ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 22, NO. 5, 2005, 730–740

A Study of the Extratropical Transformation of (1997)

ZHU Peijun∗1 (Á¢), ZHENG Yongguang2 (x[1), ZHANG Chunxi2 (ÜSU), and TAO Zuyu2 (>y) 1Department of Geoscience, University, Hangzhou 310027 2Department of Atmospheric Sciences, School of Physics, Peking University, Beijing 100871 (Received 9 March 2004; revised 5 April 2005)

ABSTRACT The complicated evolutive process of how a transforms into an is still an unresolved issue to date, especially one which arises in a weakly baroclinic environment. Typhoon Winnie (1997) is studied during its extratropical transformation stage of extratropical transition (ET) with observational data and numerical simulations. Results show that Winnie experienced its extratropical transformation to the south of the subtropical high without intrusion of the mid-latitude baroclinic zone. This is significantly different from previous studies. Analyses reveal that the cold air, which appeared in the north edge of Winnie circulation, resulted from the precipitation drag and cooling effect of latent heat absorption associated with the intense precipitation there. The cooling only happened below 3 km and the greatest cooling was below 1 km. With the cold air and its advection by the circulation of Winnie, a front was formed in the lower troposphere. The front above is related not only to the cooling in the lower level but also to the warming effect of latent heat release in the middle-upper levels. The different temperature variation in the vertical caused the temperature gradient over Winnie and resulted in the baroclinicity.

Key words: extratropical transformation, typhoon, latent heat, front

1. Introduction stages: extratropical transformation, in which the tropical cyclone evolves into a baroclinic storm; and In recent years, extratropical transition (ET) has reintensification, where the transformed storm then attracted more attention from meteorologists than deepens into an extratropical cyclone. Their study ever. Considerable characteristics associated with ET also shows that in the transformation stage, all the have been revealed (Klein et al., 2000; Harr and Els- physical processes are related to the invasion of a po- berry, 2000a, 2000b; Hart and Evans, 2001; Foley lar baroclinic zone. In the studies of Sekioka (1956, and Hanstrum, 1994; Ritchie and Elsberry, 2000; Sin- 1970), Matano (1958), Matano and Sekioka (1971), clair, 2002). However, Jones et al. (2003) noted in and Brand and Guard (1979), it has been proposed their comprehensive review that the ET process is that a typhoon will not transit into an extratropical still poorly understood and incompletely researched to cyclone, which will decay and dissipate, until a cold date in contrast to tropical cyclones and extratropical front invades into it. A study of ET processes that cyclones. There is even no universal definition of ET. have affected Australia (Foley and Hanstrum, 1994) Most previous studies have concerned those ET pro- shows that the airflows and weather during ET are cesses that occurred over oceans including the North- closely associated with the capture of a tropical cy- west Pacific, Atlantic, South Pacific, and continents clone by a cold front. In addition, in some ET case including North America and Australia. In , studies it has been shown that there is a significant there have been some landed typhoons that were able baroclinic zone or cold front that determinately causes to transit into extratropical cyclones (Chen and Ding, a tropical cyclone to transit into an extratropical cy- 1979), which may have caused intense precipitation clone (Palmen, 1958; DiMego and Bosart, 1982). and even typhoon-force wind over North China. However, typhoon Winnie (1997), which landed in In an overview of 30 ET cases that occurred over Zhejiang province in the east of China at 1300 UTC the Northwest Pacific Ocean, Klein et al. (2000) con- 18 August 1997 and caused intense precipitation over cluded that the ET process can be defined in two North and Northeast China (Huang, 1998), underwent

