VOLUME 63 JOURNAL OF THE ATMOSPHERIC SCIENCES MAY 2006

Tropical Cyclogenesis Associated with Rossby Wave Energy Dispersion of a Preexisting . Part I: Satellite Data Analyses*

TIM LI AND BING FU Department of Meteorology, and International Pacific Research Center, University of Hawaii at Manoa, Honolulu, Hawaii

(Manuscript submitted 20 September 2004, in final form 7 June 2005)

ABSTRACT

The structure and evolution characteristics of Rossby wave trains induced by (TC) energy dispersion are revealed based on the Quick Scatterometer (QuikSCAT) and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) data. Among 34 cyclogenesis cases analyzed in the western North Pacific during 2000–01 typhoon seasons, six cases are associated with the Rossby wave energy dispersion of a preexisting TC. The wave trains are oriented in a northwest–southeast direction, with alternating cyclonic and anticyclonic vorticity circulation. A typical wavelength of the wave train is about 2500 km. The TC genesis is observed in the cyclonic circulation region of the wave train, possibly through a scale contraction process. The satellite data analyses reveal that not all TCs have a Rossby wave train in their wakes. The occur- rence of the Rossby wave train depends to a certain extent on the TC intensity and the background flow. Whether or not a Rossby wave train can finally lead to cyclogenesis depends on large-scale dynamic and thermodynamic conditions related to both the change of the seasonal mean state and the phase of the tropical intraseasonal oscillation. Stronger low-level convergence and cyclonic vorticity, weaker vertical shear, and greater midtropospheric moisture are among the favorable large-scale conditions. The rebuilding process of a conditional unstable stratification is important in regulating the frequency of TC genesis.

1. Introduction have six TCs. The Arabian Sea and the sea off the Tropical cyclone (TC) genesis is the transformation northwest Australian coast have an average of three process of a tropical disturbance to a warm-core cy- cyclogenesis events each year. To explain the geo- clonic system. Except for the southeastern Pacific and graphic distribution of TC genesis, Gray (1975, 1977) the southeastern Atlantic where cold SST tongues (and investigated the environmental conditions around the excessively large vertical shears) are pronounced, TC cyclogenesis regions and found that six physical param- genesis is observed over all other tropical ocean basins. eters can be used to evaluate the potential tendency for A statistical TC distribution map (Gray 1968) showed TC genesis. Based on these parameters, the favorable that the western North Pacific (WNP) experiences the conditions for TC genesis include positive relative vor- most frequent TC activity among eight TC genesis re- ticity in lower troposphere, a location away from the gions. The average annual number of TCs in the WNP equator, a warm SST above 26.1°C, a small vertical ␪ is 22, which contributes 36% of the global total TC shear, a large equivalent potential temperature ( e) ra- numbers. Both the western North Atlantic and South dient between 500 hPa and the surface, and a relatively Pacific have an average of seven TCs, while the Bay of large value of relative humidity in the middle tropo- Bengal, South Indian Ocean and eastern North Pacific sphere. Ritchie and Holland (1999) documented environ- mental flow regimes that favor TC genesis in the WNP. * SOEST Contribution Number 6740 and IPRC Contribution They are the monsoon shear line, the monsoon conflu- Number 354. ence region, and the monsoon gyre. While these favor- able large-scale flow patterns are important in provid- ing background stages for tropical storm development, Corresponding author address: Prof. Tim Li, IPRC, and Dept. of Meteorology, University of Hawaii at Manoa, 2525 Correa Rd., they are not sufficient to lead to cyclogenesis. Although Honolulu, HI 96822. these patterns are often seen in daily weather maps, E-mail: [email protected] cyclogenesis does not occur every day. This implies that

