2350 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 14, NO. 12, DECEMBER 2017 Study on Center Monitoring BasedonHY-2andFY-2Data Tangao Hu , Xiao Wang, Dengrong Zhang, Gang Zheng , Yuzhou Zhang, Yiyue Wu, and Bin Xie

Abstract— The location of a typhoon center plays an VIS and IR images with 1-km spatial resolution such as important role in moving track monitoring and forecasting. those acquired by Geostationary Operational Environmen- More and more new satellites are used to monitor . tal Satellite (GOES-12) and Moderate Resolution Imaging Combining multiple sources of satellite data has become pop- Spectroradiometer/Advanced Very High Resolution Radiome- ular for monitoring in recent years. In this letter, we demonstrate a robust method of locating the typhoon ter and Feng Yun-2/3 satellites have been used to observe center based on meteorological satellite (Feng Yun-2 (FY-2) typhoons [6]. However, due to cloud cover and rain effects, satellite) and microwave scatterometer data (Hai Yang-2 (HY-2) the inner core structures and air–sea interaction near the ocean scatterometer). First, we locate the typhoon center using HY-2 surface cannot be directly observed with VIS or IR sensors [3]. and FY-2 data independently. Next, we combine the results of With the advance in spaceborne microwave remote sensing, the HY-2 and FY-2 analysis to produce an improved method of microwave data are also used extensively for tropical cyclone locating typhoon centers. Finally, three representative typhoons analysis [7]. The advantage of sensors such as the Special Sen- (“ChanHom,” “Soudelor,” and “DuJuan”) are used to validate our methodology. The results show that: 1) compared to FY-2 sor Microwave Imager, Tropical Rainfall Measuring Mission, satellite data, the proposed results have more information on Quick Scatterometer, Advanced Scatterometer, and Hai Yang-2 wind speed and direction and 2) compared to HY-2 satellite data, (HY-2) scatterometer is that they can see through most clouds the proposed results have higher temporal resolution. Overall, and make operational measurements at the air–sea interface. the proposed method can improve typhoon monitoring results. The spatial resolution of these measurements is usually in the Index Terms— Feng Yun-2 (FY-2) satellite, Hai Yang-2 (HY-2) range of kilometers to tens of kilometers [8], [9]. This has scatterometer, moving track, typhoon center. become a popular tool for tropical cyclone monitoring over recent years [10]–[12]. I. INTRODUCTION In this letter, a typhoon center monitoring method is pro- ROPICAL cyclones are called typhoons in the northwest posed that combines techniques for identifying typhoon center TPacific Ocean and hurricanes in the Atlantic Ocean and locations using Feng Yun-2 (FY-2) satellite imagery and HY-2 northeast Pacific Ocean, and are the most intense air–sea inter- satellite data independently. Based on the above methods, action processes on a synoptic scale [1]. Typhoons cause tor- we try to improve the location of the typhoon center and rential rainfall, floods, and inland erosion, and threaten the life the complete typhoon moving track using the advantages of and property of coastal communities worldwide. Therefore, both satellites. We primarily investigated the potential ability accurate typhoon track and intensity forecasting is important of HY-2 and FY-2 sensors to monitor typhoon centers and for minimizing the damage caused by typhoons. The location tracks based on three representative typhoons that occurred of the center of a typhoon is key information that is needed in 2015. All relevant materials are presented in Section II. for timely and accurate tropical cyclone forecasting [2], [3]. A description of HY-2 and FY-2 monitoring methods is Since the launch of the first polar-orbiting meteoro- presented in Section III, while Section IV introduces the logical satellite in the early 1960s, remote sensing tech- monitoring results of typhoons “ChanHom,” “Soudelor,” and niques have proved to be a useful method for tropical “Dujuan.” The discussion and conclusion are presented in cyclone analyses and forecasting [4]. Various visible (VIS), Section V. infrared (IR), and microwave spaceborne sensors are used II. MATERIALS to observe typhoons [5]. According to previous studies, A. Study Area Manuscript received June 18, 2017; revised September 6, 2017 and In this letter, we selected the northwest Pacific as the study October 13, 2017; accepted October 16, 2017. Date of publication November 7, 2017; date of current version December 4, 2017. This work was area because tropical disturbances in this area more easily supported in part by the National Natural Science Foundation of under develop into typhoons. There are an average of 26–28 tropical Grant 41401517, Grant 21377166, and Grant 41676167, and in part by the storms and typhoons each year, of which two or three can Science and Technology Program of Hangzhou under Grant 20150533B03. (Corresponding author: Dengrong Zhang.) reach typhoon intensity. Typhoons here are seasonal and usu- T. Hu, X. Wang, D. Zhang, Y. Zhang, Y. Wu, and B. Xie are with the ally occur from July to September. For this study, we choose Institute of Remote Sensing and Earth Sciences, College of Science, and three representative typhoons that occurred in 2015 to validate also with the Zhejiang Provincial Key Laboratory of Urban Wetlands and the effectiveness of our model. Regional Change, Hangzhou Normal University, Hangzhou 311121, China (e-mail: [email protected]; [email protected]). G. Zheng is with the State Key Laboratory of Satellite Ocean Environment B. Typhoon Descriptions Dynamics, Second Institute of Oceanography, State Oceanic Administration, 1) Typhoon “ChanHom”: It formed in the northwest Pacific Hangzhou 310012, China (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available Ocean and traveled 1650 km to by 8 P.M.on online at http://ieeexplore.ieee.org. June 30, 2015. It arrived at the Zhejiang province of Digital Object Identifier 10.1109/LGRS.2017.2764620 China at 16:40 on July 11, 2015, and then traveled 1545-598X © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. HU et al.: STUDY ON TYPHOON CENTER MONITORING BASED ON HY-2 AND FY-2 DATA 2351

