A Diagnostic Study of the Intensity of Three Tropical Cyclones in the Australian Region

Home , Central dense overcast, Cyclone Kathy, Cyclone Laurence, Maximum sustained wind, Radius of maximum wind, Rapid intensification

VOLUME 138 MONTHLY WEATHER REVIEW JANUARY 2010

A Diagnostic Study of the Intensity of Three Tropical Cyclones in the Australian Region. Part I: A Synopsis of Observed Features of Tropical Cyclone Kathy (1984)

FRANCE LAJOIE AND KEVIN WALSH School of Earth Sciences, University of Melbourne, Parkville, Australia

(Manuscript received 20 November 2008, in ﬁnal form 13 May 2009)

ABSTRACT

Objective streamline analyses and digitized high-resolution IR satellite cloud data have been used to examine in detail the changes in the environmental circulation and in the cloud structure that took place in and around Tropical Cyclone Kathy (1984) when it started to intensify, and during its intensiﬁcation and dissipation stages. The change of low-level circulation around Tropical Cyclone Kathy was measured by the

change in the angle of inﬂow (a4) at a radius of 48 latitude from the cyclone center. When Kathy started to intensify, a4 increased suddenly from 208 to 42.58 in the northerly airstream to the north and northeast of the depression, and decreased to 08 to the south of the depression. At that stage the low-level circulation around the depression appeared as a giant swirl that started some 600 km to the north and northeast of the depression and spiraled inward toward its center, while trade air, which is usually cool, dry, and stable, did not enter the

cyclonic circulation. The angle a4 remained the same during intensification. During the dissipation stage, a4 returned to 208 and trade air started to participate in the cyclonic circulation. Satellite cloud data were used to determine the origin, evolution, and importance of the feeder bands in the intensification of the cyclone, to follow the moist near-equatorial air that flowed through them and to estimate the maximum height of cumulonimbi that developed in them, to observe the changes in the convective activity in the central dense overcast (CDO) area, as well as in the area around the CDO. Most of the observed changes in Kathy have also been observed in other tropical cyclones during intensification and dissipation. Using the sequence of observed changes of the circulation and of convective activity in and around the CDO of Kathy, a mathematical model has been developed to forecast the intensity of a tropical cyclone. The model and its application to three tropical cyclones in the Australian region are described in Part II of this paper.

1. Introduction a tropical cyclone without having to consider these elusive parameters and processes. Meteorological parameters and physical atmospheric processes that can influence the intensification of a a. Tropical cyclone development processes tropical cyclone have been identified and discussed by Processes that have been shown to influence tropical many investigators. Apart from having insufficient con- cyclone development are as follows. ventional observations to accurately determine the magnitude of these parameters or processes, none of the latter 1) THE SEA SURFACE TEMPERATURE is consistent all the time as any of them can produce When considering a large number of tropical cyclones, intensification or weakening in one cyclone but not nec- the minimum central surface pressure p is found to be essarily in others. We shall briefly consider this problem c related to the SST between 268 and 308C: the greater the below. In Lajoie and Walsh (2010, hereafter Part II), we SST the lesser is p (Miller 1958; Titley and Elsberry will discuss a model that can forecast, with a lead time of c 2000). This relationship appears to break down for 15 (possibly 30) h, the central surface pressure, as well as SST . 308C (Merrill 1988; Evans 1993; DeMaria and the radial and the azimuthal wind distribution around Kaplan 1994), with only a few intense storms being observed at these temperatures (Kaplan and DeMaria 2003).

2) RELATIVE HUMIDITY Corresponding author address: Kevin Walsh, School of Earth Sciences, University of Melbourne, Parkville VIC, 3010 Australia. High relative humidity (.70%) either in the boundary E-mail: [email protected] layer (Schade and Emanuel 1999) or in the 850–700-hPa

