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Eyewall Evolution of Typhoons Crossing the Philippines and Taiwan: An 1
Observational Study 2
Kun-Hsuan Chou1, Chun-Chieh Wu2, Yuqing Wang3, and Cheng-Hsiang Chih4 3
1Department of Atmospheric Sciences, Chinese Culture University, Taipei, Taiwan 4
2Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan 5
3International Pacific Research Center, and Department of Meteorology, University of 6 Hawaii at Manoa, Honolulu, Hawaii 7
4Graduate Institute of Earth Science/Atmospheric Science, Chinese Culture University, 8
Taipei, Taiwan 9
10
11
12
13
Terrestrial, Atmospheric and Oceanic Sciences 14
(For Special Issue on “Typhoon Morakot (2009): Observation, Modeling, and 15
Forecasting Applications”) 16
(Accepted on 10 May, 2011) 17 18
___________________ 19
Corresponding Author’s address: Kun-Hsuan Chou, Department of Atmospheric Sciences, 20
National Taiwan University, 55, Hwa-Kang Road, Yang-Ming-Shan, Taipei 111, Taiwan. 21
2
Abstract 23
This study examines the statistical characteristics of the eyewall evolution induced by 24
the landfall process and terrain interaction over Luzon Island of the Philippines and Taiwan. 25
The interesting eyewall evolution processes include the eyewall expansion during landfall, 26
followed by contraction in some cases after re-emergence in the warm ocean. The best 27
track data, advanced satellite microwave imagers, high spatial and temporal 28
ground-observed radar images and rain gauges are utilized to study this unique eyewall 29
evolution process. The large-scale environmental conditions are also examined to 30
investigate the differences between the contracted and non-contracted outer eyewall cases 31
for tropical cyclones that reentered the ocean. 32
Based on the best track data of JTWC from 1945 to 2010, the statistical characteristics 33
of the eyewall evolution of tropical cyclones induced by the landfall process over the 34
Philippines and Taiwan are studied. On average, there are 3.0 and 1.3 typhoons per year 35
that were originated from the western North Pacific and crossed the Philippines and Taiwan, 36
respectively. An examination of the available microwave images of 23 typhoons crossing 37
the Philippines between 2000 and 2010 shows that most typhoons experienced this kind of 38
eyewall evolution, i.e., the radius of the eyewall for 91% of the landfalling typhoons 39
increased during landfall, while the radius of the eyewall for 57% of the cases contracted 40
when the typhoons reentered the ocean after they crossed the Philippines. For typhoons 41
3
that crossed Taiwan, based on the microwave and radar images, 89% of the cases show an 42
expansion of the eyewall during the landfall. However, the reorganization of the outer 43
eyewall and subsequent contraction were rarely observed due to the small fraction of the 44
ocean when the typhoons entered the Taiwan Strait. In addition, the observed rainfall 45
shows an expansion in the rainfall area induced by terrain. Furthermore, the analyses of 46
large-scale environmental conditions show that small vertical wind shear, high low-level 47
relative humidity and sea surface temperature are important for the reorganization of the 48
outer eyewall and the subsequent eyewall contraction when the typhoons reentered the 49
ocean. 50
51
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1. Introduction 52
The eyewall of a tropical cyclone (TC) consists of deep convective clouds surrounding 53
the eye that on the contrary contains only light winds and almost no precipitation. The 54
most severe weather, such as maximum winds and torrential rains, occurs in the eyewall. 55
It is generally believed that the evolution of the eyewall is responsible for the intensity 56
change of a TC. Therefore, the evolution of the eyewall has always been an intriguing 57
issue in TC research. Recent studies have shed some new insights into the internal 58
thermodynamics and dynamics of TCs, such as the description of eyewall features and eye 59
thermodynamics (Shapiro and Willoughby, 1982; Willoughby, 1998); the replacement of 60
concentric eyewall (Willoughby et al. 1982; Willoughby and Black, 1996); asymmetric 61
mixing of the eye and eyewall in the formation of annular hurricanes (Knaff et al., 2003); 62
the formation of mesovortices and the polygonal eyewall structure which are linked to 63
barotropic instability and the potential vorticity mixing processes between the eye and the 64
eyewall (Schubert et al. 1999; Kossin and Eastin, 2001; Kossin and Schubert, 2001); and 65
the presence of convectively coupled vortex Rossby waves in the eyewall (Montgomery and 66
Kallenbach, 1997; Wang, 2002). 67
