Formation of Winter Supertyphoons Haiyan (2013) and Hagupit (2014)

3800 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 55, NO. 7, JULY 2017 Formation of Winter Supertyphoons Haiyan (2013) and Hagupit (2014) Through Interactions With Cold Fronts as Observed by Multifunctional Transport Satellite Yung-Sheng Lee, Member, IEEE, Yuei-An Liou, Senior Member, IEEE, Ji-Chyun Liu, Ching-Tsan Chiang, and Kuan-Dih Yeh

Abstract— This paper incorporates remote sensing imagery prevention [1], [2]. The satellite data that provide cloud images and image processing techniques to analyze typhoons’ evolution may be used to analyze the cloud structure and dynamics of into supertyphoons through interactions with cold fronts in the typhoons [3]Ð[8]. When two tropical cyclones (TCs) are close Western Pacific Ocean. The purpose is to enhance understanding and predictability of the tracks and profiles of supertyphoons. to some extent, they affect and interact with each other. Such Evolutions of typhoons Haiyan (2013) and Hagupit (2014) into cycloneÐcyclone interaction is known as “Fujiwhara effect” supertyphoons were studied. The 3-D profiles of weather systems in honor of pioneer Fujiwhara [9]. Several types of cycloneÐ were reconstructed using multifunctional transport satellite IR cyclone interactions were studied [10]Ð[12]. The distance cloud images. When interactions between typhoon and cold front between two typhoons, and intensity and translation speeds of happened, enhancements of typhoons were observed causing typhoons Haiyan and Hagupit to strengthen power and evolve the binary typhoons were investigated recently. The impacts into supertyphoons. The origins of typhoons Haiyan and Hagupit of binary typhoons on the upper ocean environments that were are closely located at 152°50E/05°12N and 151°30E/04°19N, explored included physical and biological environments [12]. respectively. Both typhoons went through The Philippine Sea As environmental influences are observed and concerned, in the zone of 112°52E–146°11E. The lowest values of central the track and propagation of the cyclones must be predicted pressures of typhoons Haiyan and Hagupit occurred along their paths at the positions 128°41E/10°16N and 131°06E/11°00N, accurately. In addition, the interactions between midtropical respectively. Their distances from cold front and average speed of depression (TD) and TCs were investigated, and thus expanded their movement were dominating parameters for the formation the variety of cyclone-to-cyclone interactions. Studying the of winter supertyphoons. The supertyphoon Haiyan showed variety of cyclone-to-cyclone interactions is useful for weather higher intensity than the supertyphoon Hagupit. Each of the forecasting. For example, in previous research [13]Ð[19], in the two typhoons roughly consisted of two paths with the first path moving westerly and the second path northerly. The two typhoons North Pacific area, the mid-TS Bopha located between two evolved into supertyphoons at the turning points of the two paths. typhoons Saomai and Wukong had presented an unusual The first paths were about 2330 and 2050 km for typhoons Haiyan movement in the western pacific. This paper had quantitatively and Hagupit, respectively. evaluated TS Bopha movement southward due to the forc- Index Terms— Cold fronts, satellite imagery, supertyphoons. ing of circulation associated with typhoon Saomai, the flow field associated TS Bopha steered by typhoon Saomai, and I. INTRODUCTION typhoon Wukong affected by the motion of TS Bopha [13]. HE impacts of climate variability and global warming on When mid-TDs appeared between two cyclones, Tembin and the occurrence of tropical storms (TSs), including their T Bolaven, the TCs were still located far from each other. Due distribution and variation, are valuable information for disaster to the fact that the two TDs were smaller and lower than Manuscript received November 19, 2016; revised January 9, 2017; accepted the two TCs, the upward convection between TCs and TDs February 26, 2017. Date of publication April 5, 2017; date of current happened in addition to attraction. The interaction between version June 22, 2017. This work was supported by the Ministry of Sci- ence and Technology under Grant MOST 105-2111-M-008-024-MY2 and each set of mid-TD and mid-TC, depressionÐcyclone inter- Grant 105-2221-E-008-056-MY3. action, caused indirect cyclone-to-cyclone interaction as the Y.-S. Lee is with the Computer Science and Information Engineer- distance between the two typhoons was around 1500 km ing Department, Chien Hsin University, Taoyuan 32097, Taiwan (e-mail: [email protected]). more than the threshold distance for them to interact with Y.-A. Liou is with the Center for Space and Remote Sensing each other [14]. Existence of the mid-TD significantly com- Research, National Central University, Taoyuan 32001, Taiwan (e-mail: plicates weather forecasting owing to increased difficulty in [email protected]). J.-C. Liu, C.-T. Chiang, and K.