DOI: 10.1175/2009MWR2875.1

Ó 2010 American Meteorological Society 3 Unauthenticated | Downloaded 10/01/21 03:15 PM UTC 4 MONTHLY WEATHER REVIEW VOLUME 138 layer (Kaplan and DeMaria 2003) plays a significant role flow by an axisymmetrization process that causes the in the development and intensification rate of a tropical tangential winds at a radius outside the initial RMW to cyclone. The latter authors, however, found that only increase, thus causing the vortex to intensify and the about 66% (80%) of tropical cyclones with relative hu- RMW to expand. This process can also produce a sig- midity greater than 70% (.80%) in the 850–700-hPa nificant increase of intensification rate (Montgomery layer undergo rapid intensification, defined as either and Enagonio 1998; Moller and Montgomery 2000). On an increase of sustained mean surface wind of at least the other hand, Bister (2001) has argued that the latent 15.4 m s21 (30 kt) in 24 h or a decrease of central sur- heat released in the rainbands of the VRWs reduces the face pressure of at least 15 hPa in 24 h. radial pressure gradient and causes the tropical cyclone to weaken. 3) VERTICAL WIND SHEAR 6) LOW-LEVEL INWARD FLUX OF MOMENTUM In general, if the vertical shear of environmental horizontal winds between 850 and 200 hPa is less than Low-level environmental forcing can also produce 5ms21, a tropical cyclone may intensify, but no in- intensification of a tropical cyclone. Using conventional tensification is likely to occur if the shear is 10–15 m s21 surface wind fields, Molinari and Skubis (1985) dis- (Simpson and Riehl 1958; Gray 1979; McBride and Zehr cussed a case of wavelike perturbation, characterized by 1981; Merrill 1988; Zehr 1992; DeMaria and Kaplan a surge of inflow and upward vertical motion, that de- 1994; DeMaria 1996; Hanley et al. 2001; Kimball and veloped at a radius of 1600 km on the equatorward and Evans 2002; Kaplan and DeMaria 2003). A few tropical east side of a tropical depression, and that propagated cyclones have however been observed to intensify in inward toward the depression at 15 m s21 to reach the face of strong vertical shear (Molinari 1998). storm center 36 h after its development. Molinari and Skubis used a momentum budget to show that the in- 4) UPPER-TROPOSPHERE TROUGH INTERACTION ward flux of momentum contributed to a significant The interaction between an upper-air trough and momentum source within 440 km of the center of the a tropical cyclone can also play an important role in its depression and produced a rapid intensification of the intensification (Molinari et al. 1995, 1998; Emanuel depression when the surge reached its center. 1997; Shi et al. 1997) and in its rapid intensification 7) OTHER ATMOSPHERIC PROCESSES (Titley and Elsberry 2000; Kaplan and DeMaria 2003). Yet, using a 3-yr data sample, Hanley et al. (2001) found Vortex Rossby wave (VRW) rainbands associated that only 61% of all distant large-scale troughs and 78% with extensive cirrostratus and convective clouds (May of all small-scale troughs caused storms to intensify, 1996) do sometimes extend to 150 km from the cyclone while in 82% of no-trough cases the tropical cyclone also center (Kuo et al. 1999; Chow et al. 2002; Wang 2002). intensified. Hanley et al. (2001) also found that 30% of The vertical gradient of diabatic heating that results no-trough cases exhibit rapid intensification ($20 hPa from the release of latent heat in the stratiform cloud in 12 h), compared to 16% for small-scale troughs. and the evaporational cooling below the cloud base produces a PV concentration in the midtroposphere 5) VORTEX ROSSBY WAVES (Raymond and Jiang 1990). This PV concentration is Vortex Rossby waves can develop on both sides of the sometimes associated with midlevel jets (May 1996). radius of maximum wind (RMW) of a tropical cyclone While May and Holland (1999) have argued that this PV because of the large potential vorticity (PV) gradients production can be transported into the inner core and in these areas (Montgomery and Kallenbach 1997; Kuo causes intensification, Bister (2001) has suggested that et al. 1999; Corbosiero et al. 2006). When they evolve the latent heat released by the rainbands reduces the within the RMW, their associated convective clouds give radial pressure gradient, which then weakens the tropi- rise to asymmetries in the eyewall (Kuo et al. 1999; cal cyclone. Kossin and Schubert 2002; Chen and Yau 2001; Wang The PV concentration in the midtroposphere can also 2001, 2002), but when they evolve outside the RMW, initiate mesoscale vortices that are associated with outer they manifest themselves as spiral rainbands that de- rainbands beneath the outflowing cirrostratus bands velop within 50–80 km from the cyclone center and that (Houze 1977; Zipser 1977). Whether this process plays move cyclonically around the vortex while traveling a positive or negative role in the intensification of a trop- outward from the RMW (Kimball and Evans 2002). ical cyclone is not known yet (Zhang et al. 2002). According to Montgomery and Kallenbach (1997) and The large number of environmental parameters and Moller and Montgomery (1999), the energy of the atmospheric processes that can affect the intensity of VRWs is transferred from the asymmetry into a circular a tropical cyclone one way or another makes intensity

Unauthenticated | Downloaded 10/01/21 03:15 PM UTC JANUARY 2010 L A J O I E A N D W A L S H 5 forecasting one of the most challenging tasks of the meteorologist. Let us now consider some of the techniques that are used to predict the intensity of tropical cyclones and their general performance. b. Tropical cyclone intensity forecasting techniques The Dvorak (1975, 1984) technique for determining the intensity of a tropical cyclone is based on an em- pirically determined conceptual model of the daily development of cloud features in and around the cyclone. The cloud features are subjectively determined in visible and IR satellite cloud pictures. The technique is widely used in all tropical cyclone basins and is so far the most reliable technique to forecast the intensity of a tropical cyclone (Velden et al. 2006). By using a set of specified rules, a T number is determined which, according to a table prepared by Dvorak, corresponds to the current and 24-h forecast of pc, the central surface pressure, and FIG. 1. Best-track vs Dvorak maximum sustained surface wind estimates (kt). The solid line indicates the best-fit linear relation- Vrm, the maximum sustained mean surface winds at the radius of maximum wind. The objective Dvorak tech- ship; the dashed line is a perfect (y–x) relationship (from Brown and Franklin 2002). nique of Velden et al. (1998) showed an RMS error of estimating pc of 8.3 hPa. Errors in estimating the maximum wind from these the troposphere. The disadvantage is that the spatial techniques can be quite large, though. This is because of resolution is coarse: even in the Advanced Microwave the large scatter that exists when Vrm is plotted against Sounding Unit (AMSU) used by Spencer and Braswell pc. Shea and Gray (1973) show results from recon- (2001), Brueske and Velden (2003), and Demuth et al. naissance flight data for North Atlantic hurricanes, (2004), it varies between 48 and 100 km. Errors in cy- clearly indicating that there is not a one-to-one re- clone intensity forecasts using AMSU data are compa- lationship between pc and Vrm. Brown and Franklin rable to those by Dvorak’s technique: 72.5% of errors (2002) also observed a similar large variance when re- were within 15 kt and 13% were between 20 and 57 kt, connaissance-based ‘‘best track’’ maximum wind speeds the larger errors being associated with cyclones having in North Atlantic hurricanes were plotted against the small radius of maximum wind (Demuth et al. 2004). relevant Dvorak estimates of maximum wind speeds Factors that contribute to the large variance in the

(see Fig. 1). For a Dvorak maximum wind estimate of pc/Vrm relationship are storm motion (Schwerdt et al. 80 kt, the best-track maximum winds varied from 50 1979), intensity trend (Koba et al. 1990), size, latitude, to 100 kt. Another method for forecasting tropical cy- and rm, the radius of maximum wind (Knaff and Zehr clone intensity in operational use in the United States is 2007). According to the latter authors storm motion and the Statistical Hurricane Intensity Prediction Scheme intensity trend have a relatively small effect on the re- (SHIPS) model developed by DeMaria and Kaplan duction of the scatter, while other factors have a small (1999) and DeMaria et al. (2005), based on a statistical but significant effect. They could not, however, study the multiple regression technique. A number of climato- effect of rm because this parameter was not available or logical, persistence, and synoptic factors are obtained was not accurate enough in the archived data. But even from objective forecast fields, and after removal of the after having taken all these factors, except rm, into cyclone circulation are used as predictors. The magni- consideration they found that there was still a significant tude of errors produced by this method is similar to that scatter and concluded that this may be due to the effects of Dvorak. of other factors such as the one used to reduce flight

