THE EXTRATROPICAL TRANSITION OF TYPHOON WINNIE (1997): SELF-AMPLIFICATION AFTER LANDFALL Chih-Shin Liu *1,2 and George Tai-Jen Chen 2 1 Weather Forecast Center, Central Weather Bureau, 2 Department of Atmospheric Sciences, National Taiwan University, Taipei 1. INTRODUCTION As tropical cyclones re-curve poleward and move into the midlatitude, they often interact with baroclinic systems and transform into extratropical cyclones. Previous studies have suggested that about 40% of tropical cyclones undergo such a extratropical transition (ET) process (DiMego and Bosart 1982; Hart and Evans 2001). These systems can at times produce extremely heavy precipitation and strong winds, and cause damage of properties and loss of lives. Unfortunately, ET processes are often not well predicted by operational models (Browning et al. 1998; Fogarty 2002). Recently, the ET process has attracted considerable attention in various regions of the world. But over East Asia, the topic is rarely studied and very few articles can be found in the open literature. The present study diagnoses the ET process of Typhoon Winnie (1997) over mainland China. In contrast to many prior ET processes similar to Petterssen Type-B scenario (DiMego and Bosart 1982; Sinclair 1993; Harr and Elsberry 2000), our results suggest that the ET of Typhoon Winnie was a manifestation of the self-amplification process. *Corresponding author address: Chih-Shin Liu, Department of Atmospheric Sciences, National Taiwan University, No. 61, Ln.144, Sec. 4, Keelung Rd., Taipei e-mail: gestef@mfcsv.cwb.gov.tw 2. DATA AND METHODOLOGY In our study, six-hourly surface and 12-hourly upper level weather maps of Japan Meteorological Agency (JMA) were used to analyze the synoptic systems, to determine the track and intensity of the typhoon/cyclone, and to verify objective analyses. Visible and infrared enhanced imageries from the GMS satellite were used to analyze the characteristics of clouds associated with the cyclone undergoing ET process. 12-hourly (00 and 12 Z) gridded analyses from ECMWF were also adopted as our data source for the calculation and potential vorticity (PV) inversion. The data set has a horizontal resolution of 1.125 1.125 latitude/ longitude and is provided at nine pressure levels (1000, 925, 850, 700, 500, 400, 300, 200, and 100 hpa) in the vertical. The nonlinear balanced PV diagnose system developed by Davis and Emanuel (1991) and Davis (1992a,b) was used as our primary tool of diagnosis. In our study, the average of August 1997 was adopted as the mean field. The scheme of partitioning PV perturbations is shown in Fig. 1. The group of lb includes lower boundary potential temperature perturbations, ms includes all positive mid-level PV anomalies in the saturated air (RH>70%), so it is associated with condensational heating, mu stands for PV anomalies associated with unsaturated air at mid-levels, and ul includes all upper level PV perturbations.
