simulated impacts of a mesoscale convective system on the track

2232 VOLUME 128M O N T H L Y W E A T H E R R E V I E W

Simulated Impacts of a Mesoscale Convective System on the Track ofTyphoon Robyn during TCM-93

ELIZABETH A. RITCHIE AND RUSSELL L. ELSBERRY

Department of Meteorology, Naval Postgraduate School, Monterey, California

(Manuscript received 8 February 1999, in final form 14 September 1999)

ABSTRACT

The contribution that a mesoscale convective vortex that developed within the circulation of Typhoon Robyn(1993) may have had to a significant tropical cyclone track change is simulated with a mesoscale model usinginitial conditions that approximate the circulations measured by aircraft during the Tropical Cyclone Motion(TCM-93) field experiment. A dry version of the PSU–NCAR Mesoscale Model is used to first investigate thedynamic aspects of the interaction. A deceleration of about 2 m s21 and then more northward movement, similarto that observed for Typhoon Robyn, could have been produced by an interaction with the mesoscale convectivevortex of the type modeled in the control run. Sensitivity of the simulated track change is tested for variousaspects of the mesoscale vortex and tropical cyclone. It is found that track deflections between 67 and 130 kmin 18–24 h could be produced under a variety of realistic scenarios.

As the mesoscale vortex is advected around the tropical cyclone by the cyclonic winds, it is also beingfilamented by the horizontal shear of the tropical cyclone outer winds. Whereas the rate at which the vortex isadvected about the tropical cyclone is critical to the amount of curvature of the tropical cyclone track deflection,the timescale of the filamentation of the mesoscale vortex is critical to the longevity of the track deflection, andthus maintenance of the vortex is a crucial factor. For the same tropical cyclone and separation distance, thetrack deflections are greater for a larger, deeper, and more intense mesoscale vortex. For the same mesoscalevortex, a variety of track deflections is possible, depending on the outer wind structure of the tropical cyclonedue to both the advective effect and to the change in the gradient of vorticity. For a large tropical cyclone,positive vorticity extends farther from the core region, and the curvature of the track deflection is greater thanfor a smaller tropical cyclone where the vorticity becomes anticyclonic at smaller radii. Although a large initialeffect on the tropical cyclone path occurs for small separation distances, the mesoscale vortex is rapidly filamentedso that effects on the tropical cyclone track are negligible by 12 h. For larger separation distances, the mesoscalevortex does not filament as rapidly, and a smaller but longer lasting track deflection is simulated.

When the control simulation is extended to a b plane, it is found that the primary contribution to the tropicalcyclone track change is still due to the interaction with the mesoscale convective vortex. However, a secondaryeffect due to a nonlinear interaction between the b gyres and the mesoscale convective vortex adds a smallcomponent of propagation to the tropical cyclone that becomes significant after about 15 h of simulation.

1. Introduction

It has been recently established that mesoscale con-vective systems in the Tropics develop midlevel vorticessimilar to those that develop in the midlatitudes (e.g.,McKinley 1992; Harr et al. 1996; Simpson et al. 1997;Ritchie and Holland 1997). Simpson et al. (1997) in-vestigated the interaction between two such mesoscaleconvective systems during the development of TropicalCyclone Oliver in the Australian region. Data collectedwithin the mesoscale convective systems using two air-borne platforms clearly identified and mapped a vortexstructure associated with each system. In addition, both

Corresponding author address: Elizabeth Ritchie, Code MR/Ri,Department of Meteorology, 589 Dyer Rd., Room 254, Monterey,CA 93943-5114.E-mail: [email protected]

vortices were tracked continuously using GeostationaryMeteorological Satellite (GMS) and C-band radar im-agery. A centroid-relative diagram of the tracks indi-cated a cyclonic rotation and approach, which was sug-gestive of a vortex interaction and merger. Simpson etal. (1997) proposed that the ‘‘merger’’ of the weakervortex into the stronger vortex contributed to the sub-sequent development of Tropical Cyclone Oliver.

The possibility that significant track changes or me-dium-scale meanders on the order of tens to hundredsof kilometers over several days may result from an in-teraction between a tropical cyclone and a persistentmesoscale convective system has been hypothesized byHolland and Lander (1993). They proposed that an in-teraction between a tropical cyclone and a (hypothe-sized) midlevel vortex associated with a mesoscale con-vective system in close proximity may have been re-sponsible for some of the observed track changes. They

JULY 2000 2233R I T C H I E A N D E L S B E R R Y

showed an example of two mesoscale convective sys-tems that developed outside the core region of TropicalStorm Sarah in 1989 that may have contributed to theobserved unusual track of Sarah. Using a barotropicmodel, they simulated two separate vortex–vortex in-teractions between the tropical cyclone and a mesoscalevortex assumed to be associated with the mesoscale con-vective system, and the track was successfully repro-duced.1 If this hypothesis is valid, then mesoscale con-vective systems can be identified in satellite imagery,and short-term track changes may be more reliably fore-cast.

The interaction and merger of vortices as it relates totropical cyclone motion change has been studied bothobservationally (e.g., Dong and Neumann 1983; Landerand Holland 1993) and numerically (e.g., Smith et al.1990; Ritchie and Holland 1993; Holland and Dietach-mayer 1993; Wang and Holland 1994). The majority ofobservational studies emphasize interaction betweentwo tropical cyclones that are approximately equal inmass. Less than half of these systems merge duringinteraction (Lander and Holland 1993). Indeed, Carr etal. (1997) found the merger of two tropical cyclonesrarely occurred (less than one event per two years) inthe western North Pacific, which has the largest fre-quency of simultaneous multiple cyclones. It is morecommon for the two tropical cyclones to undergo a mu-tual rotation that terminates rapidly, generally due toexternal synoptic influences. Pronounced track deflec-tions of the order of one hundred to a few hundredkilometers are common during the orbit phase.

Numerical simulations have been used to explore arange of unequal vortex–vortex interactions and theirimpact on motion that may have more relevance to thetrack deflection hypothesis proposed by Holland andLander (1993). For example, Smith et al. (1990) useda barotropic model to show that an interaction betweenone vortex five times the intensity of the other produceda rapid rotation and dispersion of the weaker vortex,with only a short-term change in the track of the strongervortex. Ritchie and Holland (1993) used a two-dimen-sional, contour dynamics model (Zabusky et al. 1979)to study a range of motion changes that might be ex-pected when two vortices of different sizes and inten-sities, such as a tropical cyclone and a vortex generatedby a mesoscale convective system, interact (see theirFig. 7). The large vortex had a radius of maximum windsthat was three times that of the small vortex and theratio of the intensities of the two systems (IL/IS) wasvaried over a range from three (i.e., large vortex inten-sity three times that of the small vortex) to one-third(i.e., large vortex intensity one-third of the small vor-

1 However, Carr and Elsberry (1995) attribute the cyclonic relativerotation of Sarah and the first mesoscale convective system, and theunusual track motion of Sarah, to the monsoon gyre in which theywere embedded.

tex). They found that the center of the large vortexundergoes small-scale oscillations during the interactionthat increase in amplitude and period as the ratio of theintensity decreases. When the small vortex is weakerthan the large vortex, it is rapidly sheared into a filamentthat wraps around the large vortex, similar to the Smithet al. (1990) simulation, except that the track of thelarge vortex continues to oscillate until the end of thesimulation.

These results suggest that several scales of tropicalcyclone meanders may be induced owing to interactionbetween vortices of different sizes and intensities. Vary-ing the initial separation distance between the two vor-tices also affects the induced track meander. As the sep-aration distance increases, both the advective compo-nent and the horizontal shear imposed over each vortexby the adjacent vortex’s outer circulation decrease,which results in slower motion, and less distortion ofeach vortex, and the period of the imposed track me-ander increases. This indicates that a mesoscale con-vective system that forms in the outer circulation of atropical cyclone may exist longer (i.e., not be stretchedinto a filament so quickly because of the weaker hori-zontal shear in that region) and affect the tropical cy-clone track for a longer period, but with a smaller trackdeflection.