Another statistical method for forecasting the in- wind data to surface wind and rm. tensity of a tropical cyclone uses data from passive mi- Another method that has been used to determine Vrm, crowave radiometers on polar-orbiting satellites. The the maximum wind at the gradient level, is to assume advantages of using this method are that microwaves of some parametric equation to approximate the radial different wavelengths can penetrate below cloud tops proﬁle of the surface pressure and then use either the and can sense the temperature, the average amount of cyclostrophic or gradient wind equation to determine cloud liquid water, and rain rates at different levels in Vrm (see e.g., Atkinson and Holliday 1975, 1977; Holland

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1980). Then using a reduction factor the maximum sur- area extending about 1000 km from the cyclone cen- face wind is obtained. According to Love and Murphy ter. We have observed a number of characteristic time (1985), Holland’s (1980) technique underestimates max- changes of environmental circulation and of cloud imum wind by 10%–23%. On the other hand, Willoughby structure in and around a tropical cyclone when it starts and Rahn (2004) found that for North Atlantic hurri- to intensify, and during its intensification and dissipa- canes (i) the estimated maximum surface winds from the tion. These time changes have been incorporated in Holland (1980) wind profile are too strong with an RMS a simple model that was found to be capable of fore- 21 error of 4.2 m s , and (ii) the winds 2–3 re (eye radius) casting, with a good degree of accuracy, not only pc, Vrm, away from rm (radius of maximum wind) are in error by and the intensification rate, but also the time changes of about 50%. Holland (2008) has recently modified his the radial distribution of the surface pressure and of the original model to determine the surface maximum wind winds for three tropical cyclones for which some good and in so doing making bs, the shape factor of the radial ground truth data existed. These time changes of the surface pressure profile, a function of Dp (1010 2 pc), observed features are discussed in the remaining sec- (›pc/›t) the rate of change of intensity, the latitude, and tions of this paper, while the model and the forecast the speed of translation of the storm. Holland found that results are discussed in Part II. his new model ‘‘captures successfully much of the observed scatter with major errors considered to arise from local transients and major asymmetries that cannot 2. Observed features be covered by a general pressure-wind relationship.’’ Objective wind analyses and high-resolution digitized Nevertheless, the accuracy of determining V rests on rm IR satellite cloud data have been used to observe the an evaluation of Dp that in most cases is obtained by evolution in the large-scale environmental circulation Dvorak’s technique. and in the cloud structure associated with Tropical Cy- For intensity forecasting, the NWP models are not clone Kathy in March 1984. This cyclone was selected performing better than statistical techniques. According for study for the following reasons: it started to re- to Elsberry (2002), no dynamic model can show signifi- generate when crossing the coast of the Cape York cant skill in the prediction of tropical cyclone intensity Peninsula to move over water so that the time at which it until the model can correctly predict the surface mois- started to intensify is known, it reached its maximum ture flux from the ocean, the wind structure in the intensity after 63 h of intensification, it started to boundary layer, and the advection of moisture in the weaken over water while still about 100 km from land- lower layers of the atmosphere that contributes to fall, and it passed over a meteorological station that the active convective clouds and heavy precipitation in provided recorded observations of its intensity. the eyewall. The problem is compounded by the lack of data over the ocean, inadequate model resolution and a. Track and intensity of Tropical Cyclone Kathy physics, poor initial conditions, and insufficient understanding of the physical processes governing inten- Kathy was an intense tropical cyclone. Its best- sity change (Wang and Wu 2004), although there have determined track, after Thom (1984) and Falls and recently been some improvements in skill (Surgi et al. Murphy (1984), is shown in Fig. 2. The 6-figure number 2008). close to each of the 6-hourly positions of the cyclone center indicates PPddtt, where PP is the last 2 digits of c. Summary the estimated central pressure of the cyclone in hPa for It is clear that none of the factors influencing the in- the date (dd) and time (tt) in hours (UTC). The time tensity of a tropical cyclone is always accompanied by variation of the central surface pressure, which was es- either intensification or weakening and that appreciable timated using Dvorak’s (1975) technique, is shown in errors can and do occur, particularly in the forecasting of Fig. 3. To the east of Cape York Peninsula, Kathy was the maximum winds. More to the point, forecasting the in a steady-state condition for 30 h with an estimated wind profile within 100 km from the cyclone center and minimum central surface pressure of 990 hPa. Once the rate of intensification are still serious problems that over Cape York Peninsula, it lost most of its original await prompt solutions. The present study has been intensity, but kept its identity as a tropical depression made to try to improve the forecasting of all aspects of with the central surface pressure just below 1000 hPa. tropical cyclone intensity from data that are generally By about 2100 UTC 19 March, Kathy crossed the western available to the forecaster. The data used are the ob- coast of Cape York Peninsula to move over water and jective streamline and isotach analyses, digitized high- started to regenerate. Kathy kept intensifying for 63 h. At resolution IR satellite cloud data, and station data in an 1200 UTC 22 March the central surface pressure reached

