-km radius from 0000 UTC 3 Jul to 0000 UTC 5 Jul 2004 in (top to bottom) northwest, northeast, southwest, and southeast quadrants. 2 2 2 Light (dark) gray indicates values . 1.0 (3.0) K (100 km) 1 (3 h) 1 , with contour intervals of (a)–(l) 0.5 and (m)–(p) 5.0 K (100 km) 1 2 (3 h) 1. calculation of scalar frontogenesis magnitude that is upper troposphere (Fig. 5). JRA-25, FNL, and the WRF dependent on the horizontal gradients of potential simulation results show the peak increasing rates of temperature and wind fields and thus is highly de- the sum of the SFP around 0000 UTC 4 July 2004, pendent on the resolutions of the datasets. In the suggesting that the ET completion time of Mindulle northwestern quadrant, weak frontogenesis occurred determined using these data is 0000 UTC 4 July 2004. in the mid- and upper troposphere prior to 0600 UTC 4 However, it is difficult to obtain the ET completion time July 2004 (Fig. 5m). Relatively intense frontogenesis in using RSM-20km data due to there being no continu- the mid- and upper levels was found in the northeastern ously growing summed SFP (Fig. 7a), although striking quadrant, whereas notable frontolysis existed in the frontogenesis was found in the mid- to upper tropo- lower troposphere at the same time (Fig. 5n). It is noted sphere (Figs. 5i–l). that marked frontolysis was found throughout the tro- The SFPs associated with Typhoon Yagi and calcu- posphere in the southeastern quadrant before 0000 lated with the different data sources are portrayed in UTC 4 July 2004 (Fig. 5p), while the evidence of weak Fig. 6, showing distinct distribution features. Despite frontogenesis was found from FNL (Fig. 5h). the comparable horizontal resolutions, the SFP de- The temporal evolution of the addition of the 500– tails of JRA-25 and FNL somewhat differed. Significant 300-hPa SFP of Mindulle in the northeastern and south- frontogenesis existed in the northeastern quadrant with western quadrants is depicted later (Fig. 7a), showing peak SFPs at 1800 UTC 23 September and 1800 UTC clearly that frontogenesis was found in the middle and 24 September 2006 for FNL and JRA-25 (Figs. 6b and 6f),

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FIG. 6. As in Fig. 5, but for Typhoon Yagi (2006). respectively. Figures 6d and 6h suggest weak frontogen- quadrant, with frontolysis in the mid- and upper tropo- esis in the southeastern quadrant, with maximum SFPs sphere for the RSM-20km data (Fig. 6l) and frontolysis at 0000 UTC 23 September and 0600 UTC 24 September prevailing before 0000 UTC 25 September 2006 for the 2006 for FNL and JRA-25, respectively. Furthermore, WRF simulation (Fig. 5p). the distributions of the SFPs derived from RSM-20km For the rapidly reintensifying case of Yagi, it seems and the WRF simulation results were strikingly dif- that only the JRA-25 data represented an obvious in- ferent from those from the JRA-25 and FNL data, and crease in the addition of the SFP in the northeastern and the SFPs computed from the WRF results were rela- southwestern quadrants around 1800 UTC 24 September tively noisy again. Note that Figs. 6i–l only show the 2006 (Fig. 7b), indicative of the completion of ET ac- SFP distributions derived from RSM-20km in the 48-h cording to the SFP method. Other data failed to determine period prior to 0000 UTC 25 September 2006 after the end of ET via the addition of the SFP, even with which the storm circulation partly moved beyond the frontolysis for the RSM-20km and WRF results during coverage range of the data. The RSM-20km and WRF most of the period of interest (Fig. 7b). simulation results represent visible frontogenesis below c. CPS parameters the midtroposphere in the northwestern quadrant (Figs. 6i and 6m). Unlike the frontogenesis revealed from Phase-space diagrams can be used to illustrate the ET, JRA-25 and FNL in the northeastern quadrant, the subtropical and tropical transitions, and the development RSM-20km and WRF simulations indicated that front- of warm seclusions within ECs (Hart 2003). As repre- olysis was dominant in that quadrant (Figs. 6j and 6n). sented previously in the literature (Hart 2003; Evans and Additional distinction of frontogenesis characteristics Hart 2003; Hart et al. 2006), a TC should start with a rel- among these data sources occurred in the southeastern atively symmetric warm-core structure, reﬂected by the

