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Mesoscale Interactions in Tropical Cyclone Genesis

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Mesoscale Interactions in Tropical Cyclone Genesis OCTOBER 1997 SIMPSON ET AL. 2643 Mesoscale Interactions in Tropical Cyclone Genesis J. SIMPSON Earth Sciences Directorate, NASA/Goddard Space Flight Center, Greenbelt, Maryland E. RITCHIE Department of Meteorology, Naval Postgraduate School, Monterey, California G. J. HOLLAND Bureau of Meteorology Research Centre, Melbourne, Australia J. HALVERSON* Visiting Fellow, University Space Research Association, NASA/Goddard Space Flight Center, Greenbelt, Maryland S. STEWART NEXRAD OSF/OTB, National Weather Service, NOAA, Norman, Oklahoma (Manuscript received 21 November 1996, in ®nal form 13 February 1997) ABSTRACT With the multitude of cloud clusters over tropical oceans, it has been perplexing that so few develop into tropical cyclones. The authors postulate that a major obstacle has been the complexity of scale interactions, particularly those on the mesoscale, which have only recently been observable. While there are well-known climatological requirements, these are by no means suf®cient. A major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development. Observations exist for only a few forming cases. In these, the moist convection in the preformation environment is organized into mesoscale convective systems, each of which have associated mesoscale potential vortices in the midlevels. Interactions between these systems may lead to merger, growth to the surface, and development of both the nascent eye and inner rainbands of a tropical cyclone. The process is essentially stochastic, but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental in¯uences. For example a monsoon trough provides a region of reduced deformation radius, which substantially improves the ef®ciency of mesoscale vortex interactions and the amplitude of the merged vortices. Further, a strong monsoon trough provides a vertical wind shear that enables long-lived midlevel mesoscale vortices that are able to maintain, or even redevelop, the associated convective system. The authors develop this hypothesis by use of a detailed case study of the formation of Tropical Cyclone Oliver observed during TOGA COARE (1993). In this case, two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband. The eye developed on the edge of the major convective system, and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere, and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere. The importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western North Paci®c. 1. Introduction answers to several key questions about the behavior of tropical cyclones. In particular, he hypothesized that More than a decade ago, a prescient paper by Ooyama their genesis and tracks could not be treated as a de- (1982) discussed the futility of attempting deterministic terministic or initial value problem. A statistical or chaos approach was needed because of the nonlinear inter- action of widely differing scales. * Current af®liation: Science Systems and Applications, Inc., Even with perfect data, the mesoscale interactions NASA/GSFC, Lanham, Maryland. involved cannot necessarily be assumed to be deter- ministic. Ooyama, together with Schubert et al. (1980) Corresponding author address: Dr. Joanne Simpson, Earth Sci- and Schubert and Hack (1982), also noted that for gen- ences Directorate, Code 912, NASA/GSFC, Greenbelt, MD 20771. esis to occur the heat released by moist convective pro- E-mail: [email protected] cesses needed to be con®ned to a warm core on the q1997 American Meteorological Society Unauthenticated | Downloaded 10/01/21 07:31 PM UTC 2644 MONTHLY WEATHER REVIEW VOLUME 125 storm scale rather than diffused away by gravity waves. stratiform cloud region of the MCS. Warming from la- This required a substantial reduction of the Rossby ra- tent heating in the stratiform cloud combined with cool- dius of deformation or ``stiffening'' of the tropical at- ing by evaporation of rain below cloud base sharpens mosphere. the potential temperature gradient near cloud base. Since It also has been known for several decades that in- the stratiform cloud region is nearly neutral to moist tense tropical cyclones cannot develop without being processes, the local deformation radius approaches zero able to tap the ocean for an energy requirement that is and gravity waves cannot be sustained. Thus stretching vastly greater than that available in the ambient atmo- and tilting from mesoscale convergence near the cloud sphere (Riehl 1954; Malkus and Riehl 1960; Emanuel base, with condensational warming above and evapor- 1986, 1991; Holland 1997). The ambient atmosphere ational cooling below (Houze 1977), produces a poten- can only support a 10±30-hPa drop in surface pressure. tial vorticity anomaly with relative circulation on the Additional deepening requires enhanced surface energy scale of the stratiform cloud region. In suitable