3556 MONTHLY WEATHER REVIEW VOLUME 128

Large Intensity Changes in Tropical Cyclones: A Case Study of Supertyphoon Flo during TCM-90

DAVID W. T ITLEY AND RUSSELL L. ELSBERRY Department of Meteorology, Naval Postgraduate School, Monterey, California

(Manuscript received 5 November 1998, in ®nal form 5 May 1999)

ABSTRACT A unique dataset, recorded during the rapid intensi®cation and rapid decay of Typhoon Flo, is analyzed to isolate associated environmental conditions and key physical processes. This case occurred during the Tropical Cyclone Motion (TCM-90) ®eld experiment with enhanced observations, especially in the upper troposphere beyond about 300 km. These data have been analyzed with a four-dimensional data assimilation technique and a multiquadric interpolation technique. While both the ocean thermal structure and vertical wind shear are favorable, they do not explain the rapid intensi®cation or the rapid decay. A preconditioning phase is de®ned in which several interrelated factors combine to create favorable conditions: (i) a cyclonic wind burst occurs at 200 mb, (ii) vertical wind shear between 300 and 150 mb decreases 35%, (iii) the warm core is displaced upward, and (iv) 200-mb out¯ow becomes larger in the 400±1200-km radial band, while a layer of in¯ow develops below this out¯ow. These conditions appear to be forced by eddy ¯ux convergence (EFC) of angular momentum, which appears to act in a catalyst function as proposed by Pfeffer and colleagues, because the EFC then decreases to small values during the rapid intensi®cation stage. Similarly, the outer secondary circulation decreases during this stage, so that the rapid intensi®cation appears to be an internal (within 300 km) adjustment process that is perhaps triggered during the preconditioning phase. Rapid decay occurred over open ocean when the environmental factors of ocean thermal structure, and vertical wind shear, positive 200-mb EFC, and vigorous out¯ow into the midlatitudes appear favorable. However, the EFC extending down to 500 mb and inducing a second shallower secondary circulation is hypothesized to account for the rapid decay by leading to a less ef®cient energy conversion.

1. Introduction msϪ1). Such large errors in a TC intensifying close to a coast could mean that the public would not have time The problems of understanding and predicting trop- to prepare. ical cyclone (TC) intensity have been highlighted in the It is particularly important to forecast accurately the American Meteorological Society's Symposium on most intense TCs, since it has been estimated (Pielke Tropical Cyclone Intensity Change in Phoenix, Arizona, and Landsea 1998) that 80% of the damage in land- during January 1998. Thirteen keynote papers and 31 falling U.S. hurricanes is caused by TCs with Saf®r± other contributions summarized the present state of Simpson categories of 3±5. It has been known for some knowledge on TC intensity change. Other than being a time that these most intense TCs will have experienced fascinating and challenging scienti®c problem, an ac- at least one period of rapid intensi®cation. For example, curate intensity prediction is an important factor in the Holland (1984) found intense TCs in the Australian re- warnings of landfalling TCs (Marks and Shay 1998; gion typically intensify at twice the rate, and for three Elsberry and Marks 1998). In general, slow intensi®- times as long, as minimal hurricane-strength systems. cation of TCs is predicted by the U.S. National Hurri- Thus, the focus should be on understanding and pre- cane Center reasonably well (M. DeMaria 1997, per- dicting these periods of rapid intensi®cation. sonal communication), with mean 24-h intensity fore- It is also of interest to understand and predict the Ϫ1 cast errors of about 10 kt (5 m s ). However, rapid rapid decreases in TC intensity. The case of Hurricane intensity changes are not predicted well, with 5% of the Opal that rapidly weakened just prior to striking the 24-h intensity forecasts with errors exceeding 40 kt (20 Gulf of Mexico coast is the focus of many of the papers in the TC intensity change symposium. Although it is fortunate in terms of the potential damage that is avert- ed, credibility of the forecasts can suffer if such large Corresponding author address: Prof. Russell L. Elsberry, Depart- ment of Meteorology, Code MR/Es, Naval Postgraduate School, 589 intensity decreases are not predicted well. Dyer Rd., Room 254, Monterey, CA 93943-5114. The favorable environmental factors involved in TC E-mail: [email protected] intensi®cation are relatively well established (Frank

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1987; McBride 1995). More recent summaries are given ¯ux events were sometimes, but not always, correlated in the TC intensity change symposium, and the reader with future TC intensity changes. Some factors may be is referred to that preprint volume for details. In general the TC stage at the time of environmental forcing event; terms, higher sea surface temperatures (SSTs) are con- how long, and at what radius and over what depth, the sidered favorable because the energy source for the TC eddy momentum ¯ux occurs; and the intermediate storm is the ¯uxes of heat and moisture from the warm tropical structure that determines the inner-core response to a ocean [see keynote papers by Emanuel (1998) and Black remote (large radius) forcing event. Given these many and Shay (1998)]. factors by which environmental forcing may be involved Large environmental vertical wind shear is considered in TC intensi®cation, perhaps it is not surprising that unfavorable for intensi®cation, and is believed to be a diverse opinions exist as to the role of environmental major factor in the rapid decay of TCs. For example, forcing. values of vertical shear above certain threshold values A fourth factor in TC intensi®cation is the internal are believed to prevent TC intensi®cation beyond the dynamics of how the heat/moisture gained from the tropical storm (35±63 kt, 17±32 m sϪ1) stage [see key- ocean, and transferred to the atmospheric boundary lay- note papers by DeMaria and Huber (1998) and Jones er, is released. Clearly, factors such as the radial dis- (1998)]. However, Molinari (1998) states that storms tribution of clouds with various depths are important. have intensi®ed despite high vertical shear during in- One fairly well accepted internal dynamic mechanism teractions with upper-tropospheric troughs. Elsberry and for TC intensity changes is the contracting eyewall cy- Jeffries (1996) caution that vertical shear estimates from cles of Willoughby et al. (1982). As the outer eyewall operational wind analyses in the Tropics may be faulty contracts inward, the maximum surface wind (intensity) because of data-sparsity problems over developing TCs, associated with the inner eyewall decreases. As the outer and Elsberry (1998) suggests these questionable vertical eyewall replaces the inner eyewall, the intensity again shear estimates may be a primary factor in poor TC increases (see Willoughby 1995 for examples and a intensity predictions with statistical equations. Less complete description). Although these contracting cy- agreement exists as to how the TC intensity is related cles seem to occur relatively frequently in TCs with an to vertical shears below the threshold value. Zehr (1998) intensity greater than 50 m sϪ1, the mechanism(s) that takes the rather extreme position that vertical shear is triggers the cycles is not known. As Holland and Wang the only atmospheric environmental in¯uence that de- (1998) note, these internal dynamic processes have spe- termines the TC intensity! DeMaria and Huber (1998) ci®c timescales and life cycles, and these dictate to some propose that the physical processes involved in TC in- extent the adjustment of the TC to external forcing, and tensity may change with the magnitude of the environ- thus affect the rate of intensi®cation. mental vertical shear and the existing storm structure. In summary, four nonlinearly interactive processes A particularly complex and controversial topic is the (ocean thermal structure changes, atmospheric boundary in¯uence of adjacent midlatitude or tropical troughs in layer and convective transports, and external factors the upper troposphere on TC intensity changes. Studies such as upper-tropospheric static stability and eddy an- back to Pfeffer (1958) have suggested that a proper gular momentum transports) are involved in TC inten- juxtaposition with an adjacent trough will lead to an sity changes. Although many numerical simulations eddy angular momentum ¯ux convergence into the TC have been made to explore the sensitivity to these var- in the upper troposphere. Molinari (1998) considers this ious factors, relatively few observational studies of rap- TC±trough interaction to be an example of a ``good id intensity changes have been published. This is par- trough'' for TC intensi®cation. Molinari et al. (1995, ticularly true in the western North Paci®c region. 1998) propose superposition of a positive potential vor- The objective of this study is to describe the rapid ticity feature in the upper troposphere with a similar intensity changes during the life cycle of Supertyphoon horizontal scale as the TC potential vorticity may be an Flo (intensity Ͼ 130 kt, 65 m sϪ1) that occurred during essential feature in some cases of rapid deepening. As the Tropical Cyclone Motion (TCM-90) ®eld experi- Wu and Cheng (1998) demonstrate using a potential ment (Elsberry 1990; Elsberry et al. 1990). Although vorticity (PV) inversion technique, and as will be il- the purpose of this experiment was to collect a com- lustrated from the analyses below, evidence for a PV prehensive dataset to study western North Paci®c TC superposition is lacking in the case of Typhoon Flo. motion, the dataset will be used here to study TC in- Juxtaposition of the trough may also increase the ver- tensity changes. In particular, the rapid intensity changes tical shear over the TC, and this would be a ``bad that led to Flo having achieved supertyphoon intensity trough'' effect on TC intensity. In a series of modeling (Fig. 1a) will be contrasted with the simultaneous in- studies culminating in a recent paper (Challa et al. tensi®cation of Typhoon Ed, which did not exceed 90 1998), Pfeffer and colleagues have studied environ- kt or 46 m sϪ1 (Fig. 1b). Maximum intensity of Flo was mental forcing of TCs. This group (and several others) achieved after a long track toward the northwest. Later have simulated TC formation/intensi®cation in response in the life cycle of Flo, a rapid decay occurred over to strong and persistent environmental forcing. DeMaria open ocean prior to landfall on Japan. Typhoon Ed had et al. (1993) found the large eddy angular momentum a long westward track along 20ЊN with only a slow