northeast to the Korean Peninsula. More than 700 houses collapsed, and over 1400 houses were damaged. 2) Typhoon “Soudelor”: It formed in the northwest Pacific Ocean on July 30, 2015. It then entered the Strait and coastal areas of the province of China. According to a report from the government, 3 078 000 people were affected. 3) Typhoon “Dujuan”: It formed in the northwest Pacific Ocean at 2 A.M. on September 23, 2015. It affected 990 000 residents in 518 villages and 56 counties in Fig. 1. Flowchart of the technical process. China.

C. Remote Sensing Data 3) Best Typhoon Track Published by Joint Typhoon Warn- 1) HY-2 Data: HY-2 was launched on August 16, 2011, ing Center (JTWC): The JTWC is a joint facility of and commenced operations in October 2011. It is a the U.S. Navy and Air Force. The center uses sur- sun-synchronous polar-orbiting satellite with a scat- face and upper air data and atmospheric models via terometer that uses pencil beams operating at a fre- satellite remote system probing and radar systems to quency of 13.225 ± 0.003 GHz (Ku-band). Two pencil predict and analyze typhoon routes, development pat- beams sweep in a circular pattern at incidence angles terns, and year-round trends covering 90% of global of 38° (horizontally polarized) and 44° (vertically polar- typhoon activity. It publishes typhoon center locations at ized) with a resolution of 25 km. The measurement 6-h intervals. range for wind speed is 2–24 m/s with an accuracy of 2 m/s (or 10%). Wind direction is measured from III. METHODS 0° to 360° with an accuracy of 20°. We used Level A. Technical Process 2B product data that average ascending and descend- In this letter, we used two different satellite (HY-2 and FY-2) ing swath data. We collected data from June 2015 to data to monitor typhoon centers. The flowchart of the technical September 2015 [13]. process is shown in Fig. 1. 2) FY-2 Data: In this letter, we used FY-2 stationary weather satellites that produce cloud images every B. HY-2 Typhoon Center Location Method half hour. These images have five channels: VIS light (0.55–0.90 µm), IR (10.30–11.30 µm), IR split win- Wind field data from a microwave scatterometer can be used dow (11.50–15.50 µm), water vapor (6.20–7.60 µm), to determine the location of typhoons. In this letter, we used and medium-wave IR (6.20–7.60 µm). The spatial HY-2 scatterometer data to obtain effective wind field data and extent of satellite imagery is between 10°S–50°N and calculate the typhoon center location. 100°E–170°E. The spatial resolution for VIS wave- 1) Regional Wind Field Interpolation: We acquired lengths is 1.25 km and that for IR wavelengths 25-km-resolution wind field grid data from HY-2 and is 5 km [14]. interpolated them. We adopted the kriging interpolation method to ensure that the interpolated values of gridded points were the same as the original data values. First, D. Reference Data Acquisition we constructed wind speed contours to acquire a regional In order to evaluate the accuracy of the proposed method, wind speed map. Then, we transformed the wind direc- we selected the best typhoon tracks published by three official tion into u and v wind components, and interpolated institutions as reference data. the components. We constructed the wind direction map 1) Best Typhoon Track Published by China Meteo- by the vector addition method. Finally, we matched rological Administration (CMA): The CMA is the the coordinates and superimposed the maps to process national center for weather forecasting, climate pre- the wind speed map and the wind direction map, diction, climate change research, and meteorological thus acquiring the regional wind field. From the above information collection and distribution services. The method, we acquired an interpolated map of regional best typhoon track is obtained by analyzing infor- wind directions [Fig. 2(a)] and regional wind speeds mation from various sources such as radiosondes, [Fig. 2(b)]. Finally, we matched geographic coordinates reconnaissance flights, dropsondes, ground-based radar, and produced a regional wind field map [Fig. 2(c)]. and various satellite products. Data are published 2) Calculation of Typhoon Center: The location of the at 6-h or 3-h intervals normally, and at 1-h intervals center, or the , of a typhoon is defined by its structure during typhoons. and characteristics. The center of a typhoon is an area of 2) Best Typhoon Track Published by Meteorological relatively low pressure; owing to this, a large pressure Agency (JMA): The JMA is responsible for collecting gradient is observed near the center and an eyewall and providing daily observations and for conducting forms around the center. The eye is usually a relatively research on meteorology, hydrology, earthquake and vol- calm area, but the eyewall is the region where winds are canic activity, and other natural phenomena in relevant the strongest. scientific fields. It publishes typhoon center locations at We extracted the typhoon center from scatterometer data 6-h intervals. by two methods: 1) by using regional wind speed maps and 2352 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 14, NO. 12, DECEMBER 2017