Another important factor affecting the eyewall evolution is the topography that a TC 68
encounters. Brand and Blelloch (1973) was the first to document the effect of the 69
Philippine Islands on the changes in the track, eye diameter, storm intensity and size of TCs 70
5
during 1960-70 based on the Annual Typhoon Reports of Joint Typhoon Warning Center 71
(with aircraft reconnaissance available during that period). It was indicated that the 72
frictional effect of land mass and the reduction in heat and moisture supply by the ocean are 73
the primary causes of the above changes (Wu et al. 2009). 74
There have been limited numbers of studies focusing on the eyewall dynamics of 75
typhoons making landfalls at the Philippine Islands, whose terrain has horizontal scales 76
about equivalent to the size of the TC. The interesting eyewall evolution of Typhoon Zeb 77
(1998) before, during, and after its landfall at Luzon was documented from both the satellite 78
observation and numerical simulation (Wu et al. 2003). It is proposed that the terrain 79
plays a critical role in such an eyewall evolution: first the eyewall contraction just before 80
landfall, its breakdown after landfall, and then the reorganization of a larger outer eyewall 81
after the storm returns to the ocean. Further detailed numerical examination of this unique 82
eyewall evolution was documented in Wu et al. (2009). It was shown that the presence of 83
Luzon plays a critical role in the observed eyewall evolution. The eyewall replacement 84
occurred in Typhoon Zeb was triggered by the mesoscale terrain that has a horizontal scale 85
similar to the core of the typhoon. Furthermore, based on several sensitivity experiments, 86
it was shown that both diabatic heating and surface friction are key factors for the 87
maintenance and the reorganization of the outer eyewall when Zeb reentered the ocean. 88
Besides Typhoon Zeb (1998) over Luzon, similar scenarios have also been observed in 89
6
other storms, such as Typhoon Dan (1999) over Luzon, Typhoon Talim (2005) over Taiwan, 90
and Hurricane Gilbert (1988), Wilma (2005) and Dean (2007) over Yucatan Peninsula 91
(Chou and Wu 2008). Figure 1 shows the topography of the Philippines and Taiwan. It 92
can be found that both the Philippines and Taiwan have terrain features with the horizontal 93
scales (several hundred km) similar to the size of TCs. In addition, it is shown that the 94
area of the Philippines is wider than the island of Taiwan, and the high mountains are 95
located in the northern Philippines (Luzon island) while similar topography is found in 96
central Taiwan. Since any changes in the eyewall structure would lead to or be 97
accompanied by change in the storm intensity, it is believed that a better understanding of 98
such terrain-induced eyewall evolution could improve our understanding of TC structure 99
and intensity changes, especially for TCs making landfall and interacting with mesoscale 100
terrains (Wang and Wu 2004). Furthermore, the issue on intensity and inner-core structure 101
change, and the associated precipitation pattern of a landfalling TC is very important for 102
operational forecasts. 103
The main objective of this study is to document the statistical characteristics of the 104
eyewall evolution induced by the landfall process and terrain interaction over Luzon Island 105
of the Philippines and Taiwan based on the advanced satellite microwave imagers and high 106
spatial and temporal radar images. The difference between the typhoons with and without 107
the reorganization of the outer eyewall when typhoons reentered the ocean will be examined. 108
7
Furthermore, the differences in eyewall evolutions between the typhoons crossing the 109
Philippines and those crossing the Taiwan are also identified. The analyses data are 110
described in section 2. The statistical characteristics of the eyewall evolution of typhoons 111
crossing the Philippines and Taiwan are discussed in sections 3 and 4, respectively. The 112
overall findings are summarized in the last section. 113
2. The Data Descriptions 114
a. Best track TC data 115
The best track data including 6-hourly estimates of position and intensity are obtained 116
from the Joint Typhoon Warning Center (JTWC). The JTWC data are utilized to 117
document the overall statistics for TCs crossing the Philippines during the period 118
1945-2010. Since the eyewall structure is well organized only in strong TCs and that its 119
evolution can be depicted using the microwave imagers, this study will primarily focus on 120
the eyewall evolution of TCs with maximum sustained surface winds larger than 64 knots 121
(namely, “typhoons” as defined in this study). 122
