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Observational Evidence for Alternate Modes of Track-Altering Binary Tropical Cyclone Scenarios

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Observational Evidence for Alternate Modes of Track-Altering Binary Tropical Cyclone Scenarios 2094 MONTHLY WEATHER REVIEW VOLUME 125 Observational Evidence for Alternate Modes of Track-Altering Binary Tropical Cyclone Scenarios LESTER E. CARR III, MARK A. BOOTHE, AND RUSSELL L. ELSBERRY Department of Meteorology, Naval Postgraduate School, Monterey, California (Manuscript received 11 July 1996, in ®nal form 13 January 1997) ABSTRACT An observational study of western North Paci®c tropical cyclones (TC) revealed many cases of two TCs whose tracks were altered by processes that were quite different from the mutual advection (Fujiwhara-type) processes. Thus, four conceptual models are proposed to describe these track alterations. A conceptual model called direct interaction is proposed that is a modi®cation of one by Lander and Holland and has three modes: 1) a one-way in¯uence in which the track of a smaller TC that is embedded in the circulation of a larger TC has a cyclonic orbiting motion, but no signi®cant track alteration of the larger TC is apparent; 2) a similar case in which a mutual advection occurs with the tracks of both the smaller and larger TCs being altered; and 3) a subset of 2) in which the mutual advection includes an attraction component such that the two similarly sized TC circulations eventually merge into a larger circulation with a single center. During the 7-yr period (1989± 95), the one-way in¯uence, mutual interaction, and merger modes were detected seven, three, and two times. A semidirect interaction conceptual model is proposed in which the two TCs have a relative cyclonic rotation as in the Lander and Holland model, but the TCs are separated by 108±208 longitude so that a direct (advective- type) interaction is excluded. Rather, the track alteration is attributed to an environmental ¯ow established by the juxtaposition of a TC on one side and a subtropical anticyclone cell on the opposite side. In an east±west orientation of the two TCs and a subtropical anticyclone cell to the east (west), the height gradient between the western (eastern) TC and the eastern (western) subtropical anticyclone establishes a poleward (equatorward) environmental steering ¯ow across the eastern (western) TC. In the 1989±95 sample, a semidirect interaction that altered the tracks of the eastern or the western TC occurred 18 and 14 times, respectively. An indirect interaction conceptual model is proposed in which the distinguishing feature is the Rossby wave dispersion-induced anticyclone to the east and equatorward of the western TC. This anticyclone imposes an equatorward (poleward) steering ¯ow across the eastern (western) TC. Several variations of the indirect interaction are possible depending on the separation distance, sizes of each TC, and their relative orientation. During the 7-yr period, an indirect interaction affecting the western TC or the eastern TC occurred 36 and 22 times, respectively. A fourth conceptual model of track alterations involving two TCs is proposed in conjunction with a reverse- oriented monsoon trough formation. The distinguishing feature of this conceptual model is the combination of the peripheral anticyclones of both TCs as the eastern TC moves into an east±west orientation and has a separation of 108±208 longitude. In the 1989±95 sample, a reverse-oriented trough formation involving two TCs occurred seven times. The frequency of track alterations whenever two TCs are present emphasizes that forecasters must give special attention to such situations. The four conceptual models proposed here emphasize that the physical mechanisms are complex and in the vast majority of cases cannot be attributed to the mutual advection (Fujiwhara-type) process implied in the Lander and Holland model. 1. Introduction cyclone interaction, for which one of the de®ning char- Anomalous tropical cyclone (TC) tracks such as non- acteristics is that the two TCs appear to rotate cyclon- climatological equatorward de¯ections, rapid poleward ically relative to an intermediate midpoint. Based on movement, and apparent rotation are often observed studies dating back to Fujiwhara (1923), the key factors when two tropical cyclones are present in a region. Such in the direct binary interaction are the separation dis- track anomalies have often been attributed by forecast- tance and the outer wind structure of each cyclone, ers to what will be referred to here as direct binary which determine the cyclonic rotation rate and the ten- dency for attraction or repulsion (Elsberry 1995). More speci®cally, two cyclones with a suitably small sepa- ration distance are presumed to be mutually advected Corresponding author address: Dr. Lester E. Carr, Department of by their cyclonic circulations. Meteorology, Naval Postgraduate School, Code MR/Cr, 589 Dyer Road, Room 254, Monterey, CA 93943-5114. Observed tracks in the western North Paci®c region, E-mail: [email protected] where multiple TCs are common (Neumann 