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Mesoscale Features Associated with Tropical Cyclone Formations in the Western North Pacific

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Mesoscale Features Associated with Tropical Cyclone Formations in the Western North Pacific 2006 MONTHLY WEATHER REVIEW VOLUME 136 Mesoscale Features Associated with Tropical Cyclone Formations in the Western North Pacific CHENG-SHANG LEE Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan KEVIN K. W. CHEUNG* National Science and Technology Center for Disaster Reduction, Taipei, Taiwan JENNY S. N. HUI Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan RUSSELL L. ELSBERRY Department of Meteorology, Naval Postgraduate School, Monterey, California (Manuscript received 24 May 2007, in final form 13 September 2007) ABSTRACT The mesoscale features of 124 tropical cyclone formations in the western North Pacific Ocean during 1999–2004 are investigated through large-scale analyses, satellite infrared brightness temperature (TB), and Quick Scatterometer (QuikSCAT) oceanic wind data. Based on low-level wind flow and surge direction, the formation cases are classified into six synoptic patterns: easterly wave (EW), northeasterly flow (NE), coexistence of northeasterly and southwesterly flow (NE–SW), southwesterly flow (SW), monsoon conflu- ence (MC), and monsoon shear (MS). Then the general convection characteristics and mesoscale convective system (MCS) activities associated with these formation cases are studied under this classification scheme. Convection processes in the EW cases are distinguished from the monsoon-related formations in that the convection is less deep and closer to the formation center. Five characteristic temporal evolutions of the deep convection are identified: (i) single convection event, (ii) two convection events, (iii) three convection events, (iv) gradual decrease in TB, and (v) fluctuating TB, or a slight increase in TB before formation. Although no dominant temporal evolution differentiates cases in the six synoptic patterns, evolutions ii and iii seem to be the common routes taken by the monsoon-related formations. The overall percentage of cases with MCS activity at multiple times is 63%, and in 35% of cases more than one MCS coexisted. Most of the MC and MS cases develop multiple MCSs that lead to several episodes of deep convection. These two patterns have the highest percentage of coexisting MCSs such that potential interaction between these systems may play a role in the formation process. The MCSs in the monsoon-related formations are distributed around the center, except in the NE–SW cases in which clustering of MCSs is found about 100–200 km east of the center during the 12 h before formation. On average only one MCS occurs during an EW formation, whereas the mean value is around two for the other monsoon-related patterns. Both the mean lifetime and time of first appearance of MCS in EW are much shorter than those developed in other synoptic patterns, which indicates that the overall formation evolution in the EW case is faster. Moreover, this MCS is most likely to be found within 100 km east of the center 12 h before formation. The implications of these results to internal mechanisms of tropical cyclone formation are discussed in light of other recent mesoscale studies. * Current affiliation: Climate Risk Concentration of Research Excellence (CORE) and Department of Physical Geography, Mac- quarie University, Sydney, Australia. Corresponding author address: Kevin K. W. Cheung, Department of Physical Geography, Macquarie University, Sydney, NSW 2109, Australia. E-mail: [email protected] DOI: 10.1175/2007MWR2267.1 © 2008 American Meteorological Society MWR2267 JUNE 2008 LEEETAL. 2007 1. Introduction thermodynamic contribution) may depend on near- surface processes where observations are rare, most of The formation of tropical cyclones (TCs) has long the proposed mechanisms are based on numerical simu- been a major area of research, and has resulted in the- lations that must be subjected to validation by obser- ories such as the convective instability of the second vations. kind (CISK; Charney and Eliassen 1964) and wind- One of these theories, the so-called top-down theory, induced surface heat exchange (WISHE; Emanuel is based on the classic MCS structure that usually de- 1986). Whereas these theories focus on the intensifica- velops in an environment with substantial low-level ver- tion process of TCs after the basic kinematic structure tical wind shear and possesses a midlevel mesoscale and, in some cases, the warm-core structure is already convective vortex (MCV) in the stratiform rain region established, the physical processes responsible for the for long-lasting MCSs. This MCV is anticipated to be development from weak or unorganized disturbances the potential focal point for TC formation. Such a struc- to the tropical depression stage are not well under- ture has been observed (e.g., Chen and Houze 1997) stood. and simulated (e.g., Zhang and Fritsch 1986, 1987; The generally accepted picture of TC formation is Chen and Frank 1993), particularly for MCSs over land. multiscale in nature (Holland 1995). That is, synoptic, The characteristic