Although these numerical simulations give insightinto the range of possible vortex interactions and theireffects on motion, considerable limitations prevent di-rect application to atmospheric vortices. For example,the simulations have usually had the same tangentialwind profile for both vortices, but this is unlikely to betrue for a tropical cyclone and a midlevel vortex as-sociated with a mesoscale convective system. In fact,tropical cyclones in the western North Pacific occur ina large variety of sizes, from midgets with a rapid de-crease in tangential wind with radius to monsoon de-pressions with a circulation that extends beyond a thou-sand kilometers (Lander 1994). It is also likely that thevertical structures of the vortices will affect the resultinginteraction; that is, a midlevel vortex associated with amesoscale convective system may not interact stronglywith either the near-surface or upper-level circulation ofthe tropical cyclone. Some recent simulations that in-vestigate the first-order effects of convection on the de-velopment of tropical cyclones using vortices as proxieshave illustrated this differential effect (Montgomery andEnagonio 1998; Ritchie and Holland 1997). In addition,the depth and elevation of the mesoscale convectivevortex will determine the magnitude of vertical shearimposed by the tropical cyclone circulation and, thus,the amount of strain on the midlevel vortex structure.The vertical structure of the midlevel vortex will alsoimpose a vertical shear on the tropical cyclone structure,albeit in a limited horizontal and vertical area. It is notobvious how these imposed vertical shears will affectthe motion of the two vortices, or the extent to whichdisruptions of the vortex structure will persist. Convec-


tive heat release and momentum transport processes thatare not represented in the above two-dimensional modelsimulations are also likely to change the timescales onwhich the vortices, particularly that associated with themesoscale convective system, exist as coherent systems(e.g., Fritsch et al. 1994). Recent studies demonstratethat a vortex in which latent heat release and transferof convective momentum are present remains unaffectedby stronger environmental vertical wind shear for longerperiods than a vortex in a dry model (e.g., Flatau et al.1994; Wang and Holland 1996; Frank and Ritchie 1999).Thus, it seems likely that a vortex in which transfer ofconvective momentum is present will resist the hori-zontal and vertical shears imposed by the tropical cy-clone circulation and affect the track for a longer period.

To describe vortex–vortex interaction between a trop-ical cyclone and a midlevel vortex, a high-resolutiondataset is needed that adequately resolves the evolutionof the horizontal and vertical structure of both the trop-ical cyclone and the mesoscale vortex before, during,and after a possible interaction. To the authors’ knowl-edge, such a dataset has never been collected. However,the Tropical Cyclone Motion (TCM-93) field experi-ment in the western North Pacific during 1993 providedtwo high-resolution datasets during a period when amesoscale convective system developed in close prox-imity to the core of Typhoon Robyn (Fig. 1). A partic-ularly intriguing feature of this case is that during a 36-hperiod that included the life cycle of the mesoscale con-vective system, the westward-moving typhoon slowed,turned to the north, then accelerated northward (Fig. 2).Harr and Elsberry (1996) suggest two processes mayhave possibly influenced the track of Robyn at the timeof the deceleration and northward turn. First, an inter-action between the tropical cyclone and a potential vor-ticity anomaly associated with the mesoscale convectivesystem may have produced the observed track change,as has been proposed by Holland and Lander (1993) forTropical Storm Sarah. However, Harr and Elsberry(1996) could not validate the vortex–vortex interactionmechanism. Difficulties included a spatial resolution ofboth the in situ observations and the final reanalyzedgrids that did not resolve the wind structure of bothvortices clearly enough to test this hypothesis. Anotherobstacle was an uncertainty as to how to separate thesymmetric wind structure of both the storm and themesoscale convective vortex from the environmentalflow.

The second track alteration mechanism proposed byHarr and Elsberry (1996) was that the large-scale en-vironment in which Robyn was embedded may havechanged in a manner consistent with the observed mo-tion of Robyn. They concluded this was the more likelymechanism as the storm was moving from a trade east-erly environment into the eastern region of the monsoontrough. Radial averages of the environmental winds atvarious steering levels provide some support for thismechanism (Harr and Elsberry 1996). However, they

did conclude that changes in the environmental steeringvector calculated from the reanalysis data gave a betterindication of the future motion of Typhoon Robyn thanthe original analyses. These data included a better rep-resentation of both the environment and the mesoscaleconvective vortex through the special TCM-93 dataset.In addition, it is notable that the data used to calculatethe environmental steering vector include the mesoscaleconvective vortex. This suggests that an interaction be-tween Typhoon Robyn and the mesoscale convectivesystem may have added to the environmental flowchanges to produce the observed track deflection of Ty-phoon Robyn.

The objective of this paper is to investigate morethoroughly conditions in which the hypothesized inter-action between Typhoon Robyn and this impressive me-soscale convective system (Fig. 1b) may contribute sig-nificantly to a tropical cyclone track change. The ap-proach here is to use the information provided by thespecial TCM-93 dataset and other conventional datasources to produce balanced vortices that realisticallyrepresent both Typhoon Robyn and the mesoscale con-vective system. A dry version of the Pennsylvania StateUniversity–National Center for Atmospheric Research(PSU–NCAR) Mesoscale Model is then initialized withthese idealized fields and integrated to simulate a varietyof interactions between a tropical cyclone and a me-soscale convective system and the resulting effects onthe track of the tropical cyclone. The range of tropicalcyclone–mesoscale convective system configurationswill be based on the dataset of the Typhoon Robyninteraction and observational studies of tropical andmidlatitude mesoscale convective systems (e.g., Menardand Fritsch 1989; Bartels and Maddox 1991; Fritsch etal. 1994; Simpson et al. 1997; Bartels et al. 1997).

An analysis of the mesoscale convective system andTyphoon Robyn interaction is given in section 2. Char-acteristics of the model and experimental design aredescribed in section 3. The model results and their re-lation to the actual track change in Typhoon Robyn arepresented in section 4. A summary and conclusions areprovided in section 5.

2. The interaction between Typhoon Robyn andthe mesoscale convective system

Typhoon Robyn developed from a tropical distur-bance at a very low latitude (78N) in late July 1993 andwas upgraded to a tropical storm (17 m s21) at 0600UTC 2 August. Based on aircraft reconnaissance as-sociated with the TCM-93 field experiment, the stormwas upgraded to typhoon strength (33 m s21) at 0000UTC 4 August. During the development period (1–3August), Robyn propagated toward the west-northwestat a speed that varied between 5 and 7.5 m s21 (Fig. 2).The organization of convection improved throughoutthis period, and a discrete central dense overcast asso-ciated with the core of Robyn was evident in satellite


FIG. 1. Enhanced infrared satellite sequence of Typhoon Robyn. The black region in the center of the typhoon near 108N, 1428Edefines temperatures below 2808C, and the adjacent dark gray, gray, and then black regions represent temperature increases in 58Cintervals. A standard grayscale is used for temperatures between 2658 and 2308C, and temperatures warmer than 2308C are set toblack: (a) 0930 UTC 3 Aug 1993, (b) 1530 UTC 3 Aug 1993. The mesoscale convective system is located at 138N, 1418E, and thecenter of Robyn is at 108N, 142.58E: (c) 2030 UTC 3 Aug 1993. Regions 1 and 2 are the convective and stratiform regions of themesoscale convective system and region 3 is centered on Typhoon Robyn. Line A–B is the line that the cross section in Fig. 3 is takenalong: (d) 0930 UTC 4 Aug 1993 (adapted from Harr and Elsberry 1996).

imagery along with several convective bands in thenorthwest quadrant (Fig. 1a). Between 1200 and 1500UTC 3 August, the convection in the northwest quadrantbecame enhanced, forming a large mesoscale convectivesystem directly north of the central dense overcast as-sociated with Robyn (Fig. 1b). The mesoscale convec-tive system moved toward the west, and realigned froman east–west orientation to a north–south orientation(Fig. 1c) with convection continuing to redevelop at the

upstream (north) end of the mesoscale convective sys-tem. The first set of aircraft data was collected duringa 10-h period centered at the time of Fig. 1c. The seconddataset was collected around the time of Fig. 1d, ap-proximately 10 h after the end of the first dataset, whenthe cold cloud shield associated with the mesoscale con-vective system was decaying. Between 1200 UTC on 3August and 1200 UTC 4 August, the motion of Robynslowed to less than 2 m s21 and began to turn toward


FIG. 2. Track of Typhoon Robyn in 6-h intervals (symbols) between0000 UTC 1 Aug and 0000 UTC 7 Aug 1993. Direction and speedchanges are indicated, and the period of interest is circled.

FIG. 3. Vertical cross section of potential vorticity (1026 m2 s21 Kkg21, thick lines) and potential temperature (K, thin lines) along 1388Evalid for 2000 UTC 3 Aug 1993 (from Harr and Elsberry 1996).

the northwest (Fig. 2). The storm translation subse-quently increased to about 6 m s21.