*E-mail: [email protected] NO. 5 ZHU ET AL. 731 extratropical transition in a weakly baroclinic environ- the temperature distribution changes from symmetri- ment. At 0000 UTC 20 August, typhoon Winnie be- cal to asymmetrical. Figure 1a shows the sea level came a baroclinic cyclone with an evident temperature temperature at 0000 UTC 20 August 1997 when Win- gradient. Later on it intensified as a result of baro- nie had evolved into a baroclinic cyclone. The tem- clinic instability with a maximal deepening rate of perature gradient over the cyclone is evident with cold −1 5 hPa (6 h) . According to Klein et al.’s (2000) defi- air in the west and warm air in the east, which implies nition, the evolutionary process of Winnie can be de- that the front had formed. Figure 1b is the IR image fined in two stages: extratropical transformation and taken at the same time as Fig. 1a. The cloud pat- reintensification. In the transformation stage of Win- tern is quite different from that of a tropical cyclone. nie, it is interesting to note that there is no visible Most of the convections are located in the north and cold air intruding into the typhoon from the environ- east, which suggests a warm frontogenesis (Klein et ment. We wonder how the extratropical transforma- al., 2000). In the west of the cyclone, there is lower tion could have occurred without a front or baroclinic cloud only, while in the south there is a narrow cloud zone, which are required according to previous studies. band, which suggests a weak cold frontogenesis. The Where did the cold air come from to cause Winnie to asymmetry shown in Fig. 1b illustrates that Winnie transform then? And how did the front form in the tropical cyclone? has become a baroclinic cyclone at that time, which To solve these problems, first the fact and charac- is consistent with the temperature distribution in teristics of extratropical transformation are reviewed Fig. 1a. with conventional observation data and GMS satellite 2.2 Characteristics during extratropical trans infrared (IR) images during 18–20 August. Then, us- formation ing the mesoscale model MM5, Winnie is simulated and a sensitivity test is performed to illustrate the Some interesting characteristics can be found dur- mechanism of extratropical transformation. ing the extratropical transformation of Winnie. Due to the reintensification stage of ET regard- Figure 2 shows the weather maps at 850 hPa during ing Winnie, the following process to the extratropical Winnie’s extratropical transformation. The transfor- transformation will be studied in a future paper. mation occurred to the south of the subtropical high, whereas the mid-latitude baroclinic zone is located to 2.Observational Description of Typhoon Winnie its north. The relative situation between the baroclinic zone and Winnie implies that there is no direct rela- 2.1 Extratropical transformation tionship between them. This differs significantly from Extratropical transformation is the first stage of previous studies and general knowledge in this area. the ET process in which the tropical cyclone evolves It is due to the southerly and southwesterly wind over into a baroclinic cyclone. That is, it is an obvious the baroclinic zone to the north of the subtropical high characteristic during extratropical transformation that ridge, from which no cold air will be advected to cause

Fig. 1. (a) Sea level pressure and temperature (shaded) and (b) GMS satellite IR image at 0000 UTC 20 August 1997. 732 THE EXTRATROPICAL TRANSFORMATION OF TYPHOON WINNIE (1997) VOL. 22

Fig. 2. Synoptic maps for 850 hPa at (a) 0000 UTC 19 August, and (b) 0000 UTC 20 August. the typhoon’s transformation. It can be demonstrated to cause typhoon Winnie’s transformation? And how from the trajectories of air particles originating from can the front form in the cyclone? the baroclinic zone (Fig. 3). To reveal these questions, first the thermodynamic Figure 3 shows the 48-h trajectories of some air structure on the conventional pressure levels is ana- particles, which were produced using the PC-Vis5d lyzed. It is at 925 hPa that the temperature changes software developed by Wang et al. (1998). The grid most with time. Figure 4 shows the station temper- data used in the trajectories are results of objective ature variation during the past 24 hours at 925 hPa. analysis from observations. The dots in Figs. 3a and The temperature varies little at 0000 UTC 19 August 3c are the original locations of the air particles in the (Fig. 4a). However, at 1200 UTC 19 August, an evi- mid-latitude baroclinic zone at 0000 UTC 18 August dent decrease appears in the north circulation of Win- on the levels of 850 hPa and 925 hPa respectively. The 48-h trajectories are shown in Figs. 3b and 3d, in which nie (Fig. 4c); then at 0000 UTC 20 August, the de- we can see that none of them intrudes into Winnie, i.e. creasing area moves northward slightly (Fig. 4e). The the cold air causing Winnie to transform is not from temperature in the area of negative variation is much the mid-latitude baroclinic zone. lower than that at the last observational time and that The phenomenon presented above leads to the fol- in the nearby area. This indicates that the tempera- lowing questions: where does the cold air come from ture decreases in situ in the north of Winnie and that NO. 5 ZHU ET AL. 733

