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  • View in gallery

    Best track of the two cases studied (line with dots), (a) Prapiroon (2000) and (b) Olga (1999), and the weekly mean SST centered on the date shown above the figures. The numbers along the track indicate the date

  • View in gallery

    Maximum sustained wind speed (m s−1) of the two cases studied: (a) Prapiroon (2000) and (b) Olga (1999). Different symbols denote the best-track intensity from different centers: CMA (triangles), RSMC Tokyo (circles), and JTWC (squares). The 1-min-averaged wind speed from JTWC has been converted to 10-min average by multiplying by a factor of 0.871 (Holland 1993)

  • View in gallery

    The 500-hPa geopotential height field. The line with circles is the TC track. The typhoon symbol shows the TC position at the time. (a) 30/12 Aug 2000 and (b) 02/00 Aug 1999

  • View in gallery

    SST beneath the TC center (circles), EFC at 200 hPa averaged over the 300–600-km radial bands (squares), and vertical wind shear between 850 and 200 hPa (triangles)

  • View in gallery

    Radial–time cross section of EFC at 200 hPa calculated using (left) GDAPS and (right) NCEP analyses (unit: m s−1 day−1): (a), (b) Prapiroon (2000) and (c), (d) Olga (1999). The thick black contour indicates a value of 10 m s−1 day−1

  • View in gallery

    (a)–(f) The 200-hPa flow pattern and isotachs (thick contour) from 29/18 to 31/00 Aug 2000 according to GDAPS analyses. The line with circles is the track of Prapiroon. The typhoon symbol shows the TC position of the time

  • View in gallery

    Same as in Fig. 6, but for Olga from 02/00 to 03/06 Aug 1999

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    Vertical wind shear between 850 and 200 hPa averaged within a circle of 3°, 4°, 5°, and 6° latitude radius around Prapiroon’s center

  • View in gallery

    Change of wind velocity with height at (a) 30/06 Aug 2000 and (b) 31/06 Aug 2000. The line with triangles represents GDAPS, and the line with squares represents NCEP

  • View in gallery

    Vertical wind shear between different levels for Prapiroon

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Effect of TC–Trough Interaction on the Intensity Change of Two Typhoons

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  • 1 Shanghai Typhoon Institute, Shanghai, China
  • | 2 Typhoon Research Center, Department of Atmospheric Science, Kongju National University, Kongju, Chungnam, Korea
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Abstract

Using large-scale analyses, the effect of tropical cyclone–trough interaction on tropical cyclone (TC) intensity change is readdressed by studying the evolution of upper-level eddy flux convergence (EFC) of angular momentum and vertical wind shear for two TCs in the western North Pacific [Typhoons Prapiroon (2000) and Olga (1999)]. Major findings include the following: 1) In spite of decreasing SST, the cyclonic inflow associated with a midlatitude trough should have played an important role in Prapiroon’s intensification to its maximum intensity and the maintenance after recurvature through an increase in EFC. The accompanied large vertical wind shear is concentrated in a shallow layer in the upper troposphere. 2) Although Olga also recurved downstream of a midlatitude trough, its development and maintenance were not strongly influenced by the trough. A TC could maintain itself in an environment with or without upper-level eddy momentum forcing. 3) Both TCs started to decay over cold SST in a large EFC and vertical wind shear environment imposed by the trough. 4) Uncertainty of input adds difficulties in quantitative TC intensity forecasting.

Corresponding author address: Ms. Hui Yu, Shanghai Typhoon Institute, No. 166, Pu Xi Road, Shanghai, 200030, China. Email: yuh@mail.typhoon.gov.cn

Abstract

Using large-scale analyses, the effect of tropical cyclone–trough interaction on tropical cyclone (TC) intensity change is readdressed by studying the evolution of upper-level eddy flux convergence (EFC) of angular momentum and vertical wind shear for two TCs in the western North Pacific [Typhoons Prapiroon (2000) and Olga (1999)]. Major findings include the following: 1) In spite of decreasing SST, the cyclonic inflow associated with a midlatitude trough should have played an important role in Prapiroon’s intensification to its maximum intensity and the maintenance after recurvature through an increase in EFC. The accompanied large vertical wind shear is concentrated in a shallow layer in the upper troposphere. 2) Although Olga also recurved downstream of a midlatitude trough, its development and maintenance were not strongly influenced by the trough. A TC could maintain itself in an environment with or without upper-level eddy momentum forcing. 3) Both TCs started to decay over cold SST in a large EFC and vertical wind shear environment imposed by the trough. 4) Uncertainty of input adds difficulties in quantitative TC intensity forecasting.

