School of Earth Sciences, University of Melbourne, Parkville, Australia. (Manuscript received 20 November 2008, in final form 13 May 2009) ABSTRACT

Transcription

1 VOLUME 138 M O N T H L Y W E A T H E R R E V I E W 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 (a 4 ) at a radius of 48 latitude from the cyclone center. When Kathy started to intensify, a 4 increased suddenly from 208 to 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 a 4 remained the same during intensification. During the dissipation stage, a 4 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 Meteorological parameters and physical atmospheric processes that can influence the intensification of a tropical cyclone have been identified and discussed by many investigators. Apart from having insufficient conventional observations to accurately determine the magnitude of these parameters or processes, none of the latter is consistent all the time as any of them can produce intensification or weakening in one cyclone but not necessarily in others. We shall briefly consider this problem below. In Lajoie and Walsh (2010, hereafter Part II), we will discuss a model that can forecast, with a lead time of 15 (possibly 30) h, the central surface pressure, as well as the radial and the azimuthal wind distribution around Corresponding author address: Kevin Walsh, School of Earth Sciences, University of Melbourne, Parkville VIC, 3010 Australia. kevin.walsh@unimelb.edu.au a tropical cyclone without having to consider these elusive parameters and processes. a. Tropical cyclone development processes Processes that have been shown to influence tropical cyclone development are as follows. 1) THE SEA SURFACE TEMPERATURE When considering a large number of tropical cyclones, the minimum central surface pressure p c is found to be related to the SST between 268 and 308C: the greater the SST the lesser is p c (Miller 1958; Titley and Elsberry 2000). This relationship appears to break down for SST. 308C (Merrill 1988; Evans 1993; DeMaria and Kaplan 1994), with only a few intense storms being observed at these temperatures (Kaplan and DeMaria 2003). 2) RELATIVE HUMIDITY High relative humidity (.70%) either in the boundary layer (Schade and Emanuel 1999) or in the hPa DOI: /2009MWR Ó 2010 American Meteorological Society 3

2 4 M O N T H L Y W E A T H E R R E V I E W VOLUME 138 layer (Kaplan and DeMaria 2003) plays a significant role in the development and intensification rate of a tropical cyclone. The latter authors, however, found that only about 66% (80%) of tropical cyclones with relative humidity greater than 70% (.80%) in the hPa layer undergo rapid intensification, defined as either an increase of sustained mean surface wind of at least 15.4 m s 21 (30 kt) in 24 h or a decrease of central surface pressure of at least 15 hpa in 24 h. 3) VERTICAL WIND SHEAR In general, if the vertical shear of environmental horizontal winds between 850 and 200 hpa is less than 5ms 21, a tropical cyclone may intensify, but no intensification is likely to occur if the shear is m s 21 (Simpson and Riehl 1958; Gray 1979; McBride and Zehr 1981; Merrill 1988; Zehr 1992; DeMaria and Kaplan 1994; DeMaria 1996; Hanley et al. 2001; Kimball and Evans 2002; Kaplan and DeMaria 2003). A few tropical cyclones have however been observed to intensify in face of strong vertical shear (Molinari 1998). 4) UPPER-TROPOSPHERE TROUGH INTERACTION The interaction between an upper-air trough and a tropical cyclone can also play an important role in its intensification (Molinari et al. 1995, 1998; Emanuel 1997; Shi et al. 1997) and in its rapid intensification (Titley and Elsberry 2000; Kaplan and DeMaria 2003). Yet, using a 3-yr data sample, Hanley et al. (2001) found that only 61% of all distant large-scale troughs and 78% of all small-scale troughs caused storms to intensify, while in 82% of no-trough cases the tropical cyclone also intensified. Hanley et al. (2001) also found that 30% of no-trough cases exhibit rapid intensification ($20 hpa in 12 h), compared to 16% for small-scale troughs. 5) VORTEX ROSSBY WAVES Vortex Rossby waves can develop on both sides of the radius of maximum wind (RMW) of a tropical cyclone because of the large potential vorticity (PV) gradients in these areas (Montgomery and Kallenbach 1997; Kuo et al. 1999; Corbosiero et al. 2006). When they evolve within the RMW, their associated convective clouds give rise to asymmetries in the eyewall (Kuo et al. 1999; Kossin and Schubert 2002; Chen and Yau 2001; Wang 2001, 2002), but when they evolve outside the RMW, they manifest themselves as spiral rainbands that develop within km from the cyclone center and that move cyclonically around the vortex while traveling outward from the RMW (Kimball and Evans 2002). According to Montgomery and Kallenbach (1997) and Moller and Montgomery (1999), the energy of the VRWs is transferred from the asymmetry into a circular flow by an axisymmetrization process that causes the tangential winds at a radius outside the initial RMW to increase, thus causing the vortex to intensify and the RMW to expand. This process can also produce a significant increase of intensification rate (Montgomery and Enagonio 1998; Moller and Montgomery 2000). On the other hand, Bister (2001) has argued that the latent heat released in the rainbands of the VRWs reduces the radial pressure gradient and causes the tropical cyclone to weaken. 6) LOW-LEVEL INWARD FLUX OF MOMENTUM Low-level environmental forcing can also produce intensification of a tropical cyclone. Using conventional surface wind fields, Molinari and Skubis (1985) discussed a case of wavelike perturbation, characterized by a surge of inflow and upward vertical motion, that developed at a radius of 1600 km on the equatorward and east side of a tropical depression, and that propagated inward toward the depression at 15 m s 21 to reach the storm center 36 h after its development. Molinari and Skubis used a momentum budget to show that the inward flux of momentum contributed to a significant momentum source within 440 km of the center of the depression and produced a rapid intensification of the depression when the surge reached its center. 7) OTHER ATMOSPHERIC PROCESSES Vortex Rossby wave (VRW) rainbands associated with extensive cirrostratus and convective clouds (May 1996) do sometimes extend to 150 km from the cyclone center (Kuo et al. 1999; Chow et al. 2002; Wang 2002). The vertical gradient of diabatic heating that results from the release of latent heat in the stratiform cloud and the evaporational cooling below the cloud base produces a PV concentration in the midtroposphere (Raymond and Jiang 1990). This PV concentration is sometimes associated with midlevel jets (May 1996). While May and Holland (1999) have argued that this PV production can be transported into the inner core and causes intensification, Bister (2001) has suggested that the latent heat released by the rainbands reduces the radial pressure gradient, which then weakens the tropical cyclone. The PV concentration in the midtroposphere can also initiate mesoscale vortices that are associated with outer rainbands beneath the outflowing cirrostratus bands (Houze 1977; Zipser 1977). Whether this process plays a positive or negative role in the intensification of a tropical cyclone is not known yet (Zhang et al. 2002). The large number of environmental parameters and atmospheric processes that can affect the intensity of a tropical cyclone one way or another makes intensity

