Rapid weakening of Typhoon Chan-Hom (2015) in a monsoon gyre

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1 PUBLICATIONS RESEARCH ARTICLE Key Points: Typhoon Chan-Hom (2015) underwent the rapid weakening even under a large-scale environment favorable for intensification The rapid weakening of tropical cyclone intensity can result from its interaction with a monsoon gyre The strong convection in the monsoon gyre prevented the inward energy transportation into Chan-Hom, leading to the collapsing of the eyewall Correspondence to: J. Liang, lj_keke@sina.com Citation: Liang, J., L. Wu, G. Gu, and Q. Liu (2016), Rapid weakening of Typhoon Chan-Hom (2015) in a monsoon gyre, J. Geophys. Res. Atmos., 121, , doi:. Received 11 APR 2016 Accepted 5 AUG 2016 Accepted article online 10 AUG 2016 Published online 26 AUG American Geophysical Union. All Rights Reserved. Rapid weakening of Typhoon Chan-Hom (2015) in a monsoon gyre Jia Liang 1,2, Liguang Wu 1,3, Guojun Gu 2, and Qingyuan Liu 1 1 Key Laboratory of Meteorological Disaster, Ministry of Education/Pacific Typhoon Research Center, Nanjing University of Information Science and Technology, Nanjing, China, 2 Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland, USA, 3 State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China Abstract A monsoon gyre is a low-frequency cyclonic circulation over the western North Pacific, which plays important roles in tropical cyclone formation and motion. This study shows that the interaction between a monsoon gyre and a tropical cyclone can lead to a sudden weakening of the tropical cyclone through an observational analysis of Typhoon Chan-Hom (2015). Typhoon Chan-Hom (2015) initially moved westward along ~10 N and sharply turned northeastward in the Philippine Sea at 0000 UTC 3 July. Its intensity decreased by 10.3 m s 1 within 12 h during the sudden northward turn. Such a rapid weakening event was failed to predict in all of the operational forecasts. It is found that Chan-Hom was coalescing with a large-scale monsoon gyre on the intraseasonal (15 30 day) timescale, while it experienced the sudden track change and rapid intensity weakening. The weak and loosely organized convection on the eastern side of the monsoon gyre at 1200 UTC 2 July rapidly enhanced into the well-organized convection within 6 h. The strong convection maintaining from 1800 UTC 2 July to 0600 UTC 3 July enhanced inflows outside the radius of 500 km from the tropical cyclone center, which prevented the inward transportation of mass and moisture into Chan-Hom, leading to the collapsing of the eastern part of the eyewall. As a result, Chan-Hom underwent the rapid weakening even under a large-scale environment favorable for intensification. The study suggests that the rapid weakening of a tropical cyclone can result from its interaction with a monsoon gyre. 1. Introduction Despite decades of intense research on tropical cyclone intensity and improvements in numerical models, there are still large forecasting errors in the operational forecast especially when tropical cyclones undergo sudden intensity changes [Emanuel et al., 2004; Elsberry et al., 2007; Rappaport et al., 2009; Rogers et al., 2006, 2013; Gao et al., 2014; X. Wang et al., 2015]. While the rapid intensification (RI) of tropical cyclones is regarded as one important source of the forecasting errors, which has been investigated by observational analyses and numerical simulations [e.g., Gray, 1968; Holliday and Thompson, 1979; Kaplan and DeMaria, 2003; Wang and Zhou, 2008; Kaplan et al., 2010; Chen et al., 2011; Shu et al., 2012; Chen and Zhang, 2013; X. Wang et al., 2015], forecast errors are also associated with rapid weakening of tropical cyclones, especially over the low-latitude open ocean [Brand, 1973; Titley and Elsberry, 2000; Wang, 2012; Wood and Ritchie, 2015]. Therefore, understanding of the rapid weakening is important to improve intensity forecasts of tropical cyclones. Many observational and numerical studies focused on effects of secondary eyewall and outer spiral rainbands on the weakening of tropical cyclones [e.g., Willoughby et al., 1982; Barnes et al., 1983; Powell, 1990a, 1990b; Camp and Montgomery, 2001; Houze et al., 2007; Wang, 2002, 2009, 2012; Sitkowski et al., 2011; Li and Wang, 2012a, 2012b], while the effects of environmental factors on tropical cyclone intensity were well