Corresponding author address: Ms. Hui Yu, Shanghai Typhoon Institute, No. 166, Pu Xi Road, Shanghai, 200030, China. Email: yuh@mail.typhoon.gov.cn

1. Introduction

It has long been noticed that the interaction between a tropical cyclone (TC) and a tropical upper-tropospheric trough (TUTT) or upper-level westerly trough can modulate the intensity of the TC. However, the precise manner and degree to which upper-level troughs weaken or intensify a TC is not yet well understood (Ritchie 2002). It is still a challenge for forecasters to tell whether an encounter with a midlatitude trough is “good” or “bad” for the development of a TC.

The upper-level trough could be either favorable or unfavorable to TC development. On the favorable side, it has been suggested that the asymmetric structures of the outflow layer associated with upper-level synoptic-scale systems could produce large-eddy imports of angular momentum. As a result of the small inertial stability in the upper troposphere, the response to external forcing can penetrate to the vortex center (Holland and Merrill 1984). Up until now, many composite and case studies (McBride and Zehr 1981; Molinari and Vollaro 1989; DeMaria et al. 1993; Wu and Cheng 1999; Bosart et al. 2000; Hanley et al. 2001) have shown a significant relationship between eddy flux convergence (EFC) of angular momentum and TC intensity change. The interpretation of this is that the secondary radial circulation induced by EFC might serve as a catalyst that organizes the diabatic sources in such a way as to excite internal instabilities of the system (Molinari and Vollaro 1990; Challa et al. 1998; Titley and Elsberry 2000).

An alternative explanation is that strong divergence, and implied upward motion, at the right entrance of the upper-level jet could cause a TC to intensify (Chen and Ding 1979; Hanley et al. 2001). Numerical studies by Shi et al. (1990, 1997) showed the existence of a secondary circulation at the entrance region of the outflow jet, which is thermally direct with the ascending branch located on the anticyclonic shear side and the descending branch located at the cyclonic shear side of the outflow jet. Rodgers et al. (1986, 1991, 1998) suggested that the secondary circulation at the entrance region of trough-related outflow jet could modulate the convection of the TC and might help initiate and maintain the eyewall convective bursts.

Strong vertical wind shear is considered to be the main adverse effect of an upper-level trough. The interaction between the shear associated with the upper-level trough and a TC causes “ventilation” of the TC warm core or an asymmetry in the cloud/precipitation distribution, which is less favorable for intensification than a symmetric cloud/precipitation distribution. However, less agreement exists as to the threshold values of vertical wind shear and the relationship between weak-to-moderate shear and TC intensity change (Ritchie 2002).

Compared with the Atlantic, only a few studies on TC–trough interaction have been performed on TCs in the western North Pacific. Using the European Centre for Medium-Range Weather Forecasts (ECMWF) Tropical Ocean–Global Atmosphere advanced analysis, Wu and Cheng (1999) studied the environmental influences on the intensity changes of two typhoons, Flo and Gene, in 1990. Titley and Elsberry (2000) also studied the rapid intensity changes of Typhoon Flo (1990). Both studies stressed the importance of TC–trough interaction in the intensification process of the TCs.

Even less attention has been paid to TC–trough interaction during the stage in which the TC intensity remains steady. Although it is quite common that a TC weakens quickly after it has moved to the midlatitudes or has recurved to the east of a midlatitude trough, usually due to the lower SST or large vertical wind shear, some TCs maintain their intensity almost unchanged, bringing unanticipated damage to the midlatitudes.

In this paper, large-scale analyses will be used to study the intensity change of two TCs [Prapiroon (2000) and Olga (1999)] in the western North Pacific as they interacted with a midlatitude trough. These cases are selected for two reasons: 1) both TCs recurved downstream of the midlatitude trough near the China coast so that the data coverage should be relatively good, and 2) both had similar intensities before and after recurvature. The objective of this study is to address the following questions: How did the upper-level EFC and vertical wind shear evolve as the two TCs moved closer to a midlatitude trough and recurved downstream of it? Are there characteristic values of EFC and vertical wind shear in the TCs’ maintenance stage? To what degree could previous findings about TC–trough interaction be used to interpret the intensity change in the two cases?

In section 2, the data used are introduced. A general description of the track and intensity of the two TCs, the basic features of the large-scale circulation, and the underlying surface are given in section 3. Diagnoses of the upper-level EFC and vertical wind shear and their relationship with intensity change are presented in sections 4 and 5, respectively. Conclusions and a discussion are offered in section 6.