b. Satellite microwave imagery 123
Microwave imagery allows for identification of rain and ice particle patterns within TC 124
rainbands that are normally blocked by mid- to upper-level clouds in the infrared and visible 125
imagery (Velden et al. 2006). Passive microwave digital data and imagery can provide key 126
storm structural details and offset many of the visible/infrared spectral problems (Hawkins 127
8
et al. 2001). The microwave imagers used in this study include the Special Sensor 128
Microwave Imager (SSM/I), the Special Sensor Microwave Imager Sounder (SSM/IS), 129
Tropical Rainfall Measuring Mission Microwave Imager (TRMM/TMI), and Advanced 130
Microwave Scanning Radiometer - Earth Observing System instrument on Aqua satellite 131
(AMSR-E/Aqua). The advantage of these imagers is that the cirrus clouds are transparent 132
while using the 85-91 GHz channel. Therefore, an excellent view of the eye, eyewall, and 133
rainband structure of TCs can be provided by the 85-91 GHz brightness temperature, and 134
the size of the eyewall can be well captured and estimated by these microwave imagers. 135
The archived microwave imagers from 2000 to 2010 are obtained from the Navy Research 136
Laboratory’s Tropical Cyclone Webpage 137
(http://www.nrlmry.navy.mil/tc_pages/tc_home.html). 138
c. Radar reflectivity 139
The composite radar reflectivity data are taken from the Central Weather Bureau 140
(CWB) radar network for analyzing the eyewall evolution when typhoons cross Taiwan. 141
The dataset has 0.0125o lat. by 0.0125o lon. horizontal resolution and 10 minute temporal 142
resolution. Currently, the only archived composite radar reflectivity data after 2005 are 143
available, thus only typhoons that crossed Taiwan during 2005-2010 are examined with the 144
radar reflectivity data. The locations of CWB radar stations are denoted by triangle 145
symbols in Fig. 1b. 146
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d. NCEP FNL global analysis 147
The National Centers for Environmental Prediction (NCEP) final gridded analysis 148
(FNL) datasets are on 1.0o lat. by 1.0o lon. grids prepared operationally every six hours (00, 149
06, 12 and 18 UTC). The dataset is not a re-analysis product, but is obtained from the 150
NCEP Global Data Assimilation System, which continuously collects observational data 151
from the Global Telecommunications System and other sources. The analyses are 152
available on the surface and at 26 mandatory (and other pressure) levels from 1000 to 10 153
hPa. Parameters include surface pressure, sea level pressure, geopotential height, 154
temperature, sea surface temperature, soil values, ice cover, relative humidity, zonal and 155
meridional velocity, vertical velocity, vorticity and ozone concentration 156
(http://www.ncep.noaa.gov/). 157
In this study the environmental low level relative humidity and vertical wind shear 158
near the core of TCs are calculated based on the NCEP FNL for each typhoon that crossed 159
the Philippines or Taiwan during 2000-2010. The low-level (at 700 hPa) relative humidity 160
is averaged over a 5° latitude-longitude square, centered on the best-track location of each 161
storm. The vertical wind shear was estimated based on the vector difference between the 162
200- and 850-hPa winds, averaged within the 2-8 degree radius of the storm center. The 163
method of shear calculation is determined by referring to Knaff et al. (2005) to keep the 164
shear values from being corrupted by the bogus vortex in the global analysis. 165
10
e. Sea surface temperature 166
The NCEP Real-Time Global Sea Surface Temperature analysis (RTG-SST) has 0.5o 167
lat. by 0.5o lon. horizontal resolution. The operational RTG-SST product uses in situ (ship 168
and buoy) and satellite-derived SST data (NOAA-16 SEATEMP retrievals) from the Naval 169
Oceanographic Office Major Shared Resource Center (Thiébaux et al. 2003). In this study, 170
the RTG-SST near the core of TC, i.e, averaged over a 5° latitude by 5o longitude square, 171
centered at the best-track location of each typhoon, is calculated for each typhoon that 172
crossed the Philippines during 2001-2010. Note that since the RTG-SST data are not 173
available until 2001, the RTG-SST calculation is not available for typhoons in 2000. 174
f. Automatic rain gauge data 175
The Central Weather Bureau (CWB) automatic rain gauge data are used for the rainfall 176
analysis for typhoons that crossed Taiwan during 2005-2010. More than five hundred 177
stations are deployed around the island during 2005-2010, and the rainfall data are recorded 178
hourly. The locations of CWB rain gauge stations are denoted by cross symbols in Fig. 1b. 179