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FIG. 7. Time series of the sum of (a) 500–300-hPa SFPs averaged in the northeastern and southwestern quadrants for Typhoon Mindulle (2004) and (b) 500-hPa SFPs for Typhoon Yagi (2006). thickness asymmetry parameter B closeto0m,andthe Note that, because the circulation of Yagi moved be- 2 L 2 U thermal wind parameters VT and VT show positive yond the coverage range of RSM-20km after 0000 UTC values. Accompanying poleward movement of the cyclone 25 September 2006, only part of the CPS parameter’s and the interaction with midlatitude westerlies, B will be- evolution was shown in Fig. 8. The determined ET onset gin to rise and the thermal wind parameters will decrease; times of Yagi based on FNL, JRA-25, and RSM-20km with B greater than 10 m indicative of the ET onset; and data, as well as the WRF simulation results, were about 2 L 2 U . with both VT and VT negative along with B 10 m 0900, 1200, 2000, and 1500 UTC 23 September 2006, re- showing the end of ET. spectively, with the B parameters greater than 10 m 2 L 2 U Figure 8 shows the time series of B, VT , and VT (Fig. 8d). Like Mindulle, all data deemed that the upper- values corresponding to Mindulle (Figs. 8a–c) and Yagi level core of Yagi got cold in advance of the lower-level (Figs. 8d–f) during the periods of interest. The values of core. The FNL data show that the upper core of the storm the B parameters computed from all four data sources turned cold at about 1000 UTC 23 September 2006 were accordantly greater than 10 m around 1200 UTC (Fig. 8f), and the ET finished 8 h later when the associated 2 L 3 July 2004 (Fig. 8a), indicating the ET onset time of VT became negative (Fig. 8e). JRA-25 represented Mindulle. The consistent ET onset time indicated across a relatively late process, with the upper- (lower-) level core the datasets results partly from the areal averaging turning cold at 1800 UTC 23 September (1500 UTC 2 U employed to calculate the B parameter in the CPS 24 September) 2006. Figure 8f illustrates that the VT method. Around 1200 UTC 3 July 2004, the upper-level values computed from RSM-20km and the WRF simula- 2 U thermal wind parameters ( VT ) derived from the JRA- tion became negative around 1500 UTC 24 September 25, FNL, and RSM-20km data became negative, which 2006. The WRF simulation results showed the latest ET revealed that a cold-core structure occurred. In contrast, completion time at 0200 UTC 25 September 2006 (Fig. 8e). the upper-level thermal wind parameter of the WRF d. IPV simulation result was not negative until 0600 UTC 4 July 2004 (Fig. 8c). On the other hand, the lower-level ther- The PV can be used to depict TC structure and inter- 2 L mal wind parameters ( VT ) computed from FNL and action between the TC and surrounding environments and JRA-25 turned out to be negative at approximately has been used to describe the evolution of TCs undergoing 2 L 1800 UTC 4 July 2004 (Fig. 8b), while the VT values ET (Bosart and Lackmann 1995; McTaggart-Cowan et al. from the RSM-20km and WRF simulation results became 2001; Thorncroft and Jones 2000), thus being a potentially negative about 6 and 8 h before that time, respectively, useful tool for determining the ET time (Kofron et al. suggesting that the lower-level vortex was featured a cold 2010b). Kofron et al. (2010b) demonstrated that the core and thus the ET ended. The aforementioned phase- evolving IPV might reflect the interaction of the TC and space parameters associated with Typhoon Mindulle the preexisting trough during ET. As introduced in section showed that it was likely a warm seclusion ET case 2, Kofron et al. (2010b) found that the local minimum (Shapiro and Keyser 1990; Kofron et al. 2010a). value of 330-K IPV averaged within the 500-km radius For the rapidly reintensifying case of Yagi, the B para- apart from the storm center occurs when ET begins, and meters from all data tended to rise starting at 0000 UTC the ET completion time appears to be well defined by the 23 through 1200 UTC 25 September 2006 (Fig. 8d). 330-K IPV threshold value of 1.6 PVU. Whether the ET