2. Data

Six-hourly best-track data from the China Meteorological Administration (CMA) will be used to describe the intensity change of the two TCs. It is noted that because of the lack of aircraft reconnaissance, some differences in the best-track intensity nearly always exist among different sources. To have more confidence in the intensity estimates, the best-track intensity from CMA will be compared with those from two other sources, the Regional Specialized Meteorological Center (RSMC) Tokyo and the Joint Typhoon Warning Center (JTWC). It will be shown in the next section that the general trends of intensity change from the three sources are similar.

Six-hourly, three-dimensional analyses from the Korean Meteorological Administration (KMA) were obtained on a 1.875° latitude–longitude grid at 12 standard pressure levels: 1000, 850, 700, 500, 400, 300, 250, 200, 150, 100, 70, and 50 hPa. The analyses are constructed using a Global Data Assimilation and Prediction System (GDAPS).

Inspection of the GDAPS analyses shows that two fundamental requirements (Molinari and Vollaro 1990) were met during most of the time being studied (figures not shown): 1) the location of the storm on the 1.875 latitude–longitude grid, as defined by the maximum midtropospheric relative vorticity, was at the grid point nearest to its true location in nature, and 2) the maximum vorticity occurred at the same point throughout the lower and middle troposphere, as it should in the mature stage of a TC.

Six-hourly analyses from the National Centers for Environmental Prediction (NCEP) are also used for comparison in this paper when diagnosing the vertical wind shear and EFC. The horizontal resolution is 2.5° latitude–longitude, and there are 12 vertical levels (1000, 925, 850, 700, 600, 500, 400, 300, 250, 200, 150, and 100 hPa).

The sea surface temperature (SST) data used are the Reynolds SST analyses provided by the National Oceanic and Atmospheric Administration–Cooperative Institute for Research in Environmental Sciences (NOAA-CIRES) Climate Diagnostics Center through their Web site (http://www.cdc.noaa.gov/).

3. Basic features

According to the best track from CMA, Prapiroon (2000) formed at 19°N, 132°E on 26 August 2000 (Fig. 1a). The TC moved northward at the beginning and then turned toward the northwest a few hours later. The track showed a trend turning to the southwest on the 28th but soon turned back to northwestward 12 h later. After a period of slow moving, Prapiroon sped up toward the northwest and turned northeastward near the East China coast at ∼30°N. The TC reached its maximum intensity of ∼35 m s−1 at the recurvature point at 1200 UTC on 30 August (hereafter 30/12; see Fig. 2a). A small change in intensity (2 m s−1) was reported by CMA in the 42 h from 29/18 to 31/12.

Olga (1999) formed at 12.3°N, 134.5°E on 29 July 1999 (Fig. 1b). It moved northwestward for about 3 days before turning northward at ∼ 28°N on 2 August. As reported by CMA, the TC intensified to a typhoon at 01/06 (Fig. 2b), 18 h before recurvature. Later on, its intensity remained constant for nearly 2 days until 03/00. Then, the TC weakened quickly by more than 10 m s−1 12 h before landfall.

Synoptic analyses (Fig. 3) show that both TCs recurved to the east of a midlatitude trough. To meet the objective of this study, the TC–trough interaction will only be studied during the period from 1 to 2 days before recurvature until the TC landfall after the recurvature, that is, from 29/00 to 31/18 August 2000 for Prapiroon and from 31/00 July to 03/18 August 1999 for Olga.

Also shown in Fig. 2 are the curves of intensity of the two TCs from the RSMC Tokyo and JTWC best-track datasets. It can be seen that the general trend of intensity change of the two TCs reported by CMA is similar to that of two other centers. However, during any specific period, discrepancies sometimes exist among different centers. For example, both RSMC Tokyo and JTWC suggested a ∼ 5 m s−1 increase in intensity during the 12 h between 29/18 and 30/06 August 2000 for Prapiroon, while CMA maintained the TC’s intensity. And in contrast with both CMA and RSMC Tokyo, JTWC suggested a ∼5 m s−1 decrease in Olga’s intensity during the 12 h between 02/12 and 03/00 August 1999.