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sociated with these intensity changes. The two inde- pendent analyses of the TCM-90 dataset that are used in this study will be described in section 2.

2. Methodology a. TCM-90 dataset The TCM-90 ®eld experiment was conducted in the western North Paci®c during August and September 1990. As discussed in Elsberry (1990), this experiment was in conjunction with three other TC initiatives spon- sored by the Economic and Social Commission for Asia and the Paci®c/World Meteorological Organization, the former Soviet Union, and Taiwan. The combined ex- periments produced an enhanced dataset: (i) 6-h rawin- sonde coverage during intensive observing periods (IOPs); (ii) additional over-ocean rawinsondes from four Soviet Union ships stationed along 20ЊN; (iii) special observing platforms such as radar wind pro®lers, drift- ing buoys, and especially three ¯ights of the National Aeronautics and Space Administration (NASA) DC-8 aircraft into Typhoon Flo; and (iv) postexperiment pro- cessing of the Geostationary Meteorological Satellite cloud-drift winds by the Cooperative Institute for Me- teorological Satellite Studies at the University of Wis- consin, with careful manual assignments of altitudes based on the temperatures from the enhanced rawin- sonde network. A complete list of data collected during each IOP is given in Harr et al. (1991). Examples of the observations during 0600 UTC periods, except with special DC-8 observations, are given later (Figs. 13 and 14). Many of the key conclusions in the following anal- ysis are based on the upper-tropospheric observations, especially the angular momentum ¯ux calculations and the out¯ow characteristics.

b. Two objective analyses The National Centers for Environmental Prediction (NCEP) Environmental Modeling Center prepared a FIG. 1. Track positions and intensities (kt, small numbers) each 12 four-dimensional data assimilation (4DDA) analysis of h from (a) 0000 UTC 12 Sep to 1200 UTC 19 Sep 1990 for Super- the TCM-90 dataset (Rogers et al. 1992). Because the typhoon Flo, and (b) 1200 UTC 12 Sep to 0000 UTC 20 Sep 1990 4DDA technique blends the observations with the 6-h for Typhoon Ed from JTWC (1990). forecast of the NCEP mesoscale (50-km horizontal res- olution) Eta Model, ef®cient incorporation of asynoptic intensi®cation rate. After brushing the northern tip of data, dynamical consistency within and between levels, Luzon Island, Philippines, maximum intensity was and time consistency are expected advantages. However, achieved over the South China Sea. the technique depends on prescribed statistical relation- In contrast to the Challa et al. (1998) comparisons of ships between the observations and the Eta Model fore- simulations of a supertyphoon development and a non- cast values. It is certainly a question whether these sta- developing TC, this observational study searches for tistical relationships are valid in regions with small Cor- factors that distinguish rapid intensity changes in Flo iolis values and low inertial stability such as the out¯ow from those in a nominal typhoon development. Whereas layer of a typhoon. To avoid bias in the diagnostic stud- Merrill and Velden (1996) have previously carefully an- ies with these 4DDA analyses, no synthetic TC obser- alyzed the evolution of the out¯ow channels during the vations were included to de®ne the position and struc- rapid intensi®cation and decay of Supertyphoon Flo, this ture. In the absence of observations near the center, the study will describe some of the physical processes as- Eta Model 6-h forecast is likely to be the primary de-

Unauthenticated | Downloaded 09/30/21 12:07 AM UTC OCTOBER 2000 TITLEY AND ELSBERRY 3559 terminant of the TC position and structure. Another po- tween 300 and 600 km of a TC. This azimuthal aver- tential problem is the model parameterization of latent aging of the height ®eld occurs during the ®rst guess heat release in the TC, which might constrain the ob- construction, and the averaging of the wind ®eld occurs servations to a thermal structure that differs from the after the ®nal MQ analysis. The reason for the azimuthal real atmosphere. averaging near TCs is to avoid oval-shaped ®elds where As an alternative, a multiquadric (MQ) interpolation the MQ analysis ®t closely to an observation near a TC technique is used to convert irregularly spaced obser- is not matched with well-distributed observations vations to a regular grid. This TCM-90 application fol- around the TC to maintain the typical near symmetry lows from the Nuss and Titley (1994) MQ implemen- of ®elds. Details of these procedures and examples of tation, which is a linear combination of circular hyper- the ®nal MQ analyses near TCs are given in Titley boloids. These functions are ®t closely to the obser- (1998). vations while accounting for the likely errors in the These special procedures for the MQ analyses are different observation types. These functions have con- designed to produce wind and height analyses ®t to the tinuous derivatives and they may be interpolated to the observations similar to those analyzed by a skilled hu- regular grid with an empirically derived smoothing pa- man analyst. These MQ analyses will generally be se- rameter, which determines how sharply the gradient of lected for presentation in the following sections because the circular hyperboloid changes near each observation. the MQ technique ®ts the observations more closely. Details of the parameter speci®cations, observational An advantage of the 4DDA analyses is that they are error speci®cations, and tests of the scheme are given available for other TCM-90 storms besides Flo and Ed. in Titley (1998). Several special aspects of the MQ application to the TCM-90 dataset are brie¯y summarized here. First, the 3. Preconditioning phase of Flo analysis was of increments that are the differences be- tween the observations and values interpolated with a The various favorable environmental factors for TC B spline to the observation location in the Navy Op- intensi®cation described in the introduction are ®rst ex- erational Global Atmospheric Prediction System (NO- amined for Flo. First, no ocean-related evidence could GAPS) analyses done in real time. Although these 2.5Њ be found distinguishing the more rapid intensi®cation latitude±longitude NOGAPS analyses are relatively of Flo relative to the conditions existing in Ed. Both coarse, they provide a good dynamical synoptic basis typhoons intensi®ed over a broad region with SSTs of for a higher-resolution analysis in the region of TCM- about 28.0Њ±28.5ЊC (not shown), which are within 0.5ЊC 90 observations. Zero-increment bogus values were in- of climatological values. The Philippine Sea region in serted at grid points at least 1500 km from a real ob- which both TCs formed typically has ocean mixed layer servation, which constrains the MQ analysis to return depths exceeding 100 m, so that the heat content is to the NOGAPS analysis. Since each MQ analysis is on suf®cient for intensi®cation even if either Flo or Ed had a two-dimensional pressure surface, the NOGAPS anal- been moving slowly. No correlation of intensity change yses provide a background vertical structure that is in with the passage of either Flo or Ed over a warm-core hydrostatic balance. It is emphasized that each MQ anal- ocean eddy (as in the frequently studied Hurricane Opal ysis is a ``cold start,'' that is, the NOGAPS analysis is case) was evident. a background for calculating the increments, but the MQ Calculations of vertical wind shear are possible with analysis is not a blending of observations and a short- the TCM-90 analyses each 50 mb. Deep tropospheric, term numerical model forecast as in the 4DDA analyses. Whereas the MQ analysis will not be as model physics low-level, midlevel, and upper-level differences in wind dependent as the 4DDA analysis, it will not have the vectors have been calculated (Titley 1998) and con- bene®t of time continuity and spreading the information trasted between Flo and Ed. Averages over a variety of into data-sparse areas that comes from the analysis± horizontal domains relative to the center have been cal- forecast±analysis cycle of the 4DDA, except as is pro- culated. Considering the minimum data coverage near vided by the NOGAPS assimilation system. the center, this region has been excluded. Although an The MQ analyses of geopotential heights are geo- average vertical shear over the 200±600-km radial band strophically constrained north of 35ЊN, unconstrained is presented here, other radial bands have similar var- south of 25ЊN, and a linear blend between 25Њ and 35ЊN. iations in time (Titley 1998). The MQ analyses of temperature are not constrained by Eddy ¯uxes of angular momentum as in Molinari et winds or heights. Gradient winds based on the MQ an- al. (1995) have been calculated at each 50 km in radius alyzed height ®eld are the ®rst guess for the wind anal- and 50 mb in depth for a moving coordinate system yses north of 35ЊN and within 300 km of tropical cy- using the observed translation speeds (Figs. 1a and 1b). clones. Winds and heights within 300 km of a TC are Eddy ¯ux convergences are then calculated at various replaced by an azimuthal average in radial increments radii and converted to equivalent tangential wind speed of 50 km in a moving cylindrical coordinate system, changes (m sϪ1 dayϪ1) averaged over the cylindrical and blended with the Cartesian coordinate system be- volume within that radius.

