ON THE ROLE OF UPPER-TROPOSPHERIC POTENTIAL VORTICITY ADVECTION IN TROPICAL CYCLONE FORMATION: CASE STUDIES FROM 1991

by

DANIEL HUNT REILLY

B.S. Physics, University of Virginia (1988)

Submitted to the Department of Earth, Atmospheric, and Planetary Sciences in Partial Fulfillment of the Requirements for the Degree of

MASTER OF SCIENCE IN METEOROLOGY

at the

Massachusetts Institute of Technology

September 1992

Massachusetts Institute of Technology 1992

Signature of Author Department of Earth, Atmospheric, and Planetary Science May 26, 1992

Certified by Professor Kerry A. Emanuel Thesis Supervisor

Accepted by Thomas H. Jordan, Head Dept. of Earth, Atmospheric, and Planetary Sciences

1

MA Ri

UBRAIES

ON THE ROLE OF UPPER-TROPOSPHERIC POTENTIAL VORTICITY ADVECTION IN TROPICAL CYCLONE FORMATION: CASE STUDIES FROM 1991

by

DANIEL HUNT REILLY

Submitted to the Department of Earth, Atmospheric and Planetary Sciences on May 26th, 1992 in partial fulfillment of the requirements for the Degree of Master of Science in Meteorology

ABSTRACT Several cases of western North Pacific tropical cyclogenesis from the 1991 season are studied, with special emphasis on the antecedent conditions in the upper troposphere. Specifically, I test the hypothesis that tropical cyclogenesis takes place through an interaction between a pre-existing lower-tropospheric disturbance (ITCZ or easterly wave disturbance) and an independent upper-level trough. The upper-level trough provides a source of upper-tropospheric lifting via differential potential vorticity advection which may aid in the genesis process. In this study, locations of pre-cyclone disturbances are obtained from JTWC best-track information. A measure of potential vorticity advection by the upper-tropospheric shear or the "forcing" of ascent is then calculated from NMC gridded analyses. It is found that the majority of tropical cyclones studied are subjected to periods of positive forcing during their pre-genesis stages. In addition, positive forcing is implicated in the generation of the pre-cyclone disturbances for the majority of cases. The findings support the notion of tropical cyclogenesis as an externally triggered phenomenon.

Thesis Supervisor: Kerry A. Emanuel Title: Professor of Meteorology 4 ACKNOWLEDGMENTS

I would like to thank Kerry Emanuel for suggesting this topic to me, and for his help along the way. The conversations with Michael Montgomery were stimulating and enjoyable. I also value the friendships made with the other graduate students in the department. Most of all, I thank my parents and sister for their love and support. NMC gridded analyses and access software were obtained from the Scientific Computing Division of NCAR. The efforts of Harry Edmon, who provided analyses to fill the gaps for a handfull of missing dates, are much appreciated. Best track information was provided on diskette by Joint Typhoon Warning Center (JTWC) at Guam. Frank Wells of JTWC was prompt and courteous in his responses to my inquiries. This research was supported under NSF grant number ATM-9103025. CONTENTS

Abstract 3 Acknowledgments 5 1. Introduction 7 2. Statement of the Problem of Genesis 2.1 Necessary Conditions for Genesis 8 2.2 Population of Tropical Disturbances 9 3. Review of Previous Work 3.1 Observational Studies of Genesis 11 3.2 Conceptual Models of Externally Forced Genesis 16 3.3 Conceptual Models of Genesis Through Internal 18 Processes, and Their Relationship to the External Triggering Hypothesis 4. Relevant Properties of Ertel's Potential Vorticity 19 5. Data and Methods of Analysis 5.1 The Database for and the Preparation of the NMC 22 Analyses 5.2 JTWC Best Tracks and Estimated Intensities 23 5.3 Definition of the Forcing Parameter 25 6. Case Studies from the 1991 Season 27 7. Summary and Suggestions for Future Work 44 References 46 Figures 51 1.Introduction Tropical cyclones are awesome, and potentially devastating storms. They can form quite suddenly, with disasterous consequences for mariners and coastal or island communities caught unaware. Thus it is not only of scientific interest, but also of great practical importance to better understand the birth or genesis of these storms, so that their formation can be predicted. Current practice is more of a careful monitoring of cloud clusters for signs of development into tropical cyclones, rather than an objective forecasting of such development. The scale of the tropical cyclone and of pre-cyclone disturbances is on the order of hundreds of kilometers, which is not resolved by the standard observing network over the tropical oceans. However, it is possible to view characteristics of the larger scale environment within which these disturbances are embedded. One hypothesis which can be tested is that tropical cyclogenesis occurs when a pre-existing lower-tropospheric disturbance comes under the influence of positive potential vorticity advection from an independent upper-tropospheric anomaly brought into proximity by the relative mean flow. In this study, NMC global analyses, received on 2.5 x 2.5 degree hemispheric grids, are used to examine all of the cases of genesis found to occur in the western North Pacific region between July 1st and September 6th of the 1991 season. Attention is given to the evolution of the large-scale environment surrounding the pre-cyclone disturbances, with particular emphasis on the possible role of upper-tropospheric potential vorticity advection in the genesis of these storms. In this paper, a few basic questions related to the problem of tropical cyclongensis are first stated. Section 3 contains a review of selected works relevant to this topic. A sketch of useful and relevant properties of Ertel's potential vorticity is then made in section 4. A description of the data sets, and of the methods of analysis follows in section 5. The results for several western North Pacific cases are presented in section 6, with some discussion for each of the 13 cases occurring during the nine-week period. A discussion of the overall findings of this study, and suggestions for future work conclude the paper.

2. Statement of the Problem of Genesis 2.1 Necessary Conditions for Genesis Tropical cyclones are observed to form only when certain necessary, but not sufficient, conditions are satisfied. If genesis is to occur, the environment must be characterized by (Gray, 1979; McBride and Zehr, 1981): (i) a sufficiently warm oceanic surface layer; empirically, an SST lower bound is found to be around 26.5 degrees Celsius (Palmen, 1948); (ii) a Coriolis parameter greater than some finite value (no geneses occur within a few degrees of the equator); (iii) sufficiently weak vertical wind shear directly over the center of the potential developer; significant shear will inhibit the establishment of a deep warm core, a characteristic structual feature of a mature tropical cyclone; (iv) relatively large values of low-level relative vorticity on the synoptic and larger scales.

On a climatological or seasonal basis, middle-tropospheric relative humidity and lower-to-middle tropospheric moist entropy differences are also believed to be important. However, comparisons of composite developing versus nondeveloping cloud-dusters showed little difference in the potential instability to moist convection, (McBride and Zehr, 1981). In addition, tropical cyclones are observed to form out of pre-existing initial disturbances of apparently independent origin (Riehl, 1954). One objective of genesis-related research is to understand the processes involved in the transformation of these initial disturbances into tropical cyclones more completely, and perhaps to add to or refine the current set of conditions which are known to favor this transformation.

2.2 Population of Tropical Disturbances During tropical cyclone season, the tropical atmosphere contains large numbers of candidates for genesis, which include waves in the easterly trades, disturbances that form within or near zones of cyclonic shear such as the inter-tropical convergence zone (ITCZ) or monsoon trough, and the cloud clusters associated with them. Precise definitions of terms useful for the discussion of genesis can be found in the appendix of the Joint Typhoon Warning Center's (JTWC) Annual Tropical Cyclone Report (JTWC, 1990):

(i) Tropical Disturbance: A discrete system of apparently organized convection, generally 100 to 300 nm (185 to 555 km) in diameter, originating in the tropics or subtropics, having a non-frontal, migratory character and having maintained its identity for 12 to 24 hours. It may or may not be associated with a detectable perturbation of the wind field. (ii) Tropical Cyclone: A non-frontal migratory low-pressure system, usually of synoptic scale, originating over tropical or subtropical waters, and having a definite organized circulation. iii) Tropical Depression: A tropical cyclone with maximum sustained surface wind speeds of 33 kt (17 m/s) or less;

(iv) Tropical Storm: A tropical cyclone with maximum sustained surface wind speeds of between 34 and 63 kt (17 to 32 m/s);

(v) Typhoon (or Hurricane): A tropical cyclone with maximum sustained surface wind speeds of greater than 64 kt (33 m/s).

Whereas, by the above definitions, tropical disturbances abound, tropical cyclones are relatively rare. It has been estimated that only 1% or 2% of all tropical disturbances (called hurricane seedlings in the reference) become tropical cyclones (Simpson and Riehl, 1981, p.70). A 24-year average for the Atlantic (1967-1990) indicates that out of an annual average of 58 African waves per season, 11.6 become depressions, and 5.9 become tropical storms (Avila, 1991). Thus, it is a fundamental question of genesis to discover the differences between pre-cyclone and non-developing tropical disturbances, which may involve differences in the internal structures of the disturbances themselves, and/or in the environments within which they are embedded. One proposition of the present study is that the developers are often selected by a chance interaction with an independent upper-level trough. The suggested role of the upper-trough is to provide lifting through potential vorticity advection by the shear in the flow.

10 3. Review of Previous Work 3.1 Observational Studies of Genesis Riehl in his early works (Riehl, 1948a, 1948b,1950, 1954) recognized the existence of migratory cyclones and anticyclones in the tropical upper troposphere which appeared to be moving relative to, and independent of lower to middle-tropospheric disturbances such as easterly waves or ITCZ disturbances. He also proposed the need for a "starting mechanism" for the tropical cyclone "engine" which he argued must come from an external energy source. In the western North Pacific he found that the initial deepening and formation of tropical cyclones often occurred with the onset of upper-level height falls after the passage of an upper-tropospheric anticyclone past a lower-tropospheric trough, with the upper feature moving westward relative to the lower one (Riehl, 1948a). The author draws an analogy between this scenario and that found in middle-latitudes, the difference being in the direction of the vertical shear, i.e. the upper-tropospheric features tend to move eastward relative to the lower-level troughs. Over the years, with the addition of commercial aircraft and cloud-tracked wind observations to the available database, Riehl and others were able to observe upper-level flow patterns in the vicinities of genesis events more completely, and many cases were documented. In his more recent work (Simpson and Riehl, 1981; Riehl, 1979), it is emphasized that a commonly observed precursor to genesis or intensification of tropical cyclones is the positioning of an upper tropospheric cold-pool associated with an upper-level cyclone or trough adjacent to a lower-level disturbance. The lower-level disturbance is then observed to intensify as the upper-tropospheric cyclone diminishes in intensity, or is destroyed completely. The disappearance of the upper-level trough or cyclone ("collapsing cold-dome" in Riehl's terminology) was interpreted as signaling the release of potential energy through the sinking of cold air, thereby providing the energy necessary for triggering the genesis of the storm. Other observational evidence for the importance of upper-tropospheric troughs in the geneses of tropical cyclones is found in the work of J. Sadler (Sadler 1975, 1976, 1978). He observed that genesis in the western North Pacific often appeared related to southward and westward extentions of what he termed the Tropical Upper Tropospheric Trough, or TUTT. The TUTT is a WSW-ENE oriented tongue of cyclonic vorticity, or anomalously large potential vorticity (PV), which is a persistent feature found on the equatorward side of the subtropical ridge. This channel of high-PV is observed to vary in its eastern and southern extent, as it tends to be stretched out by the flow associated with the anticyclone to the north. Occasionally, this zonally stretched trough will fracture, and a cutoff anomaly of high-PV will emerge, manifesting itself an upper tropospheric cyclone or "TUTT cell" in Sadler's terminology. Examples of these events will be seen in the results presented later in this paper. Based on aircraft and radiosonde information, as well as subjective inferences from satellite imagery, a number of cases of tropical cyclogenesis were observed to occur at the approach of such a TUTT cell toward the site of some trade-wind of monsoon trough disturbance. Often, the TUTT cell was found to position itself to the north and west of a tropical disturbance, with genesis following. The author proposed that such an upper-level flow configuration facilitates the creation of an outflow channel to the north and east of the storm, in addition to providing upper-tropospheric divergence above the incipient center. The outflow jets were considered favorable and important for cyclone