3. The eyewall evolution scenarios for typhoons crossing the Philippines 180
Figure 2 is a histogram showing the number of tropical storm cases (with the 181
maximum sustained surface winds between 34 and 63 knots) and typhoon cases (with the 182
maximum sustained surface winds larger than 64 knots) with their centers passing through 183
either the Philippines or Taiwan from 1945 to 2010. It can be found that during this period 184
11
on average about 26 TCs occurred over the western North Pacific per year, with 8.7 in the 185
tropical storm category and 16.9 reaching typhoon intensity. Figure 2a shows that among 186
all the TCs, on average each year about 2.2 tropical storms and 3.0 typhoons made landfall 187
at and crossed the Philippines, which accounts for about 20% of the total number of TCs in 188
the western North Pacific. Among all the TCs crossing Taiwan (Fig. 2b), about 0.7 189
tropical storms and 1.3 typhoons made landfall and crossed Taiwan respectively, which 190
accounts for 8% of the total number of TCs in the western North Pacific. As compared to 191
the results of TCs crossing the Philippines, the amount of lanfalling TCs in Taiwan is 192
roughly reduced by half. 193
It is our interest to examine the terrain-induced eyewall evolution as mentioned in the 194
introduction. The 85-91 GHz brightness temperature microwave images available from 195
2000 to 2010 are used to depict the eyewall evolution for storms prior to, during and after 196
landfall, after passing through the Philippines and reentering the ocean. During the 197
11-year period, 23 typhoons passed through the Philippines. Most of the storms either 198
recurved northward or continued moving westward after crossing the Philippines. Later 199
on, most of the typhoons affected Japan, Taiwan, China, and the Indochina Peninsula, 200
respectively. The information about the duration time (the time each storm crossed the 201
islands), intensity, and estimated radius of convective ring (eyewall) during the period when 202
each storm crossed the Philippines is listed in Table 1. 203
12
Figure 3 shows the microwave images of the eyewall evolution before the storm made 204
landfall, when it left the island, and after the storm reentered the ocean. Different 205
characteristics are shown: three typhoons with reorganization of a larger outer eyewall 206
(Xangsane in 2000, Cimaron in 2006, Megi in 2010), and one typhoon (Mitag in 2007) 207
without reorganization of the outer eyewall. It can be found from these microwave images 208
that the radii of the eyewalls generally reached the minimum before landfall, with radii of 209
the eyewall ranging from 20 to 100 km (Figs. 3a, 3d, and 3g). After the storms left the 210
Philippine islands, the eye became unidentifiable since the original primary eyewall 211
convection weakened considerably due to the dissipation over the land/terrain and the 212
reduction of the surface moisture supply (Figs. 3b, 3e, and 3h). In most (91%) of the cases, 213
the outer eyewall reappeared when the storms were about to leave the land and to reenter 214
the ocean. During this period, the radii of the outer eyewall were roughly between 100 215
and 200 km. After the storm reentered the ocean, the inner eyewall vanished and the outer 216
eyewall was organized and contracted with radii between 50 and 150 km (Figs. 3c, 3g, and 217
3i). According to the microwave image analyses for 23 typhoons, 57 % of them 218
experienced reorganization of the outer eyewall. The estimated radii of the eyewall as 219
storms cross the Philippines are shown in the Table 1. 220
Considering the intensity evolution based on the best-track for typhoons passing 221
through the Philippines (Table 1), 82% of the typhoons influenced by the topography 222
13
weakened during landfall, while four of them (18%) intensified because their tracks passed 223
through a rather small fraction of the landmass in the southern part of the Philippine islands. 224
When the typhoons left the islands and reentered the ocean, 11 of them (47%) reintensified. 225
Note that these reintensification cases are not exactly the cases with reorganization of the 226
outer eyewall depicted in the microwave images. Regarding both the intensity and 227
eyewall evolutions, 8 typhoons (35%) experienced both reintensification and reorganization 228
of the outer eyewall after they reentered the ocean. These cases are called the obvious 229
reorganization eyewall cases, while the others are called the non-obvious reorganization 230
eyewall cases. 231
Figure 4 shows the duration and environmental conditions of the obvious and 232
non-obvious reorganization eyewall typhoons. The duration is defined by the time 233