To avoid confusion and for simplicity, the dataset from CMA will be used at the times when the intensity change from CMA was exactly the same or at least had the same sign as that from either RSMC Tokyo or JTWC. For those times when the intensity change from CMA differed from both RSMC Tokyo and JTWC, the estimates from RSMC Tokyo and JTWC will be accepted if they are consistent with each other. Based on this consideration, the intensity change of the two TCs could be divided into three stages: the developing, maintaining, and decaying stages. In our definition, the developing stage refers to the period before TC reached its maximum intensity. The maintaining stage is the period when TC showed no or very little intensity change after it had attained its maximum intensity. The TC entered its decaying stage when its maximum sustained wind speed started to decrease notably. For Typhoon Prapiroon, the three stages are from 29/00 to 30/12, 30/12 to 31/06, and 31/06 to 31/18 August 2000, respectively (Fig. 2a). For Typhoon Olga, they are from 31/00 July to 01/06 August, 01/06 to 03/00 August, and 03/00 to 03/18 August 1999 (Fig. 2b).

Apart from the large-scale atmospheric environment, the ocean has been suggested to play a crucial role in affecting the intensity change of TCs (e.g., Chan et al. 2001). The SST field and its change beneath the TC center are shown in Figs. 1 and 4, respectively. It can be seen that, during the developing stage, SST under Prapiroon increased slightly at first and then decreased with the maximum 28.8°C and minimum 27.7°C. Later on, SST dropped continuously to 26.2°C through the maintaining stage. Then, the TC started to decay. Things are a little different for Olga, with SST decreasing from 28.3° to 27.3°C during the developing stage while rising back to 28.3°C in the first half of maintaining stage. Its decaying stage began over an SST of 24.8°C.

Although numerical studies (e.g., Chan et al. 2001) have shown that changes in TC intensity are sensitive to those of SST, this is not the case for Prapiroon and Olga except that they both started to decay over a cold sea surface. The rise in SST does not guarantee the development of TC and vice versa. Such a fact implies that environmental influences must be quite important in modulating the two TCs’ response to SST change, especially during their developing and maintaining stages.

4. Upper-level EFC

The EFC is defined following Molinari and Vollaro (1990):
i1520-0434-20-2-199-e1
where u and v are the radial and azimuthal velocity components, respectively; r is the distance from the storm center; the primes indicate the deviation from the azimuthal mean; and the subscript L refers to storm-relative flow. The gridded analyses are interpolated bilinearly in the horizontal to TC-centered cylindrical grids with Δr = 100 km and Δλ = 15°. Calculations are done in a Lagrangian coordinate system following the storm motion. Storm-relative horizontal wind components are calculated by subtracting the 6-hourly average storm motion vector from the analyzed wind velocity at each grid point.

In many previous studies (e.g., Hanley et al. 2001), EFC at 200 hPa was analyzed as a proxy of upper-level eddy momentum forcing. For the purpose of comparison, we also focus on 200-hPa EFC here (Fig. 5). It is shown that the values of maximum EFC calculated based on both GDAPS and NCEP analyses are consistent with former studies of TCs in both the Atlantic and western North Pacific. For example, in the case studies by Molinari and Vollaro (1989, 1990), EFC values of nearly 30 m s−1 day−1 were associated with the interaction of Hurricane Elena and a midlatitude trough. DeMaria et al. (1993) showed EFC values greater than 40 m s−1 day−1 for Hurricane Gabrielle as the TC moved northward into the midlatitudes (Fig. 4 in their paper). In the case studies on Supertyphoon Flo (1990), EFC values of up to 40 (Fig. 8 in Wu and Cheng 1999) and 70 m s−1 day−1 (Fig. 8 in Titley and Elsberry 2000) were found during the rapid decaying stage of the TC.

a. Prapiroon

The maximum EFC values of Typhoon Prapiroon vary from 29 (NCEP) to 39 (GDAPS) m s−1 day−1 (Figs. 5a and 5b, respectively). In general, basic features of EFC shown in Figs. 5a and 5b are consistent with each other. Both show the general trend of an inward shift of the area with EFC greater than 5 m s−1 day−1 from 29/00 to 30/06 August 2000 when the TC was in its developing stage. A temporal increase of EFC inside 800 km started at ∼29/18 and reached the maximum at 30/06 with the 10 m s−1 day−1 contour inside the radius of 400 km. Such a rise in EFC was due to the development of cyclonic inflow to the southwest of the storm ahead of the trough and westerly jet–related anticyclonic outflow to its north (Figs. 6a–c). That should have favored Prapiroon’s intensification in spite of decreasing SST (Fig. 4a). Also shown in Fig. 4a is the EFC averaged over the 300–600-km radial bands according to NCEP analyses. It illustrates in another way the increase of EFC at inner radii during that period.

By 30/12, the local EFC maximum mentioned above had shifted outward as a result of deformation of the cyclonic inflow and migration of the anticyclonic outflow simultaneously (Fig. 6d), implying the weakening of eddy forcing at inner radii. Correspondingly, Prapiroon reached its maximum intensity and entered its maintaining stage.

