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JULY 1997 BRIEGEL AND FRANK 1397 Large-Scale In¯uences on Tropical Cyclogenesis in the Western North Paci®c LISA M. BRIEGEL AND WILLIAM M. FRANK Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania (Manuscript received 12 April 1996, in ®nal form 13 September 1996) ABSTRACT Objectively analyzed data from the European Centre for Medium-Range Weather Forecasts are used to examine the large-scale aspects of the formation of tropical cyclones. It is hypothesized that tropical cyclogenesis occurs when external atmospheric forcing on the synoptic or larger scale provides uplift through a deep layer, enhancing convection, in a region with environmental conditions favorable for genesis. Emphasis is placed on the roles of upper-level troughs, low-level wind surges, preexisting tropical cyclones, and propagating wave disturbances in triggering tropical cyclogenesis. Composites of the 200-hPa and 850-hPa ¯ows reveal the presence of both upper-level troughs and low-level wind surges, respectively, prior to genesis. In the composites, the wind surges also appear to be related to the presence of a prior circulation located approximately 2000 km to the west of the genesis location. An examination of the individual cases demonstrates that approximately 85% of all storms had either an upper-level trough or an identi®able low-level feature, while 49% had both an upper- and a lower- level feature. Given the limitations of the objective analyses over the tropical oceans, this provides strong evidence for the role of large-scale external forcing in triggering tropical cyclogenesis. 1. Introduction Section 2 presents background discussion of the na- ture of tropical cyclone genesis and reviews previous Due to the sparse observational network over the trop- research in the area. The dataset and methodology are ical oceans, progress toward understanding the physical described in section 3, results are presented in section processes involved in tropical cyclone formation has 4, and section 5 contains the conclusions. been dif®cult. Several theories have been presented to explain the way in which a loosely organized area of convection ®rst becomes a tropical depression and then 2. Background intensi®es into a tropical storm or typhoon. However, the historical observational database generally has not a. Tropical cyclogenesis permitted detailed analyses of the large-scale ¯ow char- In the ®rst global climatology of tropical cycloge- acteristics associated with the formation of individual nesis, Gray (1968) established several environmental storms. A few cases of tropical cyclogenesis have been conditions as favorable for the formation of tropical sampled by aircraft, but most of those were observed cyclones: a warm ocean layer of suf®cient depth, con- intermittently, and it is dif®cult to resolve the evolution ditional instability through a deep atmospheric layer, of the large-scale environment of the storm using mea- higher than normal midtropospheric relative humidity, surements from a single platform. above-normal low-level vorticity, weak vertical wind With the introduction of global objective analyses that shear over the center of circulation, and location suf- combine model output with observations, researchers ®ciently far from the equator (signi®cant value of the have been provided with an increased ability to explore Coriolis parameter f). The ®rst three parameters are con- the physical processes involved in the formation of in- sidered to be thermodynamic variables, while the last dividual tropical cyclones. This study uses objectively three are dynamic. While the necessary thermodynamic analyzed data from the European Centre for Medium- and Coriolis conditions exist over a considerable portion Range Weather Forecasts (ECMWF) to examine the of the tropical oceans for long periods of time, the low- large-scale aspects of the development of tropical cy- level vorticity and vertical shear parameters can change clones. signi®cantly on much smaller timescales and space scales (Gray 1975; McBride 1981). Thus, it has been hypothesized that tropical cyclogenesis occurs when above-normal low-level vorticity and locally weak ver- Corresponding author address: Ms. Lisa M. Briegel, Department of Meteorology, The Pennsylvania State University, 503 Walker tical wind shear occur within a thermodynamically fa- Building, University Park, PA 16802-5013. vorable environment (Gray 1975). More recent studies E-mail: [email protected] have indicated that cyclogenesis also tends to occur with q1997 American Meteorological Society Unauthenticated | Downloaded 09/27/21 12:21 PM UTC 1398 MONTHLY WEATHER REVIEW VOLUME 125 a period of enhanced large-scale uplifting with concur- moving over land or cold water). However, a storm of rent deep convection (Zehr 1992). tropical-depression strength also tends to be a relatively Frank (1988) proposed that tropical cyclone devel- stable system, and it appears to be capable of intensi- opment incorporates two distinct phases, genesis and fying without obvious external forcing, albeit