The development of the mesoscale convective systemis discussed in some detail in Harr and Elsberry (1996).Of particular interest is the movement of the convectiveand stratiform regions of the mesoscale convective sys-tem and the implications for a possible interaction be-tween an associated mesoscale convective vortex andthe tropical cyclone. Based on the satellite imagery, themesoscale convective system is divided into two regions(Fig. 1c). The convective region, which is forced bysurface convergence, is centered near 148N, 1388E inFig. 1c and has the lowest cloud-top temperatures. Asecond region, which is immediately south of the con-vective region in Fig. 1c, is labeled the stratiform regionbased on dropwindsonde observations with near-satu-rated mid- to upper levels, and low-level drier regionscommonly associated with stratiform anvils (e.g., Zipser1977). In general, the stratiform region will developdownstream of the convective region depending on themid- to upper-level advective flow. Whereas Robyn re-mained nearly stationary during the period 1500 UTC3 August to 0100 UTC 4 August, the convective regionof the mesoscale convective system moved westward ata speed near 14 m s21 due to a combination of the strongeasterly flow to the north of Robyn and the Robyn cir-culation. However, the stratiform region of the meso-scale convective system was advected cyclonically bythe tropical cyclone circulation. By 2000 UTC 3 August,the stratiform region was located near 118N, 137.58E(Fig. 1c), whereas the convection was located at 148N,1388E after having moved directly west from its initiallocation (Fig. 1b).

Theoretical studies have demonstrated that the strat-iform anvil is crucial for the development of a mesoscaleconvective vortex (e.g., Raymond and Jiang 1990; Chenand Frank 1993). Chen and Frank (1993) hypothesizethat the Rossby radius of deformation reduces in scale

in the saturated conditions of the anvil region since themoist static stability is reduced. Vertical stretching be-tween the upward vertical motion in the stratiform cloudand mesoscale subsidence below the stratiform cloudinduces a localized rotational response by the wind field.

A mesoscale convective vortex did develop in asso-ciation with the stratiform region (labeled 2 in Fig. 1c)of the mesoscale convective system. A vertical crosssection of the analyzed fields shows a potential vorticityanomaly between 400 and 750 mb centered near 10.58N,1388E (Fig. 3), which would be expected if the meso-scale convective system had developed a vortex thatcontinued to move with the stratiform clouds.

The period from 2030 UTC August 3 to 0930 UTCAugust 4 was characterized by little movement of eitherthe convective or stratiform regions of the mesoscaleconvective system (Fig. 1d). Although the impressiveorganization that was observed in the initial stages ofthe system never redeveloped, the system did remainintact through this period. The potential vorticity anom-aly was still detectable in the reanalyzed fields in thestratiform region (Fig. 4). However, the anomaly wasboth weaker and more shallow, which indicates the hor-izontal and vertical shear associated with the strength-ening typhoon was probably causing the mesoscale con-vective vortex to disperse gradually.

The orbiting-type motion of the mesoscale convectivesystem during the period 1230–2030 UTC is similar tothat observed during vortex–vortex interactions (e.g.,Ritchie and Holland 1993). However, it is possible thata similar track would be followed by a passive particleembedded in the cyclonic flow. For a vortex–vortex in-teraction to have occurred, the tropical cyclone shouldalso be affected. Since Harr and Elsberry (1996) were


FIG. 4. As in Fig. 3 except along 1378E for 0900 UTC 4 Aug 1993(from Harr and Elsberry 1996).

not able to establish unambiguously whether the motionof Robyn (Fig. 2) was due to an interaction with themesoscale convective vortex, the modeling study willfocus on that issue. The simulations presented in section4 will be analyzed with these points in mind.

3. Methodology

a. Model description

The model used in the study is the fifth-generation,nonhydrostatic, three-dimensional PSU–NCAR Meso-scale Model version 5 (MM5) originally described byAnthes et al. (1987). It is configured such that the prim-itive equations may be solved on an f plane or a b planevalid at 108N, with Cartesian coordinates in the hori-zontal and a terrain-following s-coordinate in the ver-tical. The model has 20 layers from s 5 0 to 1, withthe vertical boundaries at 50 mb and the surface pres-sure, Psfc. Vertical velocity is defined at the interfacesof the model layers, and all other variables are carriedat the midpoint of the layers. The horizontal grid hasan Arakawa–Lamb B staggering of the momentum var-iables (u and y) with respect to all other variables.

The model domain is configured with two meshesconsisting of 121 3 121 grid points each. The coarsemesh has a uniform grid spacing of 45 km and the nestedfine mesh has a 15-km resolution. Only the fine-meshsimulations will be presented in the figures. The coarsegrid supplies boundary values to the fine mesh, whichin turn feeds information to the coarse grid over theentire fine mesh. Relaxation boundary conditions areused around the coarse domain such that the model-predicted variables near the boundary are nudged to-ward a large-scale analysis that is held constant at theinitial idealized values.

The ability to simulate accurately mesoscale convec-tive system formation and maintenance in numericalweather prediction models is in early stages of evalu-ation, and the details of moist process representationsare likely to lead to variations in the simulations. Asthe primary purpose of this study is to evaluate theunderlying vortex dynamics of the (hypothesized) in-teraction between the tropical cyclone and the mesoscaleconvective system, it is not necessary to represent theconvective processes that result in mesoscale convectivesystem development. Thus, these model simulations aredry. In addition, surface friction and fluxes of heat andmoisture are neglected to avoid an unrealistic spindownof the tropical cyclone when convective processes arenot included. Because momentum transport by convec-tive processes is not included, the mesoscale convectivesystem will be dispersed more rapidly in the model sim-ulations than may be observed in the atmosphere. Thus,the analysis of the simulations is restricted to the first24 h.

b. Experimental design

1) LARGE-SCALE CONDITIONS AND BALANCE

CONDITIONS

The balanced vortices in these simulations are con-figured on an f plane centered at 108N in an environmentat rest. The environmental temperature sounding is froma composite analysis of a western North Pacific prestormtropical depression by McBride and Zehr (1981). Theenvironment has a uniform surface pressure of 1008.7mb. The radial and vertical variations in the tangentialwind field of each vortex are analytically specified andthe mass field is then determined by solving for gradientwind and hydrostatic balance on a cylindrical grid. Theequations are solved at 1-km resolution and interpolatedto the 15- and 45-km resolution Cartesian grids. Forconvenience, the size of the tropical cyclone is definedas the radius of gale force (17 m s21) winds. The sizeof the mesoscale convective vortex is defined as theradius where the wind speed drops to less than 5 m s21.

2) TROPICAL CYCLONE VORTEX STRUCTURE

The horizontal structure of the tropical cyclone in thecontrol simulation is based on both the structure of Ty-phoon Robyn as determined from the 50-km analysis(Harr and Elsberry 1996) and on the original wind data(Fig. 5a). The horizontal profile follows the DeMaria(1987) expression with a maximum wind of 30 m s21

at a radius of 90 km, and the profile shape constant bis set such that the radius of gale-force winds is 340km. This profile reproduces the observed tangentialwinds in Robyn within reasonable limits (Fig. 5a). Thecorresponding vorticity profile becomes anticyclonic ata radius of approximately 350 km, with the gradient invorticity becoming cyclonic (i.e., increasingly cyclonicwith radius) at 530 km (Fig. 5b).


FIG. 5. Radial profiles of (a) tangential wind (m s21), and (b) relativevorticity (1025 s21), for the control (solid line), large (short dashedline), and small (long dashed line) tropical cyclones. Actual windobservations are indicated by (,).

Despite the rigor of the method used by Harr andElsberry (1996), some uncertainty was involved in ex-tracting the symmetric structure of Typhoon Robyn fromthe environment. Thus, additional profiles were gener-ated using variations of the control profile (Fig. 5a) byvarying the parameter b, which lead to radii of gale-force winds from 530 to 250 km, respectively. The cor-responding vorticity profiles are also shown in Fig. 5b.The vertical variation of the cyclonic winds is specifiedso that maximum wind shear is above 500 mb. A weak,symmetric, upper-level anticyclone with a maximumtangential wind speed of 25 m s21 at 1100-km radiusand 200 mb is added to the wind field. The anticyclonedoes not affect the results. The balanced vortex has awarm-core structure with maximum T deviation of5.58C at 500 mb.