difference between the landfall of the storm and its exit from the island. The duration is 234
random in the two groups (Fig. 4a). The mean duration of the obvious reorganization 235
eyewall cases is 17.6 hours, which is slightly longer than the non-obvious reorganization 236
eyewall cases of 16.0 hours. This is likely not statistically significant. 237
Comparisons of the environmental conditions between the obvious and non-obvious 238
reorganization eyewall cases (Figs. 4b, 4c, and 4d) indicate that the former appears to be 239
embedded in an environment with smaller vertical wind shear, higher low-level relative 240
humidity, and higher sea surface temperature than the later. The differences in mean 241
14
vertical wind shear, low-level relative humidity, and sea surface temperature between the 242
obvious and non-obvious reorganization eyewall cases are -3.0 m s-1, 6.8 %, 1.2 0C, 243
respectively. Note that the above results are based on 23 typhoons crossing the Philippine 244
islands in the past 11 years, thus making them statistically less significant. However, these 245
three environmental factors influencing the reorganization of the outer eyewall are rather 246
consistent with the necessary conditions of tropical cyclone genesis proposed by Gray 247
(1968), i.e., the favorable vertical shear pattern, moist mid-troposphere, and warm SST. 248
4. The eyewall evolution scenarios for typhoons crossing Taiwan 249
As shown in Fig. 2b, there are 19 typhoons crossing Taiwan during 2000-2010. The 250
same microwave image analyses are applied to these typhoons. Figure 5 shows six 251
particular typhoons (Talim in 2005, Sepat and Krosa in 2007, Jangmi in 2008, Morakot in 252
2009, and Fanapi in 2010) with smaller eyewalls before landfall and larger eyewalls after 253
entering the Taiwan Strait. Note that for typhoons crossing Taiwan, reorganization of the 254
outer eyewall is rarely observed because the storms generally pass through the Taiwan Strait 255
and make landfall over Mainland China within short periods of time (usually less than 36 256
hours). The information about the duration, intensity, and estimated radius of the 257
convective eyewall during the period of each storm crossing Taiwan are shown in Table 2. 258
The minimum and maximum duration of typhoons crossing Taiwan are 1 h and 46 h, 259
respectively, with an average of 12.7 h. For the intensity evolution, all typhoons weakened 260
15
during the landfall, except in 2 cases (Nari and Lekima in 2001) the typhoon intensified 261
after entering the Taiwan Strait. For the evolution of the eyewall convection, except 262
Kaitak in 2000 and Mindulle in 2004, in about 89% cases the major convective ring was 263
shifted from the central area to the outer area of the storm. 264
Figure 6 shows the radar reflectivity evolutions of the same six typhoons crossing 265
Taiwan. Following the subjective definition of the outer eyewall by Kossin and Sitkowski 266
(2009), the major convective ring is identified and plotted in the figure when there is at least 267
75% of a complete circular band with 0.5 degree width. Taking Typhoon Talim in 2005 as 268
an example (Figs. 6a and 6b), the major convective ring before landfall and after the storm 269
reentered the ocean is located at the radii of 0.6 and 1.7 degree latitude from the storm 270
center, respectively. Based on these radar images, the convective rings are usually located 271
within 1.0 degree radius from the storm center before landfall, and they shift to the outer 272
area ranging between 1.0 and 2.0 degree radii from the storm center after reentering the 273
ocean. In the cases of Talim, Sepat, Kroa, Jangmi, Morakot, and Fanapi, the landfall of the 274
typhoons in Taiwan lead to increment of the radius of the major convective ring by 0.9, 0.7, 275
1.4, 1.2, 1.4, and 0.2 degrees, respectively. These results are consistent with the eyewall 276
evolution as depicted by the microwave images in Fig. 5, while the quantitative analysis is 277
preformed based on radar images with high temporal and spatial resolution. 278
Detailed evaluations of the eyewall evolution for typhoons crossing Taiwan are 279
16
analyzed by the radius-time Hovmöller diagrams of the azimuthally-averaged radar 280
reflectivity as shown in Fig. 7. It can be found that most typhoons (53%) showed 281
contraction of the inner eyewall and the presence of concentric eyewall before landfall over 282
Taiwan (Figs. 7a, 7b, 7e, 7f, 7h, 7j, and 7l). The inner eyewalls in most cases are located 283
within 1.0 degree radius from the storm center, while the outer eyewalls are located at a 284
radius about 1.0-2.0 degrees from the storm center. Both the inner and outer eyewalls 285