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2 L 2 U FIG. 8. The temporal evolution of CPS parameters: (a) B, (b) VT , and (c) VT for Typhoon Mindulle (2004) and (d)–(f) for Typhoon Yagi (2006). onset and completion times of Typhoons Mindulle (2004) PV values around the 330-K isentropic surface for all and Yagi (2006) can be successfully determined or not with data, with higher PV values above and below, respectively. the IPV method proposed by Kofron et al. (2010b) based Therefore, following the method in Kofron et al. (2010b), on JRA-25, FNL, and RSM-20km data and WRF simu- the 330-K IPVs averaged within the radii of 300, 400, and lation results will be examined in this section. Another 500 km are drawn in Figs. 11a–c to determine the ET onset issue of interest is the sensitivity of the radius used to av- of Mindulle. The 330-K IPVs averaged within the 500-km erage the results, which is also investigated herein. radius from JRA-25, FNL, and RSM-20km data were The calculated IPV of Mindulle and Yagi is shown in characterized by a very similar pattern, with the minima Figs. 9 and 10, respectively. Figure 9 shows the average IPV present at 1200 UTC 3 July 2004 (Fig. 11a). This time was of Mindulle with the value at the initial time (0000 UTC the ET onset time of Mindulle as suggested in Kofron 3 July 2004) removed during the period 0000 UTC 3 et al. (2010b). The minimum 330-K IPV value averaged July–0000 UTC 5 July 2004. The 300-, 400-, and 500-km within the 500-km radius from the WRF results was seen at average results consistently suggest that there exist lower 0800 UTC 3 July 2004 (Fig. 11a). It is noted that the 330-K

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FIG. 9. The IPVs with the initial value removed based on (left to right) the four datasets, averaged within (top to bottom) 300, 400, and 500 km from 0000 UTC 3 Jul to 0000 UTC 5 Jul 2004. The light (dark) gray is indicative of the IPV value less than 20.4 (20.2) PVU, contoured every 0.2 PVU.

IPV patterns were somewhat changed when averaged 300-km-radius calculation. As a result, the 330-K IPV as- within the 300-km radius (Fig. 11c), although they were sociated with Yagi was featured by a rapid increase after similar to those in Fig. 11a when averaged within the 1200 UTC 25 September 2006 (Figs. 11d–f). The ET 400-km radius (Fig. 11b). Figure 11c indicates that the onset time could be clearly defined at 0000 UTC values of the 330-K IPV from FNL and JRA-25 were still 25 September 2006 in the light of the 500-km-radius minimized at about 1200 UTC 3 July 2004, while the lowest average 330-K IPVs from JRA-25 and FNL, and such values from RSM-20km and the WRF simulation appeared a time was about 3 h earlier according to the WRF at 1200 UTC 3 July and 0600 UTC 4 July 2004, respectively. simulation result (Fig. 11d). If IPV was averaged within This suggests that different average radii used to calculate a 400-km radius (Fig. 11e), the minimum 330-K IPV the mean PV value possibly lead to different IPV patterns. from FNL and the WRF results was still at 0000 UTC Figure 10 depicts the average IPV of Typhoon Yagi 25 September and 2100 UTC 24 September 2006, re- with the values at the initial time (0000 UTC 23 spectively, while the minimum 330-K IPV from JRA-25 September 2006) removed. Note that the IPV distribu- occurred at 0600 UTC 25 September 2006. In contrast, tions from RSM-20km were shown in Fig. 10 only through considering the IPV averaged within a 300-km radius, 0000 UTC 25 September 2006 due to the storm circu- the ET onset times defined from FNL data and the WRF lation beyond the coverage range of the data, and we do simulation result were shifted to 0000 and 1200 UTC not analyze them in depth herein. The distributions of 25 September 2006 (Fig. 11f), respectively. IPV associated with Yagi differed from those of Mind- ulle. JRA-25, FNL, and the WRF results notably rep- 4. Concluding discussion resented relatively low IPV values on the 325–350-K isentropic surfaces prior to 0600 UTC 25 September During ET a decaying TC frequently reintensifies as 2006 (Fig. 10), with the lowest values found in the an intense EC that may produce intense rainfall, strong

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FIG. 10. As in Fig. 8, but for Typhoon Yagi (2006) at intervals of 0.4 PVU and with the exclusion of the results from RSM-20km. winds, and hazardous seas. Therefore, determination of It was proposed that the OW and SPF methods had ET onset and completion times is important, and is a potential for determining the ET completion time challenge to forecasters. In the present study, we ex- (Demirci et al. 2007; Harr and Elsberry 2000). The OW amine four methods for determining the ET onset and method could determine the ET completion time of the completion times of two representative storms (i.e., rapidly reintensifying case of Typhoon Yagi (2006) Typhoons Mindulle and Yagi) with four datasets (i.e., based on all datasets (Fig. 12b), while it failed to indicate JRA-25, FNL, and RSM-20km, as well as WRF simu- the ET completion time of the slightly reintensifying lations) that have different horizontal resolutions. Note case of Typhoon Mindulle (2004) using RSM-20km and that, although it is impossible to quantitatively evaluate the WRF simulation results (Fig. 12a), which have finer the influence of the difference of data resolution on the resolutions. In contrast, the SFP technique was able to identification of the times, the probable role of reso- identify the ET completion time of Mindulle across the lution was qualitatively investigated in this study. A four datasets (Fig. 12a), whereas this method was suc- summarized schematic diagram of the determined ET cessful in only determining the ET completion time of onset and completion times of the two typhoons is dis- Yagi using the two coarser datasets (i.e., JRA-25 and played in Fig. 12. FNL; Fig. 12b).