3) STRUCTURE OF A CONVECTIVELY GENERATED

VORTEX

As documented by Harr and Elsberry (1996), a dis-tinct, midlevel potential vorticity anomaly developed inthe stratiform region of the mesoscale convective systemand extended from about 750 mb to 350 mb (Fig. 3).

Careful examination of the data suggests that the vortexcould not have contained winds of more than 10 m s21

or less than 7 m s21. By imposing gradient and hydro-static balance conditions, a vortex with maximum windsof 10 m s21 at a radius of 180 km resulted in a horizontalpotential vorticity structure similar to that observed(Fig. 6a). The vertical structure of the control potentialvorticity anomaly is represented with a Gaussian func-tion with maximum amplitude near 550 mb. Specifi-cation of shallow or deep vortices is controlled by thespread in the Gaussian distribution (Fig. 6b). The hor-izontal structure also follows the DeMaria (1987) ex-pression giving a radius of outer winds of about 390km (Fig. 6c). Because Fritsch et al. (1994) observed thatthe upper-level anticyclone induced during the meso-scale convective vortex formation is inertially unstableand rapidly dissipates, only the cyclonic part of the flowis specified.

The spatial resolution (50 km) of the analyzed fieldsused by Harr and Elsberry (1996) to calculate the po-tential vorticity cross section in Fig. 3 may have resultedin a weaker, more diffuse signature than actually existedin the mesoscale convective system. Thus, other doc-umented cases of the midlevel vortices that developwithin mesoscale convective systems were studied forbounds on the possible size and strength of the meso-scale convective vortex (e.g., Menard and Fritsch 1989;Bartels and Maddox 1991; Fritsch et al. 1994; Ritchie1995; Harr et al. 1996; Bartels et al. 1997). Considerablevariability is found from case to case and there is nouniform measure of size. The horizontal diameter of themidlevel vortex circulation may range from 200 to 1000km with a radius of maximum winds from 50 to 250km, respectively. The vertical extent of the circulationhas generally been observed to be between 400 and 700mb with a maximum near 550 mb but can extend overa greater depth if the mesoscale convective system existsfor longer periods.

According to these studies, the diameter of the me-soscale circulation is generally about twice the size ofthe cold cloud shield (221–240-K contour) of the me-soscale convective system at its maximum extent. Inaddition, the circulation extent (diameter) is about fourtimes the radius of maximum winds if observed, or twicethe extent of the analyzed cyclonic potential vorticityanomaly. The cold cloud shield (208-K contour) of themesoscale convective system that developed within thecirculation of Typhoon Robyn had a maximum extentof approximately 530 km by 400 km. The cross sectionof potential vorticity in Fig. 3 has a dimension of ap-proximately 48 latitude (444 km), which is reasonablegiven the cloud shield dimensions. Based on the abovestudies, this would indicate a circulation diameter of900 km with a radius of maximum wind of about 225km.

Although the calculated maximum potential vorticityvalue of 1.1 PVUs (potential vorticity units) is small


FIG. 6. Structure of the idealized mesoscale vortex: (a) vertical cross section of potential vorticity (1026 m2 s21 K kg21, thick lines) andpotential temperature (K, thin lines) for the control mesoscale vortex; (b) profile of the vertical wind variation for the control (solid line),shallow (short dashed line), and deep (long dashed line) mesoscale vortices; (c) profiles of tangential wind (m s21) and relative vorticity(1025 s21) for the control (solid line), large (long dashed line), and small (short dashed line) mesoscale vortices.

compared with other observations [e.g., 6 PVUs inFritsch et al. (1994) and 6.3 PVUs in Bartels et al.(1997)], this may be due to several factors. Most im-portantly, the 50-km resolution of the analyzed fields inHarr and Elsberry (1996) will lead to an underestimateof the maximum value of potential vorticity. In addition,the presence of horizontal and vertical shear due to Ty-phoon Robyn’s circulation is the likely cause of dis-persion of the vortex even as it develops, despite theapparent longevity of the mesoscale convective system(.15 h). In other studies, only very weak vertical shearwas generally observed. A smaller Coriolis parameterin this tropical case may also be a factor as the mainvorticity generation mechanism is believed to be con-vergence acting on the Coriolis parameter (Bartels andMaddox 1991). Mesoscale convective systems with

midlatitude vortices between 308 and 418N have a Cor-iolis parameter three times that of the latitude of interesthere. Thus, a vortex generated in a comparable meso-scale convective system at 138N is probably weaker.

Based on the above considerations, a ‘‘large’’ and a‘‘small’’ horizontal profile was generated with a radiusof outer winds of 435 and 345 km respectively (Fig.6c). The relative vorticity profiles associated with thesemesoscale convective vortices differ from that of thecontrol mesoscale convective vortex. Two other me-soscale vortex profiles were also developed based onthe horizontal profile of the control vortex, but with ashallower (420–700 mb) and a deeper (300–875 mb)potential vorticity signature to test the sensitivity of thesimulated track deflections to the depth of the mesoscalevortex (Fig. 6b).


FIG. 7. Control simulation of an interaction between a mesoscalevortex and a tropical cyclone: (a) 550-mb mesoscale vortex (v) andtropical cyclone surface positions every 3 h; potential vorticity (1026

m2 s21 K kg21, contour interval 5 0.5) at 550 mb at (b) 0, (c) 6, (d)12, (e) 18, and (f ) 24 h of simulation.

4. Results

The model is initialized with several configurationsof the primary vortex and mesoscale convective vortex.The control simulation is designed to approximate theconditions observed between Typhoon Robyn and themesoscale convective system as closely as can be de-termined from the data. The other simulations are de-signed to explore differences due to reasonable changesin the initial conditions. Both satellite imagery (Fig. 1)and aircraft observations (Fig. 3) suggest the center ofthe mesoscale convective system (and associated vortexlocation) is at a distance of approximately 400 km (3.68latitude) north of the center of Typhoon Robyn. Al-though the initial orientation of the mesoscale convec-tive system is approximately north-northwest of thetropical cyclone center, the initial configuration is north–south in the model for simplicity and to ensure that bothvortices are located on a grid point. The control sim-ulation will be described in some detail next. All sen-sitivity simulations are analyzed in terms of significantdifferences from the control run.

a. Simulated interaction between a mesoscaleconvective vortex and Typhoon Robyn

In the control case, the simulated mesoscale convec-tive vortex with maximum intensity at 550 mb is initiallylocated 400 km north of the tropical cyclone center,which corresponds to the approximate location relativeto TY Robyn of the mesoscale convective system at itsmaximum cold cloud shield extent. Two major factorsare expected to affect the mesoscale vortex during thesimulation, which may in turn impact the tropical cy-clone motion. The first is the strength of the mean ad-vecting current due to the tropical cyclone circulation,which will affect the rate at which the mesoscale vortexis advected around the tropical cyclone. This is impor-tant as the location of the mesoscale convective vortexrelative to the tropical cyclone center prescribes the di-rection of the advective flow over the tropical cyclone.The faster the rotation of the mesoscale vortex, the larg-er the changes of the tropical cyclone motion direction.The second is the horizontal shear across the mesoscalevortex due to the radial change in the tropical cyclonecirculation, which is expected to affect the rate of fi-lamentation of the mesoscale vortex. As the mesoscalevortex becomes more distorted from a circular shape,the strength of the advective flow across the tropicalcyclone is expected to weaken.

During the control simulation, a mutual rotation oc-curs and the separation between the vortices decreases(Fig. 7a) similar to that simulated with unequal, baro-tropic vortices (Smith et al. 1990; Ritchie and Holland1993). The relative motion of each vortex is primarilydue to the mutual advective flow imposed across it bythe outer circulation of the other vortex. Thus, the weak-er mesoscale convective vortex, which is initially em-

bedded in a mean (over a radius of 400 km) 550-mbflow of 12.3 m s21 and horizontal shear of 3.3 3 1025

s21, is advected farther by the stronger tropical cyclonecirculation. Given the mean advective flow of 12.3 ms21 the mesoscale vortex should be advected ;270 kmin the first 6 h of the simulation. A calculation of theactual mesoscale vortex motion shows that it moves 280km.

Because the shear flow in the simulated tropical cy-clone is sufficiently strong,2 the mesoscale convectivevortex is rapidly sheared into a filament and incorpo-rated into the circulation of the stronger vortex (Figs.7b–f). The center of the mesoscale convective vortexmay be considered a coherent region of stronger poten-tial vorticity that persists about 12 h. After 12 h, the

2 For these vortices it was found that a horizontal shear flow of,1.9 3 1025 s21 resulted in slow or little distortion of the mesoscalevortex. However, filamentation could still occur if the vortex movedinto a region of stronger shear later in the simulation.


