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1990) and Gene (1990 DECEMBER 1999 WU AND CHENG 3003 An Observational Study of Environmental In¯uences on the Intensity Changes of Typhoons Flo (1990) and Gene (1990) CHUN-CHIEH WUANDHSIU-JU CHENG Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan (Manuscript received 27 July 1998, in ®nal form 22 December 1998) ABSTRACT The European Centre for Medium-Range Weather Forecasts Tropical Ocean±Global Atmosphere advanced analysis was used to study the mechanisms that affect the intensity of Typhoons Flo (1990) and Gene (1990). The out¯ow structure, eddy momentum ¯ux convergence, and the mean vertical wind shear were examined. The evolution of potential vorticity (PV) in the out¯ow layer showed low PV areas on top of both Typhoons Flo and Gene, and the low PV areas expanded as the typhoons intensi®ed. The out¯ow pattern of the two typhoons was in¯uenced by the upper-tropospheric environmental systems. The upper-level environmental fea- tures were shown to play a crucial role in the intensi®cation of the two typhoons. The tropical upper-tropospheric trough cell east of Flo provided the out¯ow channel for the typhoon. The enhanced out¯ow, the upper-level eddy ¯ux convergence (EFC), the low vertical wind shear, and the warm sea surface temperature provided all favorable conditions for the development of Flo. On the other hand, the intensi®cation of Gene was associated with its interaction with an upper-level midlatitude trough. The approach of the trough produced upper-level EFC of angular momentum outside 108 lat radius, and the EFC shifted inward with time. As the EFC shifted into the vicinity of the storm core, Gene started to intensify steadily until the midlatitude trough passed over. The intensifying processes of the above cases indicate the importance of the upper-tropospheric systems to the intensity change of typhoons. The in¯uence of upper-level environmental systems on the tropical cyclones is prominent in the low inertial stability out¯ow layer. However, results from the piecewise PV inversion of the upper-level environmental PV anomalies showed little evidence that the intensi®cation of both typhoons were directly associated with the superposition of PV anomalies. 1. Introduction The intensity of a typhoon, as conventionally mea- Tropical cyclones are destructive weather systems. sured by its maximum surface wind or minimum surface Onslaught by a severe tropical cyclone on human res- pressure, is affected at any time by large and complex idences usually results in serious property damage and arrays of physical processes that govern the interaction sometimes heavy loss of life. Although numerical mod- of the storm both with the underlying ocean and with els have shown considerable skill (e.g., Kurihara et al. its atmospheric environment. In general, the factors af- 1998) in tropical cyclone track forecasting, the predic- fecting the intensity of tropical cyclones can be cate- tion of tropical cyclone intensity remains a highly chal- gorized into three areas: 1) upper-ocean (air±sea) inter- lenging task for meteorologists and weather forecasters. action, 2) internal eyewall (storm core) dynamics, and For example, Avila (1998) has shown that no signi®cant 3) atmospheric±environmental interaction. The back- improvement in the of®cial intensity forecast was made grounds of past studies on these factors affecting ty- at the National Hurricane Center for the Atlantic basin phoon intensity are reviewed here. between 1990 and 1997. It is clear that to improve the prediction of tropical cyclone intensity, extensive re- a. Upper-ocean (air±sea) interaction search is required for a greater understanding of the structure and evolution of a tropical cyclone and to iden- It is well known that the ocean is a large energy tify the key mechanisms controlling the change in in- reservoir for tropical cyclones. Thus sea surface tem- tensity of tropical cyclones. perature (SST) plays a crucial role in the determination of the maximum intensity of a tropical cyclone. On this basis, Merrill (1988) derived an empirical maximum intensity of tropical cyclone for a given SST. Moreover, Corresponding author address: Dr. Chun-Chieh Wu, Dept. of At- mospheric Sciences, National Taiwan University, 61, Ln. 144, Sec. a theoretical upper bound of intensity that a tropical 4, Keelung Rd., Taipei 10772, Taiwan. cyclone can achieve has been determined using SST and E-mail: [email protected] the atmospheric thermodynamic environment (Emanuel q 1999 American Meteorological Society 3004 MONTHLY WEATHER REVIEW VOLUME 127 1986), whereby a tropical cyclone is regarded as a Car- tensify after the enhanced EFC usually experienced in- not heat engine, gaining energy from the disequilibrium creasing vertical shears, moved over cool water, or be- between the air and ocean surface. This air±sea inter- came extratropical. Their results indicated that in ad- action theory for tropical cyclones has been further elab- dition to SST, EFC