-D. Yeh are with the Electrical Engineer- modeling its influence on the two cyclones. The innovative ing Department, Chien Hsin University, Taoyuan 32097, Taiwan (e-mail: generalized empirical formulas of threshold distance were [email protected]; [email protected]; [email protected]). proposed to characterize cycloneÐcyclone interactions either Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. with or without involvement of the TD(s) [15]. The empirical Digital Object Identifier 10.1109/TGRS.2017.2680418 formulas for the threshold of interaction distance between TCs 0196-2892 © 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. LEE et al.: FORMATION OF WINTER SUPERTYPHOONS HAIYAN (2013) AND HAGUPIT (2014) 3801 and TD/TS were physically related to current intensity (CI) tion channel (5.6Ð7.2 µm), and thermal infrared window number, size factor, height difference, and rotation factor. The channel (10.5Ð12.5 µm). The basis of the technique was to CI number corresponds with the maximum wind speed and determine the point around which the fluxes of the gradi- intensity. Statistical Hurricane Intensity Prediction Scheme, ent vectors of brightness temperature are converging [27]. which includes storm environmental conditions in addition The spectral features of the geostationary satellite infrared to climatology and persistence, was applied. A simplified window channel and the WV channel data were used to dynamical system for TC intensity prediction based on a calculate parameters to determine the overshooting areas in logistic growth equation was developed. The time tendency typhoon cloud systems and the centers and intensity of of the maximum sustained surface winds is proportion to the typhoons. The satellite data used to provide hourly obser- sum of the growth term and the limitations of the maximum vations of visible data by two infrared channels IR1, from wind to an upper bound [16], [17]. The interaction of dual 10.5 to 11.5 µm and IR2, from 11.5 to 12.5 µmandaset vortices in the horizontally sheared environmental flows on of WV channels, from 6.5 to 7.0 µm [28]. Since the ice- a beta plane has been examined [18]. The intensification of clouds or ice-covered surfaces on cloud were observed by Hurricane Sandy (2012) during the warm seclusion phase of using the shortwave infrared (SIR) channels [29]Ð[34], and its extratropical transition was addressed [19]. based on cloud effective temperatures and optical depths with Meanwhile, the supertyphoons, extra-TCs, and hurricanes 3.7- [29] or 3.8-µm [33] detectors, a variety of empirical were studied [20]Ð[26]. By a close examination of both active methods were applied to crudely characterize the cloud vertical and passive microwave signatures, significant scattering of structure [33]. An algorithm was proposed to extract cloud sys- radiation at a frequency of 18 GHz that occurred in the inner tems and spiral patterns of TCs in IR images. This mountain- eye wall at the altitudes of 3Ð8 km of the supertyphoon climbing algorithm defined cloud-system extraction as the was observed [20], [36]. Classification of vertical reflectivity peaks of the cloud mountain formed by a TC’s cloud system. profile of Tropical Rainfall Measuring Mission precipitation Several moving rules and stop criteria were made to define radar observations using a self-organizing map was presented. the search behaviors and constrain the traversal process [35]. Result of classification showed that hurricanes contain similar We also applied SIR images at 3.7 µm to observe the cloud characteristic profiles to those of typhoon with equal and surfaces of typhoon in depth. higher reflectivity [21]. Through an analysis of the composite At the Hong Kong observatory, the classification of TCs reflectivity, vertical wind shear, and radial velocity relative is defined in terms of wind speeds averaged over a period to the storm as observed by Doppler radar, some typical of 10 min. Supertyphoon is defined as the wind speeds properties, e.g., speed shear and a mesoscale cyclone, were of 185 km/h (pressure with 926 hPa) or above [36]. Due to discovered [22]. Several sources of satellite measurements increase in global climate variability in recent decades, many available in near-real-time: infrared measurements and night- more supertyphoons occur in the Northwestern Pacific Ocean. time composites of sea surface temperature from GOES- Considered one of the most powerful typhoons ever to make 12, gridded sea surface height maps derived from multiple- landfall in recorded history, the 600-km diameter typhoon satellite altimetry, and microwave sea surface temperature Haiyan crossed the Philippine islands, bringing widespread measurements from TRMM Microwave Imager and Advanced devastation along its path in 2103. Strong winds, heavy rain- Microwave Scanning Radiometer-E were used. The accurate fall, and storm surges caused extremely high loss of lives and prediction of hurricane track and intensity is applied for widespread damage to property. According to the Hong Kong improvement of public safety during hurricane season [23]. observatory, there were approximately eight supertyphoons QuikSCAT measurements over extra-TSs reaching hurricane in 2014 with seven of them influencing the Eastern Asia force wind strength