known [e.g., Gray, 1968, 1979; Kaplan and DeMaria, 2003; Cheung, 2004; Emanuel et al., 2004; Lowag et al., 2008; Kaplan et al., 2010; Shu et al., 2012; Y. Wang et al., 2015; Wood and Ritchie, 2015]. Willoughby et al. [1982] found that tropical cyclones experienced weakening during the process of secondary eyewall replacement. The formation of secondary eyewall prevents mass and moisture in boundary layer inflows from entering into the inner eyewall, leading to the collapsing of the inner eyewall and the weakening of tropical cyclones. The weakening associated with secondary eyewall replacement has been verified in observational and modeling studies [Camp and Montgomery, 2001; Zhu et al., 2004; Houze et al., 2007; Sitkowski and Barnes, 2009; Sitkowski et al., 2011]. Moreover, Barnes et al. [1983] argued that spiral rainbands may act as a barrier to block the inflow of tropical cyclones. Some studies suggested that outer spiral rainbands may prevent tropical cyclones from intensifying through reducing mass and moisture convergence in the inner core region, LIANG ET AL. RAPID WEAKENING IN A MONSOON GYRE 9508

2 weakening the tangential wind near the eyewall, and cooling and drying the boundary layer inflow [Powell, 1990a, 1990b; Hence and Houze, 2008; Wang, 2009; Li and Wang, 2012a, 2012b]. Titley and Elsberry [2000] found that the rapid weakening of Typhoon Flo (1990) over the midlatitude open ocean was associated with a shallower secondary circulation caused by the downward extending of the upper level eddy flux convergence. In the western North Pacific (WNP) basin, tropical cyclones are sometimes embedded in a monsoon gyre, which is a specific pattern of the low-level monsoonal circulation on the intraseasonal oscillation (ISO) timescale [Lander, 1994; Carr and Elsberry, 1995]. The important influences of monsoon gyres on tropical cyclone formation and motion have been investigated through observational analysis and numerical modeling [Lander, 1994; Carr and Elsberry, 1995; Ritchie and Holland, 1999; Chen et al., 2004; Wu et al., 2011a, 2011b, 2013; Liang et al., 2011, 2014; Liang and Wu, 2015; Bi et al., 2015]. The coalescence between a monsoon gyre and a tropical cyclone can enhance Rossby wave energy dispersion of the monsoon gyre, thus inducing strong southwesterly flows in the southeastern or eastern periphery of the monsoon gyre [Carr and Elsberry, 1995; Liang et al., 2011; Wu et al., 2011a, 2011b, 2013]. The convection will rapidly develop and strengthen in the downwind end of strong southwesterly flows [Liang et al., 2014]. When a tropical cyclone is embedded in a monsoon gyre, the intensity of the tropical cyclone can be affected due to the associated change in the eyewall structure and rainbands. In this study, we will demonstrate that Typhoon Chan-Hom (2015) experienced a rapid weakening over a tropical open ocean during its coalescence with a lowfrequency monsoon gyre. The rapid weakening event was not predicted by any of three main forecasting agencies for the WNP basin. The rest of the paper is organized as follows. The data and analysis methods used in this study are described in section 2. The intensity evolution of Typhoon Chan-Hom (2015) and operational forecasts are discussed in section 3. Sections 4 and 5 identify possible influences of environmental factors and the low-frequency monsoon gyre on the rapid weakening of Chan-Hom, respectively, followed by a brief summary in section Data and Analysis Methods In this study, the environmental fields in which Typhoon Chan-Hom was embedded are derived from the National Centers for Environmental Prediction (NCEP) Final (FNL) Operational Global Analysis data. The data are available at the surface and 26 pressure levels (from 1000 to 10 hpa) on 1 1 grids at every 6 h. The sea surface temperature data are obtained from the NOAA daily Optimum Interpolation Sea Surface Temperature (OISST) product with the spatial resolution of 0.25 [Reynolds, 2007]. The data for examining the rainbands are from the Tropical Rainfall Measuring Mission (TRMM, Version 7 3B42) real-time-derived 3-hourly precipitation product on grids and the infrared (IR-band1) satellite data derived from the Chinese geostationary meteorological Fengyun-2F (FY-2F) satellite with the spatial resolution of 5 km. We also use the NOAA Multiplatform Tropical Cyclone Surface Winds Analysis (MTCSWA) product to