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FIG. 2. Track of Tropical Cyclone Kathy, 18–24 Mar 1984. The six-ﬁgure group denotes PPddtt at 6-hourly intervals. PP is the last two digits of the central surface pressure (hPa), dd is date, and tt is time (hours UTC). From Australian Bureau of Meteorology (1984). an estimated minimum of 920 hPa. At that stage, Kathy b. The large-scale gradient-level circulation was still over the ocean, about 100 km from the north around Kathy Australian coast and yet started to weaken. At about The large-scale ‘‘gradient-level streamline analyses’’ 2000 UTC 22 March, Kathy passed over Centre Island around Tropical Cyclone Kathy for the period 18–22 where the recorded minimum surface pressure was March1984,showninFig.4,wereobtainedfrom 940 hPa at the meteorological station and 938 hPa on Davidson and McAvaney’s (1981) large-scale objective a trawler that ran aground on the east coast of the is- tropical analysis scheme using grid points 222 km apart. land (Falls and Murphy 1984). At 1930 UTC 20 March Because the analyses used all winds available within the anemometer mast at Centre Island was blown away 66 h of analysis time from the surface to the 1-km level, when the maximum gusts showed a tendency to ﬂat- it is assumed that the level of the streamline analyses is ten out, suggesting that the band of maximum winds had arrived (Murphy 1985). The recorded mean sustained surface wind just before the anemometer failed was 101 kt (52 m s21) and the maximum gust was 125 kt (64 m s21), according to the anemogram trace (not shown). The best track of Fig. 2 indicates that at 0600 UTC 22 March Kathy slightly changed its direction of motion while decelerating to a speed of about 7 km h21. Six hours later, it accelerated to 14 km h21 for 3 h and then moved at 19 km h21 after 1800 UTC 22 March. The anemometer mast failed 45 min before the minimum surface pressure was recorded at 2015 UTC 22 March (Murphy 1985; Falls and Murphy 1984). This suggests that rm was about 14 km. rm was estimated to be 15 km by the Australian Bureau of Meteorology (1984, hereafter called the Bureau) and 13 km by Love and Murphy (1985). Murphy (1985) reported that the eye passage over Centre Island lasted 50 min indicating that the eye FIG. 3. Time variation of central surface pressure of Tropical Cyclone Kathy as estimated by the Australian Bureau of Meteo- diameter was (50 3 19/60) or 16 km. A summary of the rology (solid curve) and as computed from an analytic model dis- observed and determined intensity parameters of Kathy cussed in Part II (dashed curve). The horizontal bars along the time is listed in Table 1. axis represent the times of sudden pressure falls at Centre Island.

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TABLE 1. Summary of observed or derived intensity parameters for west to north-northwest; and to the south it had changed Tropical Cyclone Kathy. from east-southeast to east or east-northeast. From about Min central surface pressure at 1200 UTC 920 hPa 440 km the flow spiralled inward toward the cyclone 22 Mar 1984 (estimated by the Bureau center. The change in the flow field at each stage of in- using Dvorak’s technique) tensification is more conspicuous in Figs. 5a–f. Each When passing over Centre Island of these diagrams shows for the specified time, the azi- Recorded min central surface pressure 940 hPa muthal variation of a , which is the angle of inflow of Recorded mean sustained surface wind 52 m s21 4 Recorded max gust ahead of cyclone 64 m s21 the gradient-level winds at 48 latitude distance from the Estimated radius of maximum wind 14 km cyclone center. The angle of inflow at any point on the Estimated radius of the eye 8 km circle of 48 latitude radius (indicated by the dashed circle) is the angle between the direction of the streamline and of the tangent to the circle at that point. Before the at the level of maximum wind where the winds are in start of intensification, the streamlines in Figs. 4a,b in- gradient balance with the surface pressure gradient, dicate that air participating in the cyclonic circulation hence the term gradient-level streamline analyses. Be- came from all sides of the depression: tropical air from cause the analysis scheme cannot resolve the smaller- the north and east of the depression and southeast trade scale circulation of the tropical cyclone, it was necessary air from the south and southwest. Also, Fig. 5a indicates to modify by hand the model analyses close to the cy- that the angle of inflow of the winds at 48 latitude radius clone center so that the location of the latter was the to the north of the depression was 208. At the start of and same as that indicated by the best-determined track of during intensification, however, the angle of inflow of Fig. 2. The area of analysis shown in Fig. 4 is between the the winds at 48 latitude radius to the north and northeast 08 and 208S and about 208 longitude wide. A circle (bold of the depression had increased from 208 to 42.58, while dashed curve) of 400-km radius whose center is at the to the south there was no inflow of trade air (see Figs. cyclone center (shown by the usual cyclone symbol) is 5b–d). The gradient-level circulation around Kathy by drawn on each analysis to give an idea of the distances this time resembled a giant swirl starting some 600 km involved. The analyses presented in Figs. 4a,b show the north and northeast of the cyclone and spiraling inward gradient-level circulation around Kathy when it was toward the center of a single vortex where a number of overland, weak, and nonintensifying; Fig. 4c shows when cumulonimbi were developing forming the central dense the cyclone was crossing the western coast of Cape York overcast (CDO) region. Peninsula and started to intensify; Figs. 4d,e show when The change in the low-level circulation that occurred it was over the Gulf of Carpentaria and was intensifying; around Kathy at the start of its regeneration and the and Fig. 4f shows when it started to dissipate. As will giant swirl circulation during its intensification appear to now be discussed, significant differences in the cyclone be characteristic features of intensifying tropical cy- circulation existed in each stage of its evolution. clones, as can be noted in the following examples and discussion. c. Nonintensifying stage Figure 6 shows two gradient-level circulation patterns around a depression that reached tropical cyclone in- Streamlines in Figs. 4a,b indicate that between 200 tensity, according to the Dvorak (1975, 1984) technique. and 600 km from the center of the tropical depression It was baptized Tropical Cyclone Winifred by the Bris- the flow was northwesterly in the sector between north bane Tropical Cyclone Warning Centre at 1800 UTC and northeast of the depression. Those between 200 and 29 January 1986. The streamline analyses were per- 600 km from the system’s center in the southeast– formed manually, using winds computed on a 2.58 lati- southwest quadrant indicate a flow of southeast trade air tude grid by Davidson and McAvaney’s (1981) tropical into the system. analysis scheme. The gridpoint winds, particularly in this case, are reliable since there were one or two upper wind d. Intensifying stage stations to the north, southwest, and southeast of the At 2100 UTC 19 March, Fig. 4c, when Kathy crossed system’s center. The solid streamlines correspond to the coast to move over water and started to regenerate, the flow pattern at 2300 UTC 28 January 1986 and the the streamline pattern of the large-scale gradient-level dashed streamlines correspond to that at 1100 UTC flow had changed considerably between 660 and 440 km 29 January 1986. The latter analysis has been moved from the cyclone center: to the north the direction of slightly to the east so that both vortex centers coincide. the flow had changed from west-northwest to north- As it can be noted, a marked change occurred in the northwest; to the northeast it had changed from north- direction of the large-scale flow to the north of the

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FIG. 4. Objective gradient-level streamline analyses around Tropical Cyclone Kathy from 18 Mar 1984. Each area extends from the equator to latitude 208S and about 208 longitude wide. The cyclone center is denoted by the cyclone symbol. The dashed circle is 400 km from the cyclone center.