During the maintaining stage, the inward shift of large EFC area continued until 31/00 or 31/06, depending on the dataset (Figs. 5a,b), accompanied by positive EFC centers reaching the maximum value. These large EFC values were also brought by cyclonic inflow to the southwest of the storm ahead of the trough and westerly jet–related anticyclonic outflow to its north (Figs. 6d–f). It is speculated that the increase of EFC at inner radii (Fig. 4a) during this period might also have helped Prapiroon withstand the continuously dropping SST and helped it not weaken after recurvature.

As Prapiroon decayed, the EFC center showed a tendency to diminish, with the values at inner radii dropping slightly.

b. Olga

Large EFC was observed for Typhoon Olga (Figs. 5c,d) as a result of a deep upper-level trough (Fig. 7). The maximum value is up to 60 or 70 m s−1 day−1 as calculated using GDAPS or NCEP analyses. The steady inward shift of large EFC area is shown clearly by both datasets (Figs. 5c,d). However, the contour of 5 m s−1 day−1 did not reach the inner radius (∼600 km) of the storm until 02/12 August 1999, 12 h before Olga entered its decaying stage. Such a phenomenon was caused by the fact that the southwesterlies ahead of the upper-level trough did not reach the inner radius of Olga until that date (Fig. 7c), although the TC had turned northward 12 h earlier.

During the developing stage of the storm, both datasets show a temporal increase of EFC extending inward to ∼500 km and outward to ∼1300 km (Figs. 5c,d). Such a local increase of EFC was caused by an enhancement of anticyclonic outflow to the storm’s northeast (not shown), while having nothing to do with the midlatitude trough.

As has been illustrated in Figs. 5c and 5d, Fig. 4b shows that, during Olga’s developing and maintaining stages, the average EFC at inner radii was almost unchanged and near zero most of the time, implying that no or quite small eddy forcing existed at inner radii even though Olga moved toward a midlatitude trough and recurved ahead of it. Hence, it is speculated that both the development and maintenance of Olga were not strongly influenced by the midlatitude trough.

The trough-induced large EFC area reached the innermost region of the storm at 03/06, when Olga was already in its decaying stage. Simultaneously, the EFC at inner radii reached a maximum of ∼50 m s−1 day−1 (Fig. 4b). Similar to that of Prapiroon, the strong cyclonic momentum convergence at inner radius during this period was caused by both cyclonic inflow to the southwest of the TC downstream of the trough and westerly jet–related anticyclonic outflow to its north (Fig. 7f). Later on, the EFC decreased quickly as Olga continued moving northward and made landfall.

Apart from the EFC averaged over the 300–600-km radial bands, those over other radial bands (such as 500–900, 700–1000 km, and so on) have also been analyzed (not shown). Results show that, although different time series occur depending on which radii the EFC is averaged over, the general trend of increasing is common as the TC moved closer to the trough, except that the value at outer radii tends to rise earlier than at inner radii, which could also be inferred from Fig. 5.

Another notable feature in Fig. 5 is that the value of EFC highly depends on the data used, although basic traits of EFC evolution from GDAPS and NCEP analyses are consistent with each other. A comparison of EFC averaged over 300–600-km radial bands demonstrates that the difference between the two analyses is on average 4.5 m s−1 day−1, with a maximum up to 12.6 m s−1 day−1, which is comparable to the EFC value itself. Results in Fig. 2 also show that discrepancies sometimes exist among different centers as to the intensity change value during any specific period even though the general trends of TC intensity change from various sources are in accord with each other. Therefore, in view of the uncertainty of the input, no effort is taken to obtain a quantitative relationship between EFC at any radial band and intensity change as has been done in several former studies (Molinari and Vollaro 1989; DeMaria et al. 1993).

Although previous studies have supported that large EFC caused by upper-level trough is favorable to TC development, the analyses above imply that the enhancement of EFC does not guarantee the intensification of a TC, which was also suggested by Wu and Cheng (1999). DeMaria et al. (1993) also pointed out that the relationship between EFC and intensity change was statistically significant only while taking into consideration the effects of vertical wind shear and SST. A notable feature is that the developing stage of both TCs was accompanied by a transient increase of EFC at inner radii, brought by the cyclonic inflow and/or the anticyclonic outflow. The EFC center showed a tendency to shift outward as Prapiroon intensified to its maximum intensity. In the case study on Typhoon Flo (1990), Wu and Cheng (1999) also found that the maximum EFC shifted outward with time in the TC’s rapid intensification stage. However, remarkable and persistent increase of EFC at inner radius started at about the end of Olga’s maintaining stage. EFC at the inner radii maintained a value over 15 m s−1 day−1 during Prapiroon’s decaying stage (Fig. 4a). It seems that a sharp increase of EFC or a persistent large EFC at inner radii might not be a favorable factor for TC development.