12 development due to their ability to remove high entropy air from the periphery of the cyclone core region, the subsidence of which would destroy the thermal gradients near the core of the cyclone, and kill the development (e.g. Sawyer, 1947). Sadler also implicated the TUTT cells in the formation of the initial low-level disturbances themselves, and thus questioned the independence of the upper and lower features in those cases. More recently, a study was made of the atmospheric conditions prior to the genesis of Hurricane Diana which occurred off the southeastern United States coast in 1984 (Bosart and Bartlo, 1991). The authors found that an upper-level cyclone, associated with the fracture of an intense trough in the westerlies, approached the genesis site in the hours prior to the formation of the storm, providing forcing of ascent (estimated from calculated advection of upper-tropospheric vorticity by the thermal wind) in the upper-troposphere near the region of genesis. The upper-tropospheric cyclone was observed to weaken as the surface cyclone developed, and the authors noted the similarity to the collapsing cold-dome scenarios described by Riehl. Another observational study relevant to this problem was made by Nitta and Takayabu, 1985, in which ECMWF level-mb gridded analyses for the FGGE year were utilized. In that study, 850 mb relative vorticity fields were filtered to pick out waves of periods between 5.7 and 10 days in the western North Pacific. They found that about half of the vorticity maxima in the filtered fields within the observed tropical cyclone generation region eventually developed into typhoons, while the other half did not. The most striking difference between the pre-typhoon and the non-developing vortices was in the horizontal structure of the 200 mb vorticity fields composited relative to the vortices. The upper-level flow

13 pattern surrounding the developing composite was characterized by the Mid-Pacific Trough (MPT) (equivalent to Sadler's TUTT) being extended westward to a position to the north of the developing vortices, while for the nondevelopers this feature was positioned much further to the east (see figure 1). Another study of western North Pacific cyclogenesis was performed by Zehr, 1991. He looked at all 50 cases of named-storm formation which occurred during 1983 and 1984, as well as 25 persistent nondevelopers. Data sources included digitized satellite radiance data in both IR and VIS channels (with 10 km, and 3 hour resolution), as well as Air Force reconnaissance, and objectively analyzed conventional (AIREPS, cloud-tracked winds, rawinsondes, ships, surface obs) data. From the IR radiance data he calculated the percentage of pixels with brightness temperatures below some specified value over an area within some specified distance from the disturbance center, and plotted this parameter versus time. He found that for 85% of pre-typhoon disturbances the time series exhibited marked peaks during the pre-cyclone stage which were termed "early convective maxima". It was found that such an early convective maximum preceded tropical storm status by 36-60 hours on average, but occasionally by several days. This finding led Zehr to propose a two-stage model of tropical cyclogenesis, in which the occurrence of such an early convective maximum signifies stage 1 (see figure 2). After the convective maximum, persistent curvature in the deep cumulonimbus and an identifyable low-level circulation center first become apparent. Aircraft reconnaissance also showed a reduction in the scale of the circulation at this time, with little change in low-level wind speed, and hence an increase in the low-level vorticity. The percent-coverage of cold cloud-tops (the time series parameter) then decreases temporarily after

14 stage 1, but increases again with the arrival of stage 2 which is characterized by decreasing central pressure, and winds increasing to tropical storm force, i.e., by the birth and early intensification of the tropical cyclone. It should be noted that the author found little difference in the separation between the developing and non-developing clusters and the axis of the TUTT, or TUTT cells, during stages 1 or 2, and suggested on this basis that the TUTT might not be important in the genesis process. A number of sounding-composite studies of developing and nondeveloping clusters have been performed, principally by the research group at Colorado State University. These composite studies involve taking whatever soundings are available from each individual disturbance or cyclone, and assigning them to a radius and sector relative to the estimated center of that disturbance or cluster. It is then possible to study quantitatively various properties of the composite disturbances. Care is taken to stratify the cases according to different stages of development, or to their ability to develop into tropical cyclones at all. One drawback of this approach is that features not having a preferred radius or sector, which may be important to genesis for each individual case, tend to be smoothed out in the composite. The smoothing will also apply in time, the degree of which is sensitive to the precision of the stratification of cases by stage of development. In one such study (McBride and Zehr, 1981; McBride, 1981a,b), developing and non-developing composite clusters were studied and compared. It was found that pre-cyclone and non-developing clusters had similar vertical and horizontal sturctures in fields of temperature and moisture, but that the pre-cyclone clusters were embedded in environments characterized by much larger lower-to middle-tropospheric relative vorticity and tangential wind fields out to

15 radii of 8 degrees latitude. The same study also showed zero 200 mb-850 mb vertical wind shear directly over, and large horizontal gradients in vertical shear in an anticyclonic sense about the centers of the pre-cyclone disturbances. In a similar study (Lee, 1986,1989) the perceived importance of a pre-genesis build-up of lower and middle-tropospheric relative vorticity on the large scales is emphasized by the author. This build-up was attributed to the agent of lower tropospheric momentum surges with their origins remote from the genesis site (Love, 1985).

3.2 Conceptual Models of Externally Forced Genesis Attempts to quantify mechanisms by which upper-tropospheric forcing can influence the development of tropical cyclones have been made in the works of Challa and Pfeffer (Challa and Pfeffer, 1980, 1990; Pfeffer and Challa, 1981, 1991). They used the equations for an axisymmetric, balanced vortex (Eliassen, 1951) to diagnose the effects of angular momentum transport by asymmetries in the flow. These were parameterized by means of an effective angular momentum source term, in the vortex equations, which depends on the radial derivative of the eddy angular momentum flux. The effect of this forcing on the secondary circulation is given by the vertical derivative of this effective momentum source. Vertical motions can arise from the adjustment of the temperature field to the forced momentum field under the prescribed balance. The authors found from the McBride composites that the Atlantic developing composites differ from the nondeveloping composites in the organization of the eddy fluxes of angular momentum in the upper-troposphere. For the composite developers, the flux convergence pattern in the upper-troposphere is such as to encourage ascent near the center of the vortex, hence favoring an in-up-and-out circulation which

16 might aid in the genesis of the storm. A series of numerical experiments were performed in which the composite fields served as initial conditions for an axisymmetric hurricane model (Sundquist, 1970) and later a 3-dimensional mesoscale primitive equation model (Madala et. al., 1987). For both models it was found that the initial conditions taken from the nondeveloping duster composites did not produce hurricanes, while the unaltered developing cluster composites did produce hurricanes in the model integrations. When the asymmetric component and hence the flux forcing was removed from the developing composite, the resultant initial conditions failed to produce a cyclone, leading the authors to suggest that proper organization of upper-tropospheric angular momentum fluxes might be an additional necessary condition for the genesis of Atlantic hurricanes. Such organization might be brought about by interactions with external agents such as independent upper-level troughs. Montgomery and Farrell (1991) draw an analogy between the forcing of ascent via flux convergence in the Pfeffer-Challa model, and that by the differential advection of potential vorticity in a 3-dimensional model of the process. They employ moist, geostrophic momentum models to investigate the response of a low-level seed field to the approach of an upper-level trough (positive PV anomaly) brought into proximity by weak upper-tropospheric shear. It was found that when the effects of moisture and a near-saturated deep column were incorporated, reducing the effective static stability, the encroaching upper-level PV anomaly acted to induce a narrow tube of strong deep ascent, and resulted in a dramatic spin-up of a surface cyclone. Also noted in this process was a collapse in the scale of the low-level circulation as the upper anomaly approached. This was contrasted with the corresponding dry experiment for which ascent was confined to the upper-troposphere, and the upper and lower

17 anomalies remained essentially decoupled.

3.3 Conceptual Models of Genesis Through Internal Processes, and Their Relationship to the External Triggering Hypothesis Other models of tropical cyclogenesis have tended to focus more on the dynamics of moist convection, and on air-sea heat exchange. Convective Instability of the Second Kind (CISK) is one such model which seeks to explain the growth of a depression-scale circulation in terms of a cooperation between convective and depression scales. The concept is that the two scales support one another, the circulation providing a favorable environment for the convection, through boundary layer convergence and Ekman pumping, and the convection, through latent heat release in a conditionally unstable atmosphere, providing energy for intensifying the vortex. In this way, a positive feedback is established, and the system intensifies. This concept led to numerous numerical experiments (for review, see Anthes, 1982) in which initial vortices were placed in conditionally unstable environments to see if hurricane-like systems would develop. Often, successful simulations would require seemingly unrealistic assumptions about the intensity and/or geometry of the initial vortex, and the vertical structure of the heating. Vortex development has also been simulated in a nonhydrostatic primitive equation model characterized by an initially conditionally neutral environment (Rotunno and Emanuel, 1987), where the suggested positive feedback is between the surface wind speed and the induced surface fluxes from the ocean, and doesn't implicitly require ambient conditional instability of the atmosphere as does CISK. This study also demonstrates the finite amplitude nature of this instability, termed WISHE (Wind Induced Surface Heat Exchange) instability, as only vortices of sufficient strength

18 and concentration would intensify. The finite amplitude nature of the WISHE instability has been explored further in Emanuel, 1989. A finite amplitude instability resembling CISK has been investigated in Handel, 1990. The question which then arises is what processes in the atmosphere might lead to the formation of the initial finite amplitude vortex. An external transient forcing might play a role in producing such an initial vortex, which might then intensify by fundamentally internal mechanisms such as CISK or WISHE. It has also been suggested (Frank and Chen, 1991) that processes involved in the formation of the initial surface vortex may be similar to those involved in the formation of lower to middle tropospheric vortices in mid-latitude MCCs (e.g. Menard and Fritcsh, 1989). A source of synoptic-scale forced ascent, such as that provided ahead of a shortwave trough, and an associated active stratiform rain area have been suggested to be necessary conditions for vortex formation in midlatitude MCCs (Chen, 1990). Thus, there is a role for external forcing in the tropics in this model as well.

4. Relevant Properties of Ertel's Potential Vorticity Plots of Ertel's potential vorticity (PV), defined by expression (1) below, are used extensively in the analyses of the genesis events presented in this paper. Ertel's theorem states that the quantity Q, defined Q4 is conserved under conditions for which (i) DX/Dt = 0, (ii) frictional effects are unimportant, and (iii) X depends only on density and pressure (i.e. is a state variable) (Pedlosky, 1987). For unsaturated flows in the atmosphere, the logical choice for 2 is potential temperature,

19 represented by the symbol 0. For this choice, Ertel's theorem states that (dry) potential vorticity is conserved following the motion for adiabatic (DO/DT=O), frictionless conditions, which are reasonable approximations for large scale, unsaturated flows outside of the boundary layer. Plots of PV on surfaces of constant potential temperature (also a conserved variable for frictionless, adiabatic flow) are useful for observing the movement of parcels or patches of air. The atmosphere is observed to contain various structures which have signatures in the PV fields; the conservation of PV allows the movements and changes in shape of these features to be followed easily. The effects of latent heating can also be addressed within the PV framework. Expressions (2) and (3) are statements concerning the Lagrangian time rate of change of PV due to heating and frictional effects, and the integral effects of the heating in the interior of a material volume (Hoskins et al., 1985):

41 DQ/Dt = (1/p) - grad(dO/dt) + (1/p) K- - grad(8) (2)

d/dt ( ijj pQ dt) = If ((dO/dt)4 + OK)-n ds) (3)

where K is the curl of the frictional force.

Concentrating on the heating terms, expression (2) states that generation or destruction of PV following a parcel depends on the dot product of absolute vorticity with the gradient in heating, divided by the density. Expression (3) states that, in the absence of heating or friction on the boundaries of a material volume, the mass-weighted PV integrated over the volume does not change with time. In a tropical cyclone, one can imagine constructing the boundary of a volume in a way such that all

20 latent heat release occurs within its interior. Then expression (2) implies that parcels below the level of maximum heating will gain PV, while parcels above will lose PV. Expression (3) offers an integral constraint on these changes, that the mass-weighted losses of PV aloft are compensated by generation in the lower layers. Such destruction of PV near the tropopause in the vicinity of intensifying cyclones will be seen in many of the results presented later. Another aspect of PV which is worth noting is that PV anomalies in quasi-balanced flow are characterized by vorticity and static stability anomalies in the same sense, i.e. positive PV anomalies are characterized by cyclonic circulations and relatively high static stabilities, and vice-versa. The circulations associated with the anomalies will penetrate some distance below the anomaly with a penetration depth dependent on the scale of the anomaly, on the effective static stability in the column below, and on the vorticity itself. Vertical shear in the horizontal wind will tend to induce upward motion on the downshear side of the anomaly. To understand this result physically, one might imagine travelling with a lower-level air parcel which is moving toward an upper-tropospheric positive PV anomaly located upshear. The parcel will be required to acquire vorticity consistent with the circulation from the anomaly above as it approaches the region beneath the anomaly. This implies vortex stretching from the time it enters the region of influence of the upper anomaly's circulation until its closest approach, and then contraction as it moves away. Translating this picture to an Eulerian frame, it follows that upper-tropospheric ascent is expected where there is positive potential vorticity advection by the relative velocity between the anomaly and the parcels below. This is the physical basis for the forcing parameter defined in the section 5.3.