examine the structure change of Typhoon Chan-Hom [Knaff et al., 2011; Wu et al., 2015]. The product centered at the tropical cyclone center contains high-resolution ( grids) wind data for each tropical cyclone at 6 h intervals. Note that the real-time data are constructed with 18 h observed and 6 h forecasted intensity and centers. Uncertainty increases when rapid changes in the tropical cyclone track and intensity occur. For this reason, we only use the wind data within 12 h before and after the rapid weakening in this study. The typhoon information is from the Joint Typhoon Warning Center (JTWC), including typhoon center positions (the latitude and longitude) and intensities (maximum sustained wind speed and minimum sea level pressure) at 6 h intervals. Operational forecasts for Typhoon Chan-Hom are from JTWC, Japan Meteorological Agency (JMA), and China Meteorological Administration (CMA) to indicate the skills of intensity forecasting for this typhoon. The forecast lead time is 72 h for JMA and 120 h for JTWC and CMA. 3. Intensity Evolution of Typhoon Chan-Hom (2015) and Operational Forecasts Typhoon Chan-Hom developed from a westerly wind burst with the counterpart Tropical Cyclone Raquel in the Southern Hemisphere. Figure 1 shows the JTWC track of Chan-Hom from 1200 UTC 30 June to 1800 UTC 5 July Chan-Hom initially moved westward along ~10 N and made a sharp northeastward turn to the east LIANG ET AL. RAPID WEAKENING IN A MONSOON GYRE 9509

3 Figure 1. The track of Typhoon Chan-Hom (2015) from the JTWC realtime data with dots indicating 6 h intervals. The red color highlights the rapid weakening period from 1800 UTC 2 July to 0600 UTC 3 July of the Philippine Island at 0000 UTC 3 July. After the sudden track change, Chan-Hom took a northwestward track and made landfall in Zhejiang Province, China in early July 11. The landfall location was roughly 140 km south of Shanghai that is the most important city in the economy of China and Chan-Hom was the strongest typhoon that affected this city over at least 35 years. More than million people were evacuated due to the approaching of the typhoon, causing about 1.5 billion U.S. dollars in damage. Figures 2a 2c depicts the intensity evolution of Typhoon Chan-Hom from 1200 UTC 30 June to 1800 UTC 5 July 2015 from the JTWC, JMA, and CMA data sets. Note that the maximum sustained wind speeds in the JTWC and CMA data sets are higher than that in the JMA data set. The difference may be due to the different average time of the sustained wind speed. The average time is 1 min and 2 min in the JTWC and CMA data sets, while it is 10 min in the JMA data set. Chan-Hom intensified into a tropical storm at 1800 UTC 1 July and a typhoon in late 2 July in the JTWC and CMA data sets. Its intensity decreased rapidly Figure 2. Observed (black lines with dots) and forecasted (color lines with dots) (a c) intensities and (d f) tracks from JTWC (Figures 2a and 2d), JMA (Figures 2b and 2e), and CMA (Figures 2c and 2f). The forecasts are started at 0000, 0600, 1200, 1800 UTC 2, and 0000 UTC 3 July and last for 120 h in Figures 2a and 2c and 72 h in Figure 2b. Vertical dashed lines in Figures 2a 2c indicate the start times of rapid weakening. LIANG ET AL. RAPID WEAKENING IN A MONSOON GYRE 9510

4 during the following 12 h. Its maximum sustained wind speed decreased 10.3 m s 1 in the JTWC and CMA data sets and 5.1 m s 1 in the JMA data set over the 12 h period. Based on the JTWC and JMA data sets, Chan-Hom degraded into a tropical storm in early 3 July and maintained the tropical storm intensity by 5 July. The weakening process of Typhoon Chan-Hom was accompanied with the evident enlargement of the eye size and the weakening of the surface wind speed. Figure 3 shows surface wind fields averaged in 12 h before and after the weakening of Chan-Hom from NOAA Multiplatform Tropical Cyclone Surface Winds Analysis (MTCSWA) data. In this study, we use the radius of the maximum sustained wind speed to define the size of the tropical cyclone eye. The eye of Chan-Hom can be identified with a radius of less than 50 km prior to the weakening with the maximum sustained wind speed exceeding 30 m s 1 (Figure 3a), whereas the radius of the eye was enlarged into about 80 km after the weakening (Figure 3b). Figure 3. Surface wind fields (shaded and vector, m s 1 ) from NOAA MTCSWA data averaged in 12 h (a) prior to and (b) after the rapid weakening of Chan-Hom, with typhoon symbols indicating centers of Chan-Hom. Black circles are 50 km away from centers. By examining intensity changes of Atlantic tropical cyclones over the open ocean from 1982 to 2010, DeMaria et al. [2012] found that the intensity decrease of 20 kt (10.3 m s 1 ) represented the nearly 5th percentile of the distribution, and thus, they defined the rapid weakening when the tropical cyclone intensity decreases by 10.3 m s 1 or more in 24 h. We can see that the weakening of Chan-Hom can be classified as a rapid weakening event since the intensity decrease reached 10.3 m s 1 only in 12 h. Such a rapid weakening event in the tropical cyclone intensity was failed to predict in the operational forecasts. The performance of the operational forecasts from JTWC, JMA, and CMA are shown in Figures 2a 2c. All of the forecasts suggested that Chan-Hom would reach the super typhoon intensity (exceeding 42 m s 1 )on4 5 July. The rapid weakening of Chan-Hom was failed to predict. The intensity forecasts were given at 0000, 0600, 1200, 1800 UTC 2 July, and 0000 UTC 3 July 2015, respectively, and valid for 120 h for the JTWC and CMA forecasts and 72 h for the JMA forecast. The JMA and CMA forecasts indicated that Chan-Hom would intensify persistently during the forecasting periods and might exceed 42 m s 1 on 4 July (Figures 2b and 2c). Even an RI event was suggested by JMA in the forecast at 1800 UTC 2 July. The intensity of Chan-Hom was predicted to increase from 28 m s 1 to 46 m s 1 from 1800 UTC 2 July to 1800 UTC 3 July. The JTWC persistently forecasted the intensification of Chan-Hom in its 120 h forecasts given at 0000, 0600, and 1800 UTC 2 July (Figure 2a), despite a weakening of 2.5 m s 1 in the first 24 h forecast at 0000 UTC 3 July. LIANG ET AL. RAPID WEAKENING IN A MONSOON GYRE 9511

5 Figure 4. Forecasted distance errors in tracks for 24 h forecasts from JTWC (black), JMA (red), and CMA (blue). The black horizontal line indicates the distance error of km. Black dashed lines outline the period of the rapid weakening and the red dashed line highlights the time of the sudden northward turn of Chan-Hom. Although the large forecasting errors in the rapid weakening, the northward turning track of Chan-Hom was predicted. Figures 2d 2f show forecasted tracks at the same time from JTWC, JMA, and CMA. All of the forecasts showed that Chan-Hom would undergo a northward turn around 10 N, 148 E in late 2 July 2015 with turning angles smaller than the observation. Figure 4 further examines distance errors for 24 h forecasts from JTWC, JMA, and CMA. The 24 h distance errors during the rapid weakening were all close to or lower than the averaged distance error of km, which was the average 24 h distance error for all of the track forecasts from NMC/CMA during suggested by Wu et al. [2013]. These small distance errors cannot make the forecasted Chan- Hom move into a region with different environmental conditions. It is suggested that the failed forecasts for the rapid weakening of Chan-Hom were not primarily associated with the forecast errors in tracks. A special feature for the rapid weakening event was that it was accompanied with a sudden northward track change around 10 N in the Philippine Sea. Previous studies have shown that such a sudden track change can result from the interaction between a tropical cyclone and a large-scale low-frequency monsoon gyre. The track change usually occurs during the coalescence of the tropical cyclone with the monsoon gyre [Carr and Elsberry, 1995; Liang et al., 2011; Wu et al., 2011a, 2011b, 2013; Liang and Wu, 2015]. To our knowledge, no attention has been paid to the influence of the coalescence on tropical cyclone intensity changes. Typhoon Chan-Hom provides a good opportunity for us to study how a monsoon gyre can affect the tropical cyclone intensity. 