in the North Pacific over a period of countries, i.e., Philippine, China, Taiwan, Korea, and Japan. seven cold seasons from 2001 to 2008 were utilized to In this paper, we studied and analyzed two supertyphoon study the average wind speed distribution within the intense cases, including supertyphoon Haiyan (November 7, 2013; cyclones [24]. The Middle Atlantic Bight had been experi- 895 hPa) and supertyphoon Hagupit (December 4, 2014; enced numerous extra-TCs and hurricanes in recent years, 905 hPa). The intensity of typhoon was partially controlled causing extensive damages along the coastlines. Understanding by the cold fronts. The cold fronts have the potential of the coastal ocean ahead of such events is critical to accurately increasing supertyphoon intensity in winter in The Philip- forecast storm intensity [25]. The extent of the flooded areas pine Sea to improve understanding of cold front-typhoon by the supertyphoon Haiyan in the Philippines was extracted interaction. It is our interpretation that winter typhoons are using Advanced Spaceborne Thermal Emission and Reflection the examples of cold front-typhoon interaction. Cold fronts Radiometer VNIR images. The phase-based analysis was happen during the fall (autumn), spring, and middle of winter. verified with damage levels obtained through visual damage Cold fronts are the leading edge of cooler air masses that inspection using high resolution satellite images [26]. exert a greater temperature gradient between the front and The typhoon features have been studied, and an analysis of typhoon circulation. In addition, we use successive two winters various typhoon images was presented. Recently, an objective with the physical responses of intensifications of typhoons technique has been developed for automatic determination Haiyan (2013) and Hagupit (2014) as examples for analysis. of center of the TC using satellite generated IR images. Both typhoons Haiyan and Hagupit occurred in The Philippine Observations were taken at 30-min interval in three channels: Sea. The associated cold fronts exerted temperature gradient visible channel (0.55Ð0.75 µm), water vapor (WV) absorp- in the northern part of the typhoons as a function of the 3802 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 55, NO. 7, JULY 2017 typhoon intensity. Therefore, except for better visualization of the intensity variety among cold fronts and typhoons, the 3-D profiling was employed to provide more information in the satellite images. The varieties in intensity and size among cold fronts and typhoons have been used to expound extra- TC interaction. When the interactions between the typhoon and cold front happened, enhancements of the typhoons were observed causing typhoons Haiyan and Hagupit to strengthen power and evolve into supertyphoons. Finally, cloud images of typhoons, tracks of typhoons, 3-D profiles of supertyphoons, and centers of supertyphoons among two supertyphoons were used for present analysis.

II. SATELLITE CLOUD IMAGES OF SUPERTYPHOONS HAIYAN (2013) AND HAGUPIT (2014) In this paper, we present case studies happening in The Philippine Sea to improve our understanding of cold front to typhoon interactions. It is our interpretation that typhoons Haiyan and Hagupit are examples of cold front to typhoon interaction. For the first example, typhoon Haiyan originated from several hundred kilometers east-southeast of Pohnpei on November 2, 2013. While generally moving westward, the system developed into a TD on the following day. It was named TS Haiyan on November 4. The system began a period of rapid intensification that brought it to become typhoon on November 5. By November 6, the system was assessed as a supertyphoon on the hurricane wind scale. Thereafter, it continued to intensify on November 7, upgraded the storm’s maximum 10-min sustained winds to 230 km/h (145 mph), with central pressure at 895 hPa, and highest relation to the cyclone. It was observed that typhoon Haiyan became the strongest TC, but gradually weakening and turning northwestward. The typhoon eventually struck northern Vietnam as a severe TS on November 10. As a second example, typhoon Hagupit was chosen. It was upgraded to a TS on December 1, 2014. On December 2, it developed to a severe TS. Later on the same day, it began to track west-northwestward along southern periphery of the subtropical ridge. Typhoon Hagupit went deepening rapidly on December 3, and became a supertyphoon when the system depicted a significant eye. The official reports forecast that typhoon Hagupit would become as strong as typhoon Haiyan, but it failed to intensify further. The failing was explained to be due to the fact that typhoon Hagupit reached peak intensity with 10-min maximum sustaining winds at 215 km/h (130 mph) with central pressure at 905 hPa. However, due to moderate easterly vertical wind shear, the system then started eye-wall replacement cycle and became more asymmetric, Fig. 1. Ten cloud images of typhoon Haiyan (2013) at (a) T1 = 00:32, = : = : when the bulk of the deep convection was displaced over November 5, (b) T2 12 32, November 5, (c) T3 00 32, = : = : the western