21 5. Data and Methods of Analysis 5.1 The Database for and the Preparation of the NMC Analyses The upper-air database used in the preparation of NMC analyses consists of radiosondes, pilot balloons, aircraft reports, cloud-tracked winds, and remotely-sensed temperature soundings. Figure 3 shows the distribution of these elements over the western North Pacific for a "typical day" (Dey, 1989). Aircraft reports generally have their best coverage in the upper troposphere and lower stratosphere, near the flight levels of commercial aircraft, which have their highest density near 250 mb. Cloud-tracked winds are derived from the identification and tracking of clouds in time loops of images obtained from geosynchronous-orbitting satellites. Crudely speaking, levels are assigned to the observations by matching the measured IR brightness temperature of the cloud to the appropriate level using some estimate of the environmental vertical temperature profile, e.g. that obtained through remotely-sensed temperature soundings. Generally, low-level cumulus tracers provide velocity measurements representative of the flow at 850 mb, while the tracking of upper-tropospheric cirrus targets provide estimates of winds near the tropopause (Elsberry, 1987). The circulations of upper-tropospheric cold-lows with associated cirrus rings about their perimeter can occasionally be well resolved by this data source (Shimamura, 1981). The remotely-sensed temperature soundings are retrieved from measurements made by passive radiometers mounted on polar-orbitting satellites. Upwelling radiation is measured in various frequency bands, each of which has a characteristic emissivity vertical profile in the atmosphere. Given the dependence of the emissions of radiation on temperature, and given the emissivity profiles and observed

22 radiances for various bands, temperatures are determined for various levels, and a sounding is retrieved. In practice, the emissivity profiles or weighting functions overlap, and the sorting out of information to obtain a vertical sounding is done by sophisticated retrieval methods (Smith, 1991). Temperature gradients are relatively weak in the lower to middle tropical troposphere, and the remotely-sensed temperature soundings are thus of little use there. However, this technique might be useful near the tropopause where temperature anomalies are more significant. For example, temperature anomalies of +8 and -5 degrees Celsius have been observed above and below upper tropospheric cold lows near 20 degrees north latitude (Shimamura, 1982). The raw observations are used to determine corrections, via optimum interpolation techniques, to background or first-guess fields which are provided by a 6-hour forecast from the NMC global spectral model (Kanamitsu, 1989; Kanamitsu et al., 1991) which had been initialized by the previous analysis. Obviously, in the absence of data, these first-guess fields will dominate the analyses. In the results which follow, attention is restricted to the 850 mb level and the 360K isentropic level (near the tropopause) where the density of observations is greatest. Presumably, the NMC analyses will accurately portray features which are well resolved by information in the database. The numerical model should then be capable of maintaining these features for some time after adequate resolution by observations has been lost.

5.2 TTWC Best Tracks and Estimated Intensities The positions and intensities of the cyclones in this study are taken from Joint Typhoon Warning Center (JTWC) post-season analyses for the 1991 season. This information includes positions of the pre-depression

23 disturbances when they were able to be determined by the JTWC analysts. Position and intensity (maximum sustained wind, 1 minute average) estimates both rely heavily on interpretation of satellite imagery. The Dvorak technique (Dvorak, 1984) is chiefly used in the intensity estimates (JTWC, 1990). The rules which characterize this technique are based on the observation that, apart from diurnal and other short term fluctuations, the patterns of deep convective clouds often follow a systematic progression during tropical cyclone intensification. The primary indicator of intensity for an immature (no eye) cyclone is the degree of coiling of curved deep convective bands around the system center (see figure 4). For a more mature cyclone, IR measurements of "eye-temperature" and cloud-top brightness temperature for the surrounding inner core region provide more objective measures of intensity (Dvorak, 1984). According to the rules of this technique, the initial development of a tropical cyclone is defined by the first appearance of curvature in a band of deep convective clouds together with an area of dense overcast of extent greater that 1.5 degrees latitude found less than 2 degrees latitude from the estimated system center. If this is observed, the disturbance is tagged "T1", and assigned an intensity of 25 kts (for table relating intensity to T-number, see figure 4). From that point, the T-number is expected to increase on average 1 per day, with the disturbance anticipated to reach tropical storm strength in 36 hours. The intensity progression expected by this Dvorak timetable is to be used as a guideline for forecasters and analysts, but is modified if the cloud-patterns clearly indicate a different intensity. In the case studies which follow later, the JTWC estimated intensities are plotted versus time. From these time series, it is possible to identify various transitions in the intensification rates of the storms. Figure 6 shows a typical example. Often the time series is initially flat, with

24 intensities of magnitude less than or equal to 25 kts, and increasing at a rate of 5 knots per day or less. During this stage, one can infer in the context of the Dvorak technique that the disturbance is lacking signs of an organized circulation (i.e. band curvature). At some point a clear transition takes place, and the system undergoes steady intensification of 10 to 25 knots or about 1 T-number per day from depression to tropical storm and perhaps to typhoon stage. Occasionally, periods of more rapid intensification are found, perhaps exceeding 30 knots per day for a limited time interval. Of particular interest in this study are the environmental conditions, and upper-tropospheric forcing during the days leading up to the transition from the very-slowly intensifying, poorly organized tropical disturbance, to the steadily intensifying well-organized tropical cyclone. Such a transition will be referred to as genesis in this paper. Also of interest are conditions at the time and place of the generation of the tropical disturbance itself, defined by the first entry for the storm in the JTWC best track. The JTWC positions are also used in the calculation of the forcing parameter to be defined below, the physical significance of which was described in the previous section. On potential vorticity and 850 mb vorticity maps in the results presented later, positions of JTWC analyzed pre-cyclone disturbances or cyclones are indicated by triangles, with the track of the disturbance and cyclone indicated by small squares.

5.3 Definition of the Forcing Parameter In this work, we are especially interested in determining the degree to which pre-genesis disturbances are fostered in environments characterized by ascent, externally forced by migrating upper-tropospheric troughs or cyclones. To quantify this effect, the forcing parameter defined below is used:

25 F = - 1/N T Y (Vijk - V40j) - grad( Q ) k i,j where N is the total number of terms in the summation

Vjk is the analysis velocity at isentropic level k, horizontal grid points ij,

-_4 V40j is the 400mb horizontal wind velocity for grid points ij,

and Q is the value of Ertel's PV at grid point ij, level k.

The summation in k is made over 5 levels of potential temperature, from 350K to 370K at 5 degree intervals. All grid points (ij) within a specified radius from the JTWC-analyzed disturbance center are included in the average. This radius is varied between 2 and 7 degrees latitude. The forcing parameter essentially measures an average potential vorticity advection near the tropopause as viewed in the frame of a parcel at 400mb. Positive values should correspond to forced ascent, negative values to descent. Time series of this forcing parameter are readily compared with those of intensity. In this work, a value of the forcing parameter prior to generation of the initial pre-cyclone disturbance is estimated by extrapolating the first entry in the JTWC best track backward 6 or 12 hours. Thus the forcing time series will begin 6 to 12 hours before the corresponding intensity plot. A second forcing parameter was also computed, with 850 mb replacing 400 mb as the lower level in the calculation. The results tended to be similar during the early stages of development, with a few exceptions which will be noted in the discussion of the results. In the time series presented later, the forcing is shown in units of 10-11 m2 K kg-1 s-2, or 0.864 PV units (Hoskins et. al., 1985) per day.

26 6. Case Studies from the 1991 Tropical Cyclone Season In the following, results from every case of tropical cyclogenesis which occurred in the western North Pacific between the dates of July 1st and September 6th of 1991, are discussed. This 9 week period saw the formation of 8 typhoons, 3 tropical storms, and 2 unnamed depressions. Emphasized in the discussion of the results is the time evolution of upper-tropospheric flow patterns and PV-structures for times leading up to the geneses of the tropical cyclones. The results for each case are presented in the following format. First, the tracks of each cyclone are shown, which are useful for determining intensity changes which may be related to a change in the lower boundary, i.e. passage of the disturbance over land or colder sea-surface temperatures. Next, the time series of JTWC estimated intensity for the cyclone is shown, from which phases in the development, characterized by markedly different average intensification rates, are identified. An attempt is made to identify the time of transition from tropical disturbance to intensifying cyclone, i.e. of genesis, on the basis of these time series. The time series for the forcing parameter for the event are then discussed, with episodes of "positive forcing" and implied forced ascent noted, and their relationship (if any) to the genesis of the tropical cyclone, and to the generation of the precursor tropical disturbance. Next, potential vorticity fields are shown on the 360K isentropic surface, which indicate the upper-level flow patterns near the time of genesis, and the structures responsible for episodes of positive forcing. For most cases, maps of 850 mb winds and relative vorticities are presented to portray the evolution of the lower-tropospheric large-scale flow environment over the days leading up to genesis. The month of July saw the formation of 4 tropical cyclones, Zeke, Amy, Brendan, and Caitlin, which each attained typhoon status. The

27 tracks of these storms, including their pre-cyclone disturbance stage, are shown in figure 5. The geneses of these storms occurred within a 2 week period, which was followed by 2 weeks of inactivity in which no tropical cyclones formed. From figure 5 note the similarity in the tracks of the storms, the exception being the dramatic turn of Caitlin toward the north on the 24th of July. Such a clustering of events in space and time has been noted by others (Gray, 1979). Each of these storms will next be discussed individually, in order of their appearance.

Typhoon Zeke The intensity time series for tropical cyclone Zeke is shown in figure 6. The following phases, characterized by nearly uniform intensification rates, are identified: Ave. Intensification Phase Time Inverval Description Rate (kts/day)

1 07/06/OOZ-07/09/18Z pre-cyclone 3 disturbance 2 07/09/18Z-07/10/18Z steadily intensifying 10 incipient cyclone 3 07/10/18Z-07/12/18Z rapidly intensifying 27 cyclone The forcing parameter time series for this case (figure 6) shows an extended period of positive forcing, and induced upper-tropospheric ascent occurring roughly between OOZ of the 6th and OOZ of the 9th, during the pre-cyclone stage of development. The actual transition between pre-cyclone disturbance and incipient cyclone, which we are calling genesis, occurs by 18Z of the 9th, a time when the forcing is actually reduced to near zero. In figure 7, the 360K PV-fields are presented every 24 hours for the days leading up to the generation of the pre-cyclone disturbance on OOZ of the 6th. Of particular note is the westward and

28 equatorward advance of the upper-level trough extending from 165E, 17N southwestward to around 145E, 7N at 12Z of the 5th, as well as the region of negative PV centered around 146E, 3N to the south of the advancing trough. The pre-cyclone disturbance associated with Zeke can initially be found just west of this feature, as shown in figure 8. Over the next 48 hours the disturbance is approached by another trough, or extension of the TUTT, which is seen by following the advance of the 3 PVU contour in figure 8, to position itself to the northeast (upshear) of the tropical disturbance by 12Z of the 7th. Figure 9 shows the subsequent movement of the pre-cyclone disturbance away from the TUTT, with nonconservative processes, i.e. destruction of PV aloft, exhibited between 12Z of the 8th and 9th. The evolution of the large-scale flow at 850 mb can be seen in the relative vorticity and wind fields shown in figures 10 and 11. Note the intensification of the equatorial trough or ITCZ over the entire 40 degree longitude span of the domain in the days prior to the generation of the pre-cyclone disturbance (figure 10), with an elliptical large-scale circulation between 130E and 145E, 5N and 1ON emerging by 12Z of the 5th. By 12Z of the 9th the pattern has evolved to a more circular shape, with genesis, or the transition to a steadily intensifying vortex, occurring around this time. In summary, the generation of the pre-cyclone disturbance related to Zeke was preceded by an intensification of the monsoon trough at 850 mb, and the approach of an upper tropospheric trough from the northeast. Once formed, the tropical disturbance remained in an environment of positive forcing up until, but not including the transition from phase 1 to phase 2, i.e. the genesis for this case. Also, the disturbance's proximity to the Phillipines during the latter part of phase 1 (figure 5) may have inhibited the onset of intensification (phase 2) until the passage of the