Fig. 3. Air particle trajectories. The particle original locations (dots) at (a) 925 hPa and (c) 850 hPa, and the potential height (blue) and temperature (red) at 0000 UTC 18 August; (b) and (d) show the trajectories ending at 0000 UTC 20 August corresponding to the particles shown in (a) and (c) respectively, where the color range from blue to red in the trajectories represents the height from the lower level to the upper. the decrease could not result from temperature advec- some extent. In Fig. 4d, the amount of precipitation tion from outside. is enhanced greatly compared to that in Fig. 4b; cor- In the studies of severe convection, it has been respondingly, the temperature decrease in Fig. 4b is found that in an intense rainfall area, a shallow cold more than that in Fig. 4a and the greatest decrease is dome always accompanies it in the planetary boundary located where the heavy rainfall is and in its down- layer (Zhang, 1999). Is the decrease of temperature in wind area. When the heavy rainfall moves northwest Figs. 4c and 4e related to rainfall? The accumulated slightly (Fig. 4f), the maximum negative temperature precipitation is contrasted with negative temperature variation also moves slightly to the northwest (Fig. 4e). variation to find whether their distributions are con- The sounding profiles of those stations with neg- sistent with each other. Figures 4b, 4d, and 4f show ative temperature variation in Fig. 4c are now been the 6-h accumulated precipitations at the same times examined. Figure 5 is the time-height cross section for as Figs. 4a, 4c, and 4e, respectively. From Fig. 4 we station 54823, which is marked with a shaded circle in can see that the area of negative temperature variation Fig. 4a. It shows the change of temperature,horizontal is consistent with that of accumulated precipitation to wind and moisture during 1200 UTC 18 August to 734 THE EXTRATROPICAL TRANSFORMATION OF TYPHOON WINNIE (1997) VOL. 22

Fig. 4. The surface temperature (plotted below station) and 24-hour temperature variation (plotted right of station) and contours of temperature variation (negative values are dashed) at (a) 0000 UTC 19 August, (b) 1200 UTC 19 August, and (c) 0000UTC 20 August. (b), (d) and (f) are the 6-h accumulated precipitation at the same times as (a), (c) and (e) respectively. NO. 5 ZHU ET AL. 735

contained in the full physics numerical model. To ver- ify the detailed relationship between the physical pro- cess accompanied with the rainfall and the tempera- ture variation, a numerical simulation and a sensitivity test without the feedback of latent heat are performed in the next section.

3. Numerical simulation and sensitivity test

A numerical simulation is performed using the meso-scale model MM5 with two nested domains (45- km and 15-km grids respectively) during 1200 UTC 18 August 1997 to 0000 UTC 20 August 1997. The simu- lated output is verified with observations of the cyclone motion path, intensity, precipitation and clouds. The result of the verifications is encouraging. The physical processes used in the simulation are similar to those in Fig. 5. The height-time cross section of temperature, hor- the simulation study of Zhu et al. (2002), which can izontal wind and T − Td during 1200 UTC 18 August to be referred to for details. 0000 UTC 20 August for station 54823. Based on the facts analyzed in section 2, a sensi- 0000 UTC 20 August. The temperature below 700 hPa tivity test is devised also, in which the model is rerun decreases with time and the greatest decrease occurs without latent heat feedback but using the same initial at 925 hPa and below, whereas the temperature above fields and the other physical options as above. With 700 hPa tends to increase, although there is an excep- the sensitivity test, it can be examined whether the tion at 500 hPa. The change of temperature shown cold air that caused Winnie to transform is related to in Fig. 5 is similar to the feedback of latent heat in- the latent heat of rainfall, and the relationship between cluding the cooling in the lower troposphere and the front formation and latent heat feedback. Hereinafter, warming in the middle-upper troposphere. Moreover, the simulation using the full physics is called the con- the value of T − Td shaded in Fig. 5 decreases with trol test (CTL), and the simulation without latent heat time in the whole troposphere. This suggests that the is called the sensitivity test (NOHL). moist layer has been extended in the vertical, which 3.1 The cold air causing Winnie’s extratropical might be the result of convective transportation. Con- transformation trasted with the changes of temperature and moisture, however, the horizontal wind changes little. There is The process of a tropical cyclone transforming to a always northeasterly wind below 500 hPa due to the baroclinic cyclone is definitely accompanied with the station located at the northwest quadrant of the cy- process of cold air emerging in the west of the cyclone. clone, while the wind speed increases with time due In the CTL simulation, the process is only present be- to the approaching of the typhoon center. Above 500 low 700 hPa (about 3 km high) and the most intensive hPa, the wind is southwesterly at first and then gets cold air appears below 900 hPa (about 1 km high). Fig- some change in direction due to Winnie’s approaching. ure 6 shows the distributions of temperature at 900 Thus the change of horizontal wind is mostly related hPa during the extratropical transformation from the to Winnie’s approaching. onset time (Jones et al., 2003) at 1800 UTC 18 Au- With these characteristics of the extratropical gust to the completion time (Jones et al., 2003) at transformation of Winnie, it is suggested that the vari- 0000 UTC 20 August (Zhu et al., 2003). At the on- ation of temperature in Winnie might be related to set time, the warm air ranging from 293 K to 296 K is the heavy rainfall in the north of the typhoon. The distributed almost symmetrically over the entire cir- processes causing the air temperature decrease may culation of Winnie, whereas the primary cold air is include: (1) the sensible heat exchange as the cooler located over the southeast coast of Siberia, consistent raindrops fall from the upper levels to the lower levels; with the mid-latitude baroclinic zone shown in Fig. (2) the melting and sublimation of solid precipitation; 2. The cold air will weaken and move northeast with and (3) the evaporation of rainwater. The first pro- time in the southwesterly wind. At this time, a weak cess is least important since the falling time of the cold air band is notable at the north of the typhoon raindrops is short. The later two processes are re- over North to the south of province, lated to the latent heat of phase change, which are though the minimum temperature is only lower than 736 THE EXTRATROPICAL TRANSFORMATION OF TYPHOON WINNIE (1997) VOL. 22