As for the maintaining stage of the two TCs, Olga’s maintenance might be due to the fact that it was in an environment with almost no upper-level forcing as implied by the near-zero EFC values at inner radii for most of the time before decaying. Although the midlatitude trough has caused the TC to recurve at 02/00, large EFC brought by the trough did not reach the inner radius at the end of the maintaining stage. The maintenance of Prapiroon was accompanied by a steady increase of EFC at inner radii. Thus, a TC’s intensity could remain unchanged in an environment with or without upper-level eddy forcing.

Previous studies have pointed out that the approach of a midlatitude trough is often accompanied by the increase of vertical wind shear, which is considered to be unfavorable to the storm’s development. Thus, vertical wind shear will be analyzed in the next section.

5. Vertical wind shear

a. Prapiroon

Although many observational studies have been carried out on vertical wind shear, the methods of calculating the shear varied in different studies. For example, the wind was averaged in a 3° latitude–longitude radius in Elsberry and Jeffries (1996) but in a 6° latitude–longitude radius in Franklin et al. (1993). A comparison of the wind shear between 850 and 200 hPa averaged within a circle of 3°, 4°, 5°, and 6° latitude radius around the TC center suggests that a small difference (less than 5 m s−1) exists among the shear values at different radii most of the time. The change of shear at different radii with time, especially, is quite consistent (see, Fig. 8 for an example). Such a result is in agreement with that of Titley and Elsberry (2000). For simplicity, only the shear averaged within a circle of 6° latitude–longitude radius will be analyzed for both TCs.

Comparison is also performed between GDAPS and NCEP analyses. The results demonstrate that discrepancy comparable to the shear value itself, which is 2.8 m s−1 on average and has 6.3 m s−1 as the maximum, exists. In any case, an increasing tendency of shear is shown clearly by both datasets as the TCs moved into the midlatitudes. Thus, only the result from one of them (NCEP) is shown in Fig. 4.

During the developing stage of Prapiroon (Fig. 4a), the shear was not more than 10 m s−1 most of the time, except that a transient large value (12 or 16 m s−1 as calculated from different datasets) appeared at 30/06. Then, the shear decreased to ∼5 m s−1 when the TC reached its maximum intensity. During the TC’s maintaining stage, the shear increased gradually and reached a second maximum of ∼15 m s−1 at the end. Then, Prapiroon started to weaken. Calculation of the average shear during the three stages of Prapiroon shows that the shear increased step by step as the TC evolved from the developing stage into the maintaining and decaying stages.

A point to note for Prapiroon is that the TC continued to intensify and reached its maximum intensity 6 h later when the vertical wind shear increased suddenly at 30/06. Later on, the TC entered its 18-h maintaining stage. Although less agreement exists as to the critical shear value where the tendency changes from intensifying to decaying TCs (Ritchie 2002), a TC should be expected to weaken after the establishment of a shear as large as 12 or 16 m s−1. One of the explanations for the intensifying and maintenance of Prapiroon after that might be the time lag between the increase in vertical shear and response of the TC that has been proposed in several studies (Gallina and Velden 2002; Frank and Ritchie, 2001). However, the time lag varied in different studies. Gallina and Velden (2002) found a time lag between 12 and 24 h with larger, more intense TCs responding slowly and weak, developing TCs responding quickly. In a model study, Frank and Ritchie (2001) found that a shear of 10 m s−1 causes the TC to decay within 24 h, while a shear of 15 m s−1 starts the decay process immediately.

To investigate the reason for such a sharp increase in wind shear, the change of wind velocity with height at 30/06 is analyzed (Fig. 9a). It can be seen that the large shear was confined in a shallow layer above 300 hPa, as a result of the onset of strong southwesterly winds over the TC in the upper troposphere (see Fig. 6c). Figure 9b shows that the shear was nearly linearly distributed over a deep layer at 31/06 when the shear reached a second maximum and Prapiroon started to weaken. Elsberry and Jeffries (1996) stated that the strong upper-level winds and vertical wind shear concentrating in a shallow layer below the tropopause could more easily be opposed or deflected by a strong, similarly concentrated outflow layer above a developing TC than the shear distributing over a deep layer. Because of the fact that the increase of shear at 30/06 was a transient phenomenon, it is speculated that the onset of large shear at the time did not cause Prapiroon to decay, because the large shear was confined in a shallow layer above 300 hPa and was deflected before the TC could respond to it.