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JAS3692 1378 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 63 there must be some triggering mechanisms for cyclo- torward, forming a synoptic-scale wave train of alter- genesis. Previous studies pointed out various synoptic nating regions of anticyclonic and cyclonic vorticity per- perturbation triggering scenarios such as the Rossby turbations. The perturbation energy, being accumu- wave energy dispersion of a preexisting TC (Frank lated in the confluence zone, helps release the 1982; Davidson and Hendon 1989; Ritchie and Holland barotropic/baroclinic instability in the ITCZ. Cumulus 1997; Briegel and Frank 1997), energy accumulation of convection is organized in the region of cyclonic circu- easterly waves in a confluent mean zonal flow (Kuo et lation, amplifying the disturbances through the convec- al. 2001), and a mixed Rossby–gravity wave packet tion–frictional convergence feedback (Wang and Li (Dickinson and Molinari 2002). Tropical cyclone gen- 1994). As the process proceeds, a TC-strength vortex esis may also involve wave–mean flow interactions may be generated. Once the newly generated vortex (Ferreira and Schubert 1997; Zehnder et al. 1999; Mo- moves away from the region, it may trigger the new linari et al. 2000) or development of synoptic-scale development of disturbances through the same energy wave trains (Lau and Lau 1990; Chang et al. 1996). dispersion/accumulation processes. Sobel and Bretherton (1999) analyzed the energy flux The role of TCED on cyclogenesis was previously convergence in the WNP and found that wave accumu- suggested based on either simple numerical models or lation may operate either on waves coming from out- indirect observational data (e.g., global model analysis side of the region or in situ. products) over open oceans. So far, no direct observa- In this study we particularly focus on the scenario tional evidence shows the detailed structure and evolu- related to TC energy dispersion (TCED). As we know, tion characteristics of the Rossby wave train induced by a mature TC is subject to Rossby wave energy disper- TCED, nor its relationship with TC genesis. Recent sion because of the change of the Coriolis force with satellite products [such as the Quick Scatterometer latitude (e.g., Anthes 1982; Flierl 1984; Luo 1994; Mc- (QuikSCAT)] provide a great opportunity to reveal the Donald 1998). While a TC moves northwestward be- detailed pattern and evolution of the Rossby wave train cause of mean flow steering and the beta drift, it emits and associated cyclogenesis processes (e.g., Li et al. Rossby wave energy southeastward, forming a synop- 2003). The objective of this paper is to document from tic-scale wave train with alternating anticyclonic and direct satellite measurements the detailed structure and cyclonic vorticity perturbations (Carr and Elsberry evolution characteristics of the Rossby wave train in 1994, 1995). Using barotropic vorticity equation mod- association with TCED and its role in cyclogenesis. els, Chan and Williams (1987) and Fiorino and Elsberry While Part I is focused on the observed aspects, Li et al. (1989) showed that a monopole, cyclonic vortex on a (2006, hereafter Part II) will simulate the TCED- beta plane experiences Rossby wave energy dispersion induced cyclogenesis in a 3D model. that generates asymmetric structure with a cyclonic Part I is organized as follows. In section 2 we briefly gyre to the west and an anticyclonic gyre to the east of describe the data and methodology. In section 3 we the vortex center. While the motion of the vortex as- present the observed structure and evolution character- sociated with the beta drift is toward the northwest, the istics of the Rossby wave train. Then in section 4 we vortex is subject to energy dispersion due to Rossby examine possible factors that control the generation of wave radiation (Flierl 1984). Luo (1994) focused on the the Rossby wave train. The large-scale dynamic and effect of Rossby energy dispersion on the motion and thermodynamic control on TCED-induced cyclogenesis structure of a TC-scale vortex on a beta plane in a is discussed in section 5. Finally, conclusions are given nondivergent barotropic model. A wavelike wake is in section 6. formed in his model as a result of Rossby energy dis- persion from the vortex. Because of the leaking of en- 2. Data and methodology ergy, the vortex weakens during its northwestward The primary data used in this study are the National movement. Carr and Elsberry (1995) found the exis- Aeronautics and Space Administration (NASA) tence of TCED-induced Rossby wave trains in both a QuikSCAT level 3 daily surface wind data and the barotropic model and Naval Operational Global Atmo- Tropical Rainfall Measurement Mission (TRMM) spheric Prediction System (NOGAPS) forecast fields. Microwave Imager (TMI) cloud liquid water data. The Holland (1995) discussed how sequential TCs might be QuikSCAT data are retrieved by a microwave scatter- generated because of TCED in the WNP. Once a TC ometer named SeaWinds that was launched on the develops in the vicinity of the monsoon trough/ITCZ, it QuikBird satellite in 1999. The primary mission of moves poleward and westward because of the planetary QuikSCAT is to measure near-surface winds. By de- vorticity gradient and mean flow steering; group energy tecting the sea surface roughness, it can use some em- associated with the TC propagates eastward and equa- pirical relationship to calculate the 10-m surface wind