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FIG. 2. Conceptual model of the thermal structure (left), vertical wind shear (middle), and circulation structure (right) changes from the beginning (top) to the end (bottom) of the precon- ditioning stage of Supertyphoon Flo. A brief summary of these changes is given on the right (see text for details). a. Preconditioning conceptual model in a storm-centered coordinate system for six times at which the storms were of similar intensities. For ®ve of A conceptual model summarizing the differences dur- these times between 0000 UTC 14 September and 1800 ing the preconditioning phase of Flo relative to that of UTC 15 September, the storms had identical intensities, Ed is shown in Fig. 2. Each of the special aspects of and the intensity of Ed was only slightly (5 kt, 2.5 m Flo will be described with selected diagrams to illustrate sϪ1) greater at the sixth time. Whereas both Flo and Ed this conceptual model. More detailed analyses are avail- intensi®ed from 45 kt (22.5 m sϪ1) to 85 kt (42.5 m able in Titley (1998). sϪ1), Flo (Ed) achieved this intensity change over only The left two panels of Fig. 2 illustrate the thermal 36 h (96 h). It is emphasized that all of the times related structure differences and emphasize the upward dis- to Flo were prior to the rapid intensi®cation, so that this placement of a warm anomaly in Flo that did not occur is considered to be a ``preconditioning effect.'' Although in Ed. These thermal structure differences may be il- these inner-region temperature differences are calculat- lustrated by azimuthal-averaged temperatures (Fig. 3) ed from the 4DDA analyses that may have some Eta Model dependencies, Ed was moving westward along 20ЊN where four Soviet Union ships were providing rawinsondes each 6 h and Flo passed near the Saipan and Guam rawinsondes. Since the key features in Fig. 3 are consistent in time, these temperature differences are believed to be real and not an artifact of the Eta Model. Notice the Flo minus Ed temperatures in Fig. 3 within 200 km of the center are ϩ1ЊC at 700 mb and Ϫ1ЊC between 300 and 200 mb. These 700- and 200- mb temperature differences indicate the inner core of Flo is less statically stable than that of Ed, which implies the midtropospheric thermal structure of Flo is more conducive to sustained convection near the center. How- ever, the upper troposphere above Flo between 150 and 100 mb is 1Њ±2ЊC higher than above Ed. That is, these FIG. 3. Azimuthal-averaged temperature (ЊC) differences between composited six analyses are consistent with Gray's Flo and Ed as a function of radius and pressure composited during (1992) hypothesis of strong, intensifying storms having the periods when both intensi®ed from 45 to 85 kt, which occurred in the 42-h preconditioning period of Flo, but over 96 h for Ed. greater convective instability in the midtroposphere and Positive (negative) values (see inset for shadings) indicate the tem- higher temperatures aloft. These temperature differences peratures in Flo were higher (lower) than in Ed. are summarized in the left panels of Fig. 2.

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FIG. 4. (a) Tangential wind difference (200±850 mb) averaged be- tween 200 and 600 km in Supertyphoon Flo calculated from the 4DDA, MQ, and NOGAPS analyses in comparison with the corre- sponding intensity (heavy solid, scale on right). (b) As in (a) except for wind differences between 150 and 300 mb for the 4DDA and FIG. 5. Azimuthal-average tangential wind (contour interval of 4 Ϫ1 MQ analyses only. ms ) as a function of radius in Flo from MQ analyses at (a) 0000 UTC 13 Sep 1990 and (b) 1200 UTC 14 Sep.

The middle panels of Fig. 2 summarize the corre- in the outer region (recall that the vertical shear is cal- sponding tangential wind structure changes. Tradition- culated between 200- and 600-km radius) of Flo, both ally, vertical wind shear has been calculated between the multiquadric and 4DDA analyses have a greater than 850 and 200 mb because these two levels typically have 35% decrease in 300±150-mb shear during the precon- the best wind coverage from low- and upper-level cloud- ditioning period. It is suggested that this rapid decrease drift winds. The 6-h estimates of 850±200-mb shear in upper-tropospheric shear is an indication of an up- (Fig. 4a) are somewhat noisy. If a 1±2±1 ®lter is applied, ward extension of the TC circulation. This decrease in the MQ estimates at 0000 and 0600 UTC 15 September 300±150-mb shear is indicated schematically in Fig. 2 are about 6 m sϪ1, which is nearly the same value as at by a upward extension of the cyclonic wind maximum 0600 and 1200 UTC 13 September, although smaller to about 150 mb, with an approximately equal increase shear values are estimated between these times. Whereas in wind at 850 and 200 mb so that the shear between the 4DDA shear estimates at the end of the precondi- those two levels remains approximately constant. tioning period are also about 6 m s-1, the initial values Azimuthal-averaged tangential wind changes during are higher and would suggest an initial decrease in shear. this period are illustrated in Fig. 5. Even disregarding Accepting the MQ estimates, the middle schematics in the tangential wind analysis within 200±300 km of the Fig. 2 re¯ect vertical shear values between 850 and 200 center, it is evident that the cyclonic circulation tends mb that decrease only slightly during the precondition- to decrease with elevation between 800 and 250 mb at ing phase. Because 6 m sϪ1 is about one-half the thresh- 0000 UTC 13 September (Fig. 5a) Only anticyclonic old shear value that may prevent TC formation (Zehr winds are present above 250 mb at this time. Although 1992), one would not expect environmental vertical the cyclonic winds have evidently not penetrated wind shear between 850 and 200 mb to be a determining throughout the troposphere (at least beyond 200±300 (inhibiting) effect for intensi®cation of Flo. km), a warm-core thermal structure may be inferred. One change during the preconditioning period of Flo Thirty-six hours later (Fig. 5b), the cyclonic tangential is the vertical shear between 300 and 150 mb (Fig. 4b). winds have increased and appear to have extended up- Although observational coverage is only fair (good) at ward at outer radii where the analyses should be more 150 (200) mb, and 300 mb winds are relatively sparse reliable. Since both the 850- and 200-mb winds have