3 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 empirically 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 p c, the central surface pressure, and V rm, the maximum sustained mean surface winds at the radius of maximum wind. The objective Dvorak technique of Velden et al. (1998) showed an RMS error of estimating p c of 8.3 hpa. Errors in estimating the maximum wind from these techniques can be quite large, though. This is because of the large scatter that exists when V rm is plotted against p c. Shea and Gray (1973) show results from reconnaissance flight data for North Atlantic hurricanes, clearly indicating that there is not a one-to-one relationship between p c and V rm. Brown and Franklin (2002) also observed a similar large variance when reconnaissance-based best track maximum wind speeds in North Atlantic hurricanes were plotted against the relevant Dvorak estimates of maximum wind speeds (see Fig. 1). For a Dvorak maximum wind estimate of 80 kt, the best-track maximum winds varied from 50 to 100 kt. Another method for forecasting tropical cyclone intensity in operational use in the United States is the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model developed by DeMaria and Kaplan (1999) and DeMaria et al. (2005), based on a statistical multiple regression technique. A number of climatological, persistence, and synoptic factors are obtained from objective forecast fields, and after removal of the cyclone circulation are used as predictors. The magnitude of errors produced by this method is similar to that of Dvorak. Another statistical method for forecasting the intensity of a tropical cyclone uses data from passive microwave radiometers on polar-orbiting satellites. The advantages of using this method are that microwaves of different wavelengths can penetrate below cloud tops and can sense the temperature, the average amount of cloud liquid water, and rain rates at different levels in FIG. 1. Best-track vs Dvorak maximum sustained surface wind estimates (kt). The solid line indicates the best-fit linear relationship; the dashed line is a perfect (y x) relationship (from Brown and Franklin 2002). the troposphere. The disadvantage is that the spatial resolution is coarse: even in the Advanced Microwave Sounding Unit (AMSU) used by Spencer and Braswell (2001), Brueske and Velden (2003), and Demuth et al. (2004), it varies between 48 and 100 km. Errors in cyclone intensity forecasts using AMSU data are comparable to those by Dvorak s technique: 72.5% of errors were within 15 kt and 13% were between 20 and 57 kt, the larger errors being associated with cyclones having small radius of maximum wind (Demuth et al. 2004). Factors that contribute to the large variance in the p c /V rm relationship are storm motion (Schwerdt et al. 1979), intensity trend (Koba et al. 1990), size, latitude, and r m, the radius of maximum wind (Knaff and Zehr 2007). According to the latter authors storm motion and intensity trend have a relatively small effect on the reduction of the scatter, while other factors have a small but significant effect. They could not, however, study the effect of r m because this parameter was not available or was not accurate enough in the archived data. But even after having taken all these factors, except r m, into consideration they found that there was still a significant scatter and concluded that this may be due to the effects of other factors such as the one used to reduce flight wind data to surface wind and r m. Another method that has been used to determine V rm, the maximum wind at the gradient level, is to assume some parametric equation to approximate the radial profile of the surface pressure and then use either the cyclostrophic or gradient wind equation to determine V rm (see e.g., Atkinson and Holliday 1975, 1977; Holland