150 hpa θ 200 hpa q 300 hpa q 400 hpa q 500 hpa q 700 hpa q 925 hpa q ul ms (R.H.>70 q >0) 963 hpa θ lb mu tropical cyclone but much resembled a extratropical one. The eye wall erosion, the multi-layer clouds that were associated with the warm frontogenesis on the poleward side, the sharp cirrus edge, and the warm-frontal clouds being much more active than the cold-frontal ones were all characteristics very similar to those of cloud development in ET cyclones over Northwestern Pacific found by Klein et al. (2000). Fig. 1 Scheme for partitioning the PV and θ purtubations. Cirrus Edge 3. ANALYSIS Figure 2 shows the track and minimum central sea-level pressure associated with the cyclone based on the analysis of JMA surface weather maps. After making landfall at about 1330 UTC 18 August, typhoon Winnie weakened rapidly. However, after 1200 UTC 20 August, the cyclone started to re-intensify and the maximum surface pressure deepening rate reached 10 hpa in 12 h between 0000 and 1200 UTC 21 August. Such a large deepening rate of cyclogenesis over land is rarely observed. Fig. 2 The track and minimum sea-level central pressure associated with the typhoon/ cyclone based on the analysis of JMA surface weather maps. Figure 3 presents the IR enhanced satellite imagery at 1200 UTC 20 August. The characteristics of clouds associated with the system were unlike the cloud signature of a Eyewall Erosion Multi-layer clouds Fig. 3 GMS IR enhanced cloud imagery at 1200 UTC 20 August, the open circle represents the cyclone center. At lower levels, the cyclone transformed from an initially symmetric system to an asymmetric one with cold and warm fronts. At 0000 UTC 19 August, a cold tongue started to move into the western side of the cyclone, Furthermore, between 0000 UTC and 1200 UTC 21 August, the area surrounding station Chi-Feng underwent strong cooling which clearly cannot be fully explained just by adiabatic processes such as temperature advection. Observational data in Chi-Feng suggest that the cooling rate at 850 hpa was 7.6 C over 12 h. Figure 4 compares skew-t, log P diagrams at this station between 0000 and 1200 UTC 20 August. During this 12-h period, winds at lower to middle troposphere strengthened, which was obviously due to the close proximity of the site to the cyclone. The atmosphere from low levels to about 350 hpa was very moist at 1200 UTC, and suggests that deep
vertically developed clouds were present at the station. The warming at middle levels, therefore, might be caused by condensational heating (Fig. 4b). The cooling at low levels was especially evident at 850 hpa, and the atmosphere was somewhat less moist than the middle levels. Thus, evaporative cooling was likely to be responsible for the additional temperature drop, except the effect of cold air advection. At 0000 and 1200 UTC 21 August, the cold tongue continued to move into the western section of the cyclone and the strong cooling caused both by cold air advection and evaporation also continuted and contributed to the frontogenesis. (a) The authors recognized that that the cooling observed at the northern sector of the cyclone could not be explain by adiabatic process alone without further exploration to its source. In our case, in which primary frontogenetic process seemed to be not associated with adiabatic processes, we suggest that evaporative cooling was the major contributing process to the fronotogensis. Other diabatic processes, such as differential heating between cloud-covered and cloud-free areas might also contribute to temperature contrast in other cases, but appears not important in our present case. After landfall, the 500 hpa cyclone center filled rapidly and moved close to the weak baroclinic zone near 34 N. At 1200 UTC 20 August (Fig. 5), the cyclone s circulation had entered the afore-mentioned baroclinic zone and started to interact with it. It appeared that there was cold air advection at the south-western sector and warm air advection at the north-eastern one. (b) Fig. 4 The skew-t, log P diagrams of Chi-Feng, Ho-Bei Province, China at (a) 0000 UTC (b) 1200 UTC 20 August In the study of Bosart and Dean (1991), the substantial cooling at lower levels near the center of another ET cyclone, Agnes, was also reported. Fig. 5 500-hPa ECMWF analysis of height, temperature, and wind at 1200 UTC 20 August. Diagnosis through piesewise PV inversion suggests that the upper level PV anamoly contributed to warm advection near the cyclone at mid-levels, so it tended to weaken the cold
advection and enhance the warm advection associated with the cyclone itself, but its effect decreased with time. The major disturbance that caused the dipole structure in temperature advection were the middle level PV anomalies associated with condensational heating. The dipole of temperature advection not only contributed to frontogenesis at middle levels, but also cause an amplification of the upper level waves through hydrostatic relationship. The latter process will be further examined in the next section. The mid-level front reached its maximum intensity at 0000 UTC 21 August. Twelve hours later, the temperature gradient of the mid-level front decreased significantly, as the rapid cyclogenesis took place. (a) Figure 6a presents the 300 hpa weather map at 1200 UTC 19 August, and it can be seen that the ex-typhoon was associated with a closed circulation even at 300 hpa, while a weak trough near 35 N, 110 E was also observed. Then, the circulation weakened and moved into the baroclinic zone to the north. After 1200 UTC 20 August, as the afore-mentioned interaction between the cyclone s circulation at mid-levels and the baroclinic zone, the upper-level trough-ridge system began to amplify as expected. Figure 6b presents the 300 hpa weather map at 0000 UTC 21 August. The low-level cyclone moved into the region with positive vorticity advection ahead of the 300 hpa trough, so with strong vertical coupling, the development of the cyclone was favored. 