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FIG. 6. Azimuthal-average tangential wind at 200 mb (contour in- terval of 4 m sϪ1; solid is cyclonic) as a function of radius from MQ analyses during 0000 UTC 13 Sep±0000 UTC 19 Sep in Flo. Notice the ``cyclonic wind burst'' with 200-mb cyclonic winds extending to about 1100 km at 1200 UTC 14 Sep, and then shrinking rapidly to FIG. 7. Multiquadric wind analysis (arrows, scale vector in upper- less than 600 km by 0600 UTC 15 Sep. right corner) and geopotential heights (lines, m) at 200 mb for (a) 0000 UTC 13 Sep 1990 and (b) 1200 UTC 14 Sep. Observations are increased in the 200±600-km radial band, the vertical included in (b) to support the cyclonic wind burst relative to Flo in the analysis. shear between these levels appears to have not changed (Fig. 4a). However, the upward extension of the cyclonic winds above 250 mb from Fig. 5a to Fig. 5b is consistent ``cyclonic wind burst'' has not been previously reported with a decreasing wind shear between 300 and 150 mb to the authors' knowledge, perhaps owing to the normal in Fig. 4b. paucity of upper-tropospheric observations all around Another perspective of the upper-tropospheric wind the TC. This cyclonic wind burst is consistent with the changes during the preconditioning phase is given by upward extension of azimuthally averaged tangential the 200-mb tangential wind evolution (Fig. 6). Although winds from below 250 mb in Fig. 5a to above 250 mb the 6-h time continuity could be better within 400 km in Fig. 5b. where separate maxima of 4 m sϪ1 are analyzed at 1200 Evolution of the 200-mb wind ®elds from the MQ UTC 13 September and following 1200 UTC 14 Sep- analyses during the preconditioning period leading up tember, an outward extension of cyclonic winds to about to the cyclonic wind burst in Flo is shown in Fig. 7. At 1100 km is indicated by more reliable outer wind ob- 0000 UTC 13 September (Fig. 7a), Flo is a tropical servations. Notice that the 200-mb cyclonic wind then depression at 14ЊN, 145ЊE (Fig. 1a) with almost-radial rapidly contracts to within 600 km of the center by 0600 200-mb out¯ow turning anticyclonic beyond about 5Њ UTC 15 September, and that the 200-mb winds then latitude from the center. At this time, Ed has an intensity become increasingly anticyclonic with time beyond this of 45 kt and is near 20ЊN, 131ЊE (Fig. 1b). Notice the radius. Since these rapid increases and decreases in cy- anticyclone trailing Ed well to the northwest of Flo, and clonic winds are documented in multiple independent a small upper-tropospheric cyclone to the north with a MQ analyses, and in the 4DDA analyses (not shown), 20-kt isotach maximum that facilitates out¯ow from Flo. they are not considered to be a spurious feature. This The major change during the subsequent 36 h leading

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values around 0600 UTC 15 September, which is when the cyclonic wind burst at 200 mb was decreasing. Al- though some maxima in the 6-h EFC calculations appear to have an inward propagation as in the Molinari et al. (1995, 1998) studies of Hurricanes Elena and Danny, it is also plausible to interpret the EFC event as being nearly simultaneous over the 500±1300-km radius, which would be consistent with the 200-mb cyclonic wind burst in this radial band (Fig. 6). Rather than a trough±TC interaction, Flo appears to have moved into an environment with an extended band of relative cy- clonic motion (Fig. 7b). Whereas some positive EFC values are also calculated at 150 mb (not shown) during 1800 UTC 13 September to 0000 UTC 14 September, it is not possible to track a coherent signal of an EFC maxima toward the center at that level. Thus, the 200- mb cyclonic wind burst appears to be related to the EFC, although not necessarily as a discrete trough propagating inward from large radii as in some previous studies (e.g., Molinari and Vollaro 1989; Molinari et al. 1995, 1998). The corresponding 200-mb radial wind evolution (Fig. 9) indicates the cyclonic wind burst period is also associated with an enhanced out¯ow. Azimuthal-aver- age out¯ows exceed 8 m sϪ1 between 400 and 1000 km early in the period and later in the cyclonic wind burst values above 6 m sϪ1 are found between 700 and 1300 km. After 1800 UTC 14 September until late in the life cycle, the 200-mb radial winds tend to be smaller at outer radii except for a temporary maximum of 8 m sϪ1 near 500 km radius at 1200 UTC 15 September. The FIG. 8. Eddy ¯ux convergence of relative angular momentum (con- vertical structure of the radial out¯ow during the 200- tour interval of 5 m sϪ1 dayϪ1; positive values are solid) at 200 mb mb cyclonic wind burst is indicated in Fig. 10. As noted from MQ analyses during 0000 UTC 13 Sep±0000 UTC 19 Sep in Ϫ1 Flo. above, azimuthal-average out¯ows exceeding 8 m s are found between 700 and 1000 km at 200 mb (and 150 mb). Although the details of the distribution within to the 200-mb cyclonic wind burst occurs to the north 300 km are not certain, a considerable radial gradient and west of Flo, which is indicated in Fig. 7b with the in the 200-mb out¯ow (Figs. 9 and 10) does exist be- supporting observations. At 1200 UTC 14 September, tween the center and the 8 m sϪ1 isotach. The coarse Flo is near 19ЊN, 138ЊE with an intensity of 55 kt (Fig. vertical resolution in these analyses, and lack of obser- 1a), and Ed is near 19ЊN, 122ЊE with an intensity of 70 vations in the planetary boundary layer, does not rep- kt (Fig. 1b). Notice the band of strong northeasterly resent well the expected large in¯ow near the surface. winds around 25Њ±30ЊN, 135ЊE to the north of Flo that While the out¯ow maximum during the early portion extends southwestward to over Ed, which is west of Flo. of the cyclonic wind burst is at 200 mb, the strongest In combination with the southeasterly winds to the east out¯ow is at 150 mb at 0000 UTC 14 September (not and northeast of Flo (also noted by Merrill and Velden shown) and then again at 200 mb by 1200 UTC 14 1996), these cyclonic winds relative to Flo lead to the September (see second maximum in Fig. 9). These radial 200-mb cyclonic wind burst in Fig. 6. However, this wind components are primarily based on the postex- interaction with the northeasterly current to the north- periment cloud-drift winds that were carefully matched west of Flo is transient and the northwestward out¯ow with rawinsonde observations by the University of Wis- from the rapidly intensifying Flo extends into the mid- consin. Merrill and Velden (1996) have analyzed the latitudes over the next 42 h (see Fig. 13). out¯ow evolution in time. The radial wind values in The 200-mb eddy ¯ux convergence of relative an- Figs. 9 and 10 are generally consistent with their in- gular momentum (EFC) based on the MQ analysis (Fig. terpretations, which were done in an isentropic coor- 8) and the 4DDA (not shown) indicate cyclonic increas- dinate system. Indeed, this consistency with the Merrill es of up to 20 m sϪ1 dayϪ1 during the cyclonic wind and Velden study is considered to be a validation of the burst. These cyclonic EFC values appear to extend in- MQ and 4DDA analyses in the upper troposphere where ward to about 500 km and outward to 1300 km. How- the TCM-90 data coverage is considered to be better ever, the EFC values then rapidly decrease to near-zero than at middle levels. Since similar good agreement with

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FIG. 10. Azimuthal-average radial wind (contour interval of 2 m sϪ1; positive/outward values are solid) as a function of radius in Flo from MQ analyses at 1800 UTC 13 Sep.

ical implications of the in¯ow would be to spin up the vortex in the midtroposphere. In summary, both the MQ and 4DDA analyses in- dicate a cyclonic wind burst in the 400±1200-km azi- muthal-average tangential wind (Fig. 12) occurred on 14 September during the preconditioning period prior

FIG. 9. Azimuthal-average radial wind at 200 mb (contour interval of2msϪ1; positive/outward values are solid) from MQ analyses during 0000 UTC 13 Sep±0000 UTC 19 Sep in Flo.

observations is found in the lower troposphere (Titley 1998), it is presumed that the middle levels are reason- ably well represented by the dynamically based anal- yses. Another consistent feature in the radial wind structure (Fig. 10) is a strong (2±3 m sϪ1), deep in¯ow just below the out¯ow layer. This compensating in¯ow in the mid- dle troposphere below the large out¯ow is reminiscent of the idealized response to an imposed angular mo- mentum ¯ux in a diagnostic study by Holland and Mer- rill (1984). The time evolution of the azimuthal-average 400-mb radial wind (Fig. 11) has in¯ow throughout the preconditioning period except for one off-time (0600 UTC 14 Sep) analysis. The horizontal extent and relative maxima of this in¯ow correspond well with the 200-mb out¯ow features. Since the 200- and 400 mb analyses are essentially done independently in the MQ, this cor- respondence is not forced by assumed vertical structure functions. The 4DDA analysis, which has a strong dy- namical constraint via the blending with the 6-h model forecast, also has this in¯ow layer below the out¯ow FIG. 11. Azimuthal-average radial wind as in Fig. 9, except at maxima during the preconditioning period. The dynam- 400 mb.