4 6 M O N T H L Y W E A T H E R R E V I E W VOLUME ). Then using a reduction factor the maximum surface wind is obtained. According to Love and Murphy (1985), Holland s (1980) technique underestimates maximum wind by 10% 23%. On the other hand, Willoughby and Rahn (2004) found that for North Atlantic hurricanes (i) the estimated maximum surface winds from the Holland (1980) wind profile are too strong with an RMS error of 4.2 m s 21, and (ii) the winds 2 3 r e (eye radius) away from r m (radius of maximum wind) are in error by about 50%. Holland (2008) has recently modified his original model to determine the surface maximum wind and in so doing making b s, the shape factor of the radial surface pressure profile, a function of Dp ( p c ), ( p c / t) the rate of change of intensity, the latitude, and the speed of translation of the storm. Holland found that his new model captures successfully much of the observed scatter with major errors considered to arise from local transients and major asymmetries that cannot be covered by a general pressure-wind relationship. Nevertheless, the accuracy of determining V rm rests on an evaluation of Dp that in most cases is obtained by Dvorak s technique. For intensity forecasting, the NWP models are not performing better than statistical techniques. According to Elsberry (2002), no dynamic model can show significant skill in the prediction of tropical cyclone intensity until the model can correctly predict the surface moisture flux from the ocean, the wind structure in the boundary layer, and the advection of moisture in the lower layers of the atmosphere that contributes to the active convective clouds and heavy precipitation in the eyewall. The problem is compounded by the lack of data over the ocean, inadequate model resolution and physics, poor initial conditions, and insufficient understanding of the physical processes governing intensity change (Wang and Wu 2004), although there have recently been some improvements in skill (Surgi et al. 2008). c. Summary It is clear that none of the factors influencing the intensity of a tropical cyclone is always accompanied by either intensification or weakening and that appreciable errors can and do occur, particularly in the forecasting of the maximum winds. More to the point, forecasting the wind profile within 100 km from the cyclone center and the rate of intensification are still serious problems that await prompt solutions. The present study has been made to try to improve the forecasting of all aspects of tropical cyclone intensity from data that are generally available to the forecaster. The data used are the objective streamline and isotach analyses, digitized highresolution IR satellite cloud data, and station data in an area extending about 1000 km from the cyclone center. We have observed a number of characteristic time changes of environmental circulation and of cloud structure in and around a tropical cyclone when it starts to intensify, and during its intensification and dissipation. These time changes have been incorporated in a simple model that was found to be capable of forecasting, with a good degree of accuracy, not only p c, V rm, and the intensification rate, but also the time changes of the radial distribution of the surface pressure and of the winds for three tropical cyclones for which some good ground truth data existed. These time changes of the observed features are discussed in the remaining sections of this paper, while the model and the forecast results are discussed in Part II. 2. Observed features Objective wind analyses and high-resolution digitized IR satellite cloud data have been used to observe the evolution in the large-scale environmental circulation and in the cloud structure associated with Tropical Cyclone Kathy in March This cyclone was selected for study for the following reasons: it started to regenerate when crossing the coast of the Cape York Peninsula to move over water so that the time at which it started to intensify is known, it reached its maximum intensity after 63 h of intensification, it started to weaken over water while still about 100 km from landfall, and it passed over a meteorological station that provided recorded observations of its intensity. a. Track and intensity of Tropical Cyclone Kathy Kathy was an intense tropical cyclone. Its bestdetermined track, after Thom (1984) and Falls and Murphy (1984), is shown in Fig. 2. The 6-figure number close to each of the 6-hourly positions of the cyclone center indicates PPddtt, where PP is the last 2 digits of the estimated central pressure of the cyclone in hpa for the date (dd) and time (tt) in hours (UTC). The time variation of the central surface pressure, which was estimated using Dvorak s (1975) technique, is shown in Fig. 3. To the east of Cape York Peninsula, Kathy was in a steady-state condition for 30 h with an estimated minimum central surface pressure of 990 hpa. Once over Cape York Peninsula, it lost most of its original intensity, but kept its identity as a tropical depression with the central surface pressure just below 1000 hpa. By about 2100 UTC 19 March, Kathy crossed the western coast of Cape York Peninsula to move over water and started to regenerate. Kathy kept intensifying for 63 h. At 1200 UTC 22 March the central surface pressure reached

