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The Impact of Dropwindsonde on Typhoon Track Forecasts in DOTSTAR and T-PARC

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The Impact of Dropwindsonde on Typhoon Track Forecasts in DOTSTAR and T-PARC 1 Eyewall Evolution of Typhoons Crossing the Philippines and Taiwan: An 2 Observational Study 3 Kun-Hsuan Chou1, Chun-Chieh Wu2, Yuqing Wang3, and Cheng-Hsiang Chih4 4 1Department of Atmospheric Sciences, Chinese Culture University, Taipei, Taiwan 5 2Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan 6 3International Pacific Research Center, and Department of Meteorology, University of 7 Hawaii at Manoa, Honolulu, Hawaii 8 4Graduate Institute of Earth Science/Atmospheric Science, Chinese Culture University, 9 Taipei, Taiwan 10 11 12 13 14 Terrestrial, Atmospheric and Oceanic Sciences 15 (For Special Issue on “Typhoon Morakot (2009): Observation, Modeling, and 16 Forecasting Applications”) 17 (Accepted on 10 May, 2011) 18 19 ___________________ 20 Corresponding Author’s address: Kun-Hsuan Chou, Department of Atmospheric Sciences, 21 National Taiwan University, 55, Hwa-Kang Road, Yang-Ming-Shan, Taipei 111, Taiwan. 22 ([email protected]) 1 23 Abstract 24 This study examines the statistical characteristics of the eyewall evolution induced by 25 the landfall process and terrain interaction over Luzon Island of the Philippines and Taiwan. 26 The interesting eyewall evolution processes include the eyewall expansion during landfall, 27 followed by contraction in some cases after re-emergence in the warm ocean. The best 28 track data, advanced satellite microwave imagers, high spatial and temporal 29 ground-observed radar images and rain gauges are utilized to study this unique eyewall 30 evolution process. The large-scale environmental conditions are also examined to 31 investigate the differences between the contracted and non-contracted outer eyewall cases 32 for tropical cyclones that reentered the ocean. 33 Based on the best track data of JTWC from 1945 to 2010, the statistical characteristics 34 of the eyewall evolution of tropical cyclones induced by the landfall process over the 35 Philippines and Taiwan are studied. On average, there are 3.0 and 1.3 typhoons per year 36 that were originated from the western North Pacific and crossed the Philippines and Taiwan, 37 respectively. An examination of the available microwave images of 23 typhoons crossing 38 the Philippines between 2000 and 2010 shows that most typhoons experienced this kind of 39 eyewall evolution, i.e., the radius of the eyewall for 91% of the landfalling typhoons 40 increased during landfall, while the radius of the eyewall for 57% of the cases contracted 41 when the typhoons reentered the ocean after they crossed the Philippines. For typhoons 2 42 that crossed Taiwan, based on the microwave and radar images, 89% of the cases show an 43 expansion of the eyewall during the landfall. However, the reorganization of the outer 44 eyewall and subsequent contraction were rarely observed due to the small fraction of the 45 ocean when the typhoons entered the Taiwan Strait. In addition, the observed rainfall 46 shows an expansion in the rainfall area induced by terrain. Furthermore, the analyses of 47 large-scale environmental conditions show that small vertical wind shear, high low-level 48 relative humidity and sea surface temperature are important for the reorganization of the 49 outer eyewall and the subsequent eyewall contraction when the typhoons reentered the 50 ocean. 51 3 52 1. Introduction 53 The eyewall of a tropical cyclone (TC) consists of deep convective clouds surrounding 54 the eye that on the contrary contains only light winds and almost no precipitation. The 55 most severe weather, such as maximum winds and torrential rains, occurs in the eyewall. 56 It is generally believed that the evolution of the eyewall is responsible for the intensity 57 change of a TC. Therefore, the evolution of the eyewall has always been an intriguing 58 issue in TC research. Recent studies have shed some new insights into the internal 59 thermodynamics and dynamics of TCs, such as the description of eyewall features and eye 60 thermodynamics (Shapiro and Willoughby, 1982; Willoughby, 1998); the replacement of 61 concentric eyewall (Willoughby et al. 1982; Willoughby and Black, 1996); asymmetric 62 mixing of the eye and eyewall in the formation of annular hurricanes (Knaff et al., 2003); 63 the formation of mesovortices and the polygonal eyewall structure which are linked to 64 barotropic instability and the potential vorticity mixing processes between the eye and the 65 eyewall (Schubert et al. 1999; Kossin and Eastin, 2001; Kossin and Schubert, 2001); and 66 the presence of convectively coupled vortex Rossby waves in the eyewall (Montgomery and 67 Kallenbach, 1997; Wang, 2002). 68 Another important factor affecting the eyewall evolution is the topography that a TC 69 encounters. Brand and Blelloch (1973) was the first to document the effect of the 70 Philippine Islands on the changes in the track, eye diameter, storm intensity and size of TCs 4 71 during 1960-70 based on the Annual Typhoon Reports of Joint Typhoon Warning Center 72 (with aircraft reconnaissance available during that period). It was indicated that the 73 frictional effect of land mass and the reduction in heat and moisture supply by the ocean are 74 the primary causes of the above changes (Wu et al. 2009). 