4. Influences of Environmental Factors We first examine the environmental factors for intensity change. Figures 5 7 show the main environmental factors associated with the intensity change of Typhoon Chan-Hom. Figure 5 shows the sea surface temperature (SST) field from the NOAA OISST product, which is average during 2 3 July. Note that the SST pattern from the NOAA OISST product shows little change during 2 3 July. During 2 3 July, Chan-Hom moved over the open ocean with the SST higher than 28 C. During the 12 h rapid weakening, it actually moved into a warmer area with the SST above 29 C. It is clear that the SST change is not responsible for the rapid weakening of Chan-Hom. Y. Wang et al. [2015] recently suggested that the probability of tropical cyclone weakening is lowest when the SST is higher than 29 C. Figure 5. The horizontal pattern of sea surface temperature (shaded and contours, C) averaged during 2 3 July The line with typhoon symbols dedicates the track of Typhoon Chan-Hom with 6 h intervals from 0000 UTC 2 July to 0000 UTC 4 July 2015, and the red color highlights the rapid weakening period from 1800 UTC 2 July to 0600 UTC 3 July The environmental humidity in the lower and middle troposphere is an important parameter for tropical cyclone LIANG ET AL. RAPID WEAKENING IN A MONSOON GYRE 9512

6 intensity change. Usually, the middle and low-level relative humidity higher than 50% is conducive to trigger the convection during the tropical cyclone formation and development [Cheung, 2004; X. Wang et al., 2015]. The composite study of Shu and Wu [2009] showed that tropical cyclones would weaken when the middle-level dry air intrudes into the southwestern region within the radius of 360 km from the tropical cyclone center and the inner core of tropical cyclones. Figure 6 shows the lowlevel and midlevel relative humidity fields at 1800 UTC 2 July and 0000 and 0600 UTC 3 July 2015, which are averaged over the layers between 850 hpa and 700 hpa and the layers between 700 hpa and 500 hpa, respectively. Chan-Hom was embedded in a moist middle- and low-level environment with relative humidity higher than 80% within 360 km from the tropical cyclone center during the rapid weakening. It indicates no dry air intrusion into Chan-Hom when it underwent the rapid weakening. The vertical wind shear is also one of the most important parameters to tropical cyclone intensity change, with a high (low) value unfavorable (favorable) for the tropical cyclone intensification [e.g., Merrill, 1988; DeMaria and Kaplan, 1996; Frank and Ritchie, 1999, 2001; Kaplan and DeMaria, 2003; DeMaria et al., 2005; Riemer et al., 2010; Y. Wang et al., 2015]. Usually, the difference of horizontal winds between 200 and 850 hpa is used to measure the vertical wind shear [e.g., Emanuel, 2000; Paterson et al., 2005; Zeng et al., 2008, 2010]. However, recent observational researches showed that the low-level vertical wind shear measured by the horizontal winds between 850 and 1000 hpa is more representative of the Figure 6. Low-level (solid) and midlevel (dashed) relative humidity (%) at (a) 1800 UTC 2 July, (b) 0000 UTC 3 July 3, and (c) 0600 UTC 3 July The blue circles are 360 km away from the center of Chan-Hom indicated by the typhoon symbol. LIANG ET AL. RAPID WEAKENING IN A MONSOON GYRE 9513

7 Figure 7. Time series of the intensity of Chan-Hom (black solid line), commonly used (blue solid line) and low-level (blue dashed line) vertical wind shear (m s 1 ). Horizontal solid and dashed lines indicate the vertical wind shear of 9 m s 1 and 2.5 m s 1, respectively. Vertical lines outline the period of the rapid weakening of Chan-Hom. tropical cyclone intensity changes than the commonly used vertical wind shear between 200 and 850 hpa over the WNP, especially during the active season (June October) of tropical cyclones [Shu et al., 2013; Y. Wang et al., 2015]. In this study, the commonly used and low-level vertical wind shears are both examined. Figure 7 shows the evolution of the intensity of Chan-Hom and the vertical wind shears. The vertical wind shears averaged in the area with the radius of 500 km are calculated as the differences of the horizontal winds between 200 hpa and 850 hpa (commonly used vertical wind shear) and between 850 hpa and 1000 hpa (lowlevel vertical wind shear), respectively. Chan-Hom moved westward in a high vertical wind shear exceeding 9ms 1 during its intensification stage and reached the typhoon intensity at 1800 UTC 2 July when the vertical wind shear increased to the peak of about 18 m s 1. From 1800 UTC 2 July to 0600 UTC 3 July, Chan-Hom was affected by the vertical wind shear decreasing from 18 m s 1 to 12 m s 1. Note that the very high vertical wind shear exceeding 12 m s 1 during 2 3 July may lead to a very short lag time of the intensity changes. Modeling results of Frank and Ritchie [2001] suggested that the tropical cyclone