Fig. 6. Temperature (shaded, the area in white is terrain, the same hereinafter) and wind vectors at 900 hPa. (a) 1800 UTC 18 August. (b) 1200 UTC 19 August. (c) 1800 UTC 19 August. (d) 0000 UTC 20 August. that of the typhoon by 3 K. Subsequently, the cold NOHL tests (Tctl −Tnohl) represents the effect of latent air is strengthened and intrudes into the west of the heat feedback. Figures 7a and 7b are the Tctl −Tnohl at typhoon in the northeasterly wind of the typhoon cir- 900 hPa at 1800 UTC 18 August and 0000 UTC 20 Au- culation (Figs. 6b–6c), and finally inhabits the west gust respectively, which are overlapped with the CTL (Fig. 6d) with a minimum value of 286 K. There is a horizontal wind at the same level. Reviewing the tem- 5–8 K difference across the center of the cyclone, which perature distribution in Fig. 6a, we can find that the implies the front formation in the lower level. weak cold area over North Korea corresponds to a neg- In the NOHL simulation, however, there is no cold ative value area in Fig. 7a, while the colder area over air appearing in the north of Winnie and there is a Southeast Siberia in Fig. 6a is positive in Fig. 7a. This sparse isotherm distribution (figure omitted). So the implies that only the weak cold air over North Korea supposition proposed from the observational analysis is related to the latent heat process. Whereafter, the is demonstrated by the numerical simulation that there negative value area extends downstream, which is an is a certain close relationship between the cold air and advective effect by the circulation in the northwest of the latent heat, which can be illustrated by the differ- Winnie. At 1200 UTC 19 August, the negative value ence between the CTL and NOHL simulations. area extends to the west with the extremum value de- The temperature difference between the CTL and creasing (figure omitted) and reaches its minimum at NO. 5 ZHU ET AL. 737

Fig. 7. Temperature difference between the CTL and NOHL tests (Tctl − Tnohl, shaded), and wind vectors in the CTL test at 900 hPa. (a) 1800 UTC 18 August. (b) 0000 UTC 20 August.