Unauthenticated | Downloaded 09/25/21 10:33 PM UTC MAY 2006 L I A N D F U 1379 speed and direction. The products from this scatterom- bation finally reached or exceeded tropical storm inten- eter include 10-m surface zonal and meridional wind sity. Among the 34 cases, 6 cyclogenesis cases are iden- speed daily data. It covers global oceans with a resolu- tified to be associated with the energy dispersion of a tion of 0.25° ϫ 0.25°. The TMI data are retrieved by a preexisting TC. Figure 1 illustrates such a case. It shows radiometer on board the TRMM satellite. It is a joint the evolution of the 3–8-day filtered QuikSCAT wind mission supported by NASA and the National Space fields from 1 to 9 August 2000. The letter “A” in Fig. 1 Development Agency of . The TMI data cover represents the location of Typhoon Jelawat that formed the tropical region between 40°S–40°N and provide on 1 August. During the first 2–3 days, because of its 10-m surface wind speed (11 and 37 GHz), sea surface weak intensity, Jelawat did not generate a visible temperature, atmospheric water vapor, cloud liquid wa- Rossby wave train in its wake. During its northwest- ter, and precipitation rates with a horizontal resolution ward journey, the intensity of Jelawat increased. With ϫ of 0.25° 0.25°. its steady intensification, the Rossby wave train became In addition, National Centers for Environmental Pre- more and more evident in its wake. On 6 August, a diction–National Center of Atmosphere Research clear wave train was observed. This Rossby wave train (NCEP–NCAR) reanalysis data are used. Having a has a zonal wavelength of about 2500 km, oriented in a ϫ horizontal resolution of 2.5° 2.5°, the reanalysis data northwest–southeast direction. are used to fill in gaps in satellite observations with a The most remarkable characteristic of this wave train linear optimal interpolation scheme. The reanalysis is that it has a large meridional wavelength. A half data are also used to construct the vertical structure of meridional wavelength is about 4000 km on 6 August. the wave train in section 3. The meridional wavelength of the wave train is greatly We focus on the cyclogenesis events associated with reduced in subsequent days, leading to generation of a TCED in the WNP during the 2000–01 typhoon seasons new TC named Ewiniar on 9 August. Note that TC (from 1 June to 30 September). The TC best-track data Ewiniar (represented by the letter “B” in Fig. 1) formed issued by Joint Typhoon Warning Center (JTWC) are in the cyclonic vorticity center of the Rossby wave train. used to determine the cyclogenesis location. To clearly The cloud liquid water fields show a similar evolution examine synoptic-scale wind structures prior to TC for- feature (Fig. 2). During the first 2–3 days, convective mation, a time filtering method is used to retain 3–8-day clouds (represented by the cloud liquid water field) signals for both QuikSCAT and TMI data. The selec- were loosely organized. On 3 August, there was a tion of the 3–8-day bandwidth is based on Lau and Lau northwest–southeast-oriented cloud band in the wake (1990, 1992), who noted a significant synoptic-scale of Jelawat. On 6 August, when the wave train pattern wave spectrum appearing in this frequency band in the WNP. The filtering scheme is that of Murakami (1979). was discerned in the wind field, convective clouds also The response function for this filter is a Gaussian func- started to be organized in such a way that the cloud tion. In the 3–8-day band, the maximum response is at liquid water concentrated in cyclonic circulation re- a period of about 5 days. A low-pass filter (with a pe- gions. With the further development of the wave train, riod greater than 20 days) was used to compose large- one can see clearly the alternating cloudy–clear–cloudy scale background conditions associated with cyclogen- pattern, corresponding well to the alternating cyclonic– esis and noncyclogenesis cases in section 5. anticyclonic–cyclonic circulation. Thus, the indepen- The TCED-induced cyclogenesis events are identi- dent satellite products provide consistent wind and fied based on the following analysis approach. The cloud patterns during the course of the wave train de- daily synoptic maps of the filtered surface wind fields velopment. from the QuikSCAT data are first examined. If a new To clearly demonstrate the role of the Rossby wave TC formed in the cyclonic circulation embedded in a energy dispersion, we calculated an E vector (an energy wave train produced by a preexisting TC (based on propagation vector) for TC Jelawat (see Fig. 3), based JTWC best-track data), then this new TC belongs to the on Trenberth (1986). Here, E ϭ ([ϪuЈuЈϩ␷Ј␷Ј]/ 2, TCED-induced cyclogenesis scenario. Note that this [ϪuЈ␷Ј]), where [ ] represents time averaging, and uЈ analysis approach neglects possible triggering processes and ␷Ј are zonal and meridional components of synop- from the upper troposphere. tic-scale wind perturbations, respectively. The E vector in Fig. 3 was calculated based on a 15-day period cen- 3. Rossby wave train structure tered on 5 August 2000. It indicates the direction of energy dispersion around 5 August, assuming that the During the summers of 2000–01, we analyzed 34 cy- wave train is a Rossby wave type. Figure 3 illustrates clogenesis cases in the WNP in which a tropical pertur- that there is indeed southeastward propagation of

