DENG Guo (邓国) , ZHOU Yu-Shu (周玉淑) , LIU Li-Ping (刘黎平) 1

Vol.16 No.2 JOURNAL OF TROPICAL METEOROLOGY June 2010

Article ID: 1006-8775(2010) 02-0154-06 USE OF A NEW STEERING FLOW METHOD TO PREDICT TROPICAL CYCLONE MOTION

1 2 3 DENG Guo (邓国), ZHOU Yu-shu (周玉淑) , LIU Li-ping (刘黎平)

(1. National Meteorological Center, Beijing 100081 China; 2. Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029 China; 3. Chinese Academy of Meteorological Sciences, Beijing 100081 China)

Abstract: A tropical cyclone is a kind of violent weather system that takes place in warmer tropical oceans and spins rapidly around its center and at the same time moves along surrounding flows. It is generally recognized that the large-scale circulation plays a major role in determining the movement of tropical cyclones and the effects of steering flows are the highest priority in the forecasting of tropical cyclone motion and track. This article adopts a new method to derive the steering flow and select a typical swerving track case (typhoon Dan, coded 9914) to illustrate the validity of the method. The general approach is to modify the vorticity, geostropical vorticity and divergence, investigate the change in the non-divergent stream function, geoptential and velocity potential, respectively, and compute a modified velocity field to determine the steering flow. Unlike other methods in regular use such as weighted average of wind fields or geopoential height, this method has the least adverse effects on the environmental field and could derive a proper steering flow which fits well with storm motion. Combined with other internal and external forcings, this method could have wide application in the prediction of tropical cyclone track.

Key words: steering flow; prediction of tropical cyclone track; vorticity and divergence CLC number: P444 Document code: A doi: 10.3969/j.issn.1006-8775.2010.02.007

1 INTRODUCTION researches have worked to extract the steering flow; the first method is to calculate the weighted average of The complexity of tropical cyclones (TCs) wind observations surrounding the TC[3, 4], the second movement results from a wide variety of external and method is to make use of spatial filters to get internal dynamical forcings and their interaction, but large-scale environmental fields of either geopotential the most dominant factor is the relative vorticity in heights or pressure gradients[2-7]. However, there are large-scale environmental flows, like a some obvious restrictions in the abovementioned several-hundred-km radius vortex embedded in and methods, such as high demand on observations, steered by a basic surrounding flow on the scale of a [1] randomness in calculation, and destruction of thousand km . Therefore, the steering flow is the main surrounding flow, creating unbalance and factor affecting the movement of TCs, and the steering incompleteness during the separation of the concept has generally been accepted and applied surrounding flow from the TC vortex. Therefore, it is wherever possible by forecasters in typhoon track [2, 3]. difficult to identify the real situation of the steering prediction However, as the actual weather flow and the TC[8]. circulation is very complex and contains the After TCs enter the South China Sea (SCS), most information of the TC itself (internal forcings), of them move westward, but some of them swerve to surrounding flow (external dynamical forcings) and the north, forming a kind of abnormal track called their interactions, how to separate the TC vortex from swerving typhoon track of SCS. The sudden turning of the surrounding flow remains a challenge. Many typhoons in SCS often makes it difficult to predict the

Received date: 2009-12-17; revised date: 2010-03-18 Foundation item: project of the Ministry of Sciences and Technology of the People’s Republic of China (GYHY200706020); projects of National Natural Science Foundation of China ((40975034, 40505009); project of State Key Laboratory of Severe Weather (2008LASW-A01) Biography: DENG Guo, Ph.D., mainly studying ensemble forecasts and tropical cyclones. E-mail for corresponding author: [email protected]