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intensity changes of Ed. This out¯ow was detected by the cirrus cloud-drift winds to rise as high as 150 mb, so the absence of a 200-mb cyclonic wind burst to large radii in Ed evidently is not attributable to absence of deep convection. Ambiguous values of EFC from the 4DDA and MQ analyses of Ed make conclusions re- garding this potential contribution somewhat tentative. Whereas the 4DDA analysis suggests an energetic, but short lived, 150-mb EFC event at 1200 UTC 14 Sep- tember that was extended downward to 200 mb by the vertical structure functions, the absence of supporting 200-mb observations for the MQ analysis resulted in a calculation of near-zero EFC values. Consequently, no preconditioning event of the magnitude and character FIG. 12. Azimuthal-average 200-mb tangential and radial winds of that for Flo is found for Ed, which also did not (left ordinate) and EFC (right ordinate) within 400±1200-km radial experience a rapid intensi®cation. band around Flo from 0000 UTC 13 Sep to 0000 UTC 19 Sep 1990 Titley (1998) also describes two other typhoons (Yan- as calculated from 4DDA analyses. Cyclonic winds are found from 1200 UTC 13 Sep to 1800 UTC 14 Sep. cy and Zola) that are included in the 4DDA analyses but not in the MQ analyses. In the case of Typhoon Yancy, a slow change over 54 h from 4 m sϪ1 anticy- to the rapid intensi®cation of Flo. The timing and radial clonic to 2 m sϪ1 cyclonic in the 400±1200-km radial structure of the eddy ¯ux convergence of relative an- band, and the absence of a rapid reversal to strength- gular momentum are consistent with the tangential wind ening anticyclonic winds, could not be interpreted as a increases at 200 mb during this period. An increase in cyclonic wind burst. In the case of Zola, the 400±1200- the azimuthal-average radial wind out¯ow at 200 mb km azimuthal-average tangential wind at 200 mb rapidly (Fig. 9) and in¯ow at 400 mb (Fig. 11) is contempo- increased from Ϫ6to1msϪ1 just 12 h later and then raneous with this persistent, large EFC event and the returned to large anticyclonic values over 24 h in con- change from anticyclonic to cyclonic winds in this 400± junction with an EFC forcing event. A second EFC forc- 1200-km radial band. Although the net horizontal di- ing event also led to a change from anticyclonic to about vergence implied by this out¯ow at 300-km radius (Figs. 2msϪ1 cyclonic. Although the intensity increased fol- 9 and 10) requires that compensating vertical motions lowing each of these events, the rate of increase was must occur somewhere within 300 km of the center, no not as large, or sustained for as long, as in Flo. interior observations are available to determine what that vertical motion distribution might be. Some of the c. Interpretation of the preconditioning conceptual compensating vertical motion probably originates from model the in¯ow layer (Figs. 10 and 11) just below the out¯ow layer. However, some of the compensating vertical mo- It is hypothesized that the upward displacement of tion within 300-km radius is hypothesized to be sub- the warm core in the preconditioning period conceptual sidence from above. As will be described in section 3c, model (Fig. 2), and implied in the Flo minus Ed tem- this subsidence is hypothesized to be the explanation perature differences (Fig. 3), is created in part by upper- for the warm core to be extended upward (Fig. 3) as in tropospheric subsidence in direct response to the en- the conceptual model (Fig. 2). hanced out¯ow at 200 mb during the cyclonic wind burst in Flo. An alternate explanation would be the radial out¯ow triggered more vigorous convection around the b. Other cases during TCM-90 center and that indirectly created compensating subsi- Similar tangential and radial winds, vertical wind dence and warming inside an ``eye.'' The thermal struc- shear, and EFC calculations were also made for Ty- ture differences between Flo and Ed (Fig. 3) are con- phoons Ed, Yancy, and Zola during the intensive ob- sistent with maintaining a more vigorous deep convec- serving periods of TCM-90. In contrast to Flo, no 200- tion near the center. Evidence of higher out¯ows (150 mb cyclonic wind burst or steady increase in the outer mb) is available from the geostationary satellite imagery radii anticyclonic winds is detected for Typhoon Ed (not (see also Merrill and Velden 1996). shown). Rather, little evidence of any cyclonic winds Whereas the upward displacement of the warm core greater than 4 m sϪ1 is found above 250 mb after 1200 above 200 mb and the decrease in the vertical shear UTC 14 September when cyclonic 200-mb winds only between 150 and 300 mb (Fig. 4b) are consistent, cause extend to a radius of 500±650 km. The anticyclonic and effect relationships cannot be resolved with this winds beyond this radius were quasi-steady at 4±8 m dataset. It is hypothesized (Fig. 2) that the 200-mb cy- sϪ1. Similarly, the azimuthal-average radial wind is also clonic wind burst in association with the EFC forcing quasi-steady at 4±6 m sϪ1, with no correlation with the event decreased the vertical shear in the upper tropo-