It is then assumed that, for such a situation, it might be better to analyze the shear between the middle and lower troposphere rather than the upper and lower troposphere if any statistical relationship between shear and intensity change is sought. Thus, the shear between 500 and 850 hPa is calculated and shown in Fig. 10. It can be seen that it indeed corresponds better to Prapiroon’s intensity change than the shear between 200 and 850 hPa. Both the maintaining and decaying stages of Prapiroon started when the shear between 500 and 850 hPa increased markedly relative to the values in the preceding stage, and there was no exceptionally large shear in the developing stage.

b. Olga

As mentioned in the last section, the upper-level midlatitude trough did not reach the inner part of Olga until 02/12, accompanied by the increase of vertical wind shear (Fig. 4b). The shear reached ∼15 m s−1 12 h later, and Olga started to weaken quickly. The scenario of strong winds being confined to a narrow layer below the tropopause never occurred in the case of Olga as the TC interacted with the midlatitude trough (not shown).

Before Olga interacted with the midlatitude trough directly, the vertical wind shear remained small during the developing stage and moderate during the maintaining stage. Thus, Olga developed in a low shear environment with some temporal eddy momentum forcing irrelevant to the trough, maintained itself in a moderate shear environment with almost no EFC, and weakened in a high shear environment with large eddy forcing.

6. Conclusions and discussion

a. Conclusions

As has been stated, it is quite common for a TC to weaken quickly after it has moved to the midlatitudes or has recurved to the east of a midlatitude trough, usually as a result of the lower SST or large vertical wind shear. That is apparently not the case for the two TCs studied, both of which maintained their intensity until nearly 1 day after recurvature.

Based on the analyses in this paper, the following can be concluded:

  1. In spite of decreasing SST, the cyclonic inflow associated with a midlatitude trough should have played an important role in Prapiroon’s intensification to its maximum intensity and the maintenance after recurvature through an increase in EFC. The accompanied large vertical wind shear is concentrated in a shallow layer in the upper troposphere.
  2. Although Olga also recurved downstream of a midlatitude trough, its development and maintenance were not strongly influenced by the trough. A TC could maintain itself in an environment with or without upper-level eddy momentum forcing.
  3. Both TCs started to decay over cold SST in a large EFC and vertical wind shear environment imposed by the trough.
  4. Discrepancies comparable to the EFC or shear value itself exist between the outputs from GDAPS and NCEP analyses, although the basic characteristics of EFC or shear evolution are quite consistent with each other. Such an uncertainty of input adds more difficulties in quantitative TC intensity forecasting.

b. Discussion

The importance of upper-level forcing has been a contentious issue in the study of TCs. Although many researchers have attributed EFC forcing to fast TC intensification and found significant relationship between EFC and TC intensity change, the results of this study confirm once again previous findings that the enhancement of EFC does not guarantee the intensification of a TC, and decreases in SST or increases in shear can offset positive influences of EFC (DeMaria et al. 1993; Wu and Cheng 1999).

Different from the result for the Atlantic Ocean (DeMaria and Kaplan 1999), Fitzpatrick (1997) pointed out that EFC is not an important input to the TC prediction as proposed in a Typhoon Intensity Prediction Scheme (TIPS) for the western North Pacific Ocean. However, our results show that a significant relationship might be obtained between EFC and intensity change when taking other factors (e.g., SST and vertical wind shear) into consideration for TCs similar to Prapiroon. However, for those TCs like Olga, that might not happen. Thus, it may be better to classify all the TCs into different groups synoptically similar to what has been done by Hanley et al. (2001) for the Atlantic Ocean. In their study, all the TCs were separated into three groups based on their relationship with trough: superposition, distant interaction, and no trough. It seems that the two TCs studied here would fall into a composite of their distant interaction (Prapiroon) and no-trough (Olga) categories. respectively (during their developing and maintaining stages). Although it is uncertain as to whether the statistics and critical values proposed by Hanley et al. are suitable for the western North Pacific before pertinent analyses are carried out, it is speculated that forecast skill for intensity change might be improved if the models are set up separately for different groups, with EFC being important in some while not in others.