29 islands. Typhoon Amy The intensity time series associated with Amy (figure 12) suggests the following divisions: Ave. Intensification Phase Time Inverval Description Rate (kts/day) 1 07/12/18Z-07/15/12Z pre-cyclone 3 disturbance 2 07/15/12Z-07/17/06Z intensifying vortex, 23 incipient cyclone 3 07/17/06Z-07/18/12Z rapidly intensifying 48 cyclone The forcing parameter time series (figure 12) again indicates a bias toward positive forcing during the pre-cyclone stage (phase 1) and through phase 2 as well, with maximum forcing occurring between OOZ of the 15th and OOZ of the 17th. The forcing reduces to near zero during and after the rapid intensification stage of the cyclone. The corresponding PV maps are shown in figures 13 through 15. Note the evolution of the TUTT over the days preceding the generation of the pre-cyclone disturbance (figure 13). The TUTT appears extended from 165E, 24N to 135E, 5N as of 12Z on the 9th, and over the subsequent days, is seen to fracture, with an intense upper-level cyclone remaining in the eastern portion, centered at 156E, 19N on 12Z of the 12th. The initial disturbance associated with Amy is first detected by JTWC by 18Z of the 12th at a postion southwest of this upper-tropospheric cyclone, or TUTT cell. Over subsequent days the two features migrate westward maintaining approximately their original separation (figure 14), the upper-cyclone tending to weaken and deform. The period of maximum forcing centered around 12Z of the 15th is characterized by a strengthening northeast flow over the pre-cyclone disturbance down the axis of the upper-trough postioned to the northeast

30 (figure 18). By 12Z on the 16th, the upper-level forcing is bolstered by the arrival of another trough from the northwest. The flow at 850 mb is documented in figures 15 and 22, which show the development and genesis of Amy taking place within a poleward-displaced portion of the equatorial trough. Note the track of Amy along the monsoon trough, which is intensifying on the large scale by 12Z of the 16th. In summary, the generation of the pre-cyclone disturbance related to Amy is preceded by the formation and positioning of an intense upper-tropospheric cyclone to the northeast. The forcing at this stage is most impressive in the time series of the forcing parameter calculated using the 850 mb winds for the lower level (figure 16). Once formed, the tropical disturbance remains in a generally positive forcing regime throughout the genesis and early intensification stages of development.

Typhoon Brendan The intensity versus time graph for this storm (figure 17) suggests the division of the formation and intensification of this cyclone into the following 3 phases: Ave. Intensification Phase Time Inverval Description Rate (kts/day) 1 07/15/OOZ-07/18/06Z pre-cyclone 2 disturbance 2 07/18/06Z-07/20/18Z slow intensification, 6 genesis 3 07/20/18Z-07/22/OOZ rapidly intensifying 27 cyclone The time series of the forcing parameter shows positive and increasing values during phase 1, with a substantial peak at 12Z of the 18th. A lull in the forcing ensues, until the onset of a second episode by OOZ of the 21st

31 which is correlated with the beginning of the rapid intensification phase. The corresponding PV maps are shown in figures 18 through 21. The generation of the pre-cyclone disturbance occurs to the southeast of an upper-tropospheric trough similar to the case of Zeke, although the forcing is apparently negligible at this time. Figure 19 shows the first encounter of the pre-cyclone disturbance with an upper-tropospheric trough on 12Z of the 18th as a tongue of high-PV is drawn equatorward by northerly and northeasterly flow between OOZ of the 17th and 12Z of the 18th. At the same time, another PV anomaly is situated well to the north centered along 30N between 125 and 140E. Over the next 48 hours (see figure 20) this feature is observed to be advected southward and westward on a collision course with the lower-level disturbance. The upper-level feature is seen to encounter the low-level disturbance by OOZ of the 21st (fig. 21) thus beginning another extended period of positive forcing. The rapid intensification phase of the development begins at this time. Note that the abrupt halt to intensification at OOZ of the 22nd is likely due to the landfall made on OOZ of the 22nd (see figure 6) at the northern extreme of the Phillipine Islands. Figures 22, 26, and 27 show the large-scale flow at 850 mb for this period. The pre-cyclone disturbance appears to be associated with a migrating circulation identifyable in the NMC analyses as a vorticity maximum at least 48 hours prior to the JTWC's identification of the pre-cyclone disturbance (figure 22). In summary, the upper-tropospheric flow pattern around the pre-cyclone disturbance at its generation resembles that for the case of Zeke with a trough and region of negative PV positioned to the northeast and east respectively. A negligible forcing parameter at this time, however, makes a link between positive forcing and generation of the disturbance questionable. The pre-cyclone disturbance encounters upper-level troughs

32 on 12Z of the 18th, and beginning on OOZ of the 21st, which correspond to transitions in the intensification rates of the disturbance/incipient cyclone. The second episode, in particular, appears to be implicated in the genesis and early intensification of cyclone Brendan.

Typhoon Caitlin The intensity time series for typhoon Caitlin (figure 23) shows 2 distinct phases in the development: Ave. Intensification Phase Time Inverval Description Rate (kts/day)

1 07/18/12Z-07/23/18Z pre-cyclone 3 disturbance 2 07/23/18Z-07/27/12Z intensifying 19 tropical cyclone

The forcing appears positive but relatively weak for much of the pre-cyclone phase with peaks around 12Z of the 18th and OOZ of the 22nd. The early peak may suggest a role of positive forcing in the generation of the initial disturbance. The episode of positive forcing around 12Z of the 25th occurs well into phase 2 by which time the cyclone has attained an intensity of 65 knots, clearly after the genesis of the storm. Potential vorticity maps (figures 24 and 25) do show an upper-level trough positioned to the northwest of the low-level disturbance during much of phase 1, with the lower-level disturbance apparently overtaking the upper-level trough by OOZ 23rd, but this event is not reflected strongly in the forcing time series. For this case, on the basis of the quantitative measure, we must conclude that a link between upper-tropospheric potential vorticity advection and tropical cyclogenesis is not readily apparent. It is worthy of note that the transition from stage 1 to 2 is also accompanied by the marked change in the track of the storm which is

33 observed to occur around 6Z of the 24th (figure 5). It is not clear how or if these events are related. At 850 mb (figures 26 and 27) the pre-cyclone disturbance is seen to form in the vicinity of a vorticity maximum migrating along the equatorial trough, similar to the case of Brendan. Subsequent panels (figure 27) show increasing vorticity about the disturbance on the large scale, prior to the genesis of the cyclone arount July 24th. Two weeks of inactivity ended with the genesis of Doug around OOZ of August 9th. This event marked the start of another active period. The next 10 days saw the geneses of 5 tropical cyclones. Their tracks are shown in figures 28 and 35. While the track of Fred was similar to that of the first 4 typhoons, the other cyclones tended to form about 10 degrees latitude further north. The clustering of events in time and space is again noted. The cyclones within this second group will now be discussed individually in order of their appearance.

Tropical Storm Doug Doug achieved maximum sustained winds of only 35 knots, minimal intensity for a tropical storm. The storm tracked quickly northward (figure 28) toward higher latitude and colder ocean, failing to intensify further. The intensity and forcing time series are shown in figure 29. The transition from tropical disturbance to weak tropical cyclone (genesis) is seen to occur around OOZ of the 9th. Forcings at this time and before are primarily positive (especially for 7 degree averaging radius) but weak, with no clear relation between genesis and positive forcing for this event. Potenital vorticity maps are shown in figures 30 and 31, with relevant 850 mb maps displayed in figure 32. The PV maps for 12Z of the 8th, just prior to genesis, do indicate high PV to the north and east of the disturbance, but

34 the forcing is apparently weak. At 850 mb (figure 32), the pre-cyclone disturbances related to Doug and Ellie are observed to form within a pre-existing region of high relative vorticity apparently associated with a frontal zone intrusion from mid-latitiudes days before. In summary, there is no clear indication of upper-level forcing of either the genesis or the generation of the initial disturbance for this case.

Typhoon Ellie The time series pertaining to Typhoon Ellie are shown in figure 33. The intensity time series is unusual in that there is no clear pre-cyclone disturbance phase, with a fairly rapid elevation to tropical storm status within the first 30 hours of JTWC detection as an entity. The following phases are defined: Ave. Intensification Phase Time Inverval Description Rate (kts/day)

1 08/09/00Z-08/10/06Z genesis 16

2 08/10/06Z-08/11/18Z slow intensification 10

3 08/11/18Z-08/12/18Z steady 0

4 08/12/18Z-08/14/18Z moderate 20 intensification

The time series of forcing shows near zero PV advection by the shear during the early stages of development and genesis of this storm. Thus, there is no apparent relationship between upper-tropospheric forcing and the generation of the pre-cyclone disturbance, or genesis of the tropical cyclone for this case. The event around 12Z of the 13th appears to be related to the approach of the tropical cyclone to a trough in the westerlies. Note the enhanced outflow to the north, and the PV destruction aloft

35 exhibited by 00Z of the 13th (see figure 34).

Typhoon Fred The intensity time series for Fred (figure 36) suggests that the development of the cyclone be divided into 3 stages: Ave. Intensification Phase Time Inverval Description Rate (kts/day)

1 08/08/OOZ-08/12/18Z pre-cyclone 2 disturbance 2 08/12/18Z-0/15/12Z intensifying 18 cyclone 3 08/15/12Z-08/16/OOZ brief period of 40 rapid intensification The time series of the forcing parameter (figure 36) shows impulses of positive forcing at the onsets of both phases 1 and 2 (around OOZ of the 8th and 13th respectively), with another more extended period of forcing during the pre-cyclone disturbance phase, peaking at 12Z of the 11th. The first impulse is implicated in the generation of the initial disturbance, while the second and third episodes are implicated in the transition from phase 1 to phase 2, i.e. in the genesis of the tropical cyclone. After OOZ of the 13th, the forcing parameter is seen to hover around zero. It should be noted that the brief landfall of Fred at the Phillipines during the 12th of August may have also played a role in the timing of the genesis (figure 35). The 360K PV maps show the structures responsible for the episodes of positive forcing, and implied upper-tropospheric ascent. Figure 37 portrays the events surrounding the generation of the pre-cyclone disturbance which occurs by OOZ of the 8th. Note the upper-tropospheric positive PV anomaly centered around 147.5E, 7N as of 12Z on the 7th, moving westward with the strong easterlies of about 50 kts, which converts to about 10 degrees longitude per 12 hours, consistent with the

36 observed displacement between 12Z of the 7th and OOZ of the 8th. Generation of the pre-cyclone disturbance is seen to occur just downshear of this anomaly on OOZ of the 8th. Figures 38 and 39 show the events leading to the genesis of the cyclone. As of 12Z on the 9th, an upper-tropospheric anomaly is positioned near 23N, 147.5E, and is being advected west-southwestward. By 12Z on the 11th, the anomaly is positioned to the northeast of the pre-cyclone disturbance. This configuration remains fixed to some extent for the next few days as the two features drift toward the west, with the flow configuration corresponding to the peak in the forcing at OOZ on the 13th shown in figure 39. Also noteworthy is the approach of the intensifying cyclone toward an intense but decaying upper-level trough, whose axis is along 110E as of OOZ on the 14th. The lower-left panel of figure 39 shows the situation just prior to the onset of phase 3, a period of rapid intensification just prior to landfall. The configuration appears conducive to the establishment of an outflow channel to the north (Sadler, 1978). Also, the demise of the upper-trough is reminiscent of the collapsing-cold-dome scenarios described by Riehl (Riehl, 1979). At 850 mb (figures 40 and 41) an intensification of the equatorial westerlies and the monsoon trough is observed to precede and accompany the genesis of the storm. In summary, the pre-cyclone disturbance associated with Fred was generated downshear of a rapidly moving upper-tropospheric PV anomaly. The tropical disturbance then experienced positive forcing, from another anomaly situated to the northeast, with a peak in the forcing at the onset of steady intensification, or the genesis of the tropical cyclone. The brief landfall of the disturbance on 12th of August may have also influenced the timing of genesis.

37 Tropical Depression 13W This disturbance was first detected on 12Z of August 11th, reaching its maximum intensity of 25 kts by 6Z of the 12th. The forcing time series for this disturbance is shown in figure 42, with positive forcing apparent at the generation of the disturbance, and zero or negative forcing during its lifetime as a depression. The relevant PV maps (figure 43) show the generation of the pre-cyclone disturbance occurring downstream of an upper-level trough situated just to the east. Tropical cyclone Ellie can be seen situated to the northwest. The negative forcing appears to result from the relative movement of the depression toward the low-PV air in the upper-troposphere left in the wake of cyclone Ellie. At 850 mb, the disturbance is seen to develop on the cyclonic shear side of a belt of 25 kt southeast winds (figure 44). Note also the motion of TD 13W around cyclone Ellie. In summary, positive forcing is clearly indicated in the generation of the initial tropical disturbance for this case. Negative forcing prevailed during the depression stage, with the system failing to intensify beyond 25 knots.