Unauthenticated | Downloaded 09/30/21 12:07 AM UTC 3566 MONTHLY WEATHER REVIEW VOLUME 128 sphere. Based on thermal wind considerations, the de- and beyond 300-km radius, where data coverage is bet- crease in 150±300-mb tangential wind shear would re- ter. Although the ®rst NASA DC-8 mission did provide quire a decrease in radial temperature gradient, which detailed observations at a ¯ight level of about 195 mb might simply be accomplished by displacement of the with dropwindsondes to describe some inner-core ver- warm core to higher elevations. From the hypsometric tical structures, this mission was centered around 0600 equation, an elevated warm core would imply a deeper UTC 16 September and, thus, was not until the end of sea level pressure, if no compensating cooling effects the rapid intensi®cation stage. A second DC-8 mission occur in the intervening layer. While the hypothesized was centered on 0600 UTC 17 September and provided process alone does not guarantee subsequent rapid in- a dropwindsonde that documented a 891-mb central sea tensi®cation, it may provide an indirect mechanism to level pressure, which corresponds to a maximum surface allow rapid intensi®cation in the future. wind estimate of 72 m sϪ1 (Fig. 4). Considering the Based on composites of rawinsondes, Fitzpatrick maximum ¯ight-level (195 mb) wind measured by the (1993) found that rapidly intensifying tropical cyclones DC-8 was 56 m sϪ1, the surface wind estimate seems had much deeper cyclonic ¯ow and, thus, argued that reasonable (if not conservative) and indicates that the the upper-tropospheric temperatures would be relatively rapid intensi®cation period led to a supertyphoon. cooler than in less rapidly intensifying cases. This ar- The evolution of the 200-mb tangential wind (Fig. 6) gument is consistent with the cool anomaly in the 300± in Flo indicates that the cyclonic wind burst had already 200-mb layer (Fig. 3) as the cyclonic wind layer was terminated by 1200 UTC 15 September, so that the az- displaced upward (Fig. 5) in Flo in response to the cy- imuthal-average cyclonic winds then extended to about clonic wind burst. 550 km. At radii beyond about 700 km, the anticyclonic Molinari and Vollaro (1990; Fig. 1) detected a de- winds increased rapidly, with a brief maximum of 20 crease in anticyclonic 200-mb wind 48±60 h before msϪ1 at 1500-km radius at the time of maximum in- deepening occurs. In their case, this cyclonic wind tensity. Although the discrete cyclonic maxima in the change was due to an approach of a midlatitude trough. inner region at the times of the two DC-8 missions The existence of the enhanced EFC event during the (described above) have validity, the remainder of the preconditioning period (rather than during the rapid in- analyses within about 300-km radius is not considered tensi®cation period) seems signi®cant. Inspection of reliable. Figs. 13 and 15 of Challa et al. (1998) suggests the eddy The primary features of the 200-mb circulation at the ¯uxes of angular momentum that provide the lateral time of the ®rst DC-8 mission are shown in Fig. 13. boundary condition for their numerical simulations have Notice the inner-core cyclonic circulation is con®ned to a decreasing trend 24 h prior to the start of the inte- about 450-km radius. Several regions of concentrated gration. They use the word ``catalyst'' to imply that the out¯ow are illustrated in the MQ analysis. Large cross- enhanced eddy ¯uxes may be part of the initiation of contour ¯ow is indicated to the north in a region of good the event, but are not essential during later stages (R. data coverage over the Japanese island network. The Pfeffer 1998, personal communication). This catalyst strong anticyclonic ¯ow exceeding 30 m sϪ1 about 15Њ concept is consistent with an idealized axisymmetric longitude to the east is between a trailing anticyclone simulation by Challa and Pfeffer (1980) in which only and a large upper low farther to the east. Anticyclonic 12 h of angular momentum forcing at the lateral bound- ¯ow is also found at larger radii on the equatorward ary was suf®cient to initiate vortex modi®cations that side. This dual out¯ow pattern toward the pole and to- ultimately led to an intense hurricane. Such a catalyst ward the equator is consistent with forecaster rules of role may be an appropriate interpretation for the EFC thumb (C. Guard, former director, JTWC, 1993, per- event during the preconditioning period of Flo. At least sonal communication) for more rapid intensi®cation. in this case, rapid intensi®cation followed the EFC One of the surprising aspects of this out¯ow pattern event, and the EFC values were then about an order of is that the azimuthal-average radial wind (Fig. 9) has a magnitude smaller during the rapid intensi®cation pe- relative minimum during the rapid intensi®cation stage riod. This contrasts with the continual eddy angular mo- relative to the preconditioning phase (or to the subse- mentum ¯ux used to force development of a hurricane- quent midlatitude interaction stage). Except for a tem- like vortex in some numerical simulations (e.g., Mont- porary maximum of 8 m sϪ1 near 400-km radius at 1800 gomery and Farrell 1993). UTC 15 September, the average radial winds are about 4msϪ1 in the outer region during the rapid intensi®- cation. Notice also that the midtropospheric in¯ow also 4. Rapid intensi®cation stage of Flo disappears during this stage (Fig. 11). Merrill and Vel- A distinct increase in the intensi®cation rate occurred den (1996) have previously noted this decrease in av- following 1200 UTC 15 September (Fig. 4). The inten- erage radial motion during the rapid intensi®cation stage sity of Flo increased 23 m sϪ1 in only 18 h, from 44 m of Flo. Thus, a simple secondary circulation model in sϪ1 at 1800 UTC 15 September to 67 m sϪ1 by 1200 which the strength of the out¯ow branch increases in UTC 16 September. The discussion here will concen- proportion to the central intensity does not apply in this trate on the changes analyzed in the upper troposphere case.

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FIG. 13. Observations (wind barbs, with long barb5msϪ1, and geopotential heights in m) and MQ wind analysis (arrows, scale vector in upper-right corner) at 200 mb for 0600 UTC 16 Sep 1990. Isotachs (dashed) are at 10 m sϪ1 intervals and a single height contour of 12 480 m is shown on this 5Њ lat and 5Њ long grid. The dense coverage of observations through the central region of Flo are from the ®rst NASA DC-8 mission.

Vertical wind shears between 850 and 200 mb (Fig. ample, Molinari and Vollaro (1989) suggested that EFC 4a) and 300 and 150 mb (Fig. 4b) are even smaller values within 300 km of the center were questionable. during the rapid intensi®cation stage than in the pre- However, both the MQ and 4DDA (not shown) EFC conditioning phase. Except for two MQ values of about evolutions clearly indicate that the angular momentum 6msϪ1, all 850±200-mb values are less than 5 m sϪ1. forcing in the radii beyond 400 km decreases by more Very consistent 300±150-mb values are found in the than 50% prior to the rapid intensi®cation. At least in MQ and 4DDA analyses, with a trend toward decreasing this one case of rapid intensi®cation, concurrent external vertical shear from the beginning of the rapid intensi- forcing is not a requirement. ®cation period following 1200 UTC 15 September to A summary of the azimuthal-average tangential and the maximum intensity at 1200 UTC 17 September. The radial winds and the EFC averaged in the 400±1200- small vertical 850±200-mb wind shears are well below km radial band (Fig. 12) con®rms these evolutions. A the Zehr (1992) criterion of 12.5 m sϪ1 that are believed considerable decrease in the radial wind and the EFC to inhibit TC formation. Even though these shear values after the preconditioning period is again evident. The are below this threshold value, it may not be concluded cyclonic wind increase during the preconditioning pe- that smaller shear values are necessarily more favorable riod is clearly followed by increasingly anticyclonic for intensi®cation. For example, the mean 850±200-mb winds in this radial band during the rapid intensi®cation vertical shears during Typhoon Ed from the 4DDA anal- stage. yses was 4.5 m sϪ1 compared to 6.1 m sϪ1 for Flo, and One interpretation of this lack of a direct correlation Ed did not experience rapid intensi®cation. Indeed, the of secondary circulation enhancement with intensity in- vigorous deep convection in the rapidly developing creases may be that an internal (within 300 km) read- storm may reduce the vertical shear, which may suggest justment is responsible. Perhaps the external forcing that vertical shear reduction is a symptom, rather than during the preconditioning period initiated contraction a precursor, of rapid intensi®cation. of an outer rainband of the Hurricane Danny type de- As hinted earlier, the maximum in 200-mb EFC is scribed by Willoughby (1995, Fig. 2.9), which is also during the preconditioning phase, and then decreases proposed by Molinari et al. (1998). Unfortunately, the during the rapid intensi®cation stage (Fig. 8). Given the near-continuous monitoring with airborne radar avail- variability in such an eddy calculation, caution is ad- able in the Atlantic is not available in western North vised in giving too much credence to short-lived EFC Paci®c. Whereas the DC-8 did have a weather radar, the maxima. This is especially true near the center; for ex- images were not recorded for later analysis. Although

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FIG. 14. Observations and MQ wind analysis as in Fig. 13 except at 0600 UTC 18 Sep, and during the third NASA DC-8 mission. the science leader (G. Dunnavan 1990, personal com- 4DDA analysis 850±200-mb vertical shear values (Fig. munication) on this mission did note concentric eye- 4a) in the 200±600-km radial band oscillated between walls near the center, the origin or evolution of these Ͻ4msϪ1 (1200 UTC) and Ͻ8msϪ1 (0600 and 1800 features prior to the mission (recall this was near the UTC), and the MQ interpolation values remained less end of the rapid intensi®cation) is unknown. than 4.5 m sϪ1. It is assumed here the 1200 and 0000 Another alternative focuses on the EFC event during UTC 4DDA analyses are more reliable than at 0600 and the preconditioning triggering radial winds, and the up- 1800 UTC (off synoptic) as they agree in magnitude ward extension of cyclonic winds that led to a thermal with all of the MQ analyses that draw closely to the structure adjustment (Fig. 2). A possible interpretation observations. Thus, vertical shear does not seem to be is that an upward displacement or modi®cation of the a primary factor in the rapid intensity decrease. One upper-tropospheric thermal structure permits a higher interpretation is that the intense cyclonic circulation is maximum potential intensity as described by Emanuel able to resist the inhibiting effects of relatively weak (1988) and Holland (1997). environmental shear until the intensity and central con- Unfortunately, the absence of continual internal data vection are diminished by other processes. in Flo during this critical rapid intensi®cation stage does The interaction of the out¯ow of Flo with the mid- not allow resolution of the alternate mechanisms. Both latitude jet at 0600 UTC 16 September (Fig. 13) con- external region dropwindsondes and concurrent aircraft tinued and intensi®ed. At the time (0600 UTC 18 Sep- radar in the inner region as in the Hurricane Research tember) of the third NASA DC-8 mission during the Division Synoptic Flow Experiments (Franklin et al. period of rapid intensity decrease, the cyclonic out¯ow 1993) are required during the rapid intensi®cation stage. was con®ned to a relatively small radius (Fig. 14). Ex- treme cross-isobaric ¯ow toward lower 200-mb heights, and thus rapid accelerations downstream, occurred in 5. Rapid decay stage the out¯ow toward the pole. Another out¯ow branch As indicated in Fig. 4, the intensity of Flo also de- was toward the southeast, part of which curved cyclon- creased after having reached maximum intensity at ically around the large upper low that had nearly over- 0600±1200 UTC 17 September, with a decrease of 50 taken Flo. Another portion of the southern out¯ow con- kt in 36 h. Fortunately, the decrease occurred over open tinued southward and joined the tropical easterly jet over ocean prior to Flo striking Japan, or even greater damage the Philippines (not shown). From an evacuation of mass would have occurred. In this sense, Flo was a similar perspective, this out¯ow pattern would seem to be fa- case to Hurricane Opal that also ®lled rapidly before vorable for intensi®cation, rather than a rapid intensity striking the Gulf of Mexico coast. decrease. While the rapid intensity decrease was occurring, the The azimuthal-average anticyclonic winds at radii be-