1993), are Unauthenticated | Downloaded 09/27/21 04:45 AM UTC SEPTEMBER 1997 CARR ET AL. 2095 frequently not compatible with a direct binary interaction explanation. Many of the anomalous track de¯ections oc- cur at separation distances that are too large to be a mutual advection, especially when one or both of the TC circu- lations are small. Dong and Neumann (1983) attributed some of the anomalous rotation rates or separation distance tendencies to environmental horizontal wind shear in the monsoon trough, although similar shear effects might be expected near the subtropical ridge. Based on a 7-yr compilation of western North Paci®c TCs, it is proposed that many of these anomalous track de¯ections may be explained by two other modes of binary TC interaction that are fundamentally different from the direct type. Two conceptual models that will be termed semidirect and indirect binary cyclone inter- action will be introduced and described with case stud- ies. Another mode of sometimes almost simultaneous FIG. 1. Conceptual model of binary cyclone interaction by Lander track alterations of two suitably separated TCs is a re- and Holland (1993) indicating the approach and capture, mutual orbit, verse-oriented monsoon trough formation (Carr and Els- the release and escape, and the merger stages. berry 1994; Lander 1996). This mode is not necessarily an actual TC interaction, because it just may be that an east±west juxtaposition of the two TCs happens to be tween the two vortices is indeed the centroid). They favorable for setting up the circulation that leads to near- emphasize that the track changes associated with the ly simultaneous poleward track changes. ``capture'' and ``release'' events typically occur sud- For the semidirect and indirect interactions and the denly prior to and subsequent to the period of mutual reverse trough formation (RTF), the track-altering in- cyclonic rotation (orbit). Thus, the successful forecast ¯uence is in conjunction with an adjacent anticyclone. of the tracks during and following the binary cyclone The key factor in the semidirect type is an advection interaction period will critically depend on the predic- established by a pressure gradient between one of the tion of the times of capture and release. Examples of TCs and a favorably aligned cell in the subtropical an- actual merger of the two cyclones, as would be expected ticyclone. A preliminary interpretation of the indirect from the attraction component of the Fujiwhara effect, type is offered based on the Carr and Elsberry (1995) is found by Lander and Holland (1993) to be rare. Land- numerical simulations of monsoon gyres and large TCs er (1995) has documented such a merger case in which that generate a trailing anticyclone via Rossby wave separate cloud patterns can be identi®ed from the ge- dispersion. The presence and strength of such an anti- ostationary satellite imagery as the two centers complete cyclone between the two TCs is hypothesized to be the more than one cyclonic orbit. critical factor in the indirect interaction. In the RTF, the A more typical scenario is that a smaller cyclone is two TCs are part of a trough oriented southwest to north- swept into the horizontally and vertically sheared ¯ow east, and their trailing anticyclones combine to form a of the larger cyclone. This mode of direct interaction is nearly parallel anticyclonic circulation on the equator- more appropriately considered as a one-way in¯uence ward side that together establish a more poleward en- (Fig. 2a) because the motion of the larger TC is little vironmental steering ¯ow across one or both TCs. affected by the presence of the smaller TC. In terms of The extensive literature on the direct type will be the barotropic vortex stabilization mechanism of Carr brie¯y reviewed in section 2 to provide a contrast with and Williams (1989), the smaller vortex circulation is the semidirect and indirect types to be introduced in equivalent to a high azimuthal wavenumber (short sections 3 and 4, respectively. Following the description wavelength) perturbation on the larger vortex circula- of the RTF in section 5, the frequency and some char- tion. The horizontal shear of the larger vortex distorts acteristics of each of these track-altering situations based the smaller circulation, and elongation along the stream- on the 7-yr compilation will be given in section 6. lines must be accompanied by contraction in the cross-stream direction to conserve absolute vorticity in a barotropic sense (Smith et al. 1990). In nature, the 2. Direct binary interaction additional effect of vertical wind shear of the larger Lander and Holland (1993) have documented anom- (stronger) vortex may also contribute to the dissipation alous tracks associated with binary tropical cyclones. of the smaller vortex if it remains under the in¯uence They propose a conceptual model of direct interaction of the larger vortex for very long. (Fig. 1) with cyclonic rotation of the two vortices rel- A related mode of direct interaction called mutual in- ative to the centroid (generally assuming the two cir- teraction (Fig.
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