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FIG. 5. Variations of a4, the angle of inflow at 48 latitude radius at different times. vortex center at distances extending to about 108 latitude from the vortex center between 2300 UTC 28 January and 1100 UTC 29 January. The angle of inflow to the north and north-northeast of the depression center at 48 latitude radius was about 208 at 2300 UTC 28 January and 458 at 1100 UTC 29 January. This change in the low- level circulation was similar to that that occurred in Kathy at the start of intensification. Note the giant swirl that started far to the north and northeast of the cyclone and that spiraled into the cyclone center. Note also that trade air to the south of the cyclone did not penetrate within about 100 km from the cyclone center at 1100 UTC 29 January. Another example of a giant swirl around a tropical cyclone is shown in Fig. 7. The latter shows the circulation at the surface around intensifying tropical cyclone Edwina in the southwest Indian Ocean. The winds in Fig. 7 were deduced from the European Remote Sensor Satellite-1 (ERS-1) and published in the 1992–93 cyclone season in the southwest Indian Ocean by Reunion Me- FIG. 6. Gradient-level streamline analyses around Tropical Cy- teorological Services (1993). The giant swirl started at clone Winifred at 2300 UTC 28 Jan (solid streamlines) and at 1100 UTC 29 Jan 1986 (dashed streamlines). They were produced least some 440 km to the north and northeast of the from objectively determined vector winds at grid points 2.58 lati- cyclone and spiraled into its center. The absence of dry tude apart. The second analysis was displaced slightly so that the trade air inflow during intensification of other tropical two cyclone centers coincide.

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FIG. 7. An example of a giant swirl. The winds associated with severe Tropical Cyclone Edwina were deduced from the ERS-1 (Reunion Meteorological Services 1993). depressions is a feature that has been noted before by and during intensification the large-scale environmental Simpson (1971). flow in the midtroposphere was similar to that at gra- It would appear then that the change in the low-level dient level (cf. Figs. 4a,c), particularly in the sector be- circulation at the start of intensification, particularly the tween north-northwest and north-northeast where air increase in the angle of inflow to the north and northeast of a tropical cyclone, and the giant swirl spiraling from about 440 km from the cyclone toward its center might be characteristic features of an intensifying tropical cyclone. e. Change of the large-scale circulation at 700-hPa level The changes in the gradient-level circulation of Kathy when the depression started to regenerate also occurred at midtropospheric levels. Two objective 700-hPa streamline analyses are shown in Fig. 8. The solid streamlines represent the large-scale flow at 2300 UTC 18 March when the system was nonintensifying. At 1100 UTC 19 March the flow pattern was the same as that of 2300 UTC 18 March. The dashed streamlines correspond to the flow at 2300 UTC 19 March when the system had just crossed the western coast of Cape York FIG. 8. Objective 700-hPa streamline analyses at 2300 UTC Peninsula and had started to intensify. The latter anal- 18 Mar (solid curves) and 2300 UTC 19 Mar 1984 (dotted curves). ysis has been slightly displaced to the east so that the The latter has been shifted slightly to the east so that the cyclone system’s centers coincide. Figure 8 shows that before centers coincide.

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FIG. 9. The 2300 UTC objective streamline and isotach at 200 hPa on (a) 19 and (b) 20 Mar 1984. The cyclone symbol in the Gulf of Carpentaria indicates the position of the cyclone at analysis time. parcels, from about 800 km, spiraled inward toward the angle of inflow of the airstream at 48 latitude radius to cyclone center. Thus, in this case the inward-spiraling the north of the cyclone changed from 42.58 to 338, then swirl mentioned in section 2d was not confined to the to 208 (see Figs. 5d,e), the same value before in- gradient-level only but extended vertically to at least the tensification started. As will be shown in the next sec- 700-hPa level. tion, this change in circulation happened concurrently with a significant change of cloud structure around the f. 850- and 200-hPa analyses cyclone, both changes occurring 12 h before the cyclone Because of the effect on the development and rate of reached its peak intensity (see Fig. 10) and started to intensification of a tropical cyclone by an upper-air weaken. Also, in Dvorak’s (1975) forecasting technique trough on the one hand, and by the tropospheric vertical a tropical cyclone is forecast to lose intensity 12 h after wind shear on the other, two 200-hPa objective stream- the cloud structure shows a sign of weakening. line and isotach analyses are presented in Fig. 9, one at 2300 UTC 19 March when Kathy had just crossed the 3. Movement of moist ‘‘near-equatorial air’’ coast to move over water and started to regenerate and toward the center of Tropical Cyclone Kathy the other at 2300 UTC 20 March when it had already started its rapid intensification. At both times Kathy was a. Satellite imagery under a ridge that extended from a large anticyclone A series of 3-hourly cloud-top temperature (TBB) south west of the cyclone. There was also an upper-air analyses of GMS digitized IR data associated with trough some 500 km east-southeast of the cyclone cen- Tropical Cyclone Kathy is shown in Fig. 11. The TBB ter but it was moving in an easterly direction away from isopleths of 2358 and 2608C, unless otherwise indicated, the westerly moving cyclone. This upper-air trough is are drawn in each analysis. A scale is provided in unlikely to have influenced the intensification of Kathy. Fig. 11a to give an idea of distances and distortion in the The reason is that upper-air troughs that interact with cloud map. The space and temperature resolution of the tropical cyclones are to the west of the cyclone and move data in Fig. 11 are 8 km and 38C, respectively. In each east to approach the cyclone (Hanley et al. 2001). analysis the cyclone center is indicated by the usual cyclone symbol. g. Dissipating stage At 2100 UTC 19 March, Fig. 11a, a narrow cloud band

Figure 10 shows that at 2300 UTC 21 March a low- (narrow cold band in the TBB ﬁeld) developed far to the level trough had just developed to the south of the cy- north east of the cyclone. It extended from a large cloud clone. It is at that time that the circulation at the gradient mass near the equator. The narrow cloud band was level started to change (see Figs. 4e,f) and when the about 40 km wide and its southern end was about

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FIG. 10. The 850-hPa analyses: (a) shows a ridge (line of crosses) at 2300 UTC 20 Mar 1984 to the south of the cyclone; (b) shows a trough (dashed line) has replaced the ridge by 2300 UTC 21 Mar 1984. A few streamlines to the south of the cyclone have been added to emphasize the positions of the ridge and the trough.