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FIG. 15. Eddy ¯ux convergence of angular momentum (contour FIG. 16. Azimuthal-average radial wind as in Fig. 10 except at interval of 5 m sϪ1 dayϪ1; positive values are solid) as function of 1800 UTC 17 Sep 1990. pressure and radius from MQ analysis at 1200 UTC 17 Sep 1990.

action stage is evident in Fig. 9 at all radii beyond 300 yond 900 km decreased during the period of rapid in- km. Values exceed 12 m sϪ1 at the center time of rapid tensity decrease (Fig. 6). As indicated in Fig. 14, the intensity decrease. Here again, the upper branch of the out¯ow toward the north was more radial without spi- secondary circulation implied by the azimuthal average raling anticyclonically. Another major change in the out- is counterintuitive, in that the out¯ow is increasing when ¯ow pattern from that during the most intense stage (Fig. the central intensity of the primary circulation is de- 13) was the approach of the upper low. Whereas pre- creasing. viously the out¯ow curved anticyclonically as it ¯owed A summary of these 200-mb azimuthal-average southward to the west of this upper low, the out¯ow (400±1200-km radial band) wind components and EFC was now more directly toward and around the upper trends during the rapid intensity decrease is given on low (Fig. 14), so that between 10Њ and 15Њ longitude to the right side of Fig. 12. After having only small EFC the east the ¯ow was cyclonic relative to the tropical values during the rapid intensi®cation, the rapid climb cyclone. Only weakly anticyclonic ¯ow was to the south in EFC forcing during the rapid intensity decrease pe- or to the west of the center beyond 10Њ latitude radius. riod is quite striking. The radial wind appears to become A major change in the 200-mb EFC (Fig. 8) occurs increasingly outward concurrent with the EFC forcing. during the midlatitude interaction stage. Whereas the However, the azimuthal-average 200-mb tangential EFC values had been small positive or slightly negative wind in this radial band is essentially constant through- throughout the rapid intensi®cation stage, large positive out the decreasing intensity period, rather than increas- EFC values appear ®rst at outer radii and seem to prop- ing in response to the layer out¯ow. agate inward. However, these eddy angular momentum The vertical and radial structure of the EFC (Fig. 15) ¯uxes are dominated by large (positive) radial com- at the time of maximum intensity (1200 UTC 17 Sep) ponents in the out¯ow jet to the north (Figs. 13 and 14) provides a clue to the subsequent intensity decrease. The that are slightly anticyclonic (negative). That is, nega- 200-mb EFC maximum found at about 1200-km radius tive angular momentum is being exported from the cyl- has a limited radial extent that is related to the northern inder around the cyclone. The EFC magnitudes increase out¯ow interacting with the midlatitude jet, which was in time as the tropical cyclone approaches the midlat- about to begin already at 0600 UTC 16 September (Fig. itude jet, not because a midlatitude trough approaches 13) and was very strong by 0600 UTC 18 September the tropical cyclone as in the Molinari and Vollaro (Fig. 14). The inward displacement of this concentrated (1989) case. In the deep Tropics away from a trough 200-mb EFC forcing is also indicated in Fig. 8. How- interaction, the Coriolis torque would be expected to ever, the special feature in Fig. 15 is that the maximum cause the tropical cyclone out¯ow to curve anticyclon- EFC forcing extends down to 500 mb, which suggests ically into an out¯ow jet. In this case with a large 200- an interaction with a midlatitude jet over a rather large mb height gradient to the north, the out¯ow from Flo vertical extent. was a downstream acceleration in an almost radial di- As in previous ®gures, a response to this EFC forcing rection, so that the EFC increased (Fig. 8), but the tan- is found in the azimuthal-average radial wind structure gential winds actually became less anticyclonic (Fig. 7). 6 h later (Fig. 16). Rather than an in¯ow as in the 400- A simple model of large EFC forcing causing an in- mb radial winds during the preconditioning period (Fig. crease in the inner cyclonic winds (intensity) clearly 11), an out¯ow is initiated in the middle troposphere. does not apply during this rapid ®lling stage. Although the maximum values are only2msϪ1 in the The increase in the azimuthal-average out¯ow from outer regions where the EFC forcing is located, positive the rapid intensi®cation stage to the midlatitude inter- (outward) values are found as far inward as 500-km

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FIG. 17. Schematic of the deep secondary (in±up±out) circulation at time of maximum intensity (left) being changed to a shallower secondary circulation in response to midtropospheric out¯ow induced by the tropical cyclone outer circulation interacting with the midlatitude ¯ow to create an EFC that extends downward to the middle troposphere. radius. Although the 400-mb level was perhaps not op- action with a deeper midlatitude trough would be con- timal early in the midlatitude interaction period, maxima sidered a bad trough. of2msϪ1 occur later during the rapid decay period This hypothesized sequence as the outer tropical cy- (Fig. 11). clone circulation interacts with the midlatitude circu- In summary, the EFC values and radial winds indicate lation may explain how the storm can weaken without signi®cant midtropospheric interaction with the midlat- the central region reaching cold water or passing over itude trough to the west and north of Flo while Flo land. Neither is it necessary for the inner-core convec- reaches peak intensity. It is hypothesized that the mid- tion to be exposed to the large vertical shear of the tropospheric out¯ow starts to degrade the ef®cient in± midlatitude jet for rapid weakening to occur, as the hy- up±out secondary circulation. Clearly the 200-mb out- pothesized process begins in the outer regions, and in- ¯ow is accelerated as the parcels ¯ow down the geo- creases while the center is still well equatorward of the potential height gradient (Fig. 14). However, the deeper midlatitude jet. layer of EFC forcing (Fig. 15) and the midtropospheric out¯ow (Fig. 16) are not considered to be favorable. It is proposed that the midtropospheric out¯ow will be 6. Summary and discussion part of a more shallow secondary circulation in which a. Data and analyses latent heat release will be at lower elevations and at larger radii (Fig. 17). One interpretation is that convec- This case of rapid intensi®cation and rapid decay of tively induced warming at outer radii decreases the ra- Flo occurred at the same time and in same area as an dial temperature gradient between the central region and ``ordinary'' intensi®cation of Typhoon Ed, which then the near environment so that the radial pressure gradient provides a contrasting case. These storms occurred dur- is decreased, and the maximum winds decrease. An al- ing TCM-90 and three other ®eld experiments, which ternative interpretation is that such an outer secondary provided the best dataset for studying tropical cyclones circulation will tend to ``rob'' some of the deeper sec- in the western North Paci®c. Having two high resolution ondary circulation that has maximum in¯ow near the in space (50 km, each 50 mb) and time (6 h) analyses surface and, thus, gains heat and moisture from the of this dataset has allowed more con®dence in the in- ocean to achieve highest ␪e, deepest ascent at the eye- terpretations presented here. First, the so-called ®nal wall, and out¯ow at the highest elevations. As stated TCM-90 analyses were produced with the NCEP me- above, ¯ow across the 200-mb geopotential gradient soscale four-dimensional data assimilation system that accelerates the EFC and out¯ow at that level. However, used the Eta Model as a basis for continued 6-h assim- the deep EFC that is initiated when the outer circulation ilation of each intensive observing period during TCM- of Flo begins to ``feel'' the midlatitude pressure gra- 90. Second, a special set of multiquadric analyses was dients down to the midtroposphere also creates midtro- prepared during the TCM-90 intensive observing period pospheric out¯ow. As Flo continues northward, the mid- that included Flo and Ed. The MQ technique provides tropospheric out¯ow will come closer and closer to the a regional reanalysis that draws closely to observations center, and more of the secondary circulation will have where available and is bounded to the NOGAPS analysis ascent over a shallower depth. Because this ascent is at in data-sparse regions away from the TCM-90 special smaller ␪e values, and latent heat will be released over observations. Another special feature of the MQ anal- shallower depths and at larger radii, the storm will weak- yses is insertion of synthetic observations to better de- en. In the terminology of Molinari (1998), this inter- ®ne the TC center location and structure (as can be