semicircle. Typhoon Hagupit weakened over the November 6, (d) T4 12 32, November 6, (e) T5 00 32, November 7, (f) T6 = 12:32, November 7, (g) T7 = 00:32, November 8, Philippines on December 7. As typhoon Hagupit slowed (h) T8 = 12:32, November 8, (i) T9 = 00:32, November 9, and = : down and continued a weakening trend, the eye was cloud- (j) T10 12 32, November 9. filled early on December 5. Then, typhoon Hagupit weakened further and downgraded to a typhoon on December 6. A half of leading edge of cooler air masses. They have stronger tem- day later, the system turned west-northwestward. Meanwhile, perature changes at the boundary layer. Since typhoons form cold fronts occurring above typhoons Haiyan and Hagupit when cooler and warmer airs are in convection, the enhanced came from west-northwestward. Cold fronts happen during the interactions between cold fronts and typhoons could cause the fall (autumn), spring, and middle of winter. Cold fronts are the typhoons to become supertyphoons. LEE et al.: FORMATION OF WINTER SUPERTYPHOONS HAIYAN (2013) AND HAGUPIT (2014) 3803

The 3-D proﬁles of typhoons were utilized for investigating in-depth distribution of the cloud top from a surface cloud image [14], [37]. Fig. 1(a)Ð(j) shows the ten-time cloud images of supertyphoon Haiyan (2013) taken at different time periods: at T1 = 00:32, November 5; at T2 = 12:32, November 5; at T3 = 00:32, November 6; at T4 = 12:32, November 6; at T5 = 00:32, November 7; at T6 = 12:32, November 7; at T7 = 00:32, November 8; at T8 = 12:32, November 8; at T9 = 00:32, November 9; and at T10 = 12:32, November 9. Fig. 2(a)Ð(j) shows the ten-time cloud images of supertyphoon Hagupit (2014) taken at different time periods: at T1 = 12:32, December 2; at T2 = 00:32, December 3; at T3 = 12:32, December 3; at T4 = 00:32, December 4; at T5 = 12:32, December 4; at T6 = 00:32, December 5; at T7 = 12:32, December 5; at T8 = 00:32, December 6; at T9 = 12:32, December 6; and at T10 = 00:32, December 7. From Fig. 1(a) and (b), it is evident that the cold fronts appeared above typhoons Haiyan at T1 and T2 at distances about 3300 and 3100 km, respectively, while the interactions straining out typhoon Haiyan started at T3 and T4,asshown in Fig. 1(c) and (d), respectively. Therefore, typhoon Haiyan was enhanced to become stronger typhoon in the period of T5 and T6, as shown in Fig. 1(e) and (f). The supertyphoon Haiyan could be found by inspecting the whole eyes from the time-series images. In Fig. 1(g), the typhoon Haiyan touched the Philippine islands at T7. During the period of T8 and T9, the typhoon Haiyan weakened as observed in Fig. 1(h) and (i), and became a weaker typhoon at T10. Typhoon Haiyan moved toward west within T1ÐT9. The direction of the movement of typhoon Haiyan changed to the northwest after T10. In Fig. 2(a), the cold front located above typhoon Hagupit at the distance about 1400 km at T1. The interactions straining out typhoon Hagupit started at T2, as demonstrated in Fig. 2(b). The typhoon Hagupit was enhanced and formed a stronger typhoon during the period of T3 and T4,asshown in Fig. 2(c) and (d). Till T5, the supertyphoon Hagupit exhibited the whole eye very clearly as seen in Fig. 2(e). In Fig. 2(f), the typhoon Hagupit touched the Philippine islands and declined in intensity at T6. During the period of T7ÐT9, the typhoon Hagupit weakened in Fig. 2(g)Ð(i), and became a weaker typhoon at T10, as given in Fig. 2(j). The tracks of typhoons Haiyan and Hagupit were shown in Fig. 3 in which landforms include Philippines Taiwan, China, Japan, Korea, and Guam with legend. Typhoon Hagupit moved toward west within T1ÐT10. The direction of the movement of typhoon Hagupit continuously remained toward the west after T10.InFig.3,atT0, the origins of typhoons Haiyan and Hagupit located at 152¡50E/05¡12N and 151¡30E/04¡19N, respectively, which were very closed. The cold fronts appeared in northwest of typhoons Haiyan and Hagupit and moved for Fig. 2. Ten cloud images of typhoon Hagupit (2014) at (a) T1 = 12:32, = : = : southeast direction. Both typhoons went through The Philip- December 2, (b) T2 00 32, December 3, (c) T3 12 32, December 3, (d) T4 = 00:32, December 4, (e) T5 = 12:32, December 4, (f) T6 = 00:32, pine Sea in the zone of 112¡52EÐ146¡11E. The lowest values December 5, (g) T7 = 12:32, December 5, (h) T8 = 00:32, December 6, of central pressures of typhoons Haiyan and Hagupit happened (i) T9 = 12:32, December 6, and (j) T10 = 00:32, December 7. within the path at the positions of 128¡41E/10¡16N (at T6) and 131¡06E/11¡00N (at T5), respectively. Both typhoons impact the area of Manila (120¡58E/14¡35N). The traveling of central pressure was 895 hPa (T6) for typhoon Haiyan, distances in the period of T1ÐT10 were 2000 km for typhoon and 905 hPa (T5) for typhoon Hagupit. The very low central Haiyan and 1680 km for typhoon Hagupit. The lowest value pressure implies very high wind speed with typhoon intensity 3804 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 55, NO. 7, JULY 2017