5 JANUARY 2010 L A J O I E A N D W A L S H 7 FIG. 2. Track of Tropical Cyclone Kathy, Mar 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 was still over the ocean, about 100 km from the north Australian coast and yet started to weaken. At about 2000 UTC 22 March, Kathy passed over Centre Island where the recorded minimum surface pressure was 940 hpa at the meteorological station and 938 hpa on a trawler that ran aground on the east coast of the island (Falls and Murphy 1984). At 1930 UTC 20 March the anemometer mast at Centre Island was blown away when the maximum gusts showed a tendency to flatten 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 s 21 ) and the maximum gust was 125 kt (64 m s 21 ), 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 h 21. Six hours later, it accelerated to 14 km h 21 for 3 h and then moved at 19 km h 21 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 r m was about 14 km. r m 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 diameter was ( /60) or 16 km. A summary of the observed and determined intensity parameters of Kathy is listed in Table 1. b. The large-scale gradient-level circulation around Kathy The large-scale gradient-level streamline analyses around Tropical Cyclone Kathy for the period March 1984, shown in Fig. 4, were obtained from Davidson and McAvaney s (1981) large-scale objective tropical analysis scheme using grid points 222 km apart. Because the analyses used all winds available within 66 h of analysis time from the surface to the 1-km level, it is assumed that the level of the streamline analyses is FIG. 3. Time variation of central surface pressure of Tropical Cyclone Kathy as estimated by the Australian Bureau of Meteorology (solid curve) and as computed from an analytic model discussed in Part II (dashed curve). The horizontal bars along the time axis represent the times of sudden pressure falls at Centre Island.

6 8 M O N T H L Y W E A T H E R R E V I E W VOLUME 138 TABLE 1. Summary of observed or derived intensity parameters for Tropical Cyclone Kathy. Min central surface pressure at 1200 UTC 920 hpa 22 Mar 1984 (estimated by the Bureau using Dvorak s technique) When passing over Centre Island Recorded min central surface pressure 940 hpa Recorded mean sustained surface wind 52 m s 21 Recorded max gust ahead of cyclone 64 m s 21 Estimated radius of maximum wind 14 km Estimated radius of the eye 8 km at the level of maximum wind where the winds are in gradient balance with the surface pressure gradient, hence the term gradient-level streamline analyses. Because the analysis scheme cannot resolve the smallerscale circulation of the tropical cyclone, it was necessary to modify by hand the model analyses close to the cyclone center so that the location of the latter was the same as that indicated by the best-determined track of Fig. 2. The area of analysis shown in Fig. 4 is between the 08 and 208S and about 208 longitude wide. A circle (bold dashed curve) of 400-km radius whose center is at the cyclone center (shown by the usual cyclone symbol) is drawn on each analysis to give an idea of the distances involved. The analyses presented in Figs. 4a,b show the gradient-level circulation around Kathy when it was overland, weak, and nonintensifying; Fig. 4c shows when the cyclone was crossing the western coast of Cape York Peninsula and started to intensify; Figs. 4d,e show when it was over the Gulf of Carpentaria and was intensifying; and Fig. 4f shows when it started to dissipate. As will now be discussed, significant differences in the cyclone circulation existed in each stage of its evolution. c. Nonintensifying stage Streamlines in Figs. 4a,b indicate that between 200 and 600 km from the center of the tropical depression the flow was northwesterly in the sector between north and northeast of the depression. Those between 200 and 600 km from the system s center in the southeast southwest quadrant indicate a flow of southeast trade air into the system. d. Intensifying stage At 2100 UTC 19 March, Fig. 4c, when Kathy crossed the coast to move over water and started to regenerate, the streamline pattern of the large-scale gradient-level flow had changed considerably between 660 and 440 km from the cyclone center: to the north the direction of the flow had changed from west-northwest to northnorthwest; to the northeast it had changed from northwest to north-northwest; and to the south it had changed from east-southeast to east or east-northeast. From about 440 km the flow spiralled inward toward the cyclone center. The change in the flow field at each stage of intensification is more conspicuous in Figs. 5a f. Each of these diagrams shows for the specified time, the azimuthal variation of a 4, which is the angle of inflow of the gradient-level winds at 48 latitude distance from the cyclone center. The angle of inflow at any point on the 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 start of intensification, the streamlines in Figs. 4a,b indicate that air participating in the cyclonic circulation came from all sides of the depression: tropical air from the north and east of the depression and southeast trade air from the south and southwest. Also, Fig. 5a indicates that the angle of inflow of the winds at 48 latitude radius to the north of the depression was 208. At the start of and during intensification, however, the angle of inflow of the winds at 48 latitude radius to the north and northeast of the depression had increased from 208 to 42.58, while to the south there was no inflow of trade air (see Figs. 5b d). The gradient-level circulation around Kathy by this time resembled a giant swirl starting some 600 km north and northeast of the cyclone and spiraling inward toward the center of a single vortex where a number of cumulonimbi were developing forming the central dense overcast (CDO) region. The change in the low-level circulation that occurred around Kathy at the start of its regeneration and the giant swirl circulation during its intensification appear to be characteristic features of intensifying tropical cyclones, as can be noted in the following examples and discussion. Figure 6 shows two gradient-level circulation patterns around a depression that reached tropical cyclone intensity, according to the Dvorak (1975, 1984) technique. It was baptized Tropical Cyclone Winifred by the Brisbane Tropical Cyclone Warning Centre at 1800 UTC 29 January The streamline analyses were performed manually, using winds computed on a 2.58 latitude grid by Davidson and McAvaney s (1981) tropical analysis scheme. The gridpoint winds, particularly in this case, are reliable since there were one or two upper wind stations to the north, southwest, and southeast of the system s center. The solid streamlines correspond to the flow pattern at 2300 UTC 28 January 1986 and the dashed streamlines correspond to that at 1100 UTC 29 January The latter analysis has been moved slightly to the east so that both vortex centers coincide. As it can be noted, a marked change occurred in the direction of the large-scale flow to the north of the