experiences the time lag of 3 h at most when it is embedded in a very high vertical wind shear. Y. Wang et al. [2015] exhibited that the low-level vertical wind shear lower than 2.5 m s 1 makes the tropical cyclones more likely intensify than weaken. In Figure 7, it shows a low-level vertical shear lower than 2.5 m s 1 prior to the rapid weakening, which sustained around 2.5 m s 1 when Chan-Hom underwent the rapid weakening. The commonly used and the low-level vertical wind shears examined by using NCEP/NCAR Reanalysis 1 wind product with the spatial resolution of 2.5 have similar evolutions with slightly smaller Figure 8. Unfiltered wind speeds (contour, m s 1 ) and day timescale wind fields (vector, m s 1 ) and the day timescale relative vorticity (shaded, 1.0e 5s 1 ) at 850 hpa. At (a) 0000 UTC 1 July 2015, (b) 0000 UTC 2 July 2015, (c) 0000 UTC 3 July, and (d) 0000 UTC 4 July Black and red dots represent centers of the monsoon gyre and Typhoon Chan-Hom, respectively. LIANG ET AL. RAPID WEAKENING IN A MONSOON GYRE 9514

8 Figure 9. Infrared brightness temperature (shaded, C) from FY2F satellites and 850 hpa wind fields on the day timescale (vector, m s 1 ) at (a) 1230 UTC 2 July, (b) 1830 UTC 2 July, (c) 0030 UTC 3 July, and (d) 0630 UTC 3 July The blue area denotes temperature less than 60 C. The purple lines and green dots denote the track and center of Typhoon Chan-Hom from 1200 UTC 3 June to 1800 UTC 5 July 2015, respectively. Blue circles are 500 km away from the center of Chan-Hom. Green dashed lines roughly outline the monsoon gyre. values (figure not shown). Thus, the rapid weakening of Typhoon Chan-Hom was not accompanied with an increase in the vertical wind shear. The low-latitude oceanic region usually has a suitable environment for tropical cyclone intensification [e.g., Gray, 1968, 1979; Kaplan and DeMaria, 2003; Cheung, 2004; Emanuel et al., 2004; Lowag et al., 2008; Kaplan et al., 2010; Shu et al., 2012; Y. Wang et al., 2015]. The analysis of the environmental factors in the section suggests that the changes of large-scale factors including the high SST, the moist middle- and low-level environment and the decreasing vertical wind shear do not have primary effects on the rapid weakening of Chan-Hom. The coincidence of the rapid weakening with the sudden track change motivates us to examine the possible influence of the monsoon gyre. 5. Influence of the Low-Frequency Monsoon Gyre We first conducted a spectral analysis of the TRMM V7 3B42RT derived daily precipitation product and found a clear spectral peak band of days (figure not shown). It is suggested that intraseasonal timescale perturbations were involved in the activity of Typhoon Chan-Hom. Following Wu et al. [2011a, 2013], we applied a Lanczos filter [Duchon, 1979] to the wind field with a period ranging from 15 days to 30 days. The synopticscale component is derived from the difference between the unfiltered field and the 10 day low-pass field, in LIANG ET AL. RAPID WEAKENING IN A MONSOON GYRE 9515

9 Figure 10. (a) Time-longitude cross section of TRMM 3B42RT 6 h accumulated rainfall (shaded and contour, mm) along the latitude of Typhoon Chan-Hom's center and (b) time-radius cross section of the azimuthal averaged radial wind speed at 1000 hpa (shaded and contour, m s 1 ). The black dots and two solid lines in Figure 10a indicate positions of Chan- Hom and 500 km away from the typhoon center, respectively. The horizontal red dashed lines outline the period of the rapid weakening of Typhoon Chan-Hom. which the cyclonic circulation of Chan- Hom can be distinguishable when it underwent the rapid weakening (figure not shown). Figure 8 shows the day timescale wind and relative vorticity fields at 850 hpa from 1 to 4 July The coalescence of Chan-Hom and the monsoon gyre can be seen. The monsoon gyre was located around 10 N, 148 E and had a zonal elongated axis of about 2500 km with dominant westerlies along the southern periphery, together with an anticyclonic circulation and negative relative vorticity to its northwest. In this study, the centers of the monsoon gyre and Chan-Hom are defined as the position of the maximum positive relative vorticity in the vicinity of the circulation center at 850 hpa and obtained from the typhoon information of JTWC data set, respectively. On 1 July (Figure 8a), Chan-Hom was located to the east of the monsoon gyre center. Then