FIG. 8. Potential vorticity (1026 m2 s21 K kg21, contour interval 50.5) after 12 h in the control simulation at (a) 750 and (b) 350 mb.

mesoscale convective vortex has become difficult to dis-cern as a coherent vortex. Rather, it exists as an asym-metry (or perturbation) in the midlevel winds of thetropical cyclone. This asymmetry is gradually dampedover time by the swirling winds of the tropical cycloneas the vorticity filament wraps around the tropical cy-clone center, which is commonly known as axisym-metrization (Melander et al. 1988). In an environmentwithout shear, a similar barotropic stabilization process(Carr and Williams 1989) will damp small perturbationsor asymmetries in the mesoscale vortex circulation andmaintain its initial circular flow. As the horizontal shearof the cyclone circulation is imposed over the mesoscalevortex, it is deformed from axisymmetry, and this bar-otropic stabilization is the only restoring mechanism inthe dry simulation by which circular flow may be re-established. Filamentation occurs because the shearingflow of the cyclone vortex over the mesoscale convec-tive vortex is stronger than the restoring force of themesoscale vortex circulation. In general, a region ofstrong winds will be associated with the filament andthus a radial cross section through the filament will havesome qualitative similarities to secondary wind maximaobserved by Willoughby et al. (1982) during eyewallreplacement cycles.

In contrast to the barotropic simulations (e.g., Smithet al. 1990; Ritchie and Holland 1993), the present bar-oclinic simulations include realistic cyclone and me-soscale vortex vertical structures so that some variationsat different heights may be expected. Although the me-soscale convective vortex is most intense at middle lev-els, the tropical cyclone circulation is much stronger,and the vortex is sheared into a filament as illustratedin Figs. 7b–f. The tropical cyclone is more intense atlower levels and the mesoscale convective vortex isweaker than at middle levels, so the mesoscale vortexis sheared into a filament slightly more rapidly althoughit is not easy to see (cf. Fig. 7d and Fig. 8a). A com-parison of the eccentricity of the mesoscale vortex pro-vides a measure of the filamentation. A value of oneindicates a circular vortex, whereas a value near zeroindicates a filament. At 12 h, the eccentricity of themesoscale vortex at 550 mb (750 mb) is 0.08 (0.07).Thus, the mesoscale vortex is slightly more filamentedat lower than at middle levels. In addition, the mesoscaleconvective vortex has rotated slightly farther around thetropical cyclone by the stronger tropical cyclone cir-culation at lower levels. At 850 mb and lower, no me-soscale convective vortex filament is detected. At highertropospheric levels, both the tropical cyclone and themesoscale convective vortex are weaker and the inter-action differs from the lower levels (Fig. 8b). By 12 h,the mesoscale convective vortex has not rotated as fararound the tropical cyclone and the filament is not elon-gated as much (eccentricity 5 0.21). It is difficult todiscern the mesoscale convective vortex at levels above300 mb. Although the mesoscale vortex also initiallyimposes a limited vertical shear over the tropical cyclone

circulation, the tropical cyclone remains vertically co-herent throughout the 24-h simulation.

Asymmetric components of the wind fields (Fig. 9)at the three pressure levels corresponding to Figs. 7d,8a, and 8b are calculated by subtracting from the totalwind field a symmetric component of the wind calcu-lated relative to the minimum 550-mb wind speed centerof the cyclone. The effects of varying vertical structuresare apparent by tracking the maximum in the asym-metric component of the relative vorticity field, whichis almost entirely associated with the mesoscale con-vective vortex. The mesoscale convective vortex hasbeen advected farther at lower levels and middle levelsthan at upper levels (Fig. 9), similar to that shown inFig. 8. The average asymmetric flow within a 350-km


FIG. 9. Storm-centered, asymmetric wind vectors and contours of relative vorticity (1025 s21, contour interval 5 5, shadingindicates values .5) for the control simulation at 12 h and (a) 350, (b) 550, and (c) 750 mb. (d) The 950–200-mb deep-layer meansteering vector and steering vectors at various pressure levels, averaged within 350-km radius of the tropical cyclone center at 12h of simulation.

radius of the tropical cyclone center (Fig. 9d) is stron-gest from about 450 through 750 mb. The rotation ofthe average steering vector with height is another in-dication of the changing rate of rotation of the mesoscalevortex with height where the vortex has rotated less athigher levels and more at lower levels. In addition, theadvective flow across the tropical cyclone at higher andlower levels is weak and, thus, is expected to have lesseffect on the tropical cyclone displacement.

In response to the changes in the mesoscale convec-tive vortex circulation, the tropical cyclone initiallymoves east, then northeast, and then north, and finallynorthwest during the 24-h period (Fig. 7a) covering a

total distance of about 109 km. Because the environmentis uniform, there is no propagation of the tropical cy-clone due to the environment. Thus, the location of themesoscale vortex relative to the tropical cyclone centerdetermines the direction of motion of the tropical cy-clone and the cyclonic rotation of the mesoscale vortexresults in a rapidly changing directional component ofadvection over the tropical cyclone. Furthermore, theimpact of the mesoscale vortex on the tropical cyclonemotion diminishes as the mesoscale convective vortexis stretched into a filament (Figs. 7b–f). A deep-layer(950–200 mb) mean steering vector averaged within350-km radius of the tropical cyclone center is calcu-


FIG. 10. Three-hourly tropical cyclone motion vectors (open arrows) and 950–200-mb deep-layer mean averaged within 350-km radius of the tropical cyclone center (filled arrows) for allf -plane simulations. The 3-h tropical cyclone motion speeds (m s21) are printed below each vectorand the 24-h track deflection (km) is indicated at the right of each plot: (a) control simulation,(b) large tropical cyclone, (c) small tropical cyclone, (d) 225-km separation distance, (e) 600-kmseparation distance, (f ) deep mesoscale vortex, (g) shallow mesoscale vortex, (h) large mesoscalevortex, and (i) small mesoscale vortex.

lated every 3 h and compared to the actual 3-h tropicalcyclone motion at sea level (Fig. 10a). The speed anddirection of motion of the tropical cyclone are closelyassociated with the advective flow from the mesoscaleconvective vortex. Although the initial deflection of thetropical cyclone is about 2 m s21 to the east and thennortheast (Fig. 10a), the mesoscale vortex effect on thetropical cyclone track rapidly weakens as the mesoscalevortex filaments. After 12 h, the mesoscale filament iswest of the tropical cyclone center, and it imposes anorth-northwestward advective flow of about 1.0 m s21

over the tropical cyclone (Fig. 9d) whereas the actualtropical cyclone motion is 1.5 m s21 to the north-north-west. By 15 h into the simulation, the tropical cyclonemotion is 1.1 m s21 to the northwest, whereas the ad-vective flow due to the mesoscale vortex is 0.8 m s21

to the northwest, and by 24 h the tropical cyclone hasalmost stalled (Fig. 10a). This simulation suggests that

an initial westward motion of 5 m s21 of Robyn (Fig.2) might have been modified by an interaction with themesoscale convective system of the type simulated here.That is, as much as a 2 m s21 deceleration in Robyn’swestward motion might have resulted in the first 6–9 hof interaction, and then a more northward track de-flection, which is similar to that observed.

This simulation suggests that the effect on the stormtrack by such an interaction may only occur for a limitedtime. In this dry simulation, the effects on the stormtrack due to the interaction have become small afterabout 15 h (Fig. 10a). This result differs from that ofRitchie and Holland (1993) based on contour dynamicsmodel results, as they showed that a track perturbationimposed on a tropical cyclone–like vortex patch by asmaller nearby vortex patch could last long after thepatch had sheared into a filament that is unresolvableby normal model resolutions or observing systems. The


FIG. 11. Potential vorticity (1026 m2 s21 K kg21, contour interval5 0.5) at 550 mb after 12 h of simulation for (a) the large tropicalcyclone and (b) the small tropical cyclone.

results differ because of the stronger diffusion in grid-point models such as the one used here. Because thecontinuation of the tropical cyclone track deflection de-pends on the timescale of the filamentation of the me-soscale convective vortex (Figs. 7b–f), the maintenanceof the mesoscale convective vortex circulation againstthe shear deformation effects of the tropical cyclonecirculation becomes a critical factor. If the mesoscaleconvective vortex can be sustained longer as a pertur-bation on the tropical cyclone circulation, then the trackdeflection effect will persist longer. In the case of Ty-phoon Robyn, the mesoscale convective vortex was stillobservable as a discrete potential vorticity anomaly atleast 18 h after the mesoscale convective system firstdeveloped, albeit shallower and weaker. This indicatesthat although the horizontal and vertical shear environ-ment of the typhoon circulation may be acting to dis-perse the observed vortex similar to the simulated vor-tex, the continued development of a stratiform anvil overthe vortex probably results in continued developmentof the vortex, partially counteracting the dispersive ef-fects. Thus, the simulated track deflection might be ex-pected to persist longer in a simulation where momen-tum transport processes are accurately represented.