75 There have been limited numbers of studies focusing on the eyewall dynamics of 76 typhoons making landfalls at the Philippine Islands, whose terrain has horizontal scales 77 about equivalent to the size of the TC. The interesting eyewall evolution of Typhoon Zeb 78 (1998) before, during, and after its landfall at Luzon was documented from both the satellite 79 observation and numerical simulation (Wu et al. 2003). It is proposed that the terrain 80 plays a critical role in such an eyewall evolution: first the eyewall contraction just before 81 landfall, its breakdown after landfall, and then the reorganization of a larger outer eyewall 82 after the storm returns to the ocean. Further detailed numerical examination of this unique 83 eyewall evolution was documented in Wu et al. (2009). It was shown that the presence of 84 Luzon plays a critical role in the observed eyewall evolution. The eyewall replacement 85 occurred in Typhoon Zeb was triggered by the mesoscale terrain that has a horizontal scale 86 similar to the core of the typhoon. Furthermore, based on several sensitivity experiments, 87 it was shown that both diabatic heating and surface friction are key factors for the 88 maintenance and the reorganization of the outer eyewall when Zeb reentered the ocean. 89 Besides Typhoon Zeb (1998) over Luzon, similar scenarios have also been observed in 5 90 other storms, such as Typhoon Dan (1999) over Luzon, Typhoon Talim (2005) over Taiwan, 91 and Hurricane Gilbert (1988), Wilma (2005) and Dean (2007) over Yucatan Peninsula 92 (Chou and Wu 2008). Figure 1 shows the topography of the Philippines and Taiwan. It 93 can be found that both the Philippines and Taiwan have terrain features with the horizontal 94 scales (several hundred km) similar to the size of TCs. In addition, it is shown that the 95 area of the Philippines is wider than the island of Taiwan, and the high mountains are 96 located in the northern Philippines (Luzon island) while similar topography is found in 97 central Taiwan. Since any changes in the eyewall structure would lead to or be 98 accompanied by change in the storm intensity, it is believed that a better understanding of 99 such terrain-induced eyewall evolution could improve our understanding of TC structure 100 and intensity changes, especially for TCs making landfall and interacting with mesoscale 101 terrains (Wang and Wu 2004). Furthermore, the issue on intensity and inner-core structure 102 change, and the associated precipitation pattern of a landfalling TC is very important for 103 operational forecasts. 104 The main objective of this study is to document the statistical characteristics of the 105 eyewall evolution induced by the landfall process and terrain interaction over Luzon Island 106 of the Philippines and Taiwan based on the advanced satellite microwave imagers and high 107 spatial and temporal radar images. The difference between the typhoons with and without 108 the reorganization of the outer eyewall when typhoons reentered the ocean will be examined. 6 109 Furthermore, the differences in eyewall evolutions between the typhoons crossing the 110 Philippines and those crossing the Taiwan are also identified. The analyses data are 111 described in section 2. The statistical characteristics of the eyewall evolution of typhoons 112 crossing the Philippines and Taiwan are discussed in sections 3 and 4, respectively. The 113 overall findings are summarized in the last section. 114 2. The Data Descriptions 115 a. Best track TC data 116 The best track data including 6-hourly estimates of position and intensity are obtained 117 from the Joint Typhoon Warning Center (JTWC). The JTWC data are utilized to 118 document the overall statistics for TCs crossing the Philippines during the period 119 1945-2010. Since the eyewall structure is well organized only in strong TCs and that its 120 evolution can be depicted using the microwave imagers, this study will primarily focus on 121 the eyewall evolution of TCs with maximum sustained surface winds larger than 64 knots 122 (namely, “typhoons” as defined in this study). 123 b. Satellite microwave imagery 124 Microwave imagery allows for identification of rain and ice particle patterns within TC 125 rainbands that are normally blocked by mid- to upper-level clouds in the infrared and visible 126 imagery (Velden et al. 2006). Passive microwave digital data and imagery can provide key 127 storm structural details and offset many of the visible/infrared spectral problems (Hawkins 7 128 et al. 2001). The microwave imagers used in this study include the Special Sensor 129 Microwave Imager (SSM/I), the Special Sensor Microwave Imager Sounder (SSM/IS), 130 Tropical Rainfall Measuring Mission Microwave Imager (TRMM/TMI), and Advanced 131 Microwave Scanning Radiometer - Earth Observing System instrument on Aqua satellite 132 (AMSR-E/Aqua).
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