PDF created with pdfFactory trial version www.pdffactory.com No.2 DENG Guo (邓国), ZHOU Yu-shu (周玉淑) et al.. 155 location of landfall, possibly resulting in great loss in general approach[8] we adopt is to modify the vorticity, the southern provinces of China. Dan (Fig. 1) was a geostrophic vorticity, and divergence, then solve for the typical SCS swerving track typhoon, and also the change in the non-divergent stream function, severest TC that has ever hit the city of Xiamen, Fujian geopotential and velocity potential, respectively, and province, in the past 46 years, leaving behind a trail of compute a modified velocity field. The general death and devastation. It is obvious that case studies on approach to modifying the flow can be illustrated in the Dan will improve the understanding of the movement context of vorticity and non-divergent wind. The patterns of TCs in SCS, hence contributing to increased relationship between wind, stream function and accuracy in routine forecasting systems[2]. This article vorticity is employs an approach to separate TC vortex from the ∇2ψ = ζ , (1) surrounding flow by modifying the vorticity, then  computing a modified vorticity field to determine the vkψ=×∇ψ , (2) steering flow. To illustrate the validity of this method, where ψ is the stream function for the non-divergent we select the case of Dan and explore in detail the wind, ζ is the relative vorticity and v is the relationship between the steering flow and the ψ typhoon’s motion. non-divergent wind. To define the non-divergent wind associated with the first-guess storm, we set vorticity equal to zero outside a radius of rm , specify ψ = 0 on the lateral boundaries of the domain and solve Eq. (1) for a perturbation stream function ψ ′ on all ' pressure surfaces. From Eq. (2) vψ is calculated and subtracted from the first-guess wind field.

Fig.1 The best track of typhoon Dan.

2 DATA AND METHODOLOGY

The data adopted in this case are the 2.5°× 2.5° global analysis (first guest) from the European Center for Medium-Range Weather Forecasts (ECMWF) from October 5 to 9, 1999. The first step of the removal process is to identify the vortex corresponding to the Fig.2 Schematic diagram of the search for the vortex in the storm of interest in the analysis field. This is first guess. Solid black contours: vorticity near the accomplished by searching for the maximum vorticity surface; shaded circles: positions of the observed on the mandatory pressure level analyzed closest to the storm and those of the vortex center in the first guess. surface within a prescribed radial distance from the Best Track location of the TC (obtained from Removal of divergent wind and pressure anomalies satellite/radar monitoring). Currently the search radius associated with the first-guess storm follows Eqs. (1) is set at 400 km (Fig. 2) and the radius could be and (2), except in the case of divergence, Eqs.(1) and flexible according to the intensity of the TC). The point (2) are replaced by. of maximum vorticity then serves as the center of the 2 vortex to be removed. ∇ χ = δ , (3)

Once the first-guess vortex is located, there are vχ = ∇χ , (4) many ways one might consider of removing it. For where χ is the velocity potential, δ the divergence example, a scale-selective smoothing might be imposed to try to damp out the incorrect circulation. In the and vχ the non-rotational wind. To remove the GFDL bogussing scheme[7] a sophisticated filtering is geopotential height anomaly Eqs. (1) and (2) become used. However, smoothing can have adverse effects on 2 ∇ φ = ζ g f 0 , (5) the far field and may not remove the entire storm from  the first guess, or will likely leave significant vkg =×∇φ , (6) imbalances in the modified background field. The and we similarly set the geostrophic vorticity

