DENG Guo (邓 国) , ZHOU Yu-Shu (周玉淑) , LIU Li-Ping (刘黎平) 1
Total Page:16
File Type:pdf, Size:1020Kb
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 Ñ2y = z , (1) surrounding flow by modifying the vorticity, then computing a modified vorticity field to determine the vky=´Ñy , (2) steering flow. To illustrate the validity of this method, where y is the stream function for the non-divergent we select the case of Dan and explore in detail the wind, z is the relative vorticity and v is the relationship between the steering flow and the y 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 y = 0 on the lateral boundaries of the domain and solve Eq. (1) for a perturbation stream function y ¢ on all ' pressure surfaces. From Eq. (2) vy 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. Ñ c = d , (3) Once the first-guess vortex is located, there are vc = Ñc , (4) many ways one might consider of removing it. For where c is the velocity potential, d the divergence example, a scale-selective smoothing might be imposed to try to damp out the incorrect circulation. In the and vc 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 Ñ f = z g f 0 , (5) the far field and may not remove the entire storm from the first guess, or will likely leave significant vkg =´Ñf , (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 f 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.