The role of vertical wind shear is another topic that has attracted much attention. It is significant in statistical intensity prediction models for both Atlantic and Pacific TCs (DeMaria and Kaplan 1999; Fitzpatrick 1997). However, the problem is that some storms could intensify despite a high vertical shear environment, in a manner similar to what happened to Prapiroon before it attained its maximum intensity. Calculations in this study show that it is better to analyze the shear between the middle and lower troposphere rather than the upper and lower troposphere for such a situation. Thus, it may not be adequate to represent tropospheric shear with only the wind difference between two layers, as has been pointed out by several other studies (Elsberry and Jeffries 1996; Yu et al. 2002).

Apart from the forecast challenge of rapid intensity change, the challenge of TC maintenance should also be an important issue especially when it moves into the midlatitudes. Results of this paper show that a TC could maintain itself in an environment with or without upper-level eddy momentum forcing over either a decreasing or increasing SST. Such a fact implies that it is also quite difficult to make a correct prediction for the maintenance of a TC.

Although composite studies are helpful in discovering signatures that appear repeatedly, case studies are indispensable because the compositing might lose potentially important characteristics of individual storms. However, the above conclusions and speculations still need to be verified with more cases.

Acknowledgments

The authors thank Prof. Johnny Chan for his comments on the paper’s initial draft. Miss Seong-Hee Won of the Typhoon Research Center and Miss Pei-Yan Chen of the Shanghai Typhoon Institute prepared part of the data used. The study is sponsored by the National Natural Science Foundation of China under Grants 40333028, 49975014, and 40275018 and a Typhoon Committee research fellowship project.

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Fig. 1.
Fig. 1.

Best track of the two cases studied (line with dots), (a) Prapiroon (2000) and (b) Olga (1999), and the weekly mean SST centered on the date shown above the figures. The numbers along the track indicate the date

Citation: Weather and Forecasting 20, 2; 10.1175/WAF836.1

Fig. 2.
Fig. 2.

Maximum sustained wind speed (m s−1) of the two cases studied: (a) Prapiroon (2000) and (b) Olga (1999). Different symbols denote the best-track intensity from different centers: CMA (triangles), RSMC Tokyo (circles), and JTWC (squares). The 1-min-averaged wind speed from JTWC has been converted to 10-min average by multiplying by a factor of 0.871 (Holland 1993)

Citation: Weather and Forecasting 20, 2; 10.1175/WAF836.1

Fig. 3.
Fig. 3.

The 500-hPa geopotential height field. The line with circles is the TC track. The typhoon symbol shows the TC position at the time. (a) 30/12 Aug 2000 and (b) 02/00 Aug 1999

Citation: Weather and Forecasting 20, 2; 10.1175/WAF836.1

Fig. 4.
Fig. 4.

SST beneath the TC center (circles), EFC at 200 hPa averaged over the 300–600-km radial bands (squares), and vertical wind shear between 850 and 200 hPa (triangles)

Citation: Weather and Forecasting 20, 2; 10.1175/WAF836.1

Fig. 5.
Fig. 5.

Radial–time cross section of EFC at 200 hPa calculated using (left) GDAPS and (right) NCEP analyses (unit: m s−1 day−1): (a), (b) Prapiroon (2000) and (c), (d) Olga (1999). The thick black contour indicates a value of 10 m s−1 day−1

Citation: Weather and Forecasting 20, 2; 10.1175/WAF836.1

Fig. 6.
Fig. 6.

(a)–(f) The 200-hPa flow pattern and isotachs (thick contour) from 29/18 to 31/00 Aug 2000 according to GDAPS analyses. The line with circles is the track of Prapiroon. The typhoon symbol shows the TC position of the time

Citation: Weather and Forecasting 20, 2; 10.1175/WAF836.1

Fig. 7.
Fig. 7.

Same as in Fig. 6, but for Olga from 02/00 to 03/06 Aug 1999

Citation: Weather and Forecasting 20, 2; 10.1175/WAF836.1

Fig. 8.
Fig. 8.

Vertical wind shear between 850 and 200 hPa averaged within a circle of 3°, 4°, 5°, and 6° latitude radius around Prapiroon’s center

Citation: Weather and Forecasting 20, 2; 10.1175/WAF836.1

Fig. 9.
Fig. 9.

Change of wind velocity with height at (a) 30/06 Aug 2000 and (b) 31/06 Aug 2000. The line with triangles represents GDAPS, and the line with squares represents NCEP

Citation: Weather and Forecasting 20, 2; 10.1175/WAF836.1

Fig. 10.
Fig. 10.

Vertical wind shear between different levels for Prapiroon

Citation: Weather and Forecasting 20, 2; 10.1175/WAF836.1

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