PDF created with pdfFactory trial version www.pdffactory.com 155 156 Journal of Tropical Meteorology Vol.16 the relationship between the steering flow and TCs, and (subscript ‘g’) equal to zero outside r = rm and ' the readers could refer to the representative work of solve for a geopotential anomaly φ to be Chan and Gray[3], Chan[10] , Velden[11], Bell[12], subtracted from the background. In this method, the Holland[13], Kegin[14]. Through researches on TCs at wind and geopotential height anomaly field are all different oceans, direction and speed of movement, removed, leaving a first-guess field with only a intensity change and size, their work indicates that the steering wind where the first-guess storm was consistence between the mid-tropospheric levels and located (Fig. 3, time in UTC, the same below). TC motion is better than any single level. Based on Figure 3 shows the original analysis field and the previous studies and the character of the case typhoon background flow with the TC removed. Comparison Dan, this paper defines a vertical level between 850 between the original and modified analysis field hPa and 300 hPa as the steering level, and the shows clearly that first, the background away from algorithm is the weighted average of the consecutive TC-affecting area is almost unmodified with the five levels. The coefficients for each level are: 0.1 for above method; second, the area where Dan 850 hPa, 0.2 for 700 hPa, 0.4 for 500 hPa, 0.2 for 400 originally located is a uniform steering field, leaving hPa, and 0.1 for 300 hPa. To compare the relationship no information about the storm. Therefore, the new between the pressure-weighted steering flow obtained steering flow method can be well relied on to predict with the method in section 2 and Dan, we analyze the the track of the TC. ECMWF analysis data from October 5 to 9 (the DAT problem) and remove the typhoon vortex to get the steering flow. This period covers the whole course of Dan’s swerving track, which is a difficult challenge to the operational forecaster. Figure 4 (a through d) shows the position of Dan and the steering flow that has been separated. Figure 4 (a through d) shows the relationship between the steering flow and Dan at different periods of them that cover its major life cycle. At 0000 October 5, Dan reached its peak intensity of 110 kts and was near the northwestern coast of Luzon of the Philippines. Dan then moved across the northernmost tip of Luzon and by 1200 had entered into the SCS just west of Laoag, a city on the northwestern tip of the (a) island (Fig. 1). Figure 4a indicates that Dan was situated at a turning point of the steering flow: behind the vortex was the southeast wind, and in front of it was the eastward flow. Therefore, Dan moved in the west-northwest direction within the strong flow of a subtropical ridge (the steering flow). Another obvious feature is that Dan ran fast at the time as the wind speed of the steering flow was high. At 0000 October 6 (Fig. 4b), with a generally steering flow, Dan made an obvious turn and headed northwest. Over the next couple of days, Dan moved rather slowly as the steering current was weak. The storm was located between a subtropical ridge to the west and high surface pressures over southeastern China to the northeast, which prevented any significant movement to (b) the northwest. The storm moved slowly westward and Fig.3 Analysis field at 1000 hPa, valid at 0000 October 8, curved to the north later. Dan reached its westernmost with the TC (a) and without the TC (b). point of track around 0000 October 7 when the direction of the steering flow was south-southwest (Fig. 3 ANALYSIS OF RELATIONSHIP BETWEEN 4c). A detailed comparison of the steering flow at THE STEERING FLOW AND TC different layers and motion of the typhoon showed that the wind at lower levels were mainly from the south or A number of researches have been carried out on southeast, and the flow at mid-tropospheric levels was

PDF crea156t ed with pdfFactory trial version www.pdffactory.com No.2 DENG Guo (邓国), ZHOU Yu-shu (周玉淑) et al.. 157 from south-southwest, well consistent with the actual the period, the basically southward steering flow typhoon movement. Therefore, it proves again that the caused Dan to run directly to the north along the mid-tropospheric levels are the best choice for longitude of 118°E. Afterwards, Dan made landfall in predicting TC movement. Twenty-four hours later, Dan China near Xiamen and continued its northward was located about 300 km west of the southern tip of journey across eastern China. Due to land fraction and Taiwan Island and was moving northward into the the intrusion of drier air, the typhoon was beginning to Taiwan Strait (Fig. 4d). By 0000 October 9, Dan was weaken and finally turned into an extratropical cyclone. located just off the Chinese coast near Xiamen. During

a b

c d

Fig.4 Derived steering flow and status of typhoon Dan, valid at 0000 October 5 (a), valid at 0000 October 6 (b), valid at 0000 October 7 (c) and valid at 0000 October 8 (d).