Fig. 3. Tracks of typhoons Haiyan (2013) and Hagupit (2014) and cold fronts with geography indicated in the background. Pressures of the two typhoons were given along the tracks. Supertyphoons were formed very closed as easily observed near the lowest pressures 895 and 905 hPa for Haiyan (2013) and Hagupit (2014), respectively. Fig. 5. Cold front and typhoon Hagupit (2014) at (a) 04:32, December 3, (b) 08:32, December 3, (c) 12:32, December 3, and (d) 16:32, December 3. Haiyan was about 1300 km in Fig. 4(a), and the distance between cold front and typhoon Hagupit is about 1500 km in Fig. 5(a). The interactions straining out typhoons Haiyan and Hagupit started and were observed. As the distances were closer, the interactions straining out typhoons Haiyan and Hagupit increased, and the corresponding convective currents were presented in Figs. 4(c) and (d) and 5(c) and (d), respectively. Finally, the distance between cold front and typhoon Haiyan is about 600 km, as shown in Fig. 4(d), and the distance between cold front and typhoon Hagupit is about 700 km, as demonstrated in Fig. 5(d). The in-depth temperature distributions of cloud top from a surface cloud image presented the vertical variations of typhoons. The 3-D profiles of typhoons were constructed with 2-D cloud images and the in-depth temperature distributions. With the images in Fig. 1(f) and Fig. 2(e), the 3-D profiles of two supertyphoons were shown in Fig. 6(a) and (b), respec- Fig. 4. Cold front and typhoon Haiyan (2013) at (a) 00:32, November 6, tively. For a rectangular coordinate (x, y,andh), the projective (b) 04:32, November 6, (c) 08:32, November 6, and (d) 12:32, November 6. cloud image is located on the xy plane, and the temperature of the cloud top from a surface cloud image is presented by the increased from “very strong” to “violent.” Each of the two height along the h-axis. The scales of 2-D cloud images in the supertyphoons approximately consisted of two paths in which xy plane are km. The vertical scale of the cloud top from a the first one moved with western direction and the second surface cloud image in the h-axis represented temperature (¡C) − + − one inclined with northern direction. The supertyphoons were from 77 ¡C to 20 ¡C; 77 ¡C is the highest-elevation + formed at the turning point of the two paths. From the origin, temperature (white) on the surface of cloud top and 20 ¡C the first paths were with distances about 2330 and 2050 km is the temperature (dark) on the surface of sea water. The for supertyphoons Haiyan and Hagupit, respectively, which 500-km diameter of 3-D profile of typhoon Haiyan is went through The Philippine Sea and Philippine islands. Both observed and the 300-km diameter of 3-D profile of typhoon typhoons weakened after touching and going through The Hagupit is presented. On the top-view of supertyphoons Philippine islands. in Fig. 6(a) and (b), the eyes of supertyphoons were observed For displaying the relation between cold front and typhoon in Fig. 7(a) and (b), respectively. The eye with 15-km depth of typhoon Haiyan is shown and the eye with 12-km depth of Haiyan clearly, four-time cloud images of T3 and T4 with hourly intervals (at 00:32, 04:32, 08:32, and 12:32 on typhoon Hagupit is expressed. Due to the IR detectors satu- − November 6) before the occurrences of the lowest central rated at 77 ¡C, they are the highest intensity of supertyphoon pressure are presented in Fig. 4(a)Ð(d), respectively. On the with white area designating saturated area. other hand, four-time cloud images of T2 and T3 with hourly intervals (at 04:32, 08:32, 12:32, and 16:32 on December 3) for III. DISCUSSION AND ANALYSIS typhoon Hagupit are presented in Fig. 5(a)Ð(d), respectively. The evolutions of supertyphoons were discussed in It is observed that the distance between cold front and typhoon Section II, including the characteristics of pressure, average LEE et al.: FORMATION OF WINTER SUPERTYPHOONS HAIYAN (2013) AND HAGUPIT (2014) 3805

Fig. 6. 3-D profile of supertyphoons. (a) Haiyan at T6. (b) Hagupit at T5. Fig. 7. Top views of the centers of supertyphoons. (a) Haiyan at T6. (b) Hagupit at T5. speed of typhoon movement, distance between cold front and pressures at T5 and T7. The average speed of movement typhoon, eye depth, and saturation area. Since central pressure decreased step by step from 30 to 1.5 km/h between T1 dominates wind speed and intensity of typhoon, the effects and T9 in which two equal average speeds of movement are could happen between cold front and typhoon when the 21and13km/hduringT4 and T6, respectively, and increased environmental cold front approaches the typhoon, indicating to 5 km/h during T9 and T10. In Fig. 9(b), the time series the variation in pressure. Thus, the parameters of average of pressure versus cold front distance for typhoon Hagupit at speed of typhoon movement and distance between cold front T1ÐT10 is shown. The cold front and typhoon Hagupit located and typhoon related to pressures are analyzed in Figs. 8Ð11, closely and the distances swung from 1400 to 730 km at T4, respectively. from 730 up to 1250 km at T6, and from 1250 down to 900 km In Fig. 8(a), the time series of pressure versus average at T10. There are two enhancement intervals in the path of speed