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FIG. 1. Time sequences of synoptic-scale surface wind patterns associated with the Rossby wave energy dispersion of Typhoon Jelawat. The letter “A” represents the center location of Jelawat that formed on 1 Aug 2000 and “B” represents the center location of a new TC named Ewiniar that formed on 9 Aug 2000 in the wake of the Rossby wave train of Jelawat.

Rossby wave energy. The E vector is not sensitive to ciated with a preexisting , which formed the averaged period, say, from 9 to 21 days. on 18 August. On 22 August, Bilis was located to the The second example of the TCED scenario is the northeast of the Philippines. One day later, it made genesis of on 26 August 2000. Fig- on the southeastern coast of and rapidly ure 4 shows the evolution of Rossby wave trains asso- dissipated within 48 h. Nevertheless, its wave train still

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FIG. 2. Time sequences of the cloud liquid water field (mm) associated with the Rossby wave energy dispersion of Typhoon Jelawat. As in Fig. 1, “A” represents center location of Jelawat and “B” repre- sents center location of Ewiniar. existed. On 26 August, a new TC (Prapiroon) formed in A clear Rossby wave train was discerned in the wake of the cyclonic vorticity region of the wave train. Durian on 30 June. This wave train has a zonal wave- The third example is the formation of . length of 3000 km, tilting toward the east-southeast. On Figure 5 illustrates the time sequence of surface wind 1 July, Durian made landfall on China, near Hong patterns associated with a preexisting Kong. In the meantime, a new TC (Utor) formed in the that formed over the South China Sea on 29 June 2001. cyclonic vorticity center of the wave train.

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amine the vertical structure of the Rossby wave train for Jelawat. Figure 6 shows that the northwest– southeast-oriented wave trains are clearly evident from the surface to 500 hPa, and become less organized in the upper troposphere (200 hPa). Figure 7 illustrates the vertical cross section of the Rossby wave train along a northwest–southeast- oriented axis (i.e., the solid black line at the bottom panel of Fig. 6). From Fig. 7, one can see that the TCED-induced Rossby wave train penetrates through the entire tropospheric layer. While the divergence in the wake region exhibits a dominant first baroclinic mode structure, with maximum divergence or conver- gence centers located at 150 hPa or the surface, the vorticity has an equivalent barotropic structure. In the cyclonic vorticity center of the wave train, the maxi- mum vorticity perturbation appears at 850 hPa. The zonal and meridional wind perturbations are collocated and have the same sign, implying a northwest– southeast orientation for the wave train.