350 km northeast of the cyclone center. For the purpose The positions relative to the cyclone center of the of identifying and following the evolution of this cloud most active part of the cloud bands of Figs. 11a–e are band with time, a bold dashed curve passing through the reproduced in Fig. 12a. The dates and times are in- most active convective elements in the cloud band (the dicated along each cloud band. The southern tips of the coldest TBB inside the band) is drawn in Fig. 11a and in cloud bands in Fig. 12a are denoted by A, B, C, ... ,F, subsequent analyses. corresponding to the cloud bands of Figs. 11a–f. The

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FIG. 11. The 3-hourly analyses of TBB for Tropical Cyclone Kathy from 2100–2100 UTC 19–22 Mar. Isotherms of 2608 and 2358C are shown, as well as selected values. Bold dashed curves are drawn along the coldest part, or most convectively active part, of the narrow cloud bands. The cyclone center is denoted by the usual cyclone symbol. A length scale is provided in the ﬁrst panel to give an idea of the map distortion seen.

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FIG. 11. (Continued) positions of the cloud bands in Fig. 12a suggest that they cloud band is significant because it indicates that cu- were one and the same cloud band that originated far mulonimbi that developed within the cloud band could north-northeast of the cyclone as a short cloud band, and reach a much higher level than other cumulonimbi de- that extended southward along the northerly airstream veloping in its environment. Because the air flowing with time while moving west. During its evolution the inside the cloud band originated near the equator, we cloud band became wider and sometimes had two lines call it ‘‘near-equatorial air.’’ of well-developed clouds in it. For example, in Fig. 12a By 0900 UTC 20 March (i.e., 12 h after its formation), there were two cloud lines close to one another with the southern part of the cloud band merged with the their southern tips C and C9 at 0300 UTC 20 March and CDO (see Fig. 11e) where the minimum TBB decreased D and D9 at 0600 UTC 20 March. The change in the suddenly from 2658 to 2798C, indicating that near- positions of the solid curves CC9 and DD9 seems to in- equatorial air had moved into the CDO area where huge dicate the movement of the cloud band toward the cy- penetrative cumulonimbi [or hot towers, as Riehl and clone in the time interval between 0300 and 0600 UTC Malkus (1961) had called them] and high-level cirro- 20 March. The cloud band finally merged with the CDO stratus had developed. The time variation of aerial at 0900 UTC 20 March. (A similar wide double-line coverage of high-level cirrostratus in the CDO, repre- cloud band also occurred in Fig. 12c when the solid curve sented by the number of pixels with TBB # 2778C, is II9 moved to JJ9 between 0000 and 0300 UTC 21 March). shown in Fig. 13. The solid curve in Fig. 13 shows the

The TBBs of the cloud tops inside this cloud band were time variation of the intensiﬁcation rate (›rc/›t)of mostly 2508 to 2608C except in a few cumulus-sized Tropical Cyclone Kathy. blobs where they reached 2808C, corresponding to The following ﬁve important features can be observed a cloud top of the 100-hPa level or higher. Between between 0900 and 1200 UTC 20 March: 2100 UTC 19 March and 0900 UTC 20 March, this extremely low TBB was only observed in cloud masses close 1) At 0900 UTC 20 March when the feeder band to the equator and was not even observed inside the CDO merged with the CDO, the area of high-level cirro- of the depression where the minimum TBB was 2658C, stratus started to increase to reach a maximum at corresponding to a cloud top of about the 200-hPa level. 1200 UTC 20 March when it was about 2.5 times its

The observation of such extremely low TBB inside the original size. Part of the increase in the high-level

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was a marked suppression of convective clouds outside the CDO up to about 400 km to the north and east of the cyclone. There was no cloud band in that area; only one or two convective clouds with tops

having a TBB of 2358C at 0900 UTC 20 March and 2108C (about the 600-hPa level) at 1200 UTC 20 March (Figs. 13e,f). This indicates that the air in the lower midlevels (possibly in the 850–700-hPa layer) in the northerly airstream had become drier than before, because of strong subsidence associated with the enhanced convective activity inside the CDO and will henceforth be referred as ‘‘subsidence dried air.’’ 3) At 1200 UTC 20 March the high-level cirrostratus area started to decrease sharply, because subsidence dried air was being fed to the cyclone. 4) As can be observed in Fig. 13, from 1200 UTC 20 March high-level cirrostratus areal coverage decreased to a minimum up to 0000 UTC 21 March before increasing again, but stayed continuously around the cyclone center. This observation suggests that vigorous convection continued unabated close to the cyclone center, due partly to the supply of moist near-equatorial air below the 850-hPa level. 5) The 2nd row of Table 2 gives the approximate times of observed changes of significant cloud features during the evolution of this feeder band. Column 6 gives the times of observed sudden surface pressure fall at Centre Island. It can be noted that the first significant sudden surface pressure fall occurred within the time the cirrus canopy started to increase FIG. 12. Positions relative to the cyclone center of cloud bands or and the time it started to decrease. The changes of feeder bands of Fig. 11, showing their southward extension and the surface pressure fall at Centre Island are dis- their westward movement until they reach the outward edge of the CDO or the inner core of the cyclone. The scale of the drawing is cussed in section 4. the same as that shown in Fig. 11. The cycle of events that occurred during the evolution of the feeder band described above occurred repeatedly cirrostratus area, however, was due to the northern in a second and third feeder band during intensification part of the cloud band moving over and to the north while the spiral inflow remained unchanged. The fea- of the CDO. It is likely that the increase of high-level tures of the second and third feeder bands are summa- cirrostratus area was not only due to the cumu- rized in rows 4 and 6 of Table 2. lonimbi becoming more active, but also becoming The fourth feeder band evolved differently. It de- more numerous, as has been observed by Jorgensen veloped far northeast of the cyclone at 1600 UTC (1984) in other tropical cyclones. It is worth noting 21 March (see Figs. 11n and 12d). Part of the feeder band that between 2100 UTC 19 March and 0900 UTC merged with the CDO soon after 0000 UTC 22 March 20 March (i.e., before the arrival of near-equatorial air giving rise to an increase of high-level cirrus area be- in the CDO), the intensification rate was 10 hPa day21, tween 0000 and ;0430 UTC 22 March (see Fig. 13). The but that soon after 1200 UTC 20 March, (when, it is other part of the feeder band with cloud tops having