Typhoon Gladys The early development of Gladys, a minimal strength typhoon at its most intense level, is divided into 4 stages (figure 45): Ave. Intensification Phase Time Inverval Description Rate (kts/day)

1 08/13/12Z-08/16/OOZ pre-cyclone 6 disturbance 2 08/16/OOZ-08/18/OOZ intensifying 15 cyclone 3 08/18/OOZ-08/20/OOZ steady at 55 knots 0

4 08/20/OOZ-08/21/06Z renewed intensification 8

38 The time series of the forcing parameter shows an episode of positive forcing during the first 24 hours of the JTWC best track, followed by lesser events (peaks in the time series) over the following few days. The relevant PV maps (figures 46 and 47) show that during its generation and early lifetime, the pre-cyclone disturbance resides in an environment of upper-tropospheric northerly flow, with a broad region of high-PV situated to the north and east, and with structures within this region occasionally rotating down to positions north of the disturbance. The corresponding 850 mb maps (figures 48 and 49) reveal a complicated flow pattern, including a large-scale cyclonic circulation on the order of 30 degrees latitude in diameter centered around 150E, 20N on OOZ of the 13th. Pre-cyclone Gladys forms on the southeast extreme of this circulation, where winds are strong and southerly, implying relatively large vertical shear between the lower and upper troposphere during the early stages. Note also the increase in the voriticity about the intensifying disturbance on the large scale. By 12Z of the 16th, a large-scale circulation surrounding now tropical depression Gladys dominates the domain. For this case, positive forcing is implicated in the generation of the pre-cyclone disturbance and, to a lesser degree, in the genesis of the tropical cyclone. An increase in the low-level vorticity on the large scale occurred during the tropical disturbance stage, preceding the genesis of the tropical cyclone.

Tropical Depression 15W The tracks of the next two cyclones of the season, TD 15W and TS Harry, are shown in figure 50. The intensity and forcing time series pertaining to TD 15W are shown in figure 51. The generation of the

39 disturbance is accompanied by an impulse of positive forcing, while its later intensification from 20 kts (pre-cyclone disturbance) to 30 kts (depression) around 12Z of the 25th is also accompanied by the onset of positive forcing. The question yet to be answered for this case is why did it not intensify further. The depression skirts the southern tip of Japan by 00Z of the 28th. The associated PV maps are shown in figures 52 and 53. The forcing agent at the generation of the disturbance appears to be an eastward extension of the TUTT (figure 52). At 850 mb (figure 54), the disturbance appears to form within a large-scale, zonally elongated trough in subtropical latitudes. Note the pervasiveness of lower-tropospheric westerlies in the tropics at this time.

Tropical Storm Harry The time series related to the genesis of this weak tropical storm are shown in figure 55. An increase in estimated intensity from 25 to 40 kts is seen to occur between 12Z of the 29th, and OOZ of the 30th, so genesis is defined as occurring around then. The forcing parameter time series show a peak at OOZ of the 27th for the 7 degrees and 5 degrees latitude averaging radii, and at 12Z of the 27th when 3 degrees is used. The corresponding potential vorticity maps (figures 56 and 57) show the approach of an upper-tropospheric cyclone toward the pre-cyclone disturbance. On OOZ of the 27th the upper cyclone is centered around 143E, 17N, with the pre-cyclone disturbance found near 133E, 20N. Over the next 24 hours the northern portion of the upper-level cyclone appears to overtake the low-level disturbance, while the bulk of the cyclone appears to be destroyed and replaced by low-PV air to the south. The collapse of an adjacent cold pool is consistent with Riehl's formula for genesis. In this

40 case, upper-tropospheric potential vorticity advection is implicated, with maximum forcing leading genesis by roughly 36 to 48 hours. At 850 mb, Harry is observed to form and develop along the cyclonic shear side of a southerly and westerly jet, which disappears by 12Z of the 30th as the cyclone moves away toward the north. In summary, the forcing at the generation of the pre-cyclone disturbance is positive for the 7-degree radius average, but near zero or slighltly negative for the 3-degree average, making the connection between positive forcing and generation suggestive but quesionable. The forcing is generally positive throughout the pre-cyclone disturbance stage for the 5 and 7 degree averages, with a peak near 12Z of the 27th for the 3-degree average. Thus, the connection between forcing and genesis is suggested, although perhaps not as clearly as in other cases.

Typhoon Ivy Figure 60 shows the tracks of the remaining 2 cases of this study, Ivy and Joel. The intensity time series (figure 61) for Typhoon Ivy is divided as follows:

Ave. Intensification Phase Time Inverval Description Rate (kts/day)

1 08/31/12Z-09/01/12Z pre-cyclone 0 disturbance 2 09/01 /OOZ-09/04/06Z slow intensification/ 11 genesis 3 09/04/06Z-09/07/06Z moderate intensification, 23 tropical cyclone The forcing parameter time series indeed shows several episodes of positive forcing during stages 1 and 2. The corresponding PV maps (figures 62 and 63) show the upper-level flow patterns over the days prior

41 to generation of the pre-cyclone disturbance (figure 62), and during the genesis and early intensification of the storm. Note the southwestward approach of the TUTT between 12Z of the 30th and 12Z of the 31st toward the site of the generation of the pre-cyclone disturbance, which first appears to the southwest of the upper-trough. By OOZ of the 3rd, the intensifying disturbance appears free from the influence of the first trough, but another anomaly is shown to advect down from the north to provide another period of forcing. By OOZ of the 5th, Ivy has reached maximum sustained winds of 65 knots. Potential vorticity advection by upper-tropospheric shear is indeed implicated in this low-latitude case of genesis, as well as in the generation of the precursor tropical disturbance. The corresponding 850 mb charts (figures 64 and 65) show some evidence of a migrating vorticity maximum within the equatorial trough prior to the generation of the initial disturbance. Note also the development of a remarkable circulation on the large scale as the disturbance/incipient cyclone intensifies.

Tropical Storm Joel The intensification of Joel (figure 66) is divided into the following 4 stages: Ave. Intensification Phase Time Inverval Description Rate (kts/day)

1 09/01/OOZ-09/02/06Z pre-cyclone 4 disturbance 2 09/02/06Z-09/03/18Z slow intensification/ 10 genesis 3 09/03/18Z-09/04/18Z steady at 30 knots 0

4 09/04/18Z-09/06/12Z renewed intensification, 14 tropical cyclone The forcing parameter time series shows an initial peak at the beginning

42 of the period, OOZ of the 1st, for the 3 degree averaging radius, and 12 hours earlier for the 7 degree average. The forcing is variable but generally positive throughout the period. The associated PV maps (figures 67 and 68) show first the westward movement of a broad region of high-PV air, with the generation of the pre-Joel disturbance occuring to its east by OOZ of the 1st (figure 67). Over the next several days, the incipient cyclone remains under the influence of northeasterly flow aloft, and generally positive potential vorticity advection near the tropopause. Forcing of ascent from upper-level positive PV anomalies is implicated in both the generation of the pre-cyclone disturbance, and in the genesis of the tropical cyclone for this case. The corresponding 850 mb maps (figures 69 and 70) show a fairly weak vorticity pattern 24 hours prior to the generation of the initial disturbance. Again, a large scale circulation begins to form during the pre-genesis stages of development of the disturbance and incipient cyclone.

43 7. Summary/ Suggestions for Future Work Observations appear to support the hypothesis that upper-level structures often play a role tropical cyclogenesis. Of the 8 typhoons studied, only Ellie showed no indication of an upper-level trigger, while for Caitlin the link was not obvious. The remaining 6 cases all had in common episodes of positive potential vorticity advection by the upper-tropospheric shear during the few days and hours prior to genesis. The upper-tropospheric agents could be traced back in the analyses and were clearly initially independent of the lower-level disturbances involved in the process. The upper-level agents often appeared as elongated upper-tropospheric troughs, i.e. equatorward and westward extensions of the TUTT, or else upper-level cut-off cyclones. Of the tropical storms, both Luke and Harry showed some indication of a correlation between positive forcing and genesis, while for Doug such a relationship was not apparent. For many cases, positive forcing was also indicated at the very beginning of the JTWC best tracks, i.e. in the generation of the initial disturbances themselves. This was certainly true for each of the nonintensifying depressions, TD13W, and TD15W. As for future direction in this work, it is suggested that the incorporation of satellite radiance data might be useful. In particular, it would be interesting to see if the episodes of positive forcing found in this study could be related to any changes in the organization or in the amount of deep convection. Also, the analyses performed in this study should also be tried on nondeveloping clusters. An attempt was made by the author to identify nondevelopers from the NMC 850 mb analyses, and a few persistent nondeveloping vorticity maxima were catalogued. Forcing time series were calculated for these as well. Figure 71 shows an example. Note that the nondeveloping vorticity maximum is not immune to

44 episodes of positive PV advection aloft. It would be more desirable to use satellite imagery to identify nondevelopers as other authors have done (e.g. McBride, 1981a). Also, it is suggested that other factors such as shear over the disturbance center and large-scale low-level vorticity be incorporated into the problem, which might help differentiate between developers and nondevelopers. It should be added that although the data scarcity over the tropical oceans is well-known, standard observations are often found to resolve the upper-level features described in this paper. It would be especially useful to have access to the database which goes into any given NMC analysis, and weigh data availability in the selection of cases from which to draw conclusions. Furthermore, analyses of PV fields surrounding genesis cases in the eastern North Pacific and western North Atlantic basins for 1991 found similar configurations for the many of those cases as well, and it is not believed that the processes documented in this study are unique to the western North Pacific.

45 References

Anthes, R. A. 1982: Tropical Cyclones-Their Evolution, Structure and Effects;. Meteor. Monographs, Vol. 19, Amer. Meteor. Soc., 208pp.

Avila, L. 1991: Atlantic tropical systems of 1990. Mon. Wea. Rev. 119, pp. 2027-2033.

Bosart, L. and J. Bartlo, 1991: Tropical storm formation in a baroclinic environment. Mon. Wea. Rev. 8, pp. 1979-2013.

Challa, M. and R. Pfeffer, 1980: Effects of eddy fluxes of angular momentum on model hurricane development. J. Atmos. Sci, 37, pp. 1603-1618.

Challa, M. and R. L. Pfeffer, 1990: Formation of Atlantic hurricanes from cloud cluster and depressions. J. Atmos. Sci., 47, pp. 909-927.

Charney, J. G., and A. Eliassen, 1964: On the growth of the hurricane depression. J. Atmos. Sci., 36, pp. 2617-2630.

Chen, S., 1990: A numerical study of the genesis of extratropical convective mesovortices. PhD thesis, Pennsylvania State University, University Park, PA 16803, 178 pp.

Dey, C. H. 1989: The evolution of objective analysis methodology at the National Meteorological Center. Wea. Forecasting, 4, pp. 297-312.

Dvorak, V. F., 1984: Tropical cyclone intensity analysis using satellite data. NOAA Tech. Rep. NESDIS 11, NOAA, 47pp.

Eliassen, A., 1951: Slow thermally or frictionally controlled meridional circulation in a circular vortex. Astrophys. Norv., 5, pp. 19-60.

Elsberry, R. L. 1987: Observation and analysis of tropical cyclones. A Global View of Tropical Cyclones, R. Elsberry (ed.), pp. 1-12.

Emanuel, K. A. 1989: The finite-amplitude nature of tropical cyclogenesis. J. Atmos. Sci, 46, pp. 3431-3456.

46 Frank, W. M. 1987: Tropical cyclone formation. A Global View of Tropical Cyclones, R. Elsberry (ed.), pp. 53-90.

Frank, W. M., and S. Chen 1991: Simulations of vortex formation in convective weather systems. 19th Conference on Hurricanes and Tropical Meteorology, Amer. Met. Soc., pp. 241-244.

Gray, W. M., 1975: Tropical cyclone genesis. Dept. of Atmos. Sci. paper no. 323, Colorado State University, Ft. Collins, CO, 121 pp.

Gray, W. M., 1979: Hurricanes: their formation, structure and likely role in the tropical circulation. Meteorology Over the Tropical Oceans. Royal Meteorology Society, pp. 155-218.

Handel, M., 1990: Tropical cyclone intensification from finite amplitude disturbances. PhD thesis, Massachusetts Institute of Technology, Cambridge, MA 02139, 261pp.

Holland, G. 1987: Mature structure and structure change. A Global View of Tropical Cyclones, R. Elsberry (ed.), pp.13-52.