of typhoon Haiyan movement at T1ÐT10 is presented. typhoon Hagupit. The pressures decreased gradually from 985 hPa (T1) to Both cold front distance and average speed of movement are 895 hPa (T6), and then increased slowly to 945 hPa (T10) [38]. considered, and the time series of cold front distance versus Except the increased average speed within T5 and T6,there average speed of movement is shown in Fig. 10(a) and (b). are two average speeds of 25 and 18 km/h during T1ÐT5 and In Fig. 10(a), the cold front located apart and the cold front T7ÐT10, respectively. In Fig. 9(a), the time series of pres- distances differ a great deal and decrease from 3300 to 450 km. sure versus cold front distance from typhoon Haiyan at The cold front approaches typhoon Haiyan with approximately T1ÐT10 is shown. The cold front distances decrease from the same average speed. By the stable opposite movement 3300 to 450 km. It is evident that the cold front approaches between the cold front and typhoon Haiyan, the significant typhoon Haiyan with approximately the same average speed. enhancement could happen in the distances of 2600Ð1500 km. Due to the stable opposite movement between cold front and The average speed of movement kept constant of 25 km/h at typhoon Haiyan, significant enhancement could happen in the the start period, increased up to 30 km/h during interaction, interval of T3ÐT6. and then decreased 15 km/h. In Fig. 10(b), the cold front In Fig. 8(b), the time series of pressure versus average and typhoon Hagupit locate closely and the distances swing speed of movement of typhoon Hagupit in T1ÐT10 is presented. a narrow range from 1400 to 730 km. There are two changes The pressures decreased from 980 hPa (T1) to 905 hPa (T5), during the enhancement intervals. However, the average speeds then increased to 925 hPa (T6), decreased to 915 hPa (T7), of movement are varied obviously. The average speed of and increased to 945 hPa (T10) finally. There are two lower movement decreased step by step from 30 to 2 km/h between 3806 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 55, NO. 7, JULY 2017

Fig. 8. Time series of pressure versus average speed of movement. Fig. 9. Time series of pressure versus cold front distance. (a) Typhoon T T T T (a) Typhoon Haiyan at 1Ð 10. (b) Typhoon Hagupit at 1Ð 10. As typhoon Haiyan at T ÐT . (b) Typhoon Hagupit at T ÐT . intensity and speed of movement are critical information in terms of weather 1 10 1 10 forecast and they are very much inﬂuenced by central pressure, correlations between pressure and speed of movement of typhoon must be well understood especially at the occurrence of enhancement.

T1 and T9 in which two equal average speeds of movement are 21and13km/hduringT4 and T6, respectively, and increased to 5 km/h finally. Therefore, the cold front distance and the average speed of movement are the dominant parameters for the formation of winter supertyphoons. The interactions happen between cold front and typhoon when the environmental cold front approaches the typhoon, enhancing typhoon to evolve into supertyphoon. The enhancement can be represented by wind speed and intensity of supertyphoon. The quantified wind speed and intensity related to self-circulation of typhoons were obtained from the field measurements conventionally. With advancement in technology, the enhancement could be observed by the temperature of cloud top and the central eye configuration in the cloud images. Due to the fact that IR detector is saturated at the temperature of −77 ¡C, there is no variation below the temperature. Therefore, −77 ¡C is the highest-elevation temperature (white) on the surface of cloud top. Connecting the saturated points to a contour similar to the land-form feature of the topographic map, we designate it as a saturation area. The results of enhancement are presented in Figs. 11(a) and (b) and 12(a) and (b). Fig. 11(a) and (b) is the time series of pressure versus saturation area for typhoons Haiyan and Hagupit, Fig. 10. Time series of cold front distance versus average speed of movement. (a) Typhoon Haiyan at T1ÐT10. (b) Typhoon Hagupit at T1ÐT10. respectively, at T1ÐT10. The saturation areas of typhoon Haiyan 2 are 850Ð1150 km from T4 to T7. The saturation areas of In cloud images, the funnel-shaped convections occur in 2 typhoon Hagupit are 800Ð1050 km from T3 to T5 and the center of self-circulation associated with variation of 2 920 km at T7. The saturation areas of typhoon Haiyan are eye depth. Fig. 12(a) and (b) is the time series of pres- obviously wider than those of typhoon Hagupit. sure versus eye depths for typhoons Haiyan and Hagupit, LEE et al.: FORMATION OF WINTER SUPERTYPHOONS HAIYAN (2013) AND HAGUPIT (2014) 3807

to T8. The eye depths of typhoon Haiyan are larger than those of typhoon Hagupit. The supertyphoon Haiyan presents higher intensity than supertyphoon Hagupit. By inspecting the results presented in the literature, the cloud mountain (3-D surface) was constructed by pixels within cloud bands, leading to mountain shape with the form of a helix. By using the mountain-climbing algorithm [35], the spiral pattern was extracted.