0000 UTC 20 August (Fig. 7b). The evolution of the θe between the completion and onset times of the negative value zone is similar to that of the cold air transformation, which are overlapped with the θe iso- at the north of the typhoon in Fig. 6, which manifests pleths of 338 K and 348 K representing the front zone. that the cold air causing the typhoon to undergo ex- The temperature decreases below 3 km of height under tratropical transformation is produced by the cooling the front zone with a minimum of −8.68 K, whereas it effect of the latent heat feedback over the intense pre- increases over the front zone above 1 km with a max- cipitation area. imum of 7.35 K at 7 km. The difference in the ver- tical temperature variation results in baroclinicity in 3.2 Front formation the troposphere below 6 km. The difference of θe is As the cold air appears in the northwest of typhoon obvious also, which exhibits the change in front zone Winnie, the temperature gradient is formed, which in- denoted by the θe isopleths of 338 K and 348 K. Figure dicates the formation of the front in the lower levels. 8c makes it clear that the formation of the front below However, since the cold air appears only below 700 hPa 6 km is related to not only the cooling effect under and mainly below 900 hPa, the main front is below 900 the front but also the warming effect above the front. hPa. Then the question is, how about the front above? Using the results of the NOHL simulation, the rela- The vertical cross sections of temperature and po- tionship between the front formation and latent heat tential equivalent temperature (θe) are available to re- can be verified. veal the front characteristics in the troposphere. The Figure 8e is the vertical cross section of the NOHL baseline of the cross section is shown in Fig. 6d. The simulation at 0000 UTC 20 August. No front emerges cross section in Fig. 8a shows that at 1800 UTC 18 below 6 km, which demonstrates that without the feed- August, the baroclinic zone is located only above 6 back of latent heat, the front could not form in the km over the north of Winnie, which is the subtropical middle-lower levels. It also shows that the upper sub- front. Below 6 km, the temperature contours are dis- tropical front is similar to that in Fig. 8b, which implies tributed quasi-horizontally. A great θe gradient is lo- that latent heat has little effect on the upper subtrop- cated between Winnie and its northwest outer bound- ical front on the synoptic timescale. Figure 8d shows ary. This is due to the moisture difference. As time the difference between the CTL and NOHL simula- elapses, the dense θe isopleths below 6 km tilt north- tions in the changes of temperature and θe during the west with height and where the downward concavity extratropical transformation, which can be used to ac- in temperature contours becomes evident. This sug- count for the effect of latent heat more quantitatively. gests that a front is forming. Figure 8b shows that at It is found that the areas of warming and cooling in 0000 UTC 20 August, the front below 6 km shows an Fig. 8d and Fig. 8c are highly consistent with each evident thermodynamic structure. other. Though their extrema differ by 1–2 K, this is a Figure 8c shows the differences of temperature and fraction of the total. Accordingly, it can be concluded 738 THE EXTRATROPICAL TRANSFORMATION OF TYPHOON WINNIE (1997) VOL. 22

