Cyclogenesis Simulation of Typhoon Prapiroon (2000) Associated with Rossby Wave Energy Dispersion*

42 MONTHLY WEATHER REVIEW VOLUME 138 Cyclogenesis Simulation of Typhoon Prapiroon (2000) Associated with Rossby Wave Energy Dispersion* XUYANG GE AND TIM LI Department of Meteorology, and International Pacific Research Center, University of Hawaii at Manoa, Honolulu, Hawaii MELINDA S. PENG Naval Research Laboratory, Monterey, California (Manuscript received 16 March 2009, in final form 27 July 2009) ABSTRACT The genesis of Typhoon Prapiroon (2000), in the western North Pacific, is simulated to understand the role of Rossby wave energy dispersion of a preexisting tropical cyclone (TC) in the subsequent genesis event. Two experiments are conducted. In the control experiment (CTL), the authors retain both the previous typhoon, Typhoon Bilis, and its wave train in the initial condition. In the sensitivity experiment (EXP), the circulation of Typhoon Bilis was removed based on a spatial filtering technique of Kurihara et al., while the wave train in the wake is kept. The comparison between these two numerical simulations demonstrates that the preexisting TC impacts the subsequent TC genesis through both a direct and an indirect process. The direct process is through the conventional barotropic Rossby wave energy dispersion, which enhances the low-level wave train, the boundary layer convergence, and the convection–circulation feedback. The indirect process is through the upper-level outflow jet. The asymmetric outflow jet induces a secondary circulation with a strong divergence tendency to the left-exit side of the outflow jet. The upper-level divergence boosts large-scale ascending motion and promotes favorable environmental conditions for a TC-scale vortex development. In addition, the outflow jet induces a well-organized cyclonic eddy angular momentum flux, which acts as a momentum forcing that enhances the upper-level outflow and low-level inflow and favors the growth of the new TC. 1. Introduction 1999) and confirmed by satellite observations in more recent studies (Li and Fu 2006). While the TC moves Among different tropical cyclone (TC) genesis mech- northwestward due to the beta effect and mean flow anisms in the western North Pacific (WNP), the Rossby steering, it emits energy southeastward and results in wave energy dispersion from a previous TC is an impor- a synoptic wave train with alternating anticyclonic and tant process (Fu et al. 2007). The connection between cyclonic circulations in its wake (Chan and Williams 1987; tropical cyclogenesis and the TC energy dispersion Carr and Elsberry 1995; Holland 1995). Under favorable (TCED) was suggested earlier, based on limited obser- large-scale environmental conditions, the TCED-induced vational data (e.g., Frank 1982; Davidson and Hendon Rossby wave train may lead to new TC genesis (Ritchie 1989; Briegel and Frank 1997; Ritchie and Holland and Holland 1999; Li and Fu 2006; Li et al. 2006). Most of the previous TCED studies (Chan and Williams 1987; Carr and Elsberry 1995) were confined to a two- * School of Ocean and Earth Science and Technology Contri- bution Number 7845 and International Pacific Research Center dimensional barotropic dynamic framework. A recent Contribution Number 623. three-dimensional (3D) baroclinic modeling study by Ge et al. (2008), investigated the vertical structure of the TCED-induced waves. Due to the vertical differ- Corresponding author address: Dr. Xuyang Ge, Dept. of Meteo- rology, International Pacific Research Center, University of Hawaii ential inertial stability, the upper-level wave train is at Manoa, 2525 Correa Rd., Honolulu, HI 96822. characterized by an asymmetric outflow jet that de- E-mail: [email protected] velops much faster than its lower-level counterpart. This DOI: 10.1175/2009MWR3005.1 Ó 2010 American Meteorological Society Unauthenticated | Downloaded 10/02/21 10:21 PM UTC JANUARY 2010 G E E T A L . 43 FIG. 1. Time sequences of synoptic-scale surface wind patterns associated with the Rossby wave energy dispersion of Typhoon Bilis; ‘‘A’’ represents the center location of Bilis, and ‘‘B’’ represents the center location of TC Prapiroon in the wake of Bilis. The QuikSCAT surface wind product is used. beta effect–induced upper-level asymmetry may influence and two numerical simulations are conducted. In the the lower-level Rossby wave train by changing the in- control experiment (CTL), both the structure of the prior tensity and structure of the TC. Moreover, an easterly typhoon, Typhoon Bilis, and its wave train are retained. In (westerly) shear of the mean flow may further enhance the sensitivity experiment (EXP), the prior typhoon is (weaken) the lower-level Rossby wave train (Ge et al. removed but the wave train in its wake is kept. The 2007). The idealized numerical simulations of Li et al. disturbance in the wave train is needed in EXP, serving (2006) demonstrated the important role of TC energy as the precursor of the subsequent TC. Through this dispersion in the wave train development and cyclo- sensitivity