The 950–200-mb deep-layer mean averaged flow pro-vides a reasonable estimate of the tropical cyclone mo-tion, particularly after about 9 h of simulation. However,the simulated asymmetric circulations in Fig. 9 suggestcaution is required in choosing an appropriate layermean because the bulk of the advective flow may beconstrained to the middle levels of the atmosphere, par-ticularly in the early stages of the simulation. In fact,the deep-layer mean flow in the first 12 h of the sim-ulation is dominated by midlevel (450–750 mb) flow.However, the deep-layer mean flow by 18 h is less dom-inated by the midlevel flow, and low-level asymmetriesbecome more important. In addition, the vertical vari-ation in the advective flow over the tropical cyclonedescribed earlier may be affected when moist processesare included. Although the basic dynamics of the vor-tex–vortex interaction leading to the cyclone track de-flections are believed to be represented in these drysimulations, moist simulations that depend on an ac-curate representation of the generation of vorticity with-in the mesoscale convective system will be required tofully test this premise.

b. Sensitivity of the track deflections to the tropicalcyclone outer wind structure

Whereas the simulated tropical cyclone has been con-structed to be a proxy of Typhoon Robyn, the outerwind structure can vary considerably from storm tostorm. Thus, sensitivity tests are used to examine howthe interaction, and the resulting track of the tropicalcyclone, would be affected if the storm were larger orsmaller than in the control (Fig. 5). Although the samemesoscale convective vortex is placed at the same dis-

tance from the tropical cyclone, three factors will bechanged. First, the advecting current will be greater(less) for the larger (smaller) tropical cyclone. Second,the horizontal wind shear across the mesoscale convec-tive vortex imposed by the tropical circulation is less(greater) for the larger (smaller) tropical cyclone. Third,the vorticity in which the mesoscale convective vortexis embedded will be cyclonic (anticyclonic) for the large(small) tropical cyclone.

A similar filamentation of the mesoscale convectivevortex as in Fig. 7 occurs for both the larger and smallertropical cyclone (Fig. 11). The spread in the final 24-hpositions of the large and small tropical cyclones isapproximately 125 km.


FIG. 12. Simulated tropical cyclone surface track deflections dueto interaction with a mesoscale vortex for the control tropical cyclone,large tropical cyclone (,), and small tropical cyclone (m). Positionsare every 3 h.

FIG. 13. Storm-centered, asymmetric wind vectors and contours ofrelative vorticity (1025 s21, contour interval 5 5, shading indicatesvalues .5) at 550 mb and 12 h of simulation for (a) the large tropicalcyclone and (b) the small tropical cyclone.

The tropical cyclone with stronger outer winds movesin a tighter circle over a larger distance (136 km) thanthe control tropical cyclone (Fig. 12), because the me-soscale convective vortex is embedded in a stronger(mean 5 16.6 m s21) cyclonic flow compared with thecontrol case and is advected more rapidly around thetropical cyclone. This relatively larger rotation resultsin a more rapidly changing directional component ofadvection over the tropical cyclone (Fig. 10b, filled ar-rows). Although the initial tropical cyclone motion iseastward, it rapidly changes direction through north-ward, westward, and finally southwestward (Fig. 10b,open arrows). Similar to the control case, the tropicalcyclone translation speed decreases considerably after15 h, as the mesoscale vortex becomes filamented (Fig.11a). However, the translation speed of the larger trop-ical cyclone is faster than the control cyclone at all timesafter 3 h (Fig. 10b). Although the mesoscale convectivevortex is advected around the large tropical cyclonemore rapidly than the control case, the filamentation isslightly slower because it is embedded in weaker shear(initially 2.3 3 1025 s21). The eccentricity of the me-soscale vortex at 12 h is 0.1 compared with 0.08 forthe control. Thus, the advective component over thetropical cyclone due to the mesoscale convective vortexremains slightly stronger for a longer period and a great-er distance has been covered than for the control sim-ulation. The asymmetric 550-mb wind field (Fig. 13a)indicates an average (within 350-km radius) steeringvector of ;2.3 m s21 toward the north-northwest at themidlevels of the tropical cyclone. The average deep-layer mean steering vector within 350 km of the tropicalcyclone center is 1.0 m s21 toward the northwest, whichis similar to the surface motion vector at 12 h (Fig.10b).

For the smaller tropical cyclone, significantly less ro-tation of the cyclone path is predicted (Fig. 12). The

tropical cyclone initially moves toward the east-north-east through northeast, then maintains a north-northeastpath until the end of the simulation. This smaller di-rectional change of the small tropical cyclone occursbecause the mesoscale convective vortex is embeddedin weaker advective flow (mean 5 7.3 m s21) comparedwith the other two simulations. Thus, the mesoscalevortex is advected much more slowly around the smalltropical cyclone (Fig. 11b), resulting in a much smallerchange in the directional component of the advectionover the small tropical cyclone (Fig. 10c, solid arrows).Although the translation speed again continues to de-crease during the simulation, a smaller speed reduction


FIG. 14. Potential vorticity (1026 m2 s21 K kg21, contour interval5 0.5) at 550 mb and 12 h of simulation for (a) 225- and b) 600-km separation distance.

occurs for the small tropical, which moves a total dis-tance of 118 km in the 24 h of simulation.

The 550-mb asymmetric wind field (Fig. 13b) at 12h indicates an average (within 350-km radius) steeringvector of 2.9 m s21 to the northeast, and the deep-layermean flow is 1.0 m s21 to the northeast similar to thetropical cyclone motion (Fig. 10c). In addition, the me-soscale vortex has not filamented as much as for thelarge tropical cyclone in spite of being initially embed-ded in stronger horizontal shear (;3.9 3 1025 s21),because the small tropical cyclone has an outer regionof anticyclonic relative vorticity (Fig. 5b) at the radiusof the mesoscale convective vortex. Since the core ofthe mesoscale convective vortex is located in a regionof positive vorticity gradient (i.e., increasingly cyclonicwith radius from the tropical cyclone center; Fig. 5b),the two vortices move apart (DeMaria and Chan 1984)rather than closer together as in the other two cases, andthe mesoscale convective vortex moves into a region ofweaker shear. Thus, the mesoscale vortex becomes fi-lamented considerably more slowly than the control (ec-centricity at 12 h 5 0.18), and the effect on the smallertropical cyclone track persists longer (Fig. 12).

c. Sensitivity of the track deflections to initialseparation distance between the vortices

The mesoscale convective vortex in Fig. 3 was lo-cated approximately 400 km from the center of TyphoonRobyn. Since this separation distance may vary greatlyfrom case to case, it was varied in simulations from asmaller (225 km) to a larger (600 km) distance. Sincethe advective effect of one vortex depends on the outercirculation of the other vortex, a smaller separation dis-tance causes a larger rotation about some intermediatepoint. Given the nonlinear wind speed increase in thetropical cyclone (e.g., Fig. 6a) or the mesoscale con-vective vortex (Fig. 6c) toward the center, the increasein rotation rate will be nonlinear as the separation dis-tance decreases. Unlike previous barotropic studies(e.g., Zabusky et al. 1979; Ritchie and Holland 1993)that used Rankine vortices, an upper limit will exist onthe interaction distance for vortices with outer windspeeds that tend to zero at finite radii. This is particularlytrue in a more realistic environment as nearby synopticcirculations are also likely to affect the tropical cyclonemotion (e.g., Holland and Dietachmayer 1993; Carr andElsberry 1994). Two physical processes change as theseparation distance is varied. First, the mesoscale con-vective vortex will be embedded in anticyclonic (cy-clonic) vorticity (Fig. 5b) for the larger (smaller) sep-aration distance. Second, the horizontal wind shear (Fig.5a) across the mesoscale convective vortex will beweaker (stronger) for the larger (smaller) separation dis-tance.