IV. CONCLUSION

This paper utilized the available remote sensing imagery and image processing techniques to observe and analyze the interactions between typhoon and cold front in Western Pacific Ocean, and the evolution of typhoon into supertyphoon. The conventional typhoon was formed by convection between the warmer sea water and cold air flow in summer time. Owing to increased climate variability in recent decades, the warmer sea water occurs in winter occasionally when accessing the cold air flow to produce the first convection and the typhoon is happened at first. Then, the cold front, the leading edge of cooler air masses in typhoon environment, exerts a greater Fig. 11. Time series of pressure versus saturation area. (a) Typhoon Haiyan temperature gradient between the front and typhoon circulation at T1ÐT10. (b) Typhoon Hagupit at T1ÐT10. to produce a second convection, which increases the intensity of typhoon and affects the track of typhoon. The purpose of this paper is to improve our understanding and predictability of the tracks and profiles of supertyphoons and effects of cold fronts on typhoons in order to reduce the influence of typhoons on the regions of concern. Two supertyphoons Haiyan (2013) and Hagupit (2014) happening in successive two winters were used as examples. When the interactions between the typhoon and cold front happened, the cold fronts caused typhoons Haiyan and Hagupit to strengthen their power and evolve into supertyphoons. With similar tracks from very close origins, both typhoons went through The Philippine Sea along a narrower zone. The lowest values of their central pressures that were observed happened at positions nearby. The supertyphoon Haiyan showed higher intensity than supertyphoon Hagupit. Four parameters, average speed of typhoon movement, distance between cold front and typhoon, and saturation area and eye depth related to pressures were used for analysis. It is found that the cold front distance and average speed of movement are the dominating parameters for forming the winter supertyphoons. Also, the results showed the cold fronts concurred with typhoons in winter, and then interacted with each other through enhancement. The enhancement caused the typhoon to strengthen intensity and evolve into supertyphoon. The observation of cold front’s effects on typhoon represents a new aspect, and hence understanding of weather systems’ interaction, especially crucial for the hazardous supertyphoon. Fig. 12. Time series of pressure versus eye depth. (a) Typhoon Haiyan at Meanwhile, it shall be of great value to further investigate T ÐT . (b) Typhoon Hagupit at T ÐT . 1 10 1 10 the similar behaviors of the two typhoons concerned in this paper. Note that use of remote sensing imagery and image respectively, at T1ÐT10. The eye depths of typhoon Haiyan processing techniques is insufficient to reveal the physical are 14.5Ð15 km from T4 to T6. The eye depths of typhoon mechanism behind the behaviors. Instead, numerical modeling Hagupit are 12Ð13 km from T4 to T5 and 2Ð5 km from T6 will be required and recommended for the further studies. 3808 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 55, NO. 7, JULY 2017

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Yuei-An Liou (S’91–M’96–SM’01) received the Ji-Chyun Liu was born in Taiwan in 1951. B.S. degree in electrical engineering from National He received the B.S., M.S., and Ph.D. degrees from Sun Yat-sen University (NSYSU), Kaohsiung, the Chung-Cheng Institute of Technology, Taoyuan, Taiwan, the M.S.E. degree in electrical engineer- Taiwan, in 1974, 1981, and 1993, respectively. ing (EE), the M.S. degree in atmospheric and From 1981 to 2003, he was an Instructor, an space sciences, and the Ph.D. degree in EE and Associate Professor, and a Professor with the Chung- atmospheric, oceanic, and space sciences from the Cheng Institute of Technology. He was a Profes- University of Michigan, Ann Arbor, MI, USA, sor with the Chien Hsin University of Science in 1987, 1992, 1994, and 1996, respectively. and Technology, Taoyuan, Taiwan, from 2003 to From 1989 to 1990, he was a Research Assis- 2017. His research interests include satellite cloud tant with the Robotics Laboratory, National Taiwan images, image processing, microwave integrated cir- University, Taipei, Taiwan. From 1991 to 1996, he was a Graduate Student cuit design, wideband ﬁlter, wideband ampliﬁer, chaotic oscillator, broadband Research Assistant with the Radiation Laboratory, University of Michigan, antenna, fractal antenna, active integration antenna, broadband absorber, where he developed land-air interaction and microwave emission models ceramic metamaterial, EMI/EMC, and microwave instrumentation and for prairie grassland. He joined the Center for Space and Remote