low-level dry layer behind the major subsynoptic-scale, and mesoscale circulations may all deep convective area is also identified in observed and contribute to the formation process. However, the de- simulated MCSs in a TC formation region (Harr and termining system (so-called on–off switch in some lit- Elsberry 1996; Cheung and Elsberry 2006). If the low- erature) is still not clear and is the focus of ongoing level winds are to strengthen to become a tropical de- research. Starting from the large scale, the basic envi- pression, two theories have been proposed: a MCV is ronmental conditions favorable for formation have either advected downward by continuous rain (Bister been known for years (Gray 1968, 1998). These condi- and Emanuel 1997); or a merging of two or more MCVs tions include high sea surface temperature (SST), con- (Ritchie and Holland 1997) occurs. Whereas these pro- ditional instability and high relative humidity in the cesses have been simulated in numerical models, veri- middle troposphere, cyclonic absolute vorticity in the fication with observations is lacking. lower troposphere, anticyclonic relative vorticity in the The bottom-up theory is somewhat based on the ob- upper troposphere, and low vertical wind shear (e.g., servations of Zehr (1992) that low-level vortex intensi- Cheung 2004). The monsoon trough region in the west- fication sometimes follows bursts of intense deep con- ern North Pacific Ocean (WNP) during the summer vection. Montgomery et al. (2006) suggest this deep- usually satisfies most of the above-mentioned condi- convective, low-level vortex enhancement is taking tions and is the source of many TCs in the basin. place within MCSs well before the system-scale vortex Very often TC formations originate from distur- becomes self-sustainable, and is then further intensified bances with enhanced convection and low-level relative through mechanisms such as CISK and WISHE. Mont- vorticity, which may originate from low-frequency os- gomery et al. suggest that vortical hot towers (VHTs) cillations in the tropics such as the Madden–Julian os- on scales of 10–20 km play an important role during the cillation. These wave activities in the tropics modulate process (see also Hendricks et al. 2004; Tory et al. the global TC activity on different time scales and their 2006a,b). relationships with TC frequency, location, and intensity Two questions then arise as to the mechanisms of are the subjects of various studies (e.g., Dickinson and low-level vortex enhancement in MCSs. Is the structure Molinari 2002; Frank and Roundy 2006). of tropical MCSs, especially those that contribute to TC Mesoscale convective systems (MCSs) are often formation, different from the usual characteristics of a found to develop in association with TC formations in tilted deep convection, an extensive stratiform rain re- the monsoon trough or in tropical waves. Contributions gion, and low-level cold pool? To what extent can cur- of MCSs to the formation process have been debated in rently available remote sensing data reveal the contri- the past decade. Due to extensive coverage of all ocean bution of MCSs in TC formation? Zehr (1992) provided basins by satellite imagery, the presence of MCSs dur- one of the first detailed reports utilizing satellite data to ing TC formation has been confirmed in various studies monitor and analyze early-stage TC development, and (Simpson et al. 1997; Ritchie and Holland 1997; Cheung he proposed a two-stage formation process. The first and Elsberry 2004) and monitoring of the life cycle of stage of enhanced convection occurs one or more days these systems is feasible. Because the exact mecha- before the tropical depression forms, and produces a nism(s) by which MCSs contribute to the formation distinct midlevel circulation center. Stage 2 begins process (e.g., whether predominantly a dynamical or a when another burst of deep convection occurs with a 2008 MONTHLY WEATHER REVIEW VOLUME 136 curved pattern of organized clouds closer to the system (ECMWF) are used to analyze the synoptic patterns center than those in stage 1, which indicates that an associated with TC formations. increase in deep-column vorticity may be taking place. Tropical depression intensity or even a tropical storm is b. Definition of formation usually attained within stage 2. It is generally agreed that TC formation is a continu- The primary objective of this paper is to extend the ous atmospheric process, and the exact time of transi- work of Lee (1986), Zehr (1992), and Ritchie and Hol- tion from unorganized cloud clusters to a self- land (1999) by using the latest remote sensing data sustainable system with a certain intensity is not well available. In addition, connections between large-scale defined. In this study, the formation time is taken to be patterns of TC formation and mesoscale convection the first time in the poststorm analysis (best track) of systems are established. Section 2 describes the data each TC that the intensity reached 25 kt, which is usu- used in the study, the period of formation to be exam- ally after the release by the Joint Typhoon Warning ined, and the definition of formation.
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