7 JANUARY 2010 L A J O I E A N D W A L S H 9 FIG. 4. Objective gradient-level streamline analyses around Tropical Cyclone Kathy from 18 Mar 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.

8 10 M O N T H L Y W E A T H E R R E V I E W VOLUME 138 FIG. 5. Variations of a 4, 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 lowlevel 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 cyclone season in the southwest Indian Ocean by Reunion Meteorological Services (1993). The giant swirl started at least some 440 km to the north and northeast of the cyclone and spiraled into its center. The absence of dry trade air inflow during intensification of other tropical FIG. 6. Gradient-level streamline analyses around Tropical Cyclone Winifred at 2300 UTC 28 Jan (solid streamlines) and at 1100 UTC 29 Jan 1986 (dashed streamlines). They were produced from objectively determined vector winds at grid points 2.58 latitude apart. The second analysis was displaced slightly so that the two cyclone centers coincide.

9 JANUARY 2010 L A J O I E A N D W A L S H 11 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 Simpson (1971). It would appear then that the change in the low-level circulation at the start of intensification, particularly the 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. and during intensification the large-scale environmental flow in the midtroposphere was similar to that at gradient level (cf. Figs. 4a,c), particularly in the sector between north-northwest and north-northeast where air 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 Peninsula and had started to intensify. The latter analysis has been slightly displaced to the east so that the system s centers coincide. Figure 8 shows that before FIG. 8. Objective 700-hPa streamline analyses at 2300 UTC 18 Mar (solid curves) and 2300 UTC 19 Mar 1984 (dotted curves). The latter has been shifted slightly to the east so that the cyclone centers coincide.

10 12 M O N T H L Y W E A T H E R R E V I E W VOLUME 138 FIG. 9. The 2300 UTC objective streamline and isotach at 200 hpa on (a) 19 and (b) 20 Mar 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 cyclone center. Thus, in this case the inward-spiraling swirl mentioned in section 2d was not confined to the gradient-level only but extended vertically to at least the 700-hPa level. f and 200-hPa analyses Because of the effect on the development and rate of intensification of a tropical cyclone by an upper-air trough on the one hand, and by the tropospheric vertical wind shear on the other, two 200-hPa objective streamline and isotach analyses are presented in Fig. 9, one at 2300 UTC 19 March when Kathy had just crossed the coast to move over water and started to regenerate and the other at 2300 UTC 20 March when it had already started its rapid intensification. At both times Kathy was under a ridge that extended from a large anticyclone south west of the cyclone. There was also an upper-air trough some 500 km east-southeast of the cyclone center but it was moving in an easterly direction away from the westerly moving cyclone. This upper-air trough is unlikely to have influenced the intensification of Kathy. The reason is that upper-air troughs that interact with tropical cyclones are to the west of the cyclone and move east to approach the cyclone (Hanley et al. 2001). g. Dissipating stage Figure 10 shows that at 2300 UTC 21 March a lowlevel trough had just developed to the south of the cyclone. It is at that time that the circulation at the gradient level started to change (see Figs. 4e,f) and when the angle of inflow of the airstream at 48 latitude radius to the north of the cyclone changed from to 338, then to 208 (see Figs. 5d,e), the same value before intensification started. As will be shown in the next section, this change in circulation happened concurrently with a significant change of cloud structure around the cyclone, both changes occurring 12 h before the cyclone reached its peak intensity (see Fig. 10) and started to weaken. Also, in Dvorak s (1975) forecasting technique a tropical cyclone is forecast to lose intensity 12 h after the cloud structure shows a sign of weakening. 3. Movement of moist near-equatorial air toward the center of Tropical Cyclone Kathy a. Satellite imagery A series of 3-hourly cloud-top temperature (T BB ) analyses of GMS digitized IR data associated with Tropical Cyclone Kathy is shown in Fig. 11. The T BB isopleths of 2358 and 2608C, unless otherwise indicated, are drawn in each analysis. A scale is provided in Fig. 11a to give an idea of distances and distortion in the cloud map. The space and temperature resolution of the data in Fig. 11 are 8 km and 38C, respectively. In each analysis the cyclone center is indicated by the usual cyclone symbol. At 2100 UTC 19 March, Fig. 11a, a narrow cloud band (narrow cold band in the T BB field) developed far to the north east of the cyclone. It extended from a large cloud mass near the equator. The narrow cloud band was about 40 km wide and its southern end was about