Hoskins, B. J., M.E. McIntyre, and A.W. Robertson, 1985: On the use and significance of isentropic potential vorticity maps. Quart. J. R. Met. Soc., 111, pp. 877-946.

Joint Typhoon Warning Center, Annual Tropical Cyclone Report 1990, 278pp.

Kanamatsu, M., 1989: Description of the NMC global data assimilation and forecast system. Wea. Forecasting, 4, pp. 335-342.

Kanamatsu, M. et al., 1991: Recent changes implemented into the global forecast system at NMC. Wea. Forecasting, 6, pp. 425-435.

Lee, C. S., 1986: An observational study of tropical cloud cluster evolution and cyclogenesis in the western North Pacific, Colorado State University report no. 403, 250pp.

Lee, C. S., 1989: Observational analysis of tropical cyclogenesis in the western north Pacific. Part I: Structural evolution of cloud clusters. Journal of Atmos. Sci., 46, pp. 2580-2598.

Love, G. 1985: Cross-equatorial interactions during tropical cyclogenesis. Mon. Wea. Rev. 113, pp. 1499-1509.

47 Madala, R., S. Chang, et. al., 1987: Description of the Naval Research Laboratory Limited Area Weather Prediction Model. NRL Memorandum Report 5992.

McBride, J.L., 1981a: Observational analysis of tropical cyclone formation, Part I: Basic description of data sets. J. Atmos. Sci., 38, pp. 1117-1131.

McBride, J.L., 1981b: Observational analysis of tropical cyclone formation, Part III: Budget analysis. J. Atmos. Sci., 38, pp. 1152-1166.

McBride, J.L., and R. Zehr, 1981: Observational analysis of tropical cyclone formation. Part II: Comparison of non-developing versus developing systems. J. Atmos. Sci., 38, pp. 1132-1151.

Menard, R. and J. Fritsch, 1989: A mesoscale convective complex generated inertially stable warm core vortex. Mon. Wea. Rev. 117, pp. 1239-1261.

Montgomery, M., and B. Farrell, 1991: Tropical cyclone formation, submitted to Journal of Atmos. Sci.

Nitta, T. and Y. Takayabu 1985: Global analysis of the lower tropospheric disturbances in the tropics during northern summer of the FGGE year part II: regional characteristics of the disturbances. Pageoph 123, pp. 272-292.

Ooyama, K.V., 1982: Conceptual evolution of the theory and modelling of the tropical cyclone. J. Meteor. Soc. Japan, 60, pp. 369-380.

Palmen, E. 1948: On the formation and structure of tropical hurricanes. Geophysica 3, pp. 26-38.

Palmen, E. and C. Newton (1969): Atmospheric Circulation Systems: Their Structure and Physical Interpretation. Academic Press, 603 pp.

Pfeffer, R. and M. Challa, 1981: A numerical study of the role of eddy fluxes of momentum in the development of Atlantic hurricanes. J. Atmos. Sci. 35, pp. 2392-2398.

48 Pfeffer, R. and M. Challa, 1991: Formation of Atlantic Hurricanes from Cloud Clusters and Deptessions, 19th Conference on Hurricanes and Tropical Meteorology, Amer. Met. Soc., pp. 214-216.

Pedlosky, J., 1987: Geophysical Fluid Dynamics, 2nd ed., Springer-Verlag, New York, NY, 710 pp. Riehl, H. 1948a: On the formation of typhoons. J. Meteor. 5, pp. 247-264.

Riehl, H. 1948b: On the formation of west Atlantic hurricanes, Misc. Reports No. 24 of the Dept. of Meteorology of the University of Chicago, University of Chicago Press, pp. 1-67.

Riehl, H. 1950: A model of hurricane formation. J. Appl. Phys. 21, pp. 917-925.

Riehl, H., 1954: Tropical Meteorology, McGraw-Hill Book Co., Inc., New York, NY., 392 pp. Riehl, H., 1979: Climate and Weather in the Tropics, Academic Press, 611 pp.

Rotunno, R. and K. Emanuel, 1987: An air-sea interaction theory for tropical cyclones, Part II. J. Atmos. Sci., 44, pp. 542-561.

Sadler, J. C., 1975: Tropical cyclone initiation by the Tropical Upper Tropospheric Trough, NAVENVPREDRSCHFAC Tech. Paper No. 276., 103 pp.

Sadler, J. C., 1976: A role of the Tropical Upper Tropospheric Trough in early season typhoon development. Mon. Wea. Rev., 104, pp. 1266-1278.

Sadler, J.C., 1978: Mid-season typhoon developments and intensity changes and the tropical upper tropospheric trough

Sawyer, J. S. 1947: Notes on the theory of tropical cyclones. Quart. J. Roy. Met. Soc., 73, pp. 101-126.

Shimamura, M. 1981: The upper-tropospheric cold lows in the northwestern Pacific as revealed in the GMS satellite data. Geophys. Mag., 39, pp. 119-156.

49 Shimamura, M. 1982: An application of GMS satellite data in the analysis of upper cold lows in the western North Pacific, Geophys. Mag., 40, pp. 113-152.

Simpson, R. H. and H. Riehl 1981: The Hurricaneand Its Impact; Louisiana State University Press, Banon Rouge, Louisiane, 398pp.

Smith, W. L., 1991: Atmospheric soundings from satellites-false expectation or the key to improved numerical weather prediction? Quart. J. R. Met. Soc., 117, pp. 267-297.

Sundquist, H., 1970: Numerical simulation of the development of tropical cyclones with a ten-level model, part I. Tellus 22, pp. 359-390.

Yanai, M. 1964: Formation of tropical cyclones; Rev. Geophys. 2, 367-414.

Yanai, M. 1968: Evolution of a tropical disturbance in the Carribean Sea region. J. Met. Soc. Japan (Ser. I) 46, 86-109. Zehr, R. M., 1991: Observational investigation of tropical cyclogenesis in the western north Pacific. 19th Conference on Hurricanes and Tropical Meteorology, Amer. Met. Soc., pp. 235-240.

50 200 mb COHPOSITED VT

case .

Figure 1. Horizontal distributions of 200 mb relative vorticity in units of 10-6 s-1 referred to the composite vortex centers for (top) pre-typhoon vortices and (bottom) nondeveloping vortices (Nitta et. al., 1985). 51 DATB/TlI6 DATB/T1

RAOIUS.*-4 39CON*A %bo . Ow. C

go WM

M-uumum Sea -Level Pressure --1010

*o*

r - I axmmOeep ConknectiveSufcwtd(m) clouds -

STGE STG

Doy eoreeTropico Stormu

Figure 2. Sample of time series from two case studies exhibiting early convective maxima. Also, schematic illustrating Zehr's two-stage model for tropical cyclogenesis (Zehr, 1991). 52 0 0 &COE

dc 00 00 0 00

CI0 0

o 0 Ok I 0o 0

01 0 0

I V 0

0I 0 0 O

0 .

- -~-- 0 Iv

01

* NJ

C U

;0 0

- IN

IfI

0

:1

Figure 3. Components of database for NMC analyses on a "typical day" (Dey, 1991): (a) upper air soundings, (b) commercial aircraft reports, (c) cloud-tracked wind observations, (d) satellite temperature profile observations. Latitude and longitude lines are shown for every 30 degrees. Figure 3a. Radiosondes (circles), and Pibals (asterisks). 53 0

U

4 I I I I ___ I I P1

0

*1

0) CYD DEVELOPMENTAL PRE TROPICAL STORM HURRICANE PATTERN TYPES PATTERN TYPES STORM (Minimal) (Sirong) (Minimal) (Strong) (Super)

T1.5 :.5 T2.5 T3.5 TA.5 TS.S T6.5 . Ta CURVED BAND F'TF ) PRIMARY PATTERN TYPE

CURVED BAND EIR ONLY C______

CDO PATTERN TYPPE VIS ONLY

SHEAR PATTERN TYPE

CI MWS MSLP MSLP Number (KYnots) (Atlantic) (NW Pacific) 25 K 1.5 25 K I 2 30 K 1 1009 mb 1000 mb 2.5 35 K 1 1005 mb 997 mb 3 45 K 1000 mb 991 mb 3.5 55 K i 994 mb 984 Mb 4 65 K -987 mb 976 mb 4.5 | 77 K 1 979 mb 966 mb 5 90 K 1 970 mb 954 rb 5.5 102 K 960 mb 941 -b 6 115 K 948 mb I 927 Fb 6.5 127 K 935 mb 914 mb 7 140K 1 921 mb I 898 mb 7.5-- 155 K I 906 mb 879 mb 8 170 K I 890 mb 858 mb

Figure 4. Dvorak technique: Primary cloud patterns and associated "T-numbers" (top). Table relating Cl-numbers to intensity (bottom). For the developing stage of tropical cyclones, Cl-numbers and T-numbers are equivalent (Dvorak, 1984). 57 ------L------a- ...... 0

a a a O

a

U

------T ------a ------

a a L

------a------a aI a a a a *N a a a a a a a a aCa X X a a a a aO z ------fa - a L - 7 k ,. a- a ~1 I d a a-'0* C

0

z

C

a ~..I %J a a ~.'-' a a If a 04 a a a a a a 01, a a a a

a a a a a a a a a a ~~~~0 - a a a a a a a a a a a a a a a a

Figure 5. Tracks of typhoons Zeke (Z), Amy (x), Brendan (+), and Caitlin (*). Plotted also is the day the observation was made (in July 1991), with OOZ, 06Z, 12Z, and 18Z fixes shown sequentially for any given full day of observation. 58 INTENSITY OF ZEKE 80 75

70

5

60

55

-50

40

35

30

25

20 15 70700 70800 70900 71000 71100 71200 71300 71400 71! ORTE MOOHH FORCING. RADII 3(R)5(B),7(C) ZEKE 4.0

3.5

3.0

2.5 2.0 JV\V 1.5

1.0

.5

0

-. 5

-1.0

-1.5

-2.0 70600 70700 70800 70900 71000 71100 71200 71300 71400 71500 DATE MMOOH Figure 6. Intensity (top) and forcing parameter (bottom) versus time for typhoon Zeke. Times are of the format MDDHH where M is the month number, DD is the day, and HH is the hour. Forcing parameter series is shown for averaging radii of 3, 5, and 7 degrees latitude.

59 to to0

ON~ to 1.- to - 0 fl- 0 40

V- 0 4.

0(i

in

L0 - .,V >in) 0

71, -~OF A r ~00.FI' CD, in) -, Li

I " I f3 F C',-i *94 r r.P a 41 9P in

1-N

ON to

C,,

C)

Le-a in M, Q

Tf §v;. 9? r0 a2 itP Figure 7. Ertel's potential vorticity (Q) fields plotted on the 360K isentropic surface for the times shown at the bottom of each panel. Contour intervals are 0.5 PVU for Q less than 1.5 PVU, and 1.5 PVU for Q greater than or equal to 1.5 PVU. Negative values are indicated by thin solid lines, 0.5 and 1.0 PVU contours by dashed lines, and contours of 1.5 PVU and greater contours, by heavy solid lines. Winds are plotted in knots, with half-barbs, full-barbs and flags each representing 5, 10, and 50 knots respectively. The track of the disturbance/cyclone is indicated by the squares, with positions every 6 hours shown, beginning at OOZ of the 6th. 60 ID I-

a ION 1~1~ a aN Sn.. q.~j '-C.)

4t0U* 0 %

Sn - CSa W '-Q.

LOa LU

kno

IIt

CLF

'O

Figure 8. Ertel's potential vorticity (Q) fields plotted on the 360K isentropic surface. Fields are shown at 12 hour intervals during the pre-cyclone disturbance phase of Zeke. Large triangles denote current positions of the pre-cyclone disturbance or cyclone, and squares represent its track. Contouring conventions are as in figure 7. 61 * U

U,

0N CD

U,

0 0..