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FIG. 11. Time series of average 330-K IPV within (a),(d) 500, (b),(e) 400, and (c),(f) 300 km from the storm centers of (a)–(c) Mindulle and (d)–(f) Yagi. The IPV value at the initial time was removed.

With the exception of RSM-20km for Typhoon Yagi, completion time of around 1200 UTC 4 July 2004 (Fig. CPS succeeded in determining the ET onset and com- 12a). For Typhoon Yagi, the values of average 330-K IPV pletion times of Mindulle and Yagi (Figs. 12a and 12b). calculated from JRA-25, FNL, and the WRF simulation Also with the exception of RSM-20km for Yagi, the IPV result exceeded 1.6 PVU, suggesting the end of the ET method enabled us to suggest the ET onset times of the according to the IPV method (Fig. 12b). two storms (Figs. 12a and 12b). However, the ET onset Of interest is that the ET onset times of Mindulle times of Yagi determined by the IPV method were much suggested by the CPS and IPV methods using the four later than those by CPS. Kofron et al. (2010b) proposed datasets were quite consistent, around 1200 UTC that the ET completion time may be deﬁned as the time 3 July 2004 (Fig. 12a). The ET completion times of when the 330-K IPV exceeds the threshold value of Mindulle determined by OW, CPS, and IPV were around 1.6 PVU. Only the 330-K IPV calculated from the 10-km 1200 UTC 4 July 2004, while the SFP method indicated WRF simulation result met the above threshold for theETcompletiontimeof0000UTC4July2004 Typhoon Mindulle (2004), with the determined ET (Fig. 12a).

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FIG. 12. The schematic diagram of the determined ET onset or completion times using the OW, SFP, CPS, and IPV methods with different datasets (i.e., JRA-25, FNL, and RSM-20km data, as well as the WRF simulation results) for (a) Mindulle and (b) Yagi. The open (ﬁlled) squares indicate the onset (completion) of the ET process.

The four methods seem not to be sensitive to the data Kofron et al. (2010a) pointed out that the OW and SFP sources for the slightly reintensifying case of Mindulle methods appeared not to be feasible operational tools for (Fig. 12a), whereas they become strikingly sensitive to determining the ET completion time. In this study we the distinct datasets for the rapidly reintensifying case of also ﬁnd that the ET completion times of the two distinct Yagi (Fig. 12b). For instance, the ET completion times ET cases were not smoothly obtained with the OW and of Yagi determined by OW using JRA-25 and FNL, SFP methods based on the four data sources. For the OW which have coarser horizontal resolutions, were around method, whether the 20-m contour intervals utilized on 0000 UTC 24 September 2006, while the completion times the 500-hPa geopotential height map are appropriate for based on high-resolution RSM-20km and the WRF sim- data having different resolutions is still questionable. On ulation results were about 24 h later (Fig. 12b). Only the other hand, it is sometimes difﬁcult to illustrate the JRA-25 data succeeded in determining the ET completion complex circulation features associated with the storm time of Yagi. The ET onset times indicated by the CPS and thus the OW structure, when the cyclone interacts method using JRA-25 and FNL were approximately with ambient midlatitude systems. Furthermore, the pres- around 1200 UTC 23 September 2006. However, these two ence of OW structure appears to be dependent on a variety datasets with comparable horizontal resolutions obviously of factors, such as data resolution, storm translation speed, indicated different completion times, with the ET process and the latitude where the storm is located. As grid spacing determined based on JRA-25 data ending about 24 h later becomes smaller, it tends to become easier to see a closed than that suggested using FNL. Figure 12b shows the much contour as suggested in this study. As a storm becomes longer ET course of Yagi determined by CPS using the superimposed on a strong pressure gradient and accelerates, WRF simulation result that lasts for about 36 h. the OW signal may occur. These factors complicate the