FIG. 3. Horizontal map of E vectors calculated based on Tren- berth (1986); E ϭ ([ϪuЈuЈϩ␷Ј␷Ј]/2, [ϪuЈ␷Ј]), where [ ] represent 4. Factors that influence the wave train generation time averaging, and uЈ and ␷Ј are synoptic wind perturbations derived from the QuikSCAT data. The E vectors are summed over a 15-day period centered on 5 Aug 2000. The letter “A” The examination of all the 34 TC cases reveals that marks position of TC Jelawat on 5 Aug 2000. not all TCs have a Rossby wave train in their wake. This poses an interesting question: What determines the generation of the Rossby wave train? Previous theoret- To make sure that the wave trains derived above are ical and modeling studies suggested that TCED de- not an artifact of the filtering, we subtracted the origi- pends greatly on the size of a TC (Carr and Elsberry nal QuikSCAT wind field from a 31-day sliding-window 1995) and its wind profile (e.g., Flierl et al. 1983). A TC average (from 15 days prior to 15 days after). The re- with zero total relative angular momentum tends to sults (figures not shown) show that the wave trains are conserve its energy (Shapiro and Ooyama 1990). clearly present even with the raw data. Because of a lack of reliable observational informa- Thus, the satellite (QuikSCAT and TMI) measure- tion on TC size and wind profiles, we examine relation- ments provide the direct observational evidence of the ships between the occurrence of the Rossby wave train TCED-induced Rossby wave train, its structure and and TC intensity, based on the fact that many intense evolution characteristics, and its role in TC genesis. TCs in the WNP are often large ones. For all the 34 This confirms the previous TCED-induced cyclogenesis TCs, we have a total of 233 (snapshot) samples. We hypothesis based on either limited observational data separate the 233 samples into three groups based on TC (e.g., Frank 1982) or numerical model experiments minimum sea level pressure (MSLP). They represent (e.g., Holland 1995). strong (MSLP Ͻ 960 hPa), moderate (960 hPa Յ MSLP To reveal the vertical structure of the TCED-induced Յ 980 hPa), and weak (MSLP Ͼ 980 hPa) TC intensi- Rossby wave train, we examine the NCEP–NCAR re- ties. Figure 8 shows the percentages of occurrence of analysis fields. The same synoptic-scale filtering tech- the Rossby wave train at each group. For the strong nique was applied to the reanalysis data. By comparing TCs, the percentage is quite high (about 87%). It drops the QuikSCAT-derived wave train patterns with those quickly to 40% (30%) for moderate (weak) storms. The from the reanalysis data, we noted that the reanalysis result above shows a great sensitivity of the occurrence data in general underestimate the strength of the wave of the Rossby wave train to the TC intensity. train; in particular, it completely misses the wave train In addition, the occurrence of the Rossby wave train pattern induced by (2000). However, may also depend on the mean background flow. Figure 9 it captures the wave train pattern of Typhoon Jelawat illustrates the wave train generation and nongeneration (2000) reasonably well. Thus, in the following we ex- cases for the strong TC group (i.e., MSLP Ͻ 960 hPa).

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FIG. 4. Time sequences of synoptic-scale surface wind patterns associated with the Rossby wave energy disper- sion of Typhoon Bilis; “A” represents the center location of Bilis that formed on 18 Aug 2000 and “B” represents the center location of a new TC named Prapiroon that formed on 26 Aug 2000 in the wake of the Rossby wave train of Bilis.