I-

wn 0O

\I 1 .i

[I gi V-J Ln K~7 I-

Nh.N

1CO 0n

U, r0 0q r'.: 0j _ 'I

113 rr~

Figure 9. Ertel's potential vorticity (Q) fields plotted on the 360K isentropic surface. Fields are shown at 12 hour intervals during the pre-cyclone disturbance phase of Zeke. Contouring conventions and meaning of symbols are same as in figures 7 and 8. 62 0

0

U3 CV)0 V- u,

mr r 0

N4 ON N N Ln1 N 1 U 1

U,

In -M - an .I.-

-*M C

Figure 10. 850 mb winds and relative vorticity fields in units of 10-5 s-1. Contour interval is 2 with positive values indicated by heavy solid lines, and negative values dashed. The 1 and 3 contours are also plotted, indicated by thin solid lines. Fields shown for times shown beneath each panel. Winds are plotted in knots, with half-barbs, full-barbs and flags each representing 5, 10, and 50 knots respectively. The track of the precyclone disturbance and then cyclone Zeke is indicated by squares as in figure 7. 63 % I I I I \1 \ \k \A I -I/ In

r I,

In In

VU,

in 40 C)I

0 'V. F CI CD

R,

CD

rI Om 0 r0- '~) :3~'~ N1

~ :''Z". C

0 N

Figure 11. 850 mb winds and relative vorticity fields in units of 10-5 s-1. Contouring convention is same as in figure 10. Triangles denote position of pre-cyclone disturbance related to Zeke. Track shown as in previous figure. 64 INTENSITY OF RMY

SI i I I I i I 71300 71400 71500 71600 71700 71800 71900 72000 72100 ORTE MO0HH FORCING. RADII 3(A).5(B).7(C) AMY 2.0

1.8

1.6

1.4

1.2

S1.0

-. 8 R 0 .6

CL .4

GS - .6

-2

-. 4

71300 71400 71500 71600 71700 71800 71900 72000 DATE MMOODH

Figure 12. Intensity (top) and forcing parameter (bottom) versus time for typhoon Amy. Times are given in the format MDDHH where M is the month number, DD is the day, and HH is the hour. 65 -,.- - 1,0 %_h

.' % ~ 04.~ W tc+- Y~~..o

.00 _O k, 4.4

If VII >

0 I In

Io

In o11

In L

- It

If

'.-- gal

-rL 0 IL

71,

FigureErtel' 13. PV on 360K surfce.FedDr hw a 4hu nevl leadingu to the gnerationof h r-yloecutrreaeoAy

poition at. 1re' 8Z of theK 12h sotura conentios aeshoin fiur 7.huritevl

66 ('M

0-

AA to rC.,,

U, wU

E3C314" Cn t C

LO

0 * r

Figure 14. Ertel's PV, 360K surface. Fields shown every 12 hours during the pre-cyclone disturbance phase for Amy, whose location is indicated by the large triangle. The track of Amy is given by the squares. Track of Brendan also indicated in panel for 12Z of the 14th. Contouring conventions as in figure 7. 67 /L I V) o~)1

U1 U)

r Prr 0 r

In

in0nL

r or r /0

on18 n h 1t. otorngcnvnis as in fiueD0 M68 )V FORCING2, RROII 3(R)1 5(B),7(C) RMY 2.0

1.8

1.6

1.4

., 1.2 1.0 .8 z .6

I- .4

L. .2 CL 0 0 -. 2

-. 4

-1.0 --

-1.2' 71300 71400 71500 71600 71700 71800 71900 72000 DRTE MMOOHH

Figure 16. Forcing parameter for Amy computed with 850 mb wind velocity used to represent the lower-level in the calculation.

69 INTENSITY OF BRENDAN

5 s- 60 --

55

50-

30-

25S

25

-10 71600 71700 71800 71900 72000 72100 72072300 72400 72500 OATE MO0M FORCING, RADII 3[R),5[B).7[C) BRENDAN 3.5

3.0

2.5

15 2.0 10

1.0

-20 - --

-2.s

71500 7160 71700 71800 71900 72000 72100 72200 72300 72400 72500 ORTE MMOOH

Figure 17. Intensity (top) and forcing parameter (bottom) versus time for typhoon Brendan. Times are given in the format MDWDHH where M is the month number, DD is the day, and HH is the hour.

70 00 OQJ

~

C)

1.5

oco WD 0.5

hiL 0.5 co 135 140 145 150 155 160 165 170 135 140 145 150 155 160 ERTEL'SPV, 360K SURFACE:071500 ERTEL'SPV, 360K SURFACE:071512 6 1;? 10,

1.5

0 10-0 41S ~CD r 06-

tj 0

Awl

0

135 140 145 150 155 160 165 170 135 140 145 150 155 160 165 170 ERTEL'SPV, 360K SURFACE:071600 ERTEL'SPV, 360K SURFACE:071612 (Q S - Lo

a k- ,, o DC7 o

InI Fiur 19-re)kVo 6K ufc. Cnorcnenin si iue7 Trck0f m and Brna arMhwsi peiu iue 72O ry N LULs iLA 41 L 3 o C4 r; ,' *'u.'- a,~ I- n , 0 :; TC '10 4 % 0

).-OOUL- 0 0 (0 % InCI, N % % n 0.. C,,

% %

z; r in , %.6

-. ,- I--r. E %0 U, 0

Ui It..t- i 1W7 r I * . r VI 'i %~ It

\ LnL

~00 0 '-0 I., a - N > A 0

0 10 C., U, N> NA9 .o- '-a- SI,

'I I

LU

U,

. ,,V, 'k # -k '4 ~, :~

Figure 20. Ertel's PV, 360K surface. Fields shown every 12 hours. Asterisks denote current positions of typhoon Amy. The track (squares) and position of Brendan are indicated. Contouring conventions as in figure 7. 73 -C--A

t-* -14

0 0-k *.

lku -

% V, ,~ 0

U1%

Lj In

In'n

04 04 % Y, rr AK~~0C ,)

dlA I . Fiue2. EtlsP,30 ufc. seik eoepstoso h prcuso dstrbnct Citin rak ndpoiton o Benanasi prvosfgr. Cnornovnin si iue7

74 %o o to W r r r r ro7 r r

o a 0 (r0

covIon asi fgr 10. 75n INTENSITY OF CRITLIN 100

90

70

40.

30150 ---

20 .

OATE MOOit

FORCING, RADII 3[R),5[B),7[C) CAITLIN

2.4 tI- I.. 2.2 -- 2.0

1.8 --

1.4

z 1.2 1.0

.8-

1.0'0 - - -. 2 -l -. 4

71900 72000 72100 72200-9230 72400 725W 28 72700 728W 72900 7 ORTE 1900m

Figure 23. Intensity and forcing parameter time series for typhoon Caitlin.

76 - - - , I . , I I I Io

040

I,- 4/% I*

M %a2 L I.. %--

< IF 7t-3

4C% C Fr 24 re'V 6Ksufc.Atrssdnoepsin fcdn Breda.(qures racad osiios tranges o Catln re ndcaed CotuigcnetonVsi iue7 77 0~w %1!J I0. % '. SL 1

4I j

---- a.

Figure 25. Ertel's PV, 360K surface. Asterisks denote positions of cyclone Brendan. Track (squares) and current positions (triangles) of Caitlin are featured. Contouring conventions as in figure 7. 78 3 -2

0 o 130 135 140 145 150 155 160 165 0 910717/1200 850 MB VOROBS(*10*5) 910718/1200 850 P4flVOROBS (*10*5) -2

0

140 145 150 155 160 165 - ~ 130 135 130 135 140 145 150 155 160 165 910719/1200 850 MB VOROBS('10"5) 910720/1200 850 MB VOROBS(*10**5) N N

Figure 27. 850 mb winds and relative vorticity. Tracks (squares) and current positions (triangles) of Brendan and Caitlin are shown. Contouring conventions are as in figure 10. 80 a 7- xx xX r" X aO x

x -

x x -

Cc X

+ M -X C4

...x x -- CD 4. +

n x

+ +0

+0 --

M

* + x

O2

Figure 28. Tracks of Doug (x), Ellie (a), TD13W (*), Gladys (+). Plotted also is the day the observation was made (in August 1991), with 00Z, 06Z, 12Z, and 18Z fixes shown sequentially for any given full day of observation. 81 INTENSITY OF DUG

34

32

0 e s

ze -28

28

807M0 800 0850 80900 80950 81000 81050 81100 81150 ORTE MooHH FORCING. RRDII 3(R.5(B),7(C) 0UG

.2

0

I -. 2 C-

.4

-. 8

-. 8 80750 80800 80850 80900 80950 81000 81050 ORTE MM00HH

Figure 29. Intensity (top) and forcing parameter (bottom) time series for the case of Tropical Storm Doug. Note changes in the scale of the axes. 82 O i V

1.

21

r r

7- -- 3 0.' 1.5: o . 140 145 150 155 160 165 170 1 ERTELSPV, 360K SURFACE:080612 5 m00

3 or

ERE' 36KSUFCE 801

140 145 150 155 160 16.5 170 17 140 4§5 0 10 155 180 165 170 175 ERT L'S PW 360K SURFACE:080800 0

OQ

Oi

140 145 150 155 160 165 170 175 0 ERTEL'SPV, 360K o SURFACE:0809% 1

'1

0n

cxn

o .

I3b'%" I -'-:' i~. -I 13t! 1.5 150 155 160 165 170 13 0 140 145 150 155 1§0 165 170 175 PV, 360K SURFACE:080912 ERTEL'SPV, 360K SURFAGE:081000 oil

(n(D

n> 5. 00 - 0

- 00

0

~ 140 1A 150 155 160 1 5 170 175 0r910806/ 200 850 MB VOROBS(*10**5)

-7r

2

-14 '1 ' 150~ 5 '16'0 '163 17 175 910808/1200 850 MB VOROBS(*10**5) n O M INTENSITY OF ELLIE so

80

70

Sso

-40

30

20

10 81000 81100 81200 81300 81400 81500 81600 81700 81800 81900 82000 OATE MOOHH FORCING. RAOII 3(A).5(B).7(C) ELLIE 3.0

2.5

2.0

-1.0

-'.5

-2.0 80900 81000 81100 81200 81300 81400 81500 81800 81700 81800 81900 ORTE MM00MH

Figure 33. Intensity (top) and forcing parameter (bottom) time series for the case of Typhoon Ellie. 86 C 3

5 -

(3<13

0~0. \

o i 140 145 150 155 160 166 170 175 140 0 145 150 155 160 165 1-0 175 ERTEL'SPV, 360K WACE: 08110 ERTEL'SPV, 360K SURFACE9-%812001-5

3

N z-

11 D

't 4.5

10 0.5

o ERESP,36kSRAE:010ERTEL'SPV, 360k SURFACE:081300 0.5. 0 X

a x a u x X --0 a X

0 X

0 x

x xX

a.I

ax ;r x

-. X

In I- X I-X

... I X

X

X

-. - -

N

Figure 35. Track of typhoon Fred. 88 INTENSITY OF FRED

80900 81000 81100 81200 81300 81400 81500 81600 81700 81800 ORTE MOOHH

FORCING. RROII 3(R),5(B),7(C) FRED

-1.0

-1.5

-2.0 80800 80900 81000 81100 81200 81300 81400 81500 81800 81700 81800 DATE MMODHH

Figure 36. Intensity (top) and forcing parameter (bottom) time series for the case of Typhoon Fred. 89 0D

to

LM U,

Uf)04 oo

0 M, to

U, 0:j

Vw

U 04 Ln V_

LO LM U, LO

0 00 An ' ' 0 Go 0

.00 to 'T-0

*A-F CI,

76 i I~

Figure 37. Ertel's PV on 360 K surface. Triangles denote positions of pre-cyclone Fred, with its track indicated by squares. Contouring conventions as in figure 7. 90 .1~ 10 4 IVI x0

F cm -WC

r.. N~ K. ~ 4I Ln

cvit

Figure 3 8. Ertel's PV on 360 K surface. Positions and track of Fred are shown as in previous figure. Note gap between panels 1 and 2. Contouring conventions as in figure 7.

91 f30

M1 0.5> --

15 ~1 A 00 ofr o.

0 105 110 115 120 125 130 1 5 100 105 110 15n 120 125 1 E L'S PV, 360K SURFACI:081300 EdL'S PV, 360 RFACE:081400 CO 1 5 4.51 0 3 4-.- 1 1 110 3 1.5 30 E 1 31 S 0

p 25

0 0.5 -~0.

125' 1 10 5 0 y '10 15 120 '125- 30 31''100 1 -i 2 EITTEL'SPV, 360K SURFACE:081500 ERTEL'S PV, 36V SURFACE:0811600 t '0001))Lnr r

Ak E r- In

Po' -7 C41 CV)

IPA 04

LI ,-0 0-

/ '0-

CU41 0

931 I 1/1'Q

0

\ 1 S00 13 k

0 CA

110 115 120 125 130 135) 140 145 110 115 120 125 130 135 140 145 910809/1200 850 MB VOROBS *10**5) 910810/1200 850 MB VOROBS(*10*65)

0 0

JQQ

0

0

0

1.0

C 0

110 115 120 125 130 135 140 120 125 130 135 140 145 910811/1200 850 MB VOROBS(*10*5) 200 850 MB VOROBS(*10**5) FORCING, RROII 3(R),5(B) ,7(C) T013W 3 .0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 a 1 I 4p I 1a

2.5

2.0

1.5

1.0

.5 z .49 0 -D - -0 --

N-I

-. 5- -. 0

R- -1.5

-2.5V

-3.0

-3.5

-4.0 ' ' ' 81150 81200 81250 81300 81350 81400 81450 81500 DRTE MMDOHH

Figure 42. Forcing time series of Tropical Depression 13W.