experiment, we investigatetheroleofthepre- genesis. The objective of the present study is to in- vious TC in the subsequent cyclogenesis. The numerical vestigate the structure of the wave train in the reality results are presented in section 4. The physical inter- and the extent to which a previous TC contributes to the pretations on the role of the upper outflow jet associated large-scale environmental conditions in its wake. In par- with the previous TC are discussed in section 5. Finally, ticular, we attempt to reveal the effect of the Rossby wave the conclusions and discussion are given in section 6. energy dispersion and the upper-tropospheric influence on the TC genesis, utilizing a real-case simulation for 2. Wave train patterns prior to Prapiroon Typhoon Prapiroon (2000) in the western North Pacific. formation The paper is organized as follows. In section 2, the observed synoptic-scale wave train patterns, prior to the Typhoon Prapiroon formed over the WNP on 25 August cyclogenesis of Typhoon Prapiroon, are presented. In 2000. The synoptic-scale wind patterns, prior to the cy- section 3, the model and experiment designs are described clogenesis, are shown in Fig. 1 for the bandpass-filtered Unauthenticated | Downloaded 10/02/21 10:21 PM UTC 44 MONTHLY WEATHER REVIEW VOLUME 138 (3–8 day) surface wind fields, following Li and Fu (2006). The filtering scheme is that of Murakami (1979). The response function for this filter is a Gaussian function. In the 3–8-day band, the maximum response is at a period of about 5 days. A Rossby wave train with alternating cyclonic–anticyclonic circulation can be clearly identified in the wake of the previous super Typhoon Bilis, which formed on 18 August. This wave train was orientated in a northwest–southeast direction with a wavelength of about 2500 km, resembling a typical TCED-induced Rossby wave train (Li and Fu 2006; Ge et al. 2008). During the course of the wave train development, an alternating rainy–clear-sky–rainy pattern occurs in the wake region (Fig. 2). Rigorous convective cells are primarily confined in the cyclonic circulation region of the wake. The convective cells merge into larger mesoscale convective systems (MCSs), eventually evolving into a tropical cyclone with a well-organized structure (bottom panel). The cyclonic circulation in the wave train intensified and eventually led to the formation of a new TC Pra- piroon to the northwest of Yap Island. Prapiroon was located about 208 to the east of TC Bilis, consistent with previous studies (e.g., Frank 1982; Ritchie and Holland 1999; Li and Fu 2006). Frank (1982) noted that WNP typhoons often formed 158–208 to the east of a preexisting storm. This implies a preferred wavelength of the TCED-induced Rossby wave train. The 150-hPa wind vector and relative vorticity on 22 August 2000 are presented in Fig. 3a. An elongated outflow jet, with a maximum velocity exceeding 35 m s21, extends in a clockwise direction from the northeast to the southwest. A cyclonic shear vorticity occurred out- side of the jet core. This asymmetric horizontal pattern resembled both observational (Black and Anthes 1971; Frank 1977; McBride 1981) and numerical simulation results (Shi et al. 1990; Wang and Holland 1996; Ge et al. 2008). The asymmetric structure remained basically un- changed in the following days, until Typhoon Bilis made FIG. 2. The evolution characteristics of TRMM rainfall rate landfall. Figure 3b shows the vertical-radius cross sec- 2 (mm h 1) and lower-level wind field on (top) 23, (middle) 25, and tion of the relative vorticity field along the axis of the (bottom) 26 Aug. wave train (dashed line in Fig. 3a). The vertical structure shows a noticeable baroclinic structure—that is, a cyclonic vorticity centered near 750 hPa with an anticy- clear evidence of southeastward Rossby wave group clonic vorticity near 200 hPa, so that the wave train tilts velocity from the center of Typhoon Bilis. Thus, the northwestward with height. The vertical structure agrees observed diagnosis supports the notion that the TC with our numerical simulation (Ge et al. 2008). Prapiroon genesis is indeed associated with the Rossby To demonstrate the existence of Rossby wave energy wave energy dispersion from the previous TC Bilis. dispersion, an E vector (Trenberth 1986; Li and Fu 2006) is used to diagnose the energy propagation. The E vector 3. The model and experiments design is calculated based on synoptic-scale wind fields during an 11-day period centered on 20 August 2000. As seen The fifth-generation Pennsylvania State University– from Fig. 4, prior to the formation of Prapiroon, there is National Center for Atmospheric Research Mesoscale Unauthenticated | Downloaded 10/02/21 10:21 PM UTC JANUARY 2010 G E E T A L . 45 21 25 21 FIG. 3. (a) The observed 150-hPa wind vector (m s ) and relative vorticity (10 s ) fields at 1200 UTC 22 Aug 2000 and (b) the vertical-radius cross section of relative vorticity fields along a northwest–southeast-oriented axis (the dashed line at the top).

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