Unauthenticated | Downloaded 09/30/21 12:07 AM UTC OCTOBER 2000 TITLEY AND ELSBERRY 3571 resolved on a 50-km grid). Without any synthetic ob- tional 850±200-mb shear does not appear to be an in- servations, the 4DDA depends primarily on the 6-h hibiting factor to intensi®cation. That is, these shear model forecasts to provide the tropical cyclone location values are small relative to the Zehr (1992) threshold and inner structure where few observations exist. value. Vertical shears are reduced even further during Independent ¯ux and budget calculations in storm the rapid intensi®cation. Overall, the vertical shears dur- coordinates with 50-km radial increments have allowed ing Flo were slightly larger than during Ed, which did judgments as to the likely reality of the radial variations not rapidly intensify. That is, small 850±200-mb vertical in these calculations. All of the key features describing shear may be a necessary, but not a suf®cient, condition the rapid intensity increase and subsequent decrease are for intensi®cation. present in both analyses. In general, the MQ analyses A special aspect during the preconditioning period of have been preferred for display because that technique Flo was a 35% decrease in the 300±150-mb vertical ®t the observations more closely. For example, the MQ shear. This shear decrease occurred in conjunction with analysis had larger magnitudes of azimuthal-average a 200-mb cyclonic wind burst that extended to about tangential and radial winds, and also the derived quan- 1100-km radius. Prior to the rapid intensi®cation, the tities such as angular momentum ¯uxes in the upper radius of cyclonic tangential winds reduced to within troposphere. The azimuthal-average radial winds from 600 km of the center, and the outer winds became in- the MQ analysis also agreed well with the manual, is- creasingly anticyclonic until maximum intensity was entropic analyses by Merrill and Velden (1996) of the achieved. Three other typhoons during TCM-90 that did out¯ow from Flo. not have sustained rapid intensi®cation were checked, and these did not have such a cyclonic wind burst at 200 mb. b. Rapid intensi®cation Other variables appeared to have changes consistent A single case study such as this cannot provide nec- with a cyclonic wind burst. A concurrent 200-mb EFC essary and suf®cient conditions, or describe the multiple forcing event with magnitudes exceeding 20 m sϪ1 dayϪ1 ways in which a tropical cyclone might rapidly intensify. would seem to provide an explanation for the cyclonic Thus, the purpose here is to summarize some of the spinup. These EFC values also decreased to small values special aspects of this case and highlight some results concurrently with the end of the cyclonic wind burst. that deviate from other cases or numerical model sim- Larger azimuthal-average radial winds also occurred at ulations. This summary is organized in terms of the 200 mb in conjunction with the cyclonic wind burst. favorable environmental factors for tropical cyclone in- The radial variations in this out¯ow near the center tensi®cation in section 1. would imply a compensating vertical mass ¯ux. Some Because the SST analyses had values of 28Њ±28.5ЊC of this mass ¯ux was from below in the form of deep throughout the region of intensi®cation, the requirement convection, with tops extending to 150 mb and higher. for an SST greater than 26ЊC is not a factor. Available It is also hypothesized that some of the compensating ocean thermal structure analyses do not indicate warm vertical mass ¯ux was forced subsidence from the upper eddies, such as the Loop Current in the Gulf of Mexico, troposphere, which then lead to an upward displacement that might account for rapid intensi®cation. Because of of the warm core mentioned above as a special feature the deep ocean mixed layers in this region, the tropical of the thermal structure of Flo relative to Ed. Although cyclone-induced SST decreases were generally less the how the thermal wind balance is achieved is not clear, 1.5ЊC, which did not inhibit Flo from becoming an in- a decrease in the vertical shear across 200 mb (i.e., the tense typhoon. 300±150-mb shear) is consistent with an upward dis- Whereas a maximum potential intensity (MPI) based placement of the warm core mentioned above. Finally, only on these SSTs would suggest a value of about 70 a layer of in¯ow was established below the layer of msϪ1, Flo achieved, and probably exceeded, this value enhanced out¯ow. Such an in¯ow is consistent with a and Ed did not. While suf®ciently high SSTs are a nec- further spinup of the cyclonic winds in the middle to essary condition for intense TCs, they are not suf®cient upper troposphere. to guarantee rapid intensi®cation, or that the TC will The observed limited period of EFC forcing during eventually reach the MPI. Estimates of MPI by Emanuel the preconditioning phase of Flo seems to be analogous (1988) and Holland (1997) also depend on the upper- to the numerical simulations of Challa and Pfeffer tropospheric thermal structure. A key comparison of the (1980) and Challa et al. (1998). That is, the EFC appears thermal structures of Flo and Ed at similar intensities to have a ``catalyst role,'' rather than being required to indicates a more unstable troposphere and warmer upper be a continued forcing as in the Montgomery and Farrell troposphere over the central region of Flo prior to the (1993) simulation of tropical cyclone formation. rapid intensi®cation. Thus, a preconditioning stage may Unfortunately, many aspects of the rapid intensi®- have created a more favorable environment for later cation of Flo were not resolved because of inadequate intensi®cation as proposed by Gray (1992). aircraft observations in the inner region. Whereas some Two calculations of vertical shear in a radial band of ideas about environmental conditions during rapid in- 200±600 km provide key information. First, the tradi- tensi®cation can be con®rmed, other ideas cannot be

Unauthenticated | Downloaded 09/30/21 12:07 AM UTC 3572 MONTHLY WEATHER REVIEW VOLUME 128 substantiated. The 850±200-mb vertical wind shear did Acknowledgments. This research is a part of a dis- reach minimum values during the most intense stage of sertation and thanks go to the committee members for Flo; however, this appears to be more of a symptom their inputs, and many colleagues who contributed to rather than a requirement for intensi®cation. The rapid its completion. Pat Harr and Pete Klein reviewed the intensi®cation appeared to follow establishment of a manuscript, which was ef®ciently prepared by Mrs. Pen- second, and strong, out¯ow channel toward the mid- ny Jones. The second author's participation was funded latitudes, which forecasters have used as a favorable by the Of®ce of Naval Research Marine Meteorology indicator of intensi®cation. However, the azimuthal-av- Program, which supported the TCM-90 ®eld experiment erage radial wind actually decreased. That is, a con- and its analysis. ceptual model of the out¯ow branch of the secondary circulation (in±up±out) being directly correlated with REFERENCES the intensity is not supported. As mentioned above, the EFC values were much smaller during the rapid inten- Black, P. G., and L. K. Shay, 1998: Observations of tropical cyclone si®cation stage than during the preconditioning phase, intensity change due to air±sea interaction processes. Preprints, so a requirement for the direct EFC forcing is not sup- Symp. on Tropical Cyclone Intensity Change, Phoenix, AZ, Amer. Meteor. Soc., 161±168. ported. One interpretation is that once the precondi- Challa, M., and R. L. Pfeffer, 1980: Effects of eddy ¯uxes of angular tioning stage had established favorable conditions, an momentum on model hurricane development. J. Atmos. Sci., 37, internal process (not resolved by this dataset) resulted 1603±1618. in rapid intensi®cation. This could have been an outer , , Q. Zhao, and S. W. 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