Here closed circles (squares) in the figure represent an important question: What determines the cyclogen- the cases with (without) the appearance of the wave esis threshold in the Rossby wave train? In the follow- train. For strong TCs, the southeast–northwest- ing, we intend to address this issue from large-scale oriented Rossby wave trains were observed west of dynamics and thermodynamic points of view. 155°E. They are absent to the east of this longitude We hypothesize that the background flow and mois- Ϫ where the mean easterly trades are large (Ϫ4ms 1 or ture conditions are critical for the wave train develop- stronger). ment. To identify the difference of the background flow Physical mechanisms that give rise to this mean flow between the cyclogenesis and noncyclogenesis cases, dependence is unclear. It is speculated that it may result we conducted a composite analysis. Six genesis cases from the frequency modulation by the mean flow, as and nine nongenesis cases are composed. Figure 10 the strong easterly trades may prevent the southeast- shows the comparisons of the composite large-scale ward propagation of the perturbation kinetic energy background variables for the genesis and nongenesis emitted from a TC. cases. These variables are averaged in a 5° ϫ 5° box centered at the cyclonic vorticity center of the wave 5. Large-scale dynamic and thermodynamic train, and have been subjected to a low-pass filter to control on cyclogenesis retain signals longer than 20 days so that they reason- ably represent the large-scale environmental conditions Our synoptic analysis of the QuikSCAT wind field including the seasonal cycle and intraseasonal oscilla- shows that for 15 TCs that generated a Rossby wave tion (ISO) activities. Note that there are clear differ- train in their wake, only 6 of them eventually lead to ences between the genesis and nongenesis cases in new TC formation. The fact that some of the Rossby large-scale background conditions. For instance, the wave trains lead to cyclogenesis but others do not raises vertical motion profiles have a significant difference in

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FIG. 5. Time sequences of synoptic-scale surface wind patterns associated with the Rossby wave energy dispersion of Typhoon Durian; “A” represents the center location of Durian that formed on 29 Jun 2001 and “B” represents the center location of a new TC named Utor that formed on 1 Jul 2001 in the wake of the Rossby wave train of Durian. the midtroposphere. A stronger upward motion ap- pears in the genesis cases. Consistent with the vertical motion, stronger low-level convergence, upper-level di- vergence, and low-level cyclonic vorticity occur in the cyclogenesis cases. Vertical wind shear is also relatively smaller and the relative humidity is larger throughout the troposphere in the cyclogenesis cases. Thus, all en- vironmental variables exhibit a favorable condition in the cyclogenesis cases. FIG. 6. Rossby wave train patterns at 200, 500, 850, and 1000 hPa produced by TCED from Typhoon Jelawat on 8 Aug 2000. Figure 11a shows the horizontal patterns of the com- The wind fields are 3–8-day filtered. The thick line in the bottom posite large-scale background wind fields at 500 and panel represents the axis of the wave train. 850 hPa associated with the genesis and nongenesis cases. For the cyclogenesis cases, a background cyclonic wind shear appears at both 850 and 500 hPa. However, posite center at both 850 and 500 hPa in the cyclogen- for the nongenesis cases, an anticyclonic shear appears esis cases, compared to a near-zero or negative vorticity in lower troposphere (850 hPa), while rather uniform in the noncyclogenesis cases. The synoptic-scale wave winds cross the center at 500 hPa. These wind features train center nearly collocates with the background vor- are well reflected in the vorticity composites (see Fig. ticity center in the genesis cases. 11b), which exhibit a positive vorticity near the com- In addition to the large-scale flow pattern, back-

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FIG. 8. Percentage of occurrence of TCED-induced Rossby wave train for strong, moderate, and weak TCs.

became relatively cooler compared to its surroundings, even a few days after Bilis had moved away from the region. This led to relatively stable stratification in the region, which did not favor cyclogenesis even though the wave train had developed. Then, the at- mospheric conditional instability started to build up gradually, which can be clearly seen from the increase of both surface moisture and air temperature at 2 m from day Ϫ4 to day 0 in Fig. 12 (here day 0 refers to the genesis date for Typhoon Prapiroon). During this building-up period, surface air temperature increased by 1°C, while the surface specific humidity increased by 2gkgϪ1. The increase of both the temperature and moisture led to the increase of the surface saturation Ϫ Ϫ FIG. 7. Vertical cross section of (a) divergence (10 6 s 1), (b) vorticity (10Ϫ5 sϪ1), (c) zonal wind (m sϪ1), and (d) meridional wind (m sϪ1) along the Rossby wave train axis defined in Fig. 6. The y axis denotes pressure (hPa). The open and closed circles represent the relative position of cyclonic and anticyclonic centers of the wave train. ground thermodynamic conditions such as humidity distribution and moist static stability in lower tropo- sphere may also play a role. It is noted that the specific humidity field has a more favorable large-scale back- ground in the genesis cases, that is, large humidity cen- ters appear near the cyclonic vorticity centers of the wave trains at both 850 and 500 hPa. Figure 12 illustrates the thermodynamic control in the formation of TC Prapiroon. Both Prapiroon and the previous Typhoon Bilis formed in the same area (within FIG. 9. Geographic location of the strong TCs (MSLP Ͻ 960 10° longitude distance). Strong winds and dense clouds hPa) that do (closed circle) and do not (closed square) have a Rossby wave train in the wake. Superposed are background June– associated with Bilis cooled the ocean surface through September-averaged wind vectors at 1000 hPa. The region where enhanced upwelling/evaporation and reduction of the mean easterly wind speed is greater than 4 m sϪ1 is contoured shortwave radiation. As a result, the local ocean surface and shaded.