Chan-Hom moved westward and gradually approached the center of the monsoon gyre. The two systems were combined at 0000 UTC 3 July 2015 (Figures 8b and 8c) and subsequently the coalesced tropical cyclone-gyre system turned northward (Figure 8d). Such a typical coalescence process was documented in previous observational and numerical studies [Wu et al., 2011a, 2011b; Liang et al., 2011; Bi et al., 2015; Liang and Wu, 2015]. Figure 8 suggests that the rapid weakening happened during the coalescence process. The strong southwesterly winds in the southwestern periphery of the monsoon gyre result from the Rossby wave energy dispersion [Carr and Elsberry, 1995; Liang et al., 2011; Wu et al., 2011a, 2011b, 2013]. The southwesterly winds in the southern and southeastern periphery of the monsoon gyre retained m s 1 on 1 2 July (Figures 8a and 8b) and rapidly increased to m s 1 after the coalescence of Chan-Hom and the monsoon gyre (Figures 8c and 8d). In Liang et al. [2014], numerical results indicated that the convection can develop and strengthen to the east of the monsoon gyre center because of the enhanced southwesterly flows. Figure 9 shows the cloud top temperature from FY2F satellites overlapped with 850 hpa wind fields on the day timescale. The blue area corresponds to the cold cloud top with a temperature less than 60 C. At 1230 UTC 2 July (Figure 9a), two weak and loosely organized convective areas with the cold cloud top of 70 C (green) were observed in the east and west bottoms of the monsoon gyre, respectively, and Chan-Hom had a compact convective pattern with the deep convection with the cold cloud top of 80 C (red). Accompanying with coalescence of Chan-Hom with the monsoon gyre, the convection on the eastern side of the monsoon gyre rapidly enhanced into the well-organized convection within 6 h, which was located about 700 km away from the center of Chan-Hom (Figure 9b). The deep convection associated with the eyewall of Chan-Hom decayed, and no evident convection remained within the radius of 500 km from the typhoon center by 0630 UTC 3 July (Figures 9c and 9d). Figure 9 suggests that the coalescence of Chan-Hom and the monsoon gyre and the development of the outside convection were associated with the rapid weakening of Chan-Hom. LIANG ET AL. RAPID WEAKENING IN A MONSOON GYRE 9516

10 The strong convection to the east of the monsoon gyre center can also be seen in the distribution of rainbands. Figure 10a shows the time-longitude cross section of the TRMM 6 h accumulated precipitation along the latitude of Typhoon Chan-Hom's center. In this study, the 6 h rainfall is the accumulated rainfall in following 6 h at the given time. There were two isolated rainfall areas at 1200 UTC 2 July, about km to the east and about km to the west of the tropical cyclone center, respectively. The rainfall to the east of the tropical cyclone center rapidly increased and reached a maximum at 1800 UTC 2 July, in agreement with the enhanced convection on the eastern side of the monsoon gyre, as shown in Figure 8. This eastern rainbands maintained by the end of the rapid weakening process. Simultaneously, inner rainbands of Chan-Hom rapidly weakened and shifted westward. It was observed about 500 km to the west of the tropical cyclone center after the rapid weakening, corresponding to the enlargement of the eye size of Chan-Hom. Figure 10b further examines the lowlevel inflow of Chan-Hom, which shows the evolution of azimuthal averaged radial wind at 1000 hpa. In the intensification stage of Chan-Hom, the strong inflow occurred within the radius of 300 km from the tropical cyclone center. The peak inflow exceeded 5 m s 1 at 1200 UTC 2 July when Chan-Hom reached the typhoon intensity. The inflow 500 km away from the tropical cyclone center started to enhance from 1200 UTC 2 July, corresponding to the enhancement of the convection Figure 11. TRMM 3B42RT 6 h accumulated rainfall (mm, shaded) at (a) 1200 UTC 2 July, (b) 1800 UTC 2 July, and (c) 0000 UTC 3 July, respectively. Arrows indicate vectors of the commonly used vertical wind shear (difference between 200 and 850 hpa) averaged over an area within a radius of 500 km from the center of Chan-Hom. Black typhoon symbols represent centers of Chan-Hom. The two circles are 200 and 500 km away from the center of Chan-Hom, respectively. LIANG ET AL. RAPID WEAKENING IN A MONSOON GYRE 9517