vertical ¯uxes that are driven by lowered surface pressure and wind shear conditions, lifting along the isentropes bulg- increased winds. ing upward over the lower cold anomaly can trigger Until recently very little data have been available new convection and stratiform anvil development. This describing the initial development stages of tropical cy- cycle can reinforce the preexisting vortex or lead to new clones. Documentation has been largely limited to syn- optic and larger-scale features of the storm and its en- developments in the vicinity. vironment (McBride 1996). The importance of asym- The scale-interaction discussion by Holland (1995) is metries has been demonstrated in numerical modeling extended here to the mesoscale, leading to a hypothesis studies (Kurihara et al. 1993), but these models have for the importance of mesoscale vortex interaction in only recently had the resolution to explore the mesoscale tropical cyclone genesis. The analysis is con®ned to system interactions associated with cyclone develop- monsoon environments, where about 85% of the world's ment. tropical cyclones develop, but parts of it may be ap- In the late 1980s, the availability of mesoscale in- plicable also to trade-wind developments. Mesoscale formation, particularly from satellites, and the increased convective systems are shown to spawn mesoscale vor- application of probability and scale interaction concepts tices, whose interaction with each other and the mon- led to the recognition of the importance of mesoscale soon environment is crucial to the initial development systems in tropical cyclones. A series of studies by Hol- of the nascent eye and inner rainband. Evidence is land and colleagues (Holland 1995; Holland and Lander shown that interactions between mesoscale vortices (R 1993; Ritchie and Holland 1993; Holland and Dietach- ; 100 km) and with the monsoon environment are an mayer 1993; Lander and Holland 1993; Wang and Hol- essential component in the development of an eye and land 1995) showed that seemingly erratic tracks of trop- inner rainbands. It is useful to divide tropical cyclone ical cyclones were in¯uenced by interactions with other genesis into three stages. cyclones or mesoscale systems in their vicinity. Partic- ularly important was the ®nding that while there is a 1) Establishment of suitable climatological conditions: signi®cant stochastic component of the vortex interac- These necessary, but not suf®cient, requirements tions, many seemingly erratic developments can be both have been summarized by Gray (1968). They are understood and forecast when couched in vortex inter- met over the monsoonal genesis regions much of the action terms. The application of these ideas has helped time and are not considered further here. forecasters identify and improve forecasts of ``badly be- 2) Preformation: During this period, which may last haved'' cyclones (Carr and Elsberry 1994). several days, the local environment becomes more The rapid improvement of radar and satellite data, favorable for cyclone development, as described by and the mounting of several special ®eld experiments Holland (1995). The development and interaction of in the 1990s, has provided a number of well-documented mesoscale convective systems predominates during examples of the presence of mesoscale systems in trop- ical cyclone genesis (e.g., Stewart and Lyons 1996; Rit- this stage. chie and Holland 1997; Harr and Elsberry 1996). 3) Initial development: Formation of a nascent eye and A cloud cluster is a well-known, but indeterminate, an inner rainband as a result of the mesoscale in- precursor to tropical cyclogenesis (e.g., McBride and teractions initiated during stage 2. Zehr 1981). Such clusters typically contain several MCSs (see the appendix for a list of acronyms used in The analysis approach and datasets used in this study this paper) of 4±10 3 104 km2 or more in area (Miller are described in section 2. Section 3 proceeds through and Fritsch 1991), which are proli®c breeders of mid- a detailed study of the formation and initial development level vortices, as has been elucidated in a number of of Tropical Cyclone Oliver in the western South Paci®c. recent studies (Menard and Fritsch 1989; Raymond and The ®ndings in section 3 are con®rmed by brief case Jiang 1990; Chen and Frank 1993; Fritsch et al. 1994). studies of Supertyphoon Ed and Typhoon Irving in sec- The vortices form in midlevels near the base of the tion 4. Conclusions are presented in section 5. Unauthenticated | Downloaded 10/01/21 07:31 PM UTC OCTOBER 1997 SIMPSON ET AL. 2645 FIG. 1. Flight tracks of the NASA DC-8 and ER-2 aircraft superimposed on NOAA-12 IR imagery for 2100 UTC 4 February 1993. Also shown are the locations and times (UTC) of the DC-8 aircraft way points. 2. Analysis approach and data ER-2 aircraft are shown in Fig. 1 superimposed on the NOAA-12 IR image made at 2100 UTC (LST ; UTC A range of observational tools provided the basic 1 10 h) on 4 February.
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