Unauthenticated | Downloaded 09/28/21 06:20 AM UTC 1410 WEATHER AND FORECASTING VOLUME 27 identification of the ET completion time using the OW As a specific case study, the present paper examined method. Additionally, an objective technique for deter- only two representative cases undergoing distinct in- mining the occurrence of the OW structure is hard to ach- tensification processes after ET. Factors affecting the ieve as well. SFP may be used to dynamically characterize ET process are highly complicated they involve unique the TC–trough interaction, but it is seemingly difficult to be synoptic and TC environments. Therefore, comprehen- used for practically finding the ET completion time, be- sive studies of a large number of ET cases may provide cause of the lack of reasonable thresholds for scalar front- further insight into the operational applicability of these ogenesis to determine an ET completion time. Harr and methods across different data sources. Additionally, it is Elsberry (2000) suggested that the time when the 500-hPa impossible to absolutely examine the role of resolution scalar frontogenesis averaged in the northeastern and in determining ET onset and completion times based on southwestern quadrants within the 500-km radius shows datasets from differing models. Sensitive numerical ex- a distinct increase could potentially indicate the ET com- periments with a certain model may be an alternative pletion time. However, reasonable thresholds for the scalar approach to this problem. Corresponding results will be frontogenesis increase are not obtained in operational represented in future publications. forecasting. Additionally, scalar frontogenesis features Acknowledgments. The authors are grateful to two represent case-to-case variability. Because of the magni- anonymous reviewers for their helpful comments. This tude of scalar frontogenesis dependent on horizontal gra- study was supported through the National Basic Research dients of the variables, noisy patterns associated with scalar Program of China (2009CB421504), and the National frontogenesis are generally found in high-resolution data Natural Science Foundation of China under Grants analysis, resulting in the lack of continuity in scalar front- 40775060, 41005033, 40905029, and 40921160381. G. Fu ogenesis evolution (e.g., Fig. 7b). CPS parameters give was partly supported by the Chinese Meteorological quantitative thresholds and succeed in determining the ET Agency under Grants GYHY200706031 and GYHY times when applied to the two cases in this study. However, GYHY200906002, and the Chinese Government Pro- there are still several problems to be discussed further. gram of Introducing Talents of Discipline to Universities Since the result from the CPS method appeared to be (B07036). We also thank the Key Laboratory of Meso- sensitive to the data sources in the case of Yagi (2006), as scale Severe Weather (MOE) of Nanjing University for shown in Fig. 11b, forecasters need to take into account the furnishing the Japanese Meteorology Agency Regional dependence of the data source as they make use of the CPS Spectral Model 20-km reanalysis. technique to predict the ET onset and completion times of such a rapidly reintensifying storm. Moreover, more rap- APPENDIX idly reinforcing cases after ET should be further examined to document whether the sensitivity to data sources is or- dinary. In addition, the corresponding thresholds for the Related Formulas Used in the Determination 2 L 2 U Techniques of ET Times parameters (i.e., B, VT ,and VT ) may vary over different ocean basins (Hart 2003), which needs to be in- The three-dimensional scalar frontogenesis param- vestigated in depth. As a method proposed more recently, eter (Schultz and Doswell 1999; Harr and Elsberry the IPV method successfully determined the ET onset times 2000) is of Mindulle and Yagi using all the datasets. However, it is ›u › ›u ›y ›u difficult to suggest the ET completion times according to 21 u . F 52j$ uj 2 2 the present threshold [averaged 330-K IPV 1.6 PVU; n h ›x ›x ›x ›x ›y Kofron et al. (2010b)]. Therefore, the threshold of IPV for ›u ›u ›u ›y ›u ›u ›v ›u ›v ›u determining the ET completion time based on different 1 2 2 1 2 2 , ›y ›y ›x ›y ›y ›p ›x ›x ›y ›y data sources of different resolutions needs to be analyzed further. As indicated in Fig. 11, the average range of IPV (A1) seems to more or less influence the pattern of IPV and thus u the result. Hart (2003) and Tapiador et al. (2007) demon- where is the potential temperature. Positive (negative) 2 strated that sensitivity to radius also existed for the CPS Fn corresponds to the frontogenesis (frontolysis). method, particularly for TCs with differing sizes. These In the cyclone phase-space method (Hart 2003), pa- sensitivities should be taken into account in practical fore- rameter B is related to the 900–600-hPa thickness casting. It is also noted that the ET onset times of Typhoon asymmetry relative to the moving direction of the storm. Yagi indicated by the CPS and IPV methods differ mark- In the Northern Hemisphere, it is computed as edly from each other (Fig. 12b), which may create forecast 5 2 2 2 uncertainty. B Z600hPa Z900hPa R Z600hPa Z900hPa L , (A2)

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