very slow- intensi®cation, that are dominated by different dynam- ly. We therefore choose to de®ne tropical cyclogenesis ical and physical processes. The ®rst consists of the as the sequence of events leading to the formation of a formation of a mesoscale vortex within a loosely or- tropical depression. While there may be other suitable ganized tropical cloud cluster. This is a relatively rapid de®nitions, this one seems to ®t observations adequate- process in which external forcing is thought to be re- ly, and it has the advantage that it is relatively well quired to produce convection by perturbing the dynamic de®ned in the historical data records (e.g., best track variables over a relatively large area suf®ciently above data). It should be noted, however, that the time of de- their climatological values. Within the stratiform cloud clared tropical depression and the time at which a vortex deck associated with this deep convection, a mesoscale becomes self-sustaining may differ by as much as 12±24 vortex then forms. If the vortex reaches a threshold h. Unfortunately, there is no way currently to identify intensity, it becomes capable of intensifying into a ma- the exact moment of genesis, but the time of tropical ture tropical cyclone due to its own interactions with depression is quite adequate for the purposes of this the ocean without further external forcing. External research. We view tropical cyclogenesis as an impulsive, forcing may facilitate the process, however. We refer to multistage process that requires external forcing on this latter stage as intensi®cation, and we de®ne tropical scales signi®cantly larger than the convective distur- cyclogenesis as the process of developing a vortex ca- bance. The goal of this paper is to document the cli- pable of self-intensi®cation. matology and nature of the larger-scale circulations as- During tropical cyclogenesis, the vortex formation sociated with the formation of tropical cyclones. has been observed to occur in conjunction with the for- mation of a mesoscale convective system (MCS) within b. Large-scale forcing of genesis a preexisting tropical disturbance (Zehr 1992; Ritchie 1995). The initial vortex forms within the stratiform rain A review of the literature on tropical cyclogenesis region of the MCS, where the radius of deformation is reveals a number of different proposals for atmospheric reduced from ambient values (Chen and Frank 1993; mechanisms that might provide the external forcing Emanuel et al. 1993; Velasco and Fritsch 1987; Ray- (e.g., enhanced large-scale upward motion) necessary mond and Jiang 1990). In order for a large convective to cause an MCS, and thus an MCV, to form. Some area (on the order of a few hundred kilometers) such as researchers have proposed that the environmental forc- an MCS to develop and be maintained, low-level con- ing may occur in the lower levels. Lee et al. (1989) and vergence and deep uplifting on the synoptic or larger Zehr (1992) argued that inward-propagating regions of scales must occur, presumably due to external forcing enhanced low-level in¯ow, which they referred to as (Gray 1988). wind surges, might trigger tropical cyclogenesis. Zehr The above scenario is in agreement with a recent (1992) observed that most of the wind surges associated observational study by Zehr (1992), who examined 50 with cases of tropical cyclogenesis in the western North individual tropical storms and typhoons that occurred Paci®c had a westerly component. He hypothesized that in the western North Paci®c during 1983±84. He pro- many tropical disturbances may experience an increase posed a conceptual model of tropical cyclogenesis that in westerly winds when they ®rst encounter the far east- consists of two distinct stages: the initial development ern end of the monsoon trough in their westward move- of a mesoscale convective vortex (MCV) within a trop- ment and thus undergo genesis. ical MCS, followed by the transformation of this vortex Upper-level circulations have also been examined for into a tropical cyclone with a subsequent lowering of possible atmospheric processes that might trigger trop- the central surface pressure. He found that each stage ical cyclone formation. McBride and Keenan (1982) was preceded by a signi®cant increase in convection documented that 45% of their cases of tropical cyclo- concurrent with deep uplifting. genesis in the Australian region occurred when upper- It is dif®cult to determine exactly when a disturbance tropospheric troughs were located within 58 and 80% has crossed the boundary between genesis and inten- within 158 latitude poleward of a disturbance. Sadler si®cation. When a tropical disturbance develops a ro- (1976, 1978) also found that tropical cyclogenesis in tation center at the surface, forecasters usually classify the western North Paci®c occurred more readily when it as a tropical depression. Once the vortex achieves an upper-level trough was poleward and westward of a maximum winds of 17 m s21 or greater, it is then clas- disturbance.

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