Fig. 8. Vertical cross sections (with baseline shown in Fig. 6d). (a) and (b), θe (shaded) and temperature (white lines with increment of 5 K) in the CTL test at 1800 UTC 18 August and 0000 UTC 20 August respectively. (c), ∆θe (completion–onset) (shaded) and ∆T (completion–onset) in the CTL test (white lines with increment of 2 K, solid is positive and dashed is negative). (d), ∆θe(ctl − nohl)|completion − ∆θe(ctl − nohl)|onset (shaded) and ∆T (ctl − nohl)|completion − ∆T (ctl − nohl)|onset (white lines with increment of 2 K, solid is positive and dashed is negative), and mean wind vector every 6 h between 0000 UTC 19 August and 0000 UTC 20 August, where the 338 K and 348 K isopleths of θe are overlapped at the onset time (black dashed lines) and the completion time (black solid lines). (e), same as (b), but for the NOHL test. that the feedback of latent heat is the predominant fac- heat can result in the front formation and it can in- tor for the formation of the front over typhoon Winnie. tensify it. In the study of diabatic frontogenesis using an ideal In the time-height cross section of a particu- simulation with an initial front, Wang et al. (2002) lar station (Fig. 5), we can see that the tropo- demonstrated that the feedback of latent heat is fa- sphere is almost saturated, which is unfavorable for vor able to the strong contrast of cool and warm air evaporation/sublimation of rain droplets. However, masses across the frontal zone. The real simulation from Fig. 8, some favorable information for evapora- study about Winnie shows that the feedback of latent tion/sublimation can be found. Comparing the flows NO. 5 ZHU ET AL. 739 in Fig. 8a and Fig. 8b with those in 8e, it can be cooling is below 1 km. With the cold air and its ad- found that the circulation is changed significantly by vection by the circulation of Winnie, a front is formed the feedback of latent heat, especially the vertical mo- in Winnie in the lower troposphere. The front above tion. The downdraft, appearing in the middle and is related not only to the cooling in the lower level but upper troposphere in Fig. 8a and 8b, does not appear also to the warming effect of latent heat release in the in Fig. 8e. Now, the factors associated with rainfall middle-upper levels. The different temperature vari- that can create or enhance downdrafts are precipita- ation in the vertical forms the temperature gradient tion drag and evaporation/sublimation/fusion cooling. over Winnie, which results in the baroclinicity in the As rain falls, it tends to drag some air along with it. typhoon. Dry air entrainment into a cloud allows for evapora- In conclusion, the cold air and the front that tion/sublimation of some of the cloud’s liquid/solid emerged in typhoon Winnie are predominantly caused water. And during the fall of solid precipitation to the by the precipitation drag and feedback of latent heat ground, fusion processes will take place. The cooling over the intense precipitation at the north of the ty- due to phase changes can cause air to become nega- phoon. tively buoyant and sink. It is an interactional process The conclusion above gives rise to some other ques- among downdraft and precipitation drag and phase tions, such as: why most typhoons, which always have change cooling. So the saturation shown in Fig. 5 is heavy rainfall and are without the effect of a mid- the static state result of the dynamic change between latitude baroclinic zone, do not experience the ET pro- unsaturated and saturated air associated with precipi- cess? From the study of Winnie, we suppose that the tation drag and evaporation/sublimation/fusion cool- precipitation distribution might be important for the ing. extratropical transformation of a typhoon: namely, an intense precipitation that is mainly distributed on the 4. Conclusions and discussion north edge of the typhoon circulation, and one that moves little for a considerably long time. The former The extratropical transformation of Winnie is stud- situation can maintain a relatively dry environment ied with observational data and numerical simulations. that is favorable for the evaporation of rainwater and With the observational data, it is found that the the sublimation of solid precipitation. The latter situ- extratropical transformation of Winnie occurs at the ation can accumulate the change of temperature pro- south of the subtropical high without the mid-latitude duced by the feedback of latent heat. A mechanism study of such a precipitation distribution and a con- baroclinic zone intruding, though this is a necessary trasting study with some other cases will be favorable condition as concluded in previous studies. The cold to reveal the answer in detail, which will be expected air appearing in the north of Winnie intensifies in situ, in future work. which is consistent with the intensification of rainfall Furthermore, though the effect of latent heat feed- there. Moreover, with the height-time cross section at back (Fig. 8d) accounts for the most changes of tem- a particular station in the cold area, it can be found perature and θ (Fig. 8c), the difference between the that the temperature decreases with time below 3 km e extremal values is about 1–2 K and the difference be- and that increases above 3 km and that the moist layer tween the extrema locations is about 1–2 km in the is extended in the vertical, which is consistent with the vertical, which hints that there must be some other effect of latent heat feedback and the intense precipita- processes to change the thermodynamical structures, tion. A supposition is proposed, then, that the change though they might be feeble. of thermodynamic structure might be related to latent heat associated with the heavy rainfall in the north of Acknowledgments. This work was supported by Winnie. the National Natural Science Foundation of China (Grant Nos. 40233036, 40305004) and the Ministry of Science and Then the supposition is manifested with a numeri- Technology Project of China (Grant No. 2001CCA02200). cal simulation (CTL) and a sensitivity test without la- tent heat feedback (NOHL) using MM5. The cold air and front that emerged over the Winnie circulation in REFERENCES the CTL simulation do not appear in the NOHL test. By contrasting the two simulation outputs, it can be Brand, S., and C. P. Guard, 1979: An observational study found that the cold air that appeared in the north is of extratropical storms evolved from tropical cyclones in the western North Pacific. J Meteor. Soc. , related to the precipitation drag and cooling effect of 57,479-482 latent heat absorption in the lower troposphere below 3 Chen Lianshou, and Ding Yihui, 1979: An Overview of Ty- km over the intense precipitation and that the greatest phoon on West Pacific. 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