11 JANUARY 2010 L A J O I E A N D W A L S H 13 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 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 of identifying and following the evolution of this cloud band with time, a bold dashed curve passing through the most active convective elements in the cloud band (the coldest T BB inside the band) is drawn in Fig. 11a and in subsequent analyses. The positions relative to the cyclone center of the most active part of the cloud bands of Figs. 11a e are reproduced in Fig. 12a. The dates and times are indicated along each cloud band. The southern tips of the cloud bands in Fig. 12a are denoted by A, B, C,...,F, corresponding to the cloud bands of Figs. 11a f. The

12 14 MONTHLY WEATHER REVIEW VOLUME 138 FIG. 11. The 3-hourly analyses of TBB for Tropical Cyclone Kathy from UTC 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.

13 JANUARY 2010 LAJOIE AND WALSH 15 FIG. 11. (Continued) positions of the cloud bands in Fig. 12a suggest that they were one and the same cloud band that originated far north-northeast of the cyclone as a short cloud band, and that extended southward along the northerly airstream with time while moving west. During its evolution the cloud band became wider and sometimes had two lines of well-developed clouds in it. For example, in Fig. 12a there were two cloud lines close to one another with their southern tips C and C9 at 0300 UTC 20 March and D and D9 at 0600 UTC 20 March. The change in the positions of the solid curves CC9 and DD9 seems to indicate the movement of the cloud band toward the cyclone in the time interval between 0300 and 0600 UTC 20 March. The cloud band finally merged with the CDO at 0900 UTC 20 March. (A similar wide double-line cloud band also occurred in Fig. 12c when the solid curve II9 moved to JJ9 between 0000 and 0300 UTC 21 March). The TBBs of the cloud tops inside this cloud band were mostly 2508 to 2608C except in a few cumulus-sized blobs where they reached 2808C, corresponding to a cloud top of the 100-hPa level or higher. Between 2100 UTC 19 March and 0900 UTC 20 March, this extremely low TBB was only observed in cloud masses close to the equator and was not even observed inside the CDO of the depression where the minimum TBB was 2658C, corresponding to a cloud top of about the 200-hPa level. The observation of such extremely low TBB inside the cloud band is significant because it indicates that cumulonimbi that developed within the cloud band could reach a much higher level than other cumulonimbi developing in its environment. Because the air flowing inside the cloud band originated near the equator, we call it near-equatorial air. By 0900 UTC 20 March (i.e., 12 h after its formation), the southern part of the cloud band merged with the CDO (see Fig. 11e) where the minimum TBB decreased suddenly from 2658 to 2798C, indicating that nearequatorial air had moved into the CDO area where huge penetrative cumulonimbi [or hot towers, as Riehl and Malkus (1961) had called them] and high-level cirrostratus had developed. The time variation of aerial coverage of high-level cirrostratus in the CDO, represented by the number of pixels with TBB # 2778C, is shown in Fig. 13. The solid curve in Fig. 13 shows the time variation of the intensification rate ( rc/ t) of Tropical Cyclone Kathy. The following five important features can be observed between 0900 and 1200 UTC 20 March: 1) At 0900 UTC 20 March when the feeder band merged with the CDO, the area of high-level cirrostratus started to increase to reach a maximum at 1200 UTC 20 March when it was about 2.5 times its original size. Part of the increase in the high-level