suggested, moist near-equatorial air had reached the a minimum TBB of 2658C extended far southeast of the cyclone center) it increased to 25 hPa day21. (The cyclone and did not merge with the CDO, thus depriving changes of intensiﬁcation rates at other times are the cyclone of most of its supply of moist near-equatorial discussed in Part II of this paper.) air. This cloud band remained almost stationary until 2) Between 0900 and 1200 UTC 20 March when con- 0600 UTC 22 March and then moved eastward away vective activity was at its peak inside the CDO, there from the cyclone. This was the last active feeder band

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FIG. 13. The dashed curve shows the time variation of n/10, where n 5 numbers of pixels with

TBB # 778C in the cirrostratus canopy over Tropical Cyclone Kathy from 20 to 22 Mar 1984. The solid curve shows the time variation of the computed intensification rate (›rc/›t) of Fig. 3. The short horizontal bars along the time axis represent the time intervals of sudden surface pressure falls at Centre Island. that originated 400 km north or northeast of the cyclone b. Relevant comments and that merged with the CDO. It happened at the time when a low-level trough had developed to the south of Raghavan et al. (1980) had also observed a variation the cyclone (Fig. 10). It is worth noting that at 1200 UTC of convective activity in the CDO of other tropical cy- 22 March, 12 h after the merging with the CDO of the clones similar to that observed in Kathy. Using range– last convectively active feeder band, the cyclone reached height indicator radar observations of tropical cyclones its peak intensity, and then started to weaken even in the Bay of Bengal, they found that the tops of the though it was still over water and its center was still eyewall clouds and their precipitation rate increased 100 km from the Australian coast. substantially above their mean values for a 3–5-h period Thus, Kathy started to weaken some 12 h after (i) in a 9–12-h cycle. In one case, the height of cumulonimbus a trough had developed to the south of the cyclone, tops increased from a mean value of 15 to 21 km while the allowing cool drier trade air to the south of the cyclone precipitation rate increased from 17 to 37 mm h21. to enter the cyclonic circulation (see Fig. 4f); (ii) the Nowadays, hot towers in the eyewall with tops circulation to the north of the cyclone had changed and reaching up to 15 to 18 km are regularly observed in the the angle of inflow was back to its preintensifying stage Tropical Rainfall Measuring Mission (TRMM) Pre- of 208; and (iii) the cyclone had lost its supply of warm cipitation Radar (PR; Iguchi et al. 2000) imagery. They moist near-equatorial air. At 1600 and 1800 UTC are usually accompanied by torrential rain with a rainfall 22 March a narrow, less convectively active feeder band, rate greater than 35 mm h21. According to Kelley and having a minimum TBB of 2608C, merged with the Stout (2004), animation of TRMM satellite imagery in- CDO. It did not originate from far north or northeast of dicates that these penetrative cumulonimbi are not al- the cyclone and did not produce an increase in the high- ways present in the eyewall but develop from time to time level cirrus area. A more active cloud band (TBB 5 and are associated with an increase of the intensification 2708C) east of the feeder band extended far to the south rate of the cyclone. It will be shown in Part II that the rate east of the cyclone (see the 2100 UTC 22 March TBB of intensification of Kathy also increased appreciably analysis of Fig. 13w). during the 3 h of intense convective activity in the CDO.

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The fact that the 3-h intense convection within the CDO was accompanied by a suppression of active convective clouds outside the CDO indicates that a strong subsidence had dried up the air in the low and midlayers fall over Excess of

mean change of the atmosphere. That subsidence-dried air was then observed pressure fed to the cyclone and caused a marked decrease of km from the cyclone convective activity in the CDO, until another batch of

diurnal variation. moist near-equatorial air arrived some 9–12 h later. It is in 3 h

surface this sort of quasi-semidiurnal variation of convective Observed pressure fall e observed surface pressure fall in activity within the CDO that gives rise to the quasi- semidiurnal variation of the area of high-level cirrostratus that has been documented in many other tropical cyclones by Browner et al. (1977) and Lajoie and But- terworth (1984).