Comparison between the traveling speed of Dan direction can be better derived but at the expense of and the steering flow indicates that during the fast slightly worse motion speed, perhaps due to the effect westward course (Before October 6), the traveling of the friction at lower levels. Overall, the speed of the speed of Dan was about 6 m/s, which was close to the steering flow was somewhat higher than typhoon’s steering flow speed; after curving northward, it actual motion, but the difference is minor. Therefore, decreased dramatically in motion and the steering flow the above analysis indicates that the steering flow was a little faster than typhoon’s movement (2 m/s). adopted in this paper is valid in determining TC Furthermore, the level between 850 hPa and 300 hPa is movement. defined as the steering level here in this study, because with this level we could get a better motion consistence 4 ANALYSIS ON DIFFERENCE BETWEEN in direction than with a thicker steering flow. In fact, if THE STEERING FLOW AND TC MOTION a thinner steering flow (e.g. between 700 hPa and 500 hPa) is selected with our method, TC’s motion

PDF created with pdfFactory trial version www.pdffactory.com 157 158 Journal of Tropical Meteorology Vol.16 The comparison between the steering flow and and more on the effect of internal forcings on TCs and Dan’s motion indicates that the newly introduced the interaction between the internal and external method to derive the steering flow could serve as a forcings. The internal forcings include convection good basis to predict TC movement. Both the direction asymmetry, mesoscale systems, vertical coupling and speed of TC movement are consistent with the among vortexes between higher and lower levels, and observations on the whole. However, the steering flow dynamical instability at outflow layers. Among all the could not control or determine TC motion entirely, interactions, the most important process is the because there is some deviation between the separated interaction between the TC vortex and beta effect [1]. steering flow and actual TC movement. Due to the This interaction could create secondary asymmetric complexity of TC motion from a wide variety of circulation and lead to the steering effect on TC motion. external and internal dynamical forcings and their Furthermore, the external and internal forcings could interaction, all these factors lead to failed make the interaction more complex. Chen et al.[19] forecasting[15-18] and should be taken into account in pointed out that the asymmetric structure within the prediction besides the dominant large-scale typhoon is a factor leading to the deviation between the environmental flow. The theory of steering flow only steering flow and typhoon movement. For the detailed incorporates the background surrounding flow, while research work on exploring the structure of TCs, the factors such as the axially symmetric circulation of method of decomposing wind fields in a limited area is tropical cyclone and the planetary vorticity gradient, encouraging[20]. Figure 5 shows the distribution of the which generates secondary asymmetric flow (Beta gyre wind field when the typhoon curved (October 6 and 7), circulation), could alter TC movement and lead to track from which an obvious structure of dense isolines is forecast error [15, 16]. shown to be in the northeast quadrant while sparse When the large-scale surrounding flow is stable, isolines in the southwest quadrant. Chan and the TC movement will be normal; if the large-scale Willians[21] proved that the beta effect could cause a circulation adjusts or changes abruptly (such as the westward stretching of TCs and create an east-west advancement or retreat of the subtropical high, asymmetry in the meridional wind field; the nonlinear breaking down of the Intertropical Convergence Zone, process produces the northwestward movement and formation or subsidence of the Equatorial Buffer Zone, brings about a wind maximum to the northeast of the alternation of trade wind and monsoon), the vortex. The wind field distribution at Fig. 5 shows the predictability of storm track is low[2]. The basis for the typical structure of wind maximum to the northeast of concept of steering flows lies in the assumption that TC the vortex, which is the indicator of the existence of the is regarded as a point vortex, and the action of the beta effect. The wind fields in other periods show large-scale surrounding flow is attributed to the similar character and will be omitted here. external forcings. Recent researches emphasize more

Fig.5 ECMWF-analysis based wind field at 500 hPa, valid at 0000 October 5 (left panel), and 0000 October 6 (right panel).

Next is a summary of the major affecting factors analysis that the movement of Dan was controlled by for the lifetime of Dan. Formed on October 1, Dan comprehensive effects of external (steering flow) and moved west before making landfall in the Philippines internal dynamical forcings (beta effect, etc.) and their (Category 3 TC), and then recurved and hit China as a interaction. (i) During the process of westward motion Category 2 TC. It could be concluded from the above (before October 6), Dan was steered mainly by the