More rapid filamentation of the mesoscale convectivevortex compared with Fig. 7 occurs for the smaller sep-aration distance because of the very strong horizontal

shear (4.7 3 1025 s21) associated with the tropical cy-clone core (Fig. 14a). By 12 h, the mesoscale convectivevortex is indistinguishable from the axisymmetric trop-ical cyclone circulation. In contrast, the larger separationdistance results in an elongation of the mesoscale con-vective vortex, which is a much slower filamentation(Fig. 14b) because it is initially in relatively weak shear(1.9 3 1025 s21). The mesoscale convective vortex isalso in a cyclonic vorticity gradient (Fig. 5b), whichcauses the vortex to move away from the tropical cy-clone into a region of even weaker horizontal shear.

These separation distance variations result in a 72-km difference in the 24-h positions of the tropical cy-clone center (Fig. 15). The smaller separation distance


FIG. 15. Simulated tropical cyclone surface track deflections dueto interaction with a mesoscale vortex for the control (400 km) sep-aration distance, small (225 km) separation distance (,), and large(600 km) separation distance (m). Positions are every 3 h.

FIG. 16. Simulated tropical cyclone surface track deflections dueto interaction with a mesoscale vortex for the control mesoscale vor-tex, shallow mesoscale vortex (,), and deep mesoscale vortex (m).Positions are every 3 h.

results in a rapid acceleration of the tropical cyclone inthe first 3–6 h because of the strong initial advectiveflow (mean 5 18.1 m s21) associated with the mesoscaleconvective vortex (Fig. 10d). The direction of motionalso changes more rapidly than in any previous case,because the mesoscale vortex is quickly advectedaround the tropical cyclone by the strong core winds.As the mesoscale vortex is filamented and wrapped intothe tropical cyclone core (Fig. 14a), this advective flowrapidly decreases after 6 h (Fig. 10d), and the final weakwest-southwestward motion is maintained until the endof the simulation. Although the tropical cyclone movesa total distance of 100 km, this simulation suggests thata mesoscale convective vortex that developed very closeto the core of a tropical cyclone would not persist longenough to have a large effect on the tropical cyclonetrack.

The larger separation distance results in an eastwardthen northward tropical cyclone motion (Fig. 15) and atotal track length of 92 km. Because the mesoscale con-vective vortex is embedded in weak advective flow(mean 5 7.2 m s21), it is advected slowly around thetropical cyclone, which causes an eastward, throughnortheastward, and finally northward motion (Fig. 10e).In addition, the mesoscale vortex is embedded in weakhorizontal shear, which becomes weaker as the sepa-ration distance from the tropical cyclone increases.Thus, the mesoscale vortex filaments slowly, and theresulting advective flow over the tropical cyclone dueto the mesoscale vortex weakens very slowly throughthe simulation (Fig. 10e and Fig. 15). This result sup-ports the hypothesis that a mesoscale convective vortexthat develops in the outer, low horizontal shear regionof a tropical cyclone will filament slower and, so, affectthe tropical cyclone track for a longer period.

d. Sensitivity of the track to the vertical structure ofthe midlevel vortex

It is difficult to estimate the possible depth of themesoscale convective vortex from satellite imageryalone. Fritsch et al. (1994) speculate that the depth ofthe vortex is related to the length of time the mesoscaleconvective system exists. Thus, it is useful to examinehow the interaction, and the resulting track of the trop-ical cyclone, would be affected if the vertical structureof the mesoscale convective vortex were deeper or shal-lower than the control. The vertical structures of the‘‘deep’’ and ‘‘shallow’’ mesoscale convective vorticesin Fig. 6b are within the limits in observational studiesof mesoscale convective vortices (e.g., Menard andFritsch 1989; Fritsch et al. 1994). Although the profilesdo not look very different, the one major factor that haschanged from the control is that the integrated circu-lation associated with the deep (shallow) mesoscale con-vective vortex that will advect the tropical cyclone hasincreased (decreased).

Very little change in the filamentation occurs at anylevel compared with the control, and the 550-mb vor-ticity patterns for the deep and shallow mesoscale vortexafter 12 h of simulation are indistinguishable from thecontrol (Fig. 8). However, the vorticity is stronger(weaker) at these levels for the deeper (shallower) me-soscale vortex (not shown).

These vertical variations in the mesoscale vortexstructure result in a small (37 km) difference in the 24-hpositions of the tropical cyclone center (Fig. 16). Thedeeper mesoscale vortex forces a stronger advectiveflow over the tropical cyclone center over a greater depthof the atmosphere (not shown) compared with the con-trol, which results in a faster motion (Fig. 10f), largeroscillation, and greater length (136 km) in the tropicalcyclone track compared to the control vortex (Fig. 10a).


FIG. 17. Three-hourly tropical cyclone surface positions for thesimulations on a b plane. The tropical cyclone–only positions areindicated by a solid cyclone symbol and the tropical cyclone–me-soscale vortex positions are indicated by an open cyclone symbol.

Although the shallower mesoscale convective vortexforces an advective flow at 550 mb of similar magnitudeover the tropical cyclone center, the advective flow athigher and lower levels is much weaker (not shown).This results in a weaker deep-layer mean advective flowover the tropical cyclone center (Fig. 10g), and a totaltrack length that is about half that for the deep mesoscalevortex (Fig. 16). Overall, the resulting tropical cyclonepositions for the deep and shallow mesoscale vortex areat about a 2:1 distance ratio, which corresponds to theapproximately 2:1 ratio in the initial amount of circu-lation of each mesoscale vortex.

e. Sensitivity of the track deflections to the horizontalwind structure of the midlevel vortex

In the case of the mesoscale convective system as-sociated with Typhoon Robyn, aircraft and dropwind-sonde observations are available to describe the struc-ture. However, such observations are not generallyavailable and uncertainty is inherent in using satelliteimagery to estimate the size of the mesoscale convectivesystem. Thus, it is useful to examine the sensitivity ofthe interaction, and the resulting track of the tropicalcyclone, if the horizontal structure of the mesoscale con-vective vortex is varied within reasonable limits. Thetangential wind structure of a large and a small meso-scale convective vortex is varied (Fig. 6c) such that theobserved potential vorticity structure of the mesoscaleconvective vortex (Fig. 3) is approximately maintained.Although the mesoscale vortex will be located at thesame horizontal distance from the tropical cyclone cen-ter as in the control, two physical factors will havechanged. First, the tropical cyclone will be located with-in the cyclonic (anticyclonic) gradient in vorticity of thesmaller (larger) mesoscale convective system. Second,the initial mean advective speed over the tropical cy-clone will be weaker (stronger) for the smaller (larger)mesoscale convective system.

As in the previous section, the filamentation of thelarge and small mesoscale vortices is very similar tothat for the control, because the mesoscale vortex islocated in the same initial mean flow and horizontalshear. The larger mesoscale vortex, which initially hada stronger vorticity core than the smaller vortex, remainsslightly stronger throughout the simulation.

The variation in the mesoscale vortex sizes again re-sults in a small (34 km) difference in the simulated 24-htropical cyclone positions (not shown). As expected, thestronger outer winds of the larger mesoscale vortex ini-tially produce a slightly faster advection of the tropicalcyclone (Fig. 10h) compared with the smaller mesoscalevortex (Fig. 10i). Because the initial radial vorticity gra-dients associated with the mesoscale vortices are quicklydistorted as the mesoscale vortices become filamented,there does not seem to be any effect caused by thelocation of the tropical cyclone within the cyclonic (an-

ticyclonic) vorticity gradient of the smaller (larger) me-soscale vortex.

f. Simulated interaction between a mesoscaleconvective vortex and Typhoon Robyn on a b plane

The simulations on an f plane in section 4a suggestthat an interaction between Typhoon Robyn and a me-soscale convective vortex could have contributed to aslowing of an initially westward motion and a subse-quent more northerly path similar to that observed.Whereas the vortex dynamics are easier to interpret onan f plane, it is possible that the track modificationsmay differ on a b plane. Thus, the tropical cyclone wasfirst integrated on a b plane for 24 h to allow devel-opment of the b gyres. After the mesoscale vortex wasinserted similar to the control f -plane simulation, thesimulation was integrated for a further 24 h on a b plane.