obviously collapsed, and the convection areas shifted inward at the beginning of the landfall. 286
Hereafter, the inner eyewall gradually weakened and the outer eyewall reorganized several 287
hours later after the storms made landfall. The radii of the reorganized outer eyewall range 288
between 1.0-2.2 degrees, with the smallest and largest being in Pabuk in 2007 and Haitang 289
in 2005 (as shown in Fig.6d and Fig. 7a). Note that except for Sinlaku, Jangmi, and 290
Fanapi, the reorganized eyewall did not contract when the storms cross the Taiwan Strait. 291
Overall, results from these Hovmöller-diagram analyses are not only consistent with 292
those depicted by the microwave images, but also provide a high temporal and spatial 293
observational evidence in demonstrating the terrain-induced eyewall evolution when 294
typhoons cross islands as indicated earlier in Wu et al. (2003). Furthermore, the 295
contraction of the eyewall before the storms made landfall at Taiwan was also consistent 296
with the observational analyses (Willoughby and Black 1996; Chang et al. 2009) and the 297
numerical simulations (Wu et al. 2009). 298
17
Figure 8 shows the radius-time Hovmöller diagrams of the azimuthally-averaged 299
(relative to the storm center) rainfall rate observed hourly for typhoons crossing Taiwan 300
during 2005-2010. The azimuthally-averaged rainfall rate is calculated from the storm 301
center to 5 degree radius with a bin size of 0.1 degree radius. It should be noted that since 302
the frequency of rainfall data is observed hourly, the distance between each rain gauge and 303
the location of the storm center is calculated every hour, while the hourly location of the 304
storm center is interpolated from 3-hourly CWB best track. It can be found that before the 305
landfall and first several hours during the landfall period, except for Pabuk in 2007 and 306
Kalmaegi in 2008 (Figs. 8d and 8g), the areas with larger rainfall rate were always located 307
within 0.5 degree radius from the storm center. Then, the rainfall rate in the storm core 308
gradually vanished and the locations of the maximum rainfall rate shifted outward to 1.0-2.0 309
degree radius from the storm center when the storms left Taiwan. These results are 310
consistent with the analyses of the radius-time Hovmöller diagrams of radar reflectivity. 311
Furthermore, these rainfall analyses indicate that the topography of Taiwan not only alters 312
the evolution of the eyewall, but also modifies the rainfall pattern of landfalling typhoons. 313
Note that the Pabuk passed through the southern part of Taiwan in just one hour. 314
Therefore, the modulation of the eyewall and rainfall pattern for Pabuk may not be as 315
significant as the other cases. The unique rainfall patterns induced by Kalmaegi and 316
Morakot are related to the confluent region associated with the storm circulation and the 317
18
environmental southwesterly monsoon flows as proposed in Ge et al. (2010) and Hong et al. 318
(2010). However, the rainfall pattern expanding outward from the storm center caused by 319
the terrain effect is consistent with the process revealed in this study. 320
5. Concluding remarks 321
The statistical characteristics of the terrain-induced eyewall evolution when a typhoon 322
makes landfall, as in the discussion about Typhoon Zeb (1998) over Luzon Islandby Wu et 323
al. (2003, 2009), are examined based on the best track data, advanced satellite microwave 324
imagers, high spatial and temporal ground-observed radar images and rain gauges in this 325
study. Based on the analyses of JTWC best track from 1945-2010, it is found that 326
annuallyat least 3.0 TCs with sustained surface maximum winds larger than 64 knots passed 327
through the Philippine islands and 1.3 through Taiwan. During the eleven-year period 328
from 2000 and 2010, the eyewall evolution associated with the 23 and 19 typhoons 329
respectively making landfall in the Philippine islands and Taiwan, are detected by the 85-91 330
GHz brightness temperature satellite microwave imagers. Based on these available 331
microwave images for typhoons crossing the Philippine islands, the results indicate that the 332
radius of the eyewall increased during landfall in 91% of the landfalling typhoons, while in 333
57% of the cases the radius of the eyewall contracted when the typhoon reentered the ocean. 334
For typhoons crossing Taiwan, both the microwave images from satellites and the 335
reflectivity images from ground-based radar observations are applied to analyze the eyewall 336
19
evolution. It is found that 89% of cases show an expansion of the eyewall during the 337