14 16 M O N T H L Y W E A T H E R R E V I E W VOLUME 138 FIG. 12. Positions relative to the cyclone center of cloud bands or feeder bands of Fig. 11, showing their southward extension and 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 the same as that shown in Fig. 11. cirrostratus area, however, was due to the northern part of the cloud band moving over and to the north of the CDO. It is likely that the increase of high-level cirrostratus area was not only due to the cumulonimbi becoming more active, but also becoming more numerous, as has been observed by Jorgensen (1984) in other tropical cyclones. It is worth noting that between 2100 UTC 19 March and 0900 UTC 20 March (i.e., before the arrival of near-equatorial air in the CDO), the intensification rate was 10 hpa day 21, but that soon after 1200 UTC 20 March, (when, it is suggested, moist near-equatorial air had reached the cyclone center) it increased to 25 hpa day 21. (The changes of intensification rates at other times are discussed in Part II of this paper.) 2) Between 0900 and 1200 UTC 20 March when convective activity was at its peak inside the CDO, there 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 T BB 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 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 and the time it started to decrease. The changes of the surface pressure fall at Centre Island are discussed in section 4. The cycle of events that occurred during the evolution of the feeder band described above occurred repeatedly in a second and third feeder band during intensification while the spiral inflow remained unchanged. The features of the second and third feeder bands are summarized in rows 4 and 6 of Table 2. The fourth feeder band evolved differently. It developed far northeast of the cyclone at 1600 UTC 21 March (see Figs. 11n and 12d). Part of the feeder band merged with the CDO soon after 0000 UTC 22 March giving rise to an increase of high-level cirrus area between 0000 and ;0430 UTC 22 March (see Fig. 13). The other part of the feeder band with cloud tops having a minimum T BB of 2658C extended far southeast of the cyclone and did not merge with the CDO, thus depriving the cyclone of most of its supply of moist near-equatorial air. This cloud band remained almost stationary until 0600 UTC 22 March and then moved eastward away from the cyclone. This was the last active feeder band

15 JANUARY 2010 L A J O I E A N D W A L S H 17 FIG. 13. The dashed curve shows the time variation of n/10, where n 5 numbers of pixels with T BB # 778C in the cirrostratus canopy over Tropical Cyclone Kathy from 20 to 22 Mar The solid curve shows the time variation of the computed intensification rate ( r c / 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 and that merged with the CDO. It happened at the time when a low-level trough had developed to the south of the cyclone (Fig. 10). It is worth noting that at 1200 UTC 22 March, 12 h after the merging with the CDO of the last convectively active feeder band, the cyclone reached its peak intensity, and then started to weaken even though it was still over water and its center was still 100 km from the Australian coast. Thus, Kathy started to weaken some 12 h after (i) a trough had developed to the south of the cyclone, allowing cool drier trade air to the south of the cyclone to enter the cyclonic circulation (see Fig. 4f); (ii) the circulation to the north of the cyclone had changed and the angle of inflow was back to its preintensifying stage of 208; and (iii) the cyclone had lost its supply of warm moist near-equatorial air. At 1600 and 1800 UTC 22 March a narrow, less convectively active feeder band, having a minimum T BB of 2608C, merged with the CDO. It did not originate from far north or northeast of the cyclone and did not produce an increase in the highlevel cirrus area. A more active cloud band (T BB C) east of the feeder band extended far to the south east of the cyclone (see the 2100 UTC 22 March T BB analysis of Fig. 13w). b. Relevant comments Raghavan et al. (1980) had also observed a variation of convective activity in the CDO of other tropical cyclones similar to that observed in Kathy. Using range height indicator radar observations of tropical cyclones in the Bay of Bengal, they found that the tops of the eyewall clouds and their precipitation rate increased substantially above their mean values for a 3 5-h period in a 9 12-h cycle. In one case, the height of cumulonimbus tops increased from a mean value of 15 to 21 km while the precipitation rate increased from 17 to 37 mm h 21. Nowadays, hot towers in the eyewall with tops reaching up to 15 to 18 km are regularly observed in the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR; Iguchi et al. 2000) imagery. They are usually accompanied by torrential rain with a rainfall rate greater than 35 mm h 21. According to Kelley and Stout (2004), animation of TRMM satellite imagery indicates that these penetrative cumulonimbi are not always present in the eyewall but develop from time to time and are associated with an increase of the intensification rate of the cyclone. It will be shown in Part II that the rate of intensification of Kathy also increased appreciably during the 3 h of intense convective activity in the CDO.