Similar advection and filamentation of the mesoscalevortex occurs as in the control f -plane simulation (notshown). Because the filamentation occurs within about15–18 h on relatively small spatial scales, the directinteraction between the mesoscale vortex and the trop-ical cyclone is not changed on a b plane. However, thetropical cyclone motion induced by the interaction withthe mesoscale vortex is slightly stronger than in thef -plane simulations (Fig. 17). Although the direction ofmotion after 24 h of the b-plane simulation with themesoscale vortex is almost identical as in the tropicalcyclone–only simulation, a net poleward deflection hasoccurred. In addition, the vector difference (Fig. 18d)relative to the b-plane control (Fig. 18b) is slightly (0.2–


FIG. 18. Three-hourly tropical cyclone motion vectors (solid arrows) and 950–200-mb deep-layer mean averaged within 350-km radius of the tropical cyclone center (open arrows). The 3-htropical cyclone motion speeds (m s21) are printed below each vector: (a) f -plane control sim-ulation, (b) tropical cyclone only on a b plane, (c) tropical cyclone and mesoscale vortex on ab plane, and (d) difference between (c) and (b).

FIG. 19. Azimuthal average of the initial tangential wind for thetropical cyclone–only (solid) and tropical cyclone with a mesoscalevortex (dashed) simulations on a b plane.

0.3 m s21) faster in the simulation with the mesoscalevortex than in the f -plane simulation (Fig. 18a). Thislarger propagation speed is due to a nonlinear interactionbetween the tropical cyclone and earth vorticity gradientthat results in an energy loss from the tropical cyclonevortex via Rossby wave dispersion. Fiorino and Els-berry (1989) demonstrated that the outer wind strengthof the tropical cyclone is important in determining thestrength, and orientation, of the b gyres. Insertion ofthe mesoscale convective vortex increases (decreases)the outer (inner) winds in the tropical cyclone (Fig. 19).

Thus, the b gyres would be expected to be proportion-ally stronger and provide a faster propagation speed thanfor the b-plane control simulation. In the case of thetropical cyclone with an embedded mesoscale vortex,the short-term effect of the interaction between the twovortices is to produce a decrease in the westward motionof the storm as demonstrated in both the f - and b-planesimulations. Whereas this interaction is limited to thetimescale of the filamentation of the mesoscale vortex,a longer-term effect may be produced because the bgyres are enhanced while the mesoscale vortex is con-tributing to a stronger outer cyclone circulation. In thissimulation, a slightly different orientation and enhancedstrength of the b gyres produced a faster tropical cyclonemotion that persisted through the end of the simulation,that is, beyond when filamentation has eliminated themesoscale vortex.

5. Summary and conclusions

These dry numerical simulations illustrate possiblecontributions to track deflections of a tropical cyclone(e.g., Typhoon Robyn) because of an interaction with amesoscale convective system that develops in proximityto the cyclone core. A special dataset collected duringthe TCM-93 field experiment in the western North Pa-cific is used to develop idealized representations of thestructure of the tropical cyclone and the potential vor-ticity anomaly that was associated with the mesoscaleconvective system. These idealized vortices are thenused to examine the sensitivity of the simulated tropicalcyclone motion over a range of tropical cyclone–me-soscale vortex interactions.

The control simulation suggests that as much as a 2


m s21 deceleration in Robyn’s westward motion mighthave resulted in the first 6–9 h, and then a more north-ward track deflection, due to an interaction with a me-soscale convective system of the type simulated here.The control simulation also suggests that the direct ef-fects on the storm track due to the mesoscale vortexbecome small as the mesoscale vortex is stretched intoa filament, which occurs in about 15 h in this dry sim-ulation. An additional small, but long term, cyclonetranslation occurs when the interaction is simulated ona b plane rather than an f plane because the mesoscalevortex increases the outer wind strength and results insubtle changes in the strength and orientation of the bgyres. An implication of the b-plane simulation is thatthe first-order, direct tropical cyclone–mesoscale vortexinteractions may be examined on an f plane. Eventhough the mesoscale convective vortex induced chang-es in the tropical cyclone motion have longer timescaleson the b plane, the track changes are small because themodification of the large-scale steering environment arelimited to the time scales of the filamentation of themesoscale vortex.

Because the continuation of the tropical cyclone trackdeflection depends on the timescale of the filamentationof the mesoscale vortex, the maintenance of the me-soscale vortex circulation against the shear deformationeffects of the tropical cyclone circulation becomes acritical factor. If the mesoscale convective vortex canbe sustained longer as a perturbation on the tropicalcyclone circulation, then the track deflection effect willpersist longer. This sensitivity was illustrated by chang-ing the initial separation distance of the tropical cycloneand mesoscale vortex. For small separation distances,there is an initially very large effect on the tropicalcyclone path due to the mesoscale vortex. However, themesoscale vortex is rapidly filamented so that effectson the tropical cyclone track are negligible by 12 h. Forlarger separation distances, the mesoscale vortex doesnot filament and the storm track is affected throughoutthe simulation. In these simulations, the rate of fila-mentation of the mesoscale vortex is due to the strengthof the horizontal shear of the tropical cyclone circula-tion. Another factor that will affect the longevity of themesoscale vortex that is not represented in these drysimulations is the development and maintenance of theparent mesoscale convective system. The persistence orredevelopment of the mesoscale convective system willprobably result in maintenance of the mesoscale vortexagainst filamentation and dispersion, which would resultin a larger track deflection than in these simulations.

These simulations also suggest that variations of themesoscale vortex structure are not very important forthe resulting track deflections. When all other aspectsare the same, a mesoscale vortex with a greater circu-lation, whether horizontal or vertical, produces a slightlylarger tropical cyclone track deflection than a mesoscalevortex with less total circulation. Since the timescale of

the filamentation process does not change, the timescaleof the track deflections also varies only slightly.

The outer wind structure of the tropical cyclone is ofimportance in determining the amount and direction ofthe tropical cyclone track deflection. For the types ofwind profiles tested here, a larger tropical cyclone exertsless horizontal shear across the mesoscale vortex, so itfilaments more slowly. However, the mean advectiveflow is stronger so the mesoscale vortex is advectedaround the tropical cyclone faster, which results in arapidly changing directional component of advectiveflow over the tropical cyclone center. The resulting trackdeflection of the larger tropical cyclone is larger thanthe control. For a smaller tropical cyclone, the meso-scale vortex is initially embedded in stronger horizontalshear. Because the vortex is also embedded in the outeranticyclonic part of the tropical cyclone, it is advectedaway from the tropical cyclone into a region of weakershear. Thus, the rate of filamentation decreases and theeffect on the storm track deflection persists until the endof the simulation.

The control and sensitivity simulations with this drymodel suggest that tropical cyclone track deflections ofup to 130 km over 18–24 h could be produced by aninteraction with a mesoscale convective vortex that isembedded close to the core of the storm, or far away,and for a large or small tropical cyclone. Because thecontinuation of the track deflection depends on howquickly the mesoscale vortex filaments, these dry sim-ulations probably underestimate the persistence of thetrack deflections owing to the rapid filamentation. Al-though this needs to be simulated with a moist modelwith convective heat release and momentum transports,it is expected that real mesoscale vortices would resistthe horizontal shear deformation by the tropical cyclonecirculation for longer periods and result in larger trackdeflections. It is left for future work to determine ifexisting moist models can also simulate the develop-ment and structure of the mesoscale vortex, rather thansimply inserting the vortex as done here. The curvatureof the track deflection depends on how rapidly the me-soscale vortex is advected around the tropical cyclone,which is primarily determined by the initial separationdistance and the tropical cyclone wind profile. Whethersuch track deflections are a significant contribution tothe overall track depends on the strength and directionof the environmental flow in which the tropical cycloneis embedded. For the mesoscale vortex and cyclonestructures tested here, it appears plausible that the me-soscale convective system about 400 km north of Ty-phoon Robyn contributed to a deceleration and polewarddeflection of Typhoon Robyn through a combination ofdirect interaction and modification of the large-scalesteering.

Acknowledgments. The authors would like to thankDr. P. Harr for providing analyses from the TCM-93field experiment. Many thanks also to Dr. P. Harr, Dr.


L. E. Carr III, and two anonymous reviewers for theirinsightful comments on an earlier version of this man-uscript. The version of MM5 used in this study wasobtained from the National Center for Atmospheric Re-search, Boulder, Colorado. Financial support for thisresearch was provided by the Office of Naval ResearchMarine Meteorology Program.

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simulated impacts of a mesoscale convective system on the track

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