11 associated with the monsoon gyre. The outer inflow associated with the monsoon gyre reached its maximum higher than 4 m s 1 around at 1800 UTC 2 July and maintained for 12 h. The inner inflow simultaneously weakened in the inner core of Chan-Hom. This figure further suggests that the development of the outside convection associated with the monsoon gyre was responsible for the rapid weakening of Chan-Hom. Although the rapid weakening of Chan-Hom happened in the environment with a decreasing vertical wind shear, many previous studies showed that the vertical wind shear may have contributions on tropical cyclone intensity changes through changing the convection distribution in tropical cyclone eyewall [e.g., Wang and Holland, 1996; Bender, 1997; Frank and Ritchie, 1999, 2001; Black et al., 2002; Rogers et al., 2003; Reasor et al., 2004; Zhu et al., 2004; Wu et al., 2011a; Xu and Wang, 2013; Y. Wang et al., 2015]. Figure 11 shows the relationship between TRMM 3B42RT 6 h accumulated rainfall and the commonly used vertical wind shear prior to and during the rapid weakening. It shows that Chan-Hom was embedded in a strong easterly vertical wind shear from 1200 UTC 2 July to 0000 UTC 3 July In 6 h prior to the rapid weakening (Figure 11a), the rainfall showed a systematic downshear rainfall maximum within 200 km around the tropical cyclone center, indicating an obvious eyewall of Chan-Hom. Since 1800 UTC 2 July (Figures 11b and 11c), the eastern strong rainfall in the inner core rapidly weakened, leading to the rainfall maximum within a band of km to the west of the tropical cyclone center. Then the western rainfall gradually shifted westward accompanying with the decreasing of the easterly vertical shear. Figure 11 suggests that the westward shift of the rainfall area happened during the rapid weakening of Chan-Hom, which may be related to the easterly vertical wind shear. The easterly vertical shear may play a role in the reorganization of the convection in the eyewall to form a larger eye after the strong convection on the eastern side of the monsoon gyre led to the collapsing of the eastern eyewall of Chan-Hom. This possible role of the vertical wind shear needs further exploration. Acknowledgments This research was jointly supported by the National Basic Research Program of China (2015CB and 2013CB430103), the National Natural Science Foundation of China ( and ), the project of the specially appointed professorship of Jiangsu Province, the Natural Science Foundation of Higher Education Institutions in Jiangsu Province (14KJB170014), the open project of the State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences (2015LASW-A06), and China Scholarship Council (CSC). Earth System Modeling Center Contribution Number 122 The NCEP FNL Global Analysis data were obtained freely from the Research Data Archive at the NCAR/CISL ( rda.ucar.edu/datasets/ds083.2/). The NOAA daily OISST product was freely obtained from NOAA/NCEI ( ncdc.noaa.gov/oisst). The TRMM V7 3B42RT 3-hourly precipitation product was freely achieved from NASA/GSFC ( downloads/trmm). The FY2F satellite product was freely achieved from CMA/NSMC, FENGYUN Satellite Data Center ( The NOAA MTCSWA product was freely achieved from NOAA/NESDIS ( noaa.gov/ps/trop/mtcswa.html). 6. Summary Typhoon Chan-Hom (2015) experienced a rapid weakening from 1800 UTC 2 July to 0600 UTC 3 July 2015, when it was around 10 N to the east of the Philippines. The rapid weakening coincided with a sudden northward turn in its track. While the conventional environmental factors exhibit the negligible influence on the occurrence of the rapid weakening, the operational forecasts issued by JTWC, JMA, and CMA all failed to predict the rapid weakening event that occurred in the deep Tropics over the open ocean. Our analysis indicates that the rapid weakening event was accompanied with the coalescence process between a monsoon gyre and Typhoon Chan-Hom. The low-frequency (15 30 days) monsoon gyre was centered around 10 N, 148 E with a zonal scale of about 2500 km. Chan-Hom moved westward and combined with the monsoon gyre at 0000 UTC 3 July The coalescence process between these two systems enhanced the convection in the outer region of the tropical cyclone because of enhanced southwesterly flows in the southeastern periphery of the monsoon gyre. 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