16 18 M O N T H L Y W E A T H E R R E V I E W VOLUME 138 TABLE 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 km from the cyclone center for Tropical Cyclone Kathy. These pressure falls are assumed to coincide with pressure falls outside the CDO. Column 7 gives the magnitude of the observed surface pressure fall in the first 3 h, while column 8 gives the excess of observed surface pressure fall over the long-term mean change of surface pressure at the same times due to diurnal variation. Excess of observed pressure fall over mean change Observed surface pressure fall in 3 h Time of observed surface pressure fall Time of cloud suppression outside CDO Time cirrus area started to decrease Time cirrus area started to increase Time feeder band merged with CDO Time feeder band started to extend south 2100 UTC 19 Mar 0900 UTC 20 Mar 0900 UTC 20 Mar 1200 UTC 20 Mar UTC 20 Mar 1200 UTC 20 Mar 2250 UTC 20 Mar 0000 UTC 21 Mar UTC 21 Mar 0000 UTC 21 Mar UTC 21 Mar hpa 2100 UTC 20 Mar 105 UTC 21 Mar Slowly 0900 UTC 21 Mar UTC 21 Mar UTC 21 Mar UTC 21 Mar hpa Rapidly 1200 UTC 21 Mar 1450 UTC 21 Mar 0150 UTC 22 Mar 0000 UTC 22 Mar 0600 UTC 22 Mar UTC 22 Mar hpa 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 of the atmosphere. That subsidence-dried air was then fed to the cyclone and caused a marked decrease of convective activity in the CDO, until another batch of moist near-equatorial air arrived some 9 12 h later. It is this sort of quasi-semidiurnal variation of convective activity within the CDO that gives rise to the quasisemidiurnal 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 Butterworth (1984). 4. Time variation of surface pressure at 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 from 3-hourly observations. The distances of the cyclone center from Centre Island are indicated along the time 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 the statistics published by the Australian Bureau of Meteorology (see online at 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 longterm mean change. If it is assumed that during the time intervals when the dashed curve is well below the solid curve the observed pressure fall is significant, then there 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 UTC 21 March, UTC 21 March, and UTC 22 March, and are listed in column 6 of Table 2. The magnitude of 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 longterm pressure change due to diurnal variation are shown in column 8. It is worth noting that the onset of these three sudden falls of surface pressure at Centre Island occurred between the time the high-level cirrus area

17 JANUARY 2010 L A J O I E A N D W A L S H 19 FIG. 14. Variation of observed surface pressure at Centre Island during the approach of Tropical Cyclone Kathy from 20 to 22 Mar 1984 (dashed curve); pressure variation if the pressure changes were the same as evaluated from the long-term mean statistics (solid curve). Distances of Kathy from Centre Island are indicated along the time axis. started to increase and the time it started to decrease. These three intervals are indicated by short horizontal bars along the time axis of Fig. 13. It will be shown in Part II that this sudden and small surface pressure fall extending up to 350 km around the cyclone center plays an important role in the intensification of Kathy. 5. Discussion and conclusions The results of this study are similar in some ways to those of Molinari and Skubis (1985). Both studies show a relationship between an inward-propagating disturbance and the subsequent intensification of a tropical cyclone. In both cases, the intensification of the cyclone was only substantial when the disturbance reached a distance of only a few hundred kilometers from the cyclone center. Disturbances in both storms showed a distinct azimuthal asymmetry. Both studies hypothesize that the arrival of the disturbances close to the center of the storms triggered enhanced deep convection and that this was a possible mechanism for cyclone intensification. The present study differs from the analysis of Molinari and Skubis (1985) by explicitly making the connection between the trajectory of the disturbance and cyclone intensification. The observations discussed above indicate that at the start of intensification of the depression, the low-level circulation took the form of a giant swirl that started some 600 km to the north and northeast of the depression with the angle of inflow changing from 208 to 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 sustain the development of huge cumulonimbi. About 12 h after the change of the circulation, the moist nearequatorial 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 strong convection inside the CDO. When this subsidence-dried air entered the CDO, convective activity in the CDO decreased until a new batch of moist nearequatorial air arrived at the edge of the CDO. The cycle of events, as described above, was repeated 2 more times during the intensification stage of Kathy. Finally, because of a low-level trough that developed to the south of Kathy, the angle of inflow returned to 208 to the north of the cyclone and 308 to the south of the cyclone. The west-southwesterly winds to the north of the cyclone pushed the moist near-equatorial air away to the east of the cyclone cutting off its supply of moist air, and causing it to dissipate rather rapidly. From this analysis, two factors are proposed as being important for tropical cyclone intensification. The import of very moist near-equatorial air into the cyclone core appears to produce in the eyewall the development of hot towers associated with a sudden increase in the intensification rate. Also, the angle of inflow of the equatorial airstream appears to be important for the development and intensification of the cyclone. These concepts, along with the features described above, are used in Part II of this paper to develop a mathematical model to forecast the intensity of a tropical cyclone. Acknowledgments. We thank Noel Davidson for supplying objective analysis data and the University of Melbourne for supporting this research. We also thank three anonymous reviewers for their detailed comments. REFERENCES Atkinson, G. D., and C. R. Holliday, 1975: Tropical cyclone minimum sea-level pressure maximum sustained wind relationship for the western North Pacific. FLEWEACEN Tech. Note, JTWC, 20 pp., and, 1977: Tropical cyclone minimum sea level pressure/ maximum sustained wind relationship for the Western North Pacific. Mon. Wea. Rev., 105,