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FIG. 10. Composite vertical structure of 20-day low-pass-filtered divergence, p vertical velocity, vorticity, zonal wind, specific humidity, and relative humidity averaged over the cyclonic vorticity center of the Rossby wave trains for six genesis (open circle) and nine nongenesis (closed circle) cases.

ץ ␪ץ equilibrium potential temperature. As a result, ( se/ z) “recovery” period to rebuild conditional unstable decreased in the lower troposphere (between 1000 and stratification in the region. 850 hPa), leading to more unstable atmospheric strati- fication. 6. Conclusions The slow thermodynamic building-up process might be responsible for the observed frequency of TC gen- The structure and evolution characteristics of Rossby esis in the WNP. In an observational study, Frank wave trains induced by TC energy dispersion are re- (1982) found that the typical separation distance and vealed with direct satellite (QuikSCAT and TMI) mea- time period between the two subsequent TCs in the surements. An E vector calculation shows that the wave WNP were around 2000 km and 7–10 days. For the train is indeed associated with the TC Rossby wave Typhoon Prapiroon case, it is likely that the timing of energy dispersion. For 34 TCs analyzed in the WNP cyclogenesis was determined by both dynamic factors during 2000–01 typhoon seasons, there are 6 cyclogen- such as the strength of the wave train and background esis cases that are associated with the Rossby wave en- flow conditions and the thermodynamic accumulation ergy dispersion of a preexisting TC. In all these cases, a of the surface moist static energy. Prapiroon formed in new TC formed in the cyclonic vorticity region of a the same region as Bilis did 8 days prior. Strong winds preexisting well-defined wave train. It seems that there and thick cloud cover associated with Bilis reduced the is a scale (particularly meridional scale) contraction SST and the surface moist static energy. It required a process associated with cyclogenesis. The cause of this

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FIG. 11. (a) Composites of 20-day low-pass-filtered wind fields at 500 and 850 hPa for the cyclogenesis and noncyclogenesis cases. The vertical (horizontal) axis is relative distance in latitude (longitude) degrees to the center of cyclonic circulation of the wave train. (b) Same as (a) except for vorticity (10Ϫ5 sϪ1) fields.

scale contraction is unknown at the moment, and it is more chances to form a Rossby wave train in their speculated that it may result from the wave train–mean wakes. flow interaction and energy accumulation in a conver- We also note that not all TCs that have a Rossby gent mean background flow. wave train finally lead to new TC formation. Whether Our observational analyses indicate that not all TCs or not the Rossby wave train results in cyclogenesis have Rossby wave trains in their wakes. The occurrence depends on background dynamic and thermodynamic of the wave train seems dependent on TC intensity and conditions that are related to both the seasonal cycle the background mean flow. For stronger TCs, there are and ISO activities. The stronger low-level convergence

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discussions; anonymous reviewers for constructive comments; and Dr. Francis Tam for the E vectors cal- culation. This work was supported by NSF Grant ATM01-19490 and ONR Grants N000140310739 and N000140210532. International Pacific Research Center is partially sponsored by the Japan Agency for Marine- Earth Science and Technology (JAMSTEC).

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