95 F CY N AV j k I I ORO, 4 , I L -A CY cz. CO

co Le) LO

In r V, 90

cc LnW)> tr) C', % Cn f7 Cn:j %r LOlu Ln r r Ln

Ij DCO k , y- Nlooo Op F Owl%Ilu CD rd-, % CO

Ln %. % 13 Ir4 %

T-C3 to cn 0 LO :j % $3 elolf7 Uj inLn % LU r , r % % r r 03 f LO Ln Q C O"S OF M M Ln

Figure 43. Ertel's PV on 360 K surface. Asterisks denote positions of tropical cyclone Ellie. Positions and track of Tropical Depression 13W are indicated by triangles and squares respectively. Contouring conventions as in figure 7. 96 (Dr (D~

ot~

r)r

140 INTENSITY OF GLADYS 85

80 ss 5

50

45

-40

3s

30

25

20

15

10 81400 81500 81500 81700 81800 81900 82000 82100 82200 8230 82400 82500 DATEMM0tH FORCING. RADII 3(R).5(B).7(C) GLADYS 2.0

1.8

1.6

1.4

1.2

1.0 .8

.5

.2

0

-. 2

-. 8

-. 8

-1.0 71400 71500 71600 71700 71800 71900 72000 72100 72200 72300 DATE MMOt

Figure 45. Intensity and forcing parameter time series for Typhoon Gladys.

98 ca 0

3I , . 'I -J -+po Z5r -

w 0o -X uC ' D 4

.5 140 1450. 150 155 (160 165 170 14Q).5 150 155 160 165 1tq 175 ERTEL'S PV,360K SURFACE:081300 1P5 L'S PV, 360K SURFACE:081312 . (IQ En 0 o -01 3 1W to 0

CD0 (0

0 o 0 0CD

-145-'' 150 135' '160 -1 ERTEL'SPV, 360K SUFACE: r~t 3Q4 i --%-I & m us - .I 1. 3',

-- p .5 cu1 5

E- --

5 0 % 15 -m-

130 1 140 445 150 55 160 165 130 1350.5 140 145 150 155 160 1 6T5 L'S PV, 360K SURFACE:081512 ERTEL'SPV, 360K SURFACE081612

r), 0

- I -- z I

0 15 0 0 -O-N-.aSOr-

00Q

0

~0 4 0 10 130 15135 14CI0 145- 150 155 160 165 t30 5 -0. 40 '145 150 f5 1 160 - '166 ERTEL'SPV, 360K SURFACE06B1712 1I 0 ERTEL'SPV, 360K SURFACE:081812 0.5 1.5 o -0SOO

o'

140 145 150 - 155 160 16 170 175 910811/0000 850 MB VOROBSR&0*5)

04 0'*

.r ts 0. 0 oLA~ CDi ~CD

910813/0000 850 MB VOROBS(*10**5) 910814/0000 850 MB VOROBS(*10**5) o0 / Y-(|/ | . . 0 . . . ///\ AW

0 231

ra r - 2 0 N I r t 0

0 "-0 130 105 140 145 150 155 160 1 5 910814/1200 850 MB VOROBS(*10**5)

c

0 co

0 -2

5

/ 3 -2 82 -2 910816/140 850 MB VOROBS(*10**5) x

300

29+

21 a 2aaa I a 29 a 30 30 a a a a7 202 30------u 2 ------a ------+ 27 2 X a a + h2 a6 x a: 4 a+ 25 aa a a+ ao + 2+ 2a + + a222 t~ ++ +++ 27X xa a a X289 ax28 a

27aa

x a a 20------

13 1Aa a5

TOL514 (+19 HAIRRY (XI

Figure 50. Tracks of Tropical Depression 15W ()and Tropical Storm Harry (x).

103 FORCING AND INTENSITY VS TIME TD15W A -7 -C

t I I I I - # I I I I II I I I 1 I 82200 82300 82400 82500 82500 82700 82800 82900 83000 ORTE MMOOHH FORCING. RADII 3(A).5(B).7(C) TD15W 2.0 1.8 1.6 1.4 1.2 1.0 .8 .8 .4 .2 0 -. 2 -. 4

-. 8 -1.0

-1.2 82200 82300 82400 82500 82500 82700 82800 82900 83000 om MM00m Figure 51. Top panel shows intensity (curve B) versus time, as well as the forcing (multiplied by 40) as computed with a 5 degree latitude averaging radius (curve A). Bottom panel contains usual forcing time series for Tropical Depression 15W. 1 04 (- r. .5

* 4. 0 M 3 '

o *' 65.5 0o M 0 2- --o

Z~iWA1TI# . I .f Lt 1.5 A; r41 . .1:6- - ^*^ f- 130 135 140 145 150 155 0 130 135 140 145 150 155 160 165 ERTEL'SPV, 360K SURFACE:082012 EiTEL S PV, 360K SURFACE:082100 0 -F -F .5 1.5 .

30 3

p~*0

1 --

C) 1- .. -l 259-

20

13(0 135 140 145 150 155 160 165 ERTEL'SPV, 360K SURFACE:082200 ---- 4U o M - * I .7 1. 35 (-) 01 00 -0 35' on /~ 'r .1 0- 30 30

2K, 25

4P

12 1_1I L-- - 1AI- 16 .. ,o A , a- .53 05~--2A&S,P~~:. i e 125 0 130 135 140 145 0 150 155 160 1P5 130 1 140 145 150 155 160 ERTEL'SPV, 360K SURFACE:082300 ERTEL'SPV, 360K SURFACE:082400

<0

SE. -

S--

1. 125 130 135 140 145 150 ERTEL'SPV, 360K SURFACE:082600 0 t w-- r I I -2

) 20 --

aAll - O~

1.- Omla. -a" a; )..**

lila S.> \ IN -,I i "It NI N 125 130 135 140 145 1g0 155 160 0D'o O 910820/1200 850 MB VOROBS(*10**5)

0U o Ni

125 130 135 140 145 150 155 160 125 130 135 140 145 150 155 160 910822/1200 850 MB VOROBS(*10**5) 910823/1200 850 MB VOROBS(*10**5) INTENSITY OF HARRY

* p. . . * I..

127s50 62900 1250 32900 2950 8300 0ss 63003100 s3150 ORTE MOOM FORCING. RRDII 3(A).5(B).7(C) HARRY

DATE HWIOHH

Figure 55. Intensity and forcing parameter time series for Tropical Storm Harry. In time labels, HH=50 corresponds to 12Z, i.e. 83150 is August 31st, 12Z. 108 Q

IS

V>

40. (n3

i IP ~C I r t T' x Ix I 11 T

Cs ~1 I4 T_ In

A I r\ 'f 4 / L' MIS V'

I a -l U) r--n IV

1-I- iT 11 I'- Nf N0 cp. O 3.0 0 of IQ' in

Figure 56. Ertel's PV, 360K surface. Triangles denote positions of pre-cyclone Harry, which first appears on OOZ of the 27th near 133E, 19N. Track of Harry is also shown (squares). Contouring conventions are as in figure 7.

109 40Q 3 35 -

30 A

25 I ikk

201 I

15 .5 0 ( ( k5" 0 125 130 135 140 145 150 155 160 12% 130 135 140 145 150 155 160 ERTEL'SPV, 360K SURFACE:082812 IRTEL'S PV, 360K SURFACE:082900 1.51 1.5 40 4 -- 1. 0 35 - C0 300 ,f r A2 30 3( q JTn I, %. I :,Ile%'oft 2U I

15 EE'130 135 140 145 150 1510 -5 ERTEL'SPV, 360K SURFACE:082912 5 . S2Z1

0.

0o 90

ICO h*5~

o ul

$O M

-Q

125 130 135 140 145 150 155 2 160 135 140 145 150 155 160 0 910826/1200 850 MB VOROBS(*10**5) 8A MB VOROBS(10**5) 0 0" o o (D

JA o (n od 0 o 0

120 125 130 135 140 145 150 155 120 125 130 135 140 145 150 155 910828/1200 850 MB VOROBS(*10*5) 910829/1200 850 MB VOROBS(10**5)

0dO

9-' 0

m

0

*1

910830/1200 850 MB VOROBS(*10**5) 910831/1200 850 MB VOdbBS(*10**5) a a

+ + * aa - I a +

M +1

a + a~

Saa

+ +~ (1 X

+ X

+X i+ ------i+ + +

+ +

+ X

a a

Figure 60. Tracks of Typhoon Ivy and Tropical Storm Joel (x).

113 INTENSITY OF IVY

90100 90200 90300 90400 90500 90600 90700 90800 90900 91000 91100 ORTE MOOH FORCING. RRDII 3(R),5(B),7(C) IVY 1.5

1.4

1.2

1.0

.8

.6

.4

.2

0

-. 2

-.6

-.0

-1.0 90100 90200 90300 90400 90500 90600 90700 90800 90900 91000 ORTE MMDDHH

Figure 61. Intensity and forcing parameter time series for Typhoon Ivy.

114 0 CO

T_ 'LN- D

00

0

-M- - F,, EME/it rL2

o~ -f (ave MDO \tNO I M-U

0 U,:: EMMM LC

In

WN

0

0 V

to LnCv

U, 0 U,

'-U

Ul

Figure 62. Ertel's PV, 360 K surface. Positions of pre-cyclone Ivy denoted by triangles. Track of Ivy is also indicated. Contour conventions as in figure 7. 115 a. CDaD

p. in In n Q

Lna IT

a W

An

%0 ~ ~R

U11 o 0-. o oo

C) P- .

0 cn3

ro P1-o

CDi

CA

145 150 155 160 165 170 175 180 910831/1200 850 MB VOROBS(*10*5) o C :! 5.os << o o

o (D

o,OP..

0- 0 00

'-1

(D1 -- j

135 140 145 150 155 160 165 170 135 140 145 150 155 160 165 170 910904/0000 850 MB VOROBS(*10*5) 910905/0000 850 MB VOROBS(*10*5) INTENSITY OF JOEL 55

so

45

a

z35 6o

2S

20

15

10

DATE MOOMIH FORCING, RADII 3(A),5(B),7(C) J0EL 1.4 i~i'I'I'I'I' I 11 1.2

1.0 |-

.6

LU

0| C-

.2 LUa

0

-. 2

90100 90200 90300 90400 90500 90600 90700 90800 ORTE MOOmI

Figure 66. Intensity and forcing parameter time series pertaining to Tropical Storm Joel. 119 -"cr % In

In M ,-0 0D

C,, r

Lnl m) CI

CD)CV

0.

LMLL A~ I

In

In In LO

Ur,

V)

n U,

iC,,

00 'C/) W)1 wVV 0-

0\M -18U*l N4 ._1C~04 'U)."- '01- In

Figure 67. Ertel's PV, 360 K surface. Positions of pre-cyclone Joel are denoted by triangles. Track of Joel indicated by squares. Contour conventions as in figure 7. 120 O\

o c71

CD

00 cn CDe

g t

CD

SN N) 0

Im. 20

D02

.0

5

CD 110 115 420 125 130 1 -Oo 0 1450 UEffTSPV, 360K SURFACe.90400 00

000 0

Q3r - 0

- ~ - I L .j

jR !e - 11

I IN

r gn 4 SIN 04 04

Ln 0

M >

of Harry. Contouring conventions as infigure 10.

122 0. ~oo 0(3

(Ap

0 m~ 110 115 120 1 130 135 140 145, 110 115 120 125 130- 135 140 145 910902/0000 85 MB VOROBS(*10"5) 910903/0000 850 MB VOROBS ('10**5)

::rO

o

0

U))

inl~ 9?0904/0000 850 MB VOROBS(*10"5) 6 4 910905/0000 850 MB VOROBS(*10**5) ~ 10 FORCING, RRDII 3(A).5(B).7[C] ND4 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 .8 .6

.2

0 -. 2 -.A -. 6 -. 8 -1.0 -1.2 72400 72500 72600 72700 72800 72900 73000 73100 73200 73300 73400 ORTE MMO0HH

Figure 71. Forcing time series for nondeveloper number 4.

124