4. RESULTS OF PV DIAGNOSIS (b) Fig. 6 300-hPa ECMWF analysis of height, temperature, and wind at (a)1200 UTC 19 (b) 0000 UTC 21 August. The white dots represent the cyclone center at 850-hPa. Preliminary results of diagnosis by piesewise PV inversion during the re-intensification stage between 1200 UTC 20 and 21 August suggest that by far the most important contribution to the 1000 hpa height anomaly at cyclone center came from mid-level PV anomalies associated with condensational heating. The lower-than-average temperature caused partially by evaporative cooling near the cyclone center, on the other hand, corresponded to negative lower boundary potential temperature perturbations (θ ) at low level and contributed to dissipation of the cyclone. During the period of diagnosis, the subtropical high was exceptionally strong compared to the mean field at most levels, so the unsaturated PV anomalies at middle and upper levels caused filling of the cyclone center most of the time. However, when the upper level trough approached and amplified, it started to contribute positively to low-level cyclone development, and countered the effects of subtropical high. The upper level PV anomalies contribution to
low-level cyclone even became positive when the cyclone reached maximum intensity at 1200 UTC 21 August. It is perhaps worthwhile to note that the afore-mentioned PV diagnosis reveals only the direct effects of PV anomalies (or θ ) on the low-level height field. The contribution from underlying physical processes can be further examined through the PV prognostic system, and this will be carried out in the future to investigate further the ET process of typhoon Winnie. 5. DISCUSSION Klein et al. (2000) proposed a threedimensional conceptual model for the transformation stages of ET in Northwestern Pacific based on the satellite imagery and NOGAPS gridded analysis. Since the characteristics of clouds development are very similar in our and their cases, the question of whether the physical processes in their model also occurred in our present case over land will be further explored in the future. Preliminary results of our studty suggest that the mechanism responsible for the development of the cold front associated with the transforming cyclone at low-levels is evaporative cooling, which strengthened the cold tongue at the western sector of the cyclone. Such a frontogenetic process is very different from that of an ordinary midlatitude cold front associated with airmasses. The important factor in ET process involves the interaction between the tropical cyclone and the baroclinic wave. In addition to the important factors in the ET process, such as diabatic heating and the pre-existing vorticity associated with the remnant of the ex-tropical cyclone, many prior ET case studies stressed the importance of the approaching of a pre-existing, upstream prominent upper-level trough for the re-development of the cyclone. In the present case, the cyclone was not only just affected by the baroclinic system, but also interacted with it and contributed to its own development. The interaction led to mid-level frontogenesis and the amplification of an initially weak upper-level trough-ridge system, which in turn caused the cyclone to develop rapidly. Thus, it is suggested that the cyclones s re-intensification was a manifestation of the self-amplification process. Bosart and Lackmann (1995) proposed that the September 1979 hurricane David modified a very weak baroclinic environment into one that was favorable to the re-intensification of the tropical cyclone. The process seems to be somewhat similar to the present study. However, because the baroclinic zone in their case was so weak initially, the cyclone characteristic produced during its re-intensification, and the scenario of the cyclone-baroclinic zone interaction were both different from our case. 6. CONCLUSION After making landfall over mainland China on 18 August 1997, Typhoon Winnie weakened rapidly, re-curved, and transformed into an extratropical cyclone. Our analysis shows that the characteristics of cloud development associated with the ET process is very similar to that of ET cyclones over Northwestern Pacific found by Klein et al. (2000). The mechanism responsible for the cold front associated with the transforming cyclone at low-levels was evaporative cooling, which strengthened the cold tongue at the western section of the cyclone. When the cyclone migrated into a mid-level baroclinic zone, the interaction between them led to mid-level frontogensis and subsequent amplification of upper-level waves, which with a strong vertical coupling in turn caused the cyclone to develop rapidly. Preliminary results of diagnosis through
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