Sensing measurement. Research (CSRSR), National Central University (NCU), Taoyuan, Taiwan, as a Faculty Member, in 1996, and the Institute of Space Sciences, NCU, in 1997, where he is currently a Distinguished Professor. He served as the Division Director of the Science Research Division, National Space Organiza- tion (NSPO), Hsin-Chu, Taiwan, in 2005, and continued to serve as an Advisor to NSPO in 2006. From 2006 to 2007, he was a Chair Professor and the Dean of the College of Electrical Engineering and Computer Science, Chien Hsin University of Science and Technology, Taoyuan. From 2007 to 2010, he was the Director of CSRSR, NCU. From 2010 to 2016, he was the Founder and the President of the Taiwan Group on Earth Observations (TGEO), Hsin-Chu, Taiwan. From 2014 to 2016, he was the President of the Taiwan GIS Center. He is the Founder of TGEO, where he has been the Honorary President, Ching-Tsan Chiang was born in Taipei, since 2016. He has been the Honorary President of the Vietnamese Experts Taiwan, in 1961. He received the B.S. degree in Association in Taiwan since 2016. He is a Principal Investigator on many electrical engineering from Tamkang University, research projects sponsored by the Ministry of Science and Technology and New Taipei City, Taiwan, in 1989, and the M.S. the Council of Hakka Affairs of Taiwan. He has over 100 referral papers and and Ph.D. degrees in electrical and computer over 200 international conference papers. His research interests include GPS engineering from University of Missouri, Columbia, meteorology and ionosphere, remote sensing of the atmosphere, land surface, MO, USA, in 1992 and 1995, respectively. and polar ice, and land surface processes modeling. In 1995, he became an Associate Professor with Dr. Liou is a member of the American Geophysical Union, the American the Electrical Engineering Department, Chien Hsin Meteorological Society, and the International Association of Hydrological University of Science and Technology, Taoyuan, Sciences. He was an elected fellow of the IET in 2015. He was an Taiwan. His research interests include neural elected Corresponding Member and Member of the International Academy of network structures, neurocontrol, computational intelligence, photovoltaic, Astronautics in 2009 and 2014, respectively. He received the Honorary Life microwave measurement, and learning control systems. Member of The Korean Society of Remote Sensing in 2007. He was an elected Foreign Member, Russian Academy of Engineering Sciences in 2008. He was a recipient of Annual Research Awards from National Science Council (NSC) in 1998, 1999, and 2000, a recipient of the First Class Research Awards from NSC in 2004, 2005, and 2006, and a recipient of NCU Outstanding Research Awards in 2004, 2006, 2007, and 2008. He received the Contribution Award to FORMOSAT3 National Space Mission by NSPO in 2006. He received the Outstanding Alumni Awards by the University of Michigan Alumni Association in Taiwan and NSYSU in 2008. He received the life-time honor Distinguished Professor from National Central University in 2010. He is a Referee of the IEEE TRANSACTIONS ON GEOSCIENCE REMOTE SENSING, the International Journal of Remote Sensing, Earth, Planets, and Space,the Kuan-Dih Yeh was born in Taoyuan, Taiwan, Remote Sensing of Environment,theJournal on Geophysics Research,andthe in 1965. He received the B.S. and M.S. degrees in Annales Geophysicae. He has been a member of the Editorial Advisory Board electrical engineering from the National Taiwan Uni- for GPS Solutions since 2001 and the International Journal of Navigation versity of Science and Technology, Taipei, Taiwan, and Observation since 2011. He serves as an Editor of the Journal of in 1991 and 1993, respectively. He is currently Aeronautics, Astronautics and Aviation since 2009, an Associate Editor of the pursuing the Ph.D. degree with the Department IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS of Electrical Engineering, Chang Gung University, AND REMOTE SENSING from 2008 to 2014, and Remote Sensing Technology Taoyuan, Taiwan. and Application since 2011. He is a Lead Guest Editor for special issues In 1993, he joined the Electrical Engineering of the GPS Solutions in 2005 and 2010, the IEEE TRANSACTIONS ON Department, Chien Hsin University of Science and GEOSCIENCE AND REMOTE SENSING in 2008, the Terrestrial, Atmospheric Technology, Taoyuan, as a Faculty Member, where and Oceanic Sciences in 2014, Advances in Meteorology from 2014 to he is currently an Assistant Professor. His research interests include microwave 2015, the Atmospheric Research in 2015, and the Remote Sensing journal in circuit design, circuit model and analysis, microwave measurement, power 2015, 2016, and 2017. quality, and static series/parallel compensators.