Time of observed 4. Time variation of surface pressure at surface pressure fall Centre Island The variation of surface pressure at Centre Island between 1600 UTC 20 March and 1000 UTC 22 March during the approach of Tropical Cyclone Kathy is shown by the dashed curve of Fig. 14. It has been determined CDO from 3-hourly observations. The distances of the cyclone Time of cloud center from Centre Island are indicated along the time suppression outside axis. They indicate that Centre Island was well outside the CDO during that period. The purpose of Fig. 14 is to examine how the surface pressure outside the CDO changes during the approach of the cyclone. To do this a comparison is made between the 3-hourly pressure change due to the regular diurnal variation of surface pressure and the observed pressure change. According to Time cirrus area

started to decrease the statistics published by the Australian Bureau of Me- teorology (see online at http://www.bom.gov.au/climate/ averages/), the long-term mean changes of surface pressure for every 3-hourly interval in March at Centre Island, due mostly to diurnal variation, are used to plot the solid curve, had the pressure change been equal to the long- term mean change. If it is assumed that during the time intervals when the dashed curve is well below the solid Time cirrus area started to increase curve the observed pressure fall is signiﬁcant, then there

Rapidly 1200 UTC 21 Mar were three intervals when Centre Island was within 440 km from the cyclone center and when the surface pressure started to fall suddenly and continued falling for 3–6 h. These intervals are 0200–0800 UTC 21 March, 1400–1700 UTC 21 March, and 0200–0800 UTC 22 March, and are listed in column 6 of Table 2. The magnitude of Time feeder band

merged with CDO the observed sudden surface pressure falls within the ﬁrst 3 h (2–3 hPa) are shown in column 7, and the excess of observed pressure fall over the corresponding long-

2. Dates and times of signiﬁcant changes in the cloud features and of sudden small surface pressure falls at Centre Island when the latter was within 440 term pressure change due to diurnal variation are shown in column 8. It is worth noting that the onset of these ABLE Time feeder

band started three sudden falls of surface pressure at Centre Island T to extend south

center for Tropical Cyclone Kathy. These pressurethe falls are ﬁrst assumed 3 to coincide h, with pressure while falls column outside the 8 CDO. gives Column 7 the gives excess the magnitude of of observed th surface pressure fall over the long-term mean change of surface pressure at the same times due to 2100 UTC 19 Mar1200 UTC 20 0900 Mar UTC 202100 Mar UTC 20 2250 Mar UTC 20 0900 Mar UTC 105 20 UTC Mar 211450 0000 Mar UTC UTC 21 21 Mar Mar 0150 Slowly UTC 0900 22 UTC Mar 21 Mar 0000 UTC 1200 22 1500–1650 UTC Mar UTC 20 21 Mar Mar 0300–0450 UTC 21 1200–1600 Mar UTC 21 Mar 0000 UTCoccurred 21 1400–1700 Mar UTC 21 0900–1200 Mar UTC 20 0600 Mar UTC 22 Mar 0200–0800 2.0 UTC 21 Mar between 3.0 0.6 hPa the — time 0.6 hPa the 0200–0800 UTC 22 Mar high-level 2.5 cirrus 0.7 hPa area

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208 to 42.58. To the south of the cyclone the angle of inﬂow changed from 708 to 08 indicating that no cool trade air took part in the cyclonic circulation. The giant swirl that spiraled into the center of the depression carried with it moist near-equatorial air that could sus- tain the development of huge cumulonimbi. About 12 h after the change of the circulation, the moist near- equatorial air reached the CDO (or a radius of 18 latitude) causing the convective activity in the CDO to increase. The convective activity reached a maximum some 3 h later when the near-equatorial air apparently reached the eyewall. During these 3 h of intense convection within the CDO, convective clouds outside the CDO, up to about 400 km, were mostly suppressed, due most likely to the strong subsidence associated with the

FIG. 14. Variation of observed surface pressure at Centre Island strong convection inside the CDO. When this sub- during the approach of Tropical Cyclone Kathy from 20 to 22 Mar sidence-dried air entered the CDO, convective activity 1984 (dashed curve); pressure variation if the pressure changes in the CDO decreased until a new batch of moist near- were the same as evaluated from the long-term mean statistics equatorial air arrived at the edge of the CDO. The cycle (solid curve). Distances of Kathy from Centre Island are indicated of events, as described above, was repeated 2 more times along the time axis. during the intensification stage of Kathy. Finally, because of a low-level trough that developed started to increase and the time it started to decrease. to the south of Kathy, the angle of inflow returned to 208 These three intervals are indicated by short horizontal to the north of the cyclone and 308 to the south of the bars along the time axis of Fig. 13. cyclone. The west-southwesterly winds to the north of It will be shown in Part II that this sudden and small the cyclone pushed the moist near-equatorial air away to surface pressure fall extending up to 350 km around the the east of the cyclone cutting off its supply of moist air, cyclone center plays an important role in the intensi- and causing it to dissipate rather rapidly. fication of Kathy. From this analysis, two factors are proposed as being important for tropical cyclone intensification. The im- port of very moist near-equatorial air into the cyclone 5. Discussion and conclusions core appears to produce in the eyewall the development The results of this study are similar in some ways to of hot towers associated with a sudden increase in the those of Molinari and Skubis (1985). Both studies show intensification rate. Also, the angle of inflow of the equa- a relationship between an inward-propagating distur- torial airstream appears to be important for the devel- bance and the subsequent intensification of a tropical opment and intensification of the cyclone. These concepts, cyclone. In both cases, the intensification of the cyclone along with the features described above, are used in Part II was only substantial when the disturbance reached of this paper to develop a mathematical model to fore- a distance of only a few hundred kilometers from the cast the intensity of a tropical cyclone. cyclone center. Disturbances in both storms showed a distinct azimuthal asymmetry. Both studies hypothe- Acknowledgments. We thank Noel Davidson for size that the arrival of the disturbances close to the supplying objective analysis data and the University of center of the storms triggered enhanced deep convec- Melbourne for supporting this research. We also thank tion and that this was a possible mechanism for cyclone three anonymous reviewers for their detailed comments. intensification. The present study differs from the analysis of Molinari and Skubis (1985) by explicitly making the connection REFERENCES between the trajectory of the disturbance and cyclone Atkinson, G. D., and C. R. Holliday, 1975: Tropical cyclone min- intensification. The observations discussed above in- imum sea-level pressure–maximum sustained wind relation- dicate that at the start of intensification of the depression, ship for the western North Pacific. FLEWEACEN Tech. Note, JTWC, 20 pp. the low-level circulation took the form of a giant swirl ——, and ——, 1977: Tropical cyclone minimum sea level pressure/ that started some 600 km to the north and northeast of maximum sustained wind relationship for the Western North the depression with the angle of inflow changing from Pacific. Mon. Wea. Rev., 105, 421–427.

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