wind direction map, and FY-2 satellite image. We extracted the center using combination map, from which the minimum value of the high-speed area, and the center of vortex-like distribu- tion wind direction by HY-2 or the location where many lines intersect by FY-2 were calculated. Finally, the centers were arranged in time sequence, and the moving track of typhoon could be monitored.

Fig. 2. Regional wind field map. (a) Wind direction map. (b) Wind speed E. Accuracy Assessment map. (c) Wind field map. In this letter, we calculated the great circle distance (GCD) between monitoring results and reference data. We used the 2) by using regional wind direction maps. In the first method, mean and standard deviation values as the basis for the eval- we determined the center by finding the minimum value of uation We assessed the performance of the proposed method the high-speed area automatically. In addition, since the wind after comparing the results of CMA, JMA, and JTWC direction of typhoon has a helical structure, the center of The GCD is the shortest distance between any two points the vortex-like distribution wind direction will correspond to on the surface of a sphere. In this letter, we regard the earth as the location of typhoon center; thus, typhoon center can be a sphere and follow McCaw’s recommendation of considering calculated by observing the vortex center. However, the centers the mean radius as approximately 6371.01 km. M(E , N ) thus determined by the two methods may be different owing 1 1 and T (E , N ) (where E and E are the longitudes, and N to the strong impact of heavy rain on the backscatter in the 0 0 0 1 0 and N are the latitudes) are monitoring data and ground truth area surrounding the typhoon. 1 data. The GCD between M and T is represented as follows: In this letter, we primarily used the wind speed extraction −1 method. We extracted typhoon centers using wind speed D = R · cos [cos N1 · cos N0 · cos(E1 − E0)+sin N1 ·sin N0] maps (from which the minimum value of the high-speed area (1) can be found easily) and used the wind direction maps as an auxiliary discrimination tool in cases where we could not where D is the GCD, and R is the mean radius of the earth. calculate the minimum value of the high-speed area or where we found more than one minimum value point [13]. IV. RESULTS A. Monitoring Results of “ChanHom” C. FY-2 Typhoon Center Location Method For the whole life cycle of “ChanHom” from The first step in the process is to prepare the suitable images June 30, 2015 to July 13, 2015, the HY-2 captured 28 images. and eliminate regions of the scene that are at the extreme However, only nine images could be used to calculate the edges. Next, it is necessary to identify weather disturbances typhoon center independently. Being geostationary, the FY-2 of potential interest within the scene, as well as a central point was able to capture many more images. In order to locate within these disturbances. After applying a low-pass filter on more centers and improve accuracy, we combined the two the image to remove noise, the gradient of the brightness results. temperatures in the image is calculated from the horizontal By combining the HY-2 and FY-2 monitoring results, and vertical derivatives by using Sobel’s template. Next, lines the proposed method located 18 centers with high accu- parallel to the gradient vector at each pixel are drawn across racy throughout the entire life cycle, as shown in Fig. 3. the image and the locations where these lines cross are stored For instance, due to atmospheric environment, land surface, in an accumulator (or density) matrix. A high number in satellite transit time, and other factors, there are no HY-2 data the matrix indicates the location where many lines intersect, for July 9 and July 10. However, the FY-2 data can locate the indicating a common point that the corresponding gradients are center effectively. Therefore, we can use the proposed method directed toward (or away from). The locations of the maxima to achieve a better typhoon track. in the accumulator matrix are considered center locations [15]. B. Accuracy Assessment Results An advantage of this system is that the number of potential weather disturbances in the scene does not need to be known First, we calculated the typhoon centers and moving track in advance in order to identify them and locate their centers. of “ChanHom” by HY-2 and FY-2 independently. Second, Each satellite image was individually analyzed to ensure based on the two results and proposed method, we achieved that the center locations were consistently located within the a new typhoon track that has more center locations and is atmospheric weather disturbance. more accurate. Finally, the combined results were compared to traditional methods and best typhoon tracks (CMA, JMA, and JTWC). Because a mesoscale vortex can be regarded as D. HY-2 and FY-2 Combination Method a large-scale thunderstorm system with a horizontal extension First, we collected the continuous HY-2 data and FY-2 data of approximately 100 to 200 km, we defined mean GCD value during the life cycle of typhoon. Next, we calculated the less than 200 km as a criterion for successful monitoring [16]. centers using HY-2 and FY-2 data according to the above 1) Comparing HY-2 and FY-2: Three moving tracks are methods. If the acquisition time of HY-2 and FY-2 data was shown in Fig. 4(a). The red curve represents the same, then we compared geographic coordinates and produced proposed method, the green curve represents the HY- a combination map which included HY-2 wind speed map, 2 track, and the blue curve represents the FY-2 track. HU et al.: STUDY ON TYPHOON CENTER MONITORING BASED ON HY-2 AND FY-2 DATA 2353