and vertical wind shear are also orated in Emanuel (1988, 1991, 1995). Another ther- crucial factors that affect the intensity of a tropical cy- modynamic approach to estimating the maximum po- clone. tential intensity (MPI) of tropical cyclones has also been The work of Pfeffer and Challa (1981) focused on described by Holland (1997). The feedback of ocean on the importance of eddy momentum forcing to the initial the intensity of tropical cyclones has been examined in development of a tropical cyclone. Molinari and Vollaro the hurricane±ocean coupled model (e.g., Bender et al. (1989, 1990) found that upper-level EFC of angular mo- 1993). A case in point is the rapid intensi®cation of mentum occurred during the intensifying period of Hur- Hurricane Opal (1995) near and across a warm-core ricane Elena (1985), indicating that the upper-tropo- eddy over the warm sector of the Gulf of Mexico (Shay spheric trough played an important role in the reinten- et al. 1998; Black and Shay 1998; Bosart et al. 1998). si®cation of Elena. The European Centre for Medium- Range Weather Forecasts (ECMWF) analyses on a 2.58 lat 3 2.58 long grid were used by Molinari and Vollaro b. Internal eyewall dynamics (1990) to evaluate the response of the Eliassen's bal- Besides the SST factor, the eyewall dynamics also anced vortex to momentum and heat ¯uxes. The bal- control the intensity evolution in tropical cyclones (e.g., anced vortex solution showed an in±up±out circulation Willoughby et al. 1982; Shapiro and Willoughby 1982; that shifted inward with time, which is consistent with Willoughby and Black 1996). However, many aspects the inward propagation of the maximum out¯ow found of eyewall behavior have not been fully understood. The by Molinari and Vollaro (1989). This result also indi- response of the tropical cyclone intensity to the con- cated the environmental control on the behavior of the vective and microphysical processes in the eyewall re- storm. gion is not well explained. A recent theoretical study (Montgomery and Kallenbach 1997; Montgomery 2) OUTFLOW STRUCTURE 1998) employing the potential vorticity (PV) concept is illuminating. However, until detailed and precise mea- The inertial stability in a tropical cyclone is lower in surements within the eyewall can be obtained, the com- the out¯ow layer than that in the middle and lower layers plicated eyewall dynamics and its relation with tropical (Holland and Merrill 1984). Because of the low inertial cyclone intensity may remain a mystery. stability in the out¯ow layer, the in¯uence of upper- level environmental forcing may extend to the inner parts of a tropical cyclone more easily. Holland and c. Atmospheric±environmental interaction Merrill (1984) showed that the radial±vertical circula- tion induced by the upper-level momentum forcing has 1) THE ROLE OF EDDY FLUX CONVERGENCE (EFC) a direct and substantial effect on the core region of a While the interaction between the internal (core) dy- tropical cyclone. They also proposed that the coopera- namics and tropical cyclone intensity is more dif®cult tive interaction between a tropical cyclone and a passing to understand, both observational and numerical studies trough can enhance the out¯ow jet of a tropical cyclone. have paid much attention to the external atmospheric The enhanced out¯ow jet can invigorate core region in¯uences (especially the upper-tropospheric interac- convection and initiate the deepening process of the tion) on tropical cyclone intensity. Pfeffer and Challa cyclone. (1981) used the observations from the Atlantic devel- The asymmetric structures of the out¯ow layer re¯ect oping and nondeveloping tropical disturbances com- the in¯uences from the upper-level environmental sys- posited by McBride (1981a,b) and McBride and Zehr tems, and these structures can produce the large eddy (1981) to examine the importance of large-scale eddy imports of angular momentum. The vertical derivative ¯uxes of momentum in the development of the storms of the EFC of angular momentum acts as a forcing func- in their model. Pfeffer and Challa (1981) concluded that tion to increase in¯ow of moist air into the boundary a suf®ciently intense and properly organized EFC of layer and to pump drier air out into the out¯ow layer momentum is an essential ingredient for the develop- (Challa and Pfeffer 1980). Challa and Pfeffer (1980) ment of Atlantic tropical disturbances into hurricanes. used Sundqvist's (1970) axisymmetric model to simu- The relationship between EFC of angular momentum late the responses of a vortex to different pro®les of and tropical cyclone intensity for the named tropical momentum forcing and found that numerical simula- cyclones during the 1989±91 Atlantic hurricane seasons tions with the environmental forcing could produce was investigated by DeMaria et al. (1993). They showed more rapid intensi®cation and more intense tropical that about one-third of the storms with enhanced EFC storms.
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