landfall period. However, reorganization of the outer eyewall is seldom observed due to 338
the limited time the typhoon spends over the Taiwan Strait before making landfall in 339
Mainland China. Furthermore, based on the observed rainfall analyses, an expansion of 340
the rainfall pattern due to the effect of terrain is also revealed. These results provide a 341
conceptual model that may help improve the forecast of severe weather associated with 342
such type of landfalling storms. It should be noted that the findings of this study could 343
gain more insights to the impact of terrain on rainfall caused by landfalling TCs. In 344
traditional view, the landfalling TC provides stronger wind on the windward side, which 345
then enhances the orographic lifting, hereafter produces more rainfall (Wu and Kuo 1999). 346
The large-scale environmental conditions are examined to investigate the differences 347
between the contracted and non-contracted outer eyewall cases. The results show that 348
small vertical wind shear, high low-level relative humidity, and warm sea surface 349
temperatures are important for the reorganization of the outer eyewall after the landfalling 350
typhoons reenter the ocean. 351
The eyewall evolution examined by the satellite microwave imagers is consistent with 352
the finding by Brand and Blelloch (1973), who indicated that the islands of the Philippines 353
affected the eye diameter of passing typhoons based on aircraft reconnaissance data. 354
Although this special eyewall evolution had been identified by Brand and Blelloch (1973), 355
20
we have documented the climatological aspects of such eyewall evolution and also shown 356
that the advanced satellite microwave imagers can depict the eyewall evolution for almost 357
all cases selected. Furthermore, this study also shows that this special eyewall evolution 358
can also be found in typhoons making landfall in Taiwan from high spatial and temporal 359
radar images. 360
Although this terrain-induced eyewall evolution has been better documented, some 361
issues remain unsolved and are yet to be addressed in future studies. (1) How is such an 362
eyewall evolution influenced by the size of the land, the height of the terrain, and the size 363
and intensity of the landfalling storm? (2) What determines the timing for the inner 364
eyewall to break down after the storm makes landfall? (3) What is the relationship 365
between land with different surface roughness, latent heat flux supply, and topography? (4) 366
How does the environmental vertical wind shear affect this eyewall evolution? It is 367
expected that more physical insights can be obtained from more well-designed sensitivity 368
numerical experiments with high-resolution cloud-resolving models. 369
Acknowledgments: The authors would like to express their sincere appreciation to Dr. P.-L. 370
Chang for providing the CWB radar mosaic data. The work is supported by the National 371
Science Council of Taiwan through Grants NSC-98-2111-M-034-002, 372
NSC98-2111-M-002-008-MY3, and NSC-99-2111-M-034-004. YW has been supported in 373
part by U.S. NSF grant ATM-0754039. 374
375
21
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447
25
Table and figure captions 448
Table 1. Information for the 23 selected typhoons that crossed the Philippines. INTi (INTo) 449
represents the maximum sustained surface wind when the tropical storms moved in 450
(out) of the islands and INTm represents the maximum sustained surface wind within 451
48 h after the storm left the islands. REi, REo, and REr mean the size of the major 452
convective ring of the eyewall estimated by the satellite microwave images, for 453
typhoons before landfall, while leaving the islands, and while reentering the ocean, 454
respectively. Ring 1 indicates the area within 0.5 degree latitudes in radius from the 455
storm center, and rings 2, 3, 4 represent the distance ranges of 0.5-1, 1-1.5, 1.5-2.0 456
degrees in radius, respectively. Superscript “*” means that the major convective 457
ring expanded after landfall, and “**” denotes events in which the major 458
convective ring contracted after reentering the ocean. OEC means the obvious 459
reorganization of the outer eyewall when INTm is larger than INTo and REr is small 460
than REo. Value of“---”indicates that the scenario is not identifiable. 461
Table 2. Same as in Table 1, but for typhoons that crossed Taiwan. 462
Fig. 1. The terrain height of 30-minute USGS (United States Geological Survey) digital 463
elevation model data (unit: m) of the Philippines (a) and Taiwan (b), respectively. 464
The triangle and cross symbols in (b) represent the CWB radar and automatic rain 465