PDF crea158t ed with pdfFactory trial version www.pdffactory.com No.2 DENG Guo (邓国), ZHOU Yu-shu (周玉淑) et al.. 159 subtropical high and moved quickly. (ii) From October [4] GEORGE J E, GRAY W M. TC motion and surrounding 6 to 7, Dan was cut off from the subtropical high and parameter relationships [J]. J. Appl. Meteor., 1976, 15: suddenly swerved to the north, when the steering flow 1252-1264. [5] BRAND S, BUENAFE C A, HAMILTON H D. was weak and the beta effect was dominant. (iii) After Comparison of TC motion and environmental steering [J]. October 7, Dan moved along the longitude of 119°E, Mon. Wea. Rev., 1981, 109: 908-909 with the combination effect of the steering flow (in the [6] NEUMANN C J. On the use of deep-layer mean northward to northeast direction), beta effect (in the geopotential height fields in statistical prediction of TC motion northwestward direction), their interactions and other [J]. 6th Conf. On Probability and Statistics in Atmospheric Science, Banff Alta: Amer. Meteor. Soc., 1979: 32-38. effects. [7] KURIHARA Y, BENDER M A, ROSS R. An initialization scheme of hurricane models by vortex specification [J]. Mon. 5 SUMMARY AND DISCUSSION Wea. Rev., 1993, 121: 2030-2045. [8] DAVIS C A, LOWNAM S. The NCAR-AFWA TC TC motion is a complex process, whose complete Bogussing Scheme [R]. A Report Prepared for the Air Force description requires at least a detailed knowledge of the Weather Agency (AFWA), 2001. [9] DING Yi-hui. The Diagnostic and Analysis Method in interactions between the cyclone circulation, the Synoptic Dynamics [M]. Beijing: Science Press, 1989: 293pp. environmental wind field, the beta effect, the [10] CHAN J C L. Definition of the steering flow for TC underlying surface process, and the fields of moist motion [C]// Porc. 15th Conf. on Hurricanes and Tropical convection. This paper makes use of a new method to Meteorology, Boston: Amer. Meteor. Soc, 1984: 559-566. calculate the steering flow and tries to test its validity. [11] VELDEN C S. The relationship between TC motion, intensity, and the vertical extent of the environmental steering The general approach is to modify the vorticity, layer in the Atlantic basin [C]// Preprints, 20th Conf. on geostrophic vorticity, and divergence, solve for the Hurricanes and Tropical Meteorology, Boston: Amer. Meteor. change in the non-divergent stream function, Soc., 1993: 31-34. geopotential and velocity potential, respectively, and [12] BELL G J, LAM C Y. Departures of TC movement from then compute a modified velocity field to determine the geostrophic steering [C]// WMO Symposium on Typhoons, steering flow. This paper defines the level between 850 Shanghai, 1980: 110-115. [13] HOLLAND G J. TC motion: a comparison of theory and hPa and 300 hPa as the steering level and makes a observation [J]. J. Atmos. Sci., 1984, 41: 68-75. detailed comparison between the steering flow and [14] KEGIN D, CHARLES J N. The relationship between TC typhoon Dan’s motion. It indicates that the newly motion and environmental geostrophic flows [J]. Mon. Wea. introduced method to derive the steering flow could Rev., 1986, 14: 155-122. serve as a good basis to predict TC movement, and [15] CHEN Lian-shou, XU Xiang-de, LUO Zhe-xian, et al. Introduction to TC dynamics [M]. Beijing: China both the direction and speed of TC movement is Meteorological Press, 2002: 317pp. consistent with the observations on the whole. The [16] XU Xiang-de, CHEN Lian-shou, XIE Yi-yang, et al. The authors attributed the minor deviation to the beta effect asymmetric and dynamic structure of the ‘β-TOP’ diploe and according to the asymmetric wind structure within the ‘ventilation flow’ of the target typhoon Flo during TCM-90 typhoon, and the combination of both factors and their field experiment [J]. Acta Meteor. 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Citation: DENG Guo, ZHOU Yu-shu and LIU Li-ping. Use of a new steering flow method to predict tropical cyclone motion. J. Trop. Meteor., 2010, 16(2): 154-159.

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