Fig. 3. Combined monitoring results of “ChanHom” obtained by HY-2 and FY-2.

Fig. 4. Moving tracks of “ChanHom.” (a) Tracks comparing HY-2, FY-2, Fig. 5. Moving tracks of “Soudelor.” (a) Tracks comparing HY-2, FY-2, and and the proposed method. (b) Tracks comparing CMA, JMA, and JTWC. the proposed method. (b) Tracks comparing CMA, JMA, and JTWC.

In the early and middle periods of the typhoon cycle, HY-2 obtained nine centers with high quality. In the mid- C. Monitoring Results of “Soudelor” dle and last periods, FY-2 obtained many more centers. After combining the results, we calculated 12 centers. From July 31, 2015 to August 07, 2015, there were about 2) Comparing CMA, JMA, and JTWC: Four moving tracks 11 images of HY-2 that could be used to calculate the typhoon are shown in Fig. 4(b). The red curve represents the center independently. At the same time, FY-2 captured more proposed method, the green curve represents the best images. In order to locate more centers and improve accuracy, CMA track, the blue curve represents the best JMA we combined the two results. track, and the yellow curve represents the best JTWC 1) Comparing HY-2 and FY-2: Three moving tracks are track. GCD results were calculated using (1). shown in Fig. 5(a). The HY-2 method obtained 11 cen- a) The mean GCD between the proposed method and ters with high quality. In the middle and last periods, the CMA is 196.75 km. These results need to be the FY-2 method obtained many more centers. By com- improved. bining both results, we calculated 15 centers. b) The mean GCD between the proposed method and 2) Comparing CMA, JMA, and JTWC: Four moving tracks the JMA is 83.22 km. Importantly, only four points are shown in Fig. 5(b). GCD results were calcu- exceeded 100 km and most centers were the same lated using (1). Compared to CMA, the mean GCD as the JMA’ centers. is 180.62 km; compared to JMA, the mean GCD is c) Compared to JTWC, the mean GCD is 96 km. 84.85 km; and compared to JTWC, the mean GCD is This agreement demonstrates that the proposed 95.47 km. The accuracy assessment results show that method accurately monitored the track of typhoon the proposed method monitors the track of typhoon “ChanHom.” “Soudelor” successfully. 2354 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 14, NO. 12, DECEMBER 2017

In order to achieve better typhoon center location and moving track results, some additional problems need to be studied further: 1) how to extract the center automatically and 2) how to improve the accuracy of typhoon center locations when eyewalls are not present.

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