gauge data stations. 466
26
Fig. 2. Histogram of numbers of tropical storms (TS) and typhoons (TY) from JTWC best 467
track from 1945 to 2010. TSI and TYI (TSO and TYO) indicate the tracks of TS and 468
TY (not) crossing the Philippines (a) and Taiwan (b), respectively. 469
Fig. 3. The microwave images of 85-91-GHz brightness temperature for three typhoons 470
(Xangsane in 2000, Cimaron on 2006, and Megi in 2010), one with the outer eyewall 471
reorganized and the other without (Mitag in 2007). Each case contains three 472
individual images, showing the inner-core structure when typhoons moved into and 473
away from the Philippines, and the reorganization of the larger outer eyewall when 474
the storms reentered the ocean. The four rings showed in (a) are each within the range 475
of 0-0.5, 0.5-1, 1-1.5, 1.5-2.0 degrees from the storm center. 476
Fig. 4. The duration (a), environmental vertical wind shear (b), low-level relative humidity 477
(c), and sea surface temperature (d) for TY cases which crossed the Philippines in 478
2000-2010. The vertical wind shear is averaged between 2 to 8 degree radius from 479
the storm center and averaged within 48h after the storm left the islands. The low 480
level relative humidity is averaged in a 5o x 5o box centered at the storm center and 481
averaged within 48h after the storm left the islands. The sea surface temperature is 482
also calculated as the mean in a 5o x 5o box centered at the storm center at the same 483
time before the typhoon made landfall. 484
Fig. 5. Same as in Fig. 3, but for six typhoons (Talim in 2005, Sepat and Krosa in 2007, 485
27
Jangmi in 2008, Morakot in 2009, and Fanapi in 2010) that crossed Taiwan. Each 486
case contains two individual images, showing the inner-core structure changes when 487
typhoons moved into and away from Taiwan. 488
Fig. 6. The radar reflectivity evolutions of six particular typhoons (Talim in 2005, Sepat and 489
Krosa in 2007, Jangmi in 2008, Morakot in 2009, and Fanapi in 2010) that crossed 490
Taiwan. Each case contains three individual images, showing the inner-core 491
structure changes when typhoons moved into and away from Taiwan. Three solid 492
rings with 0.5 degree radius are shown for identifying the maximum convection 493
regions near the core of the typhoons. The locations of typhoons are interpolated 494
from 3-h best track of CWB. 495
Fig. 7. Radius-time Hovmöller diagram of the azimuthally-averaged radar reflectivity (dBZ) 496
for 12 typhoons that crossed Taiwan. The dotted (dashed) horizontal line indicates 497
the time when typhoons made landfall at (exited from) Taiwan. 498
Fig. 8. Radius-time Hovmöller diagram of the azimuthally-averaged (relative to the storm 499
center) observed rainfall rate (mm h-1) for 12 typhoons that crossed Taiwan. The 500
dotted (dashed) horizontal line indicates the time when typhoons made landfall at 501
(exited from) Taiwan. The observed rainfall rates are calculated from the CWB 502
automatic rain gauge. 503
504
28
Table 1. Information for the 23 selected typhoons that crossed the Philippines. INTi (INTo) 505 represents the maximum sustained surface wind when the tropical storms moved in (out) of 506 the islands and INTm represents the maximum sustained surface wind within 48 h after the 507 storm left the islands. REi, REo, and REr mean the size of the major convective ring of 508 the eyewall estimated by the satellite microwave images, for typhoons before landfall, 509 while leaving the islands, and while reentering the ocean, respectively. Ring 1 indicates 510 the area within 0.5 degree latitudes in radius from the storm center, and rings 2, 3, 4 511 represent the distance ranges of 0.5-1, 1-1.5, 1.5-2.0 degrees in radius, respectively. 512 Superscript “*” means that the major convective ring expanded after landfall, and 513 “**” denotes events in which the major convective ring contracted after reentering the 514 ocean. OEC means the obvious reorganization of the outer eyewall when INTm is larger 515 than INTo and REr is small than REo. Value of“---”indicates that the scenario is not 516 identifiable. 517
TC NAME DT INTi INTo INTm REi REo REr OEC
XANGSANE(2000) 26 70 55 90 1-2 2-3* 2** Yes BEBINCA(2000) 25 65 65 65 1 2* 1-2** No IMBUDO(2003) 7 130 80 90 1-2 3-4* 2-3** Yes KROVANH(2003) 7 90 70 85 1-2 2* 2 No MELOR(2003) 11 65 65 65 1-2 2* 2 No MUIFA(2004) 18 115 60 90 1-2 4* 2-3** Yes NANMADOL(2004) 4 130 100 100 1-2 2-3* 4 No ROKE(2005) 20 80 40 40 1 --- --- No CHANCHU(2006) 36 75 75 125 1-2 2-3* 2** Yes XANGSANE(2006) 24 65 100 115 1-2 2-3* 2** Yes CIMARON(2006) 10 140 95 100 1 3-4* 2-3** Yes CHEBI(2006) 11 125 95 95 1-2 2* 1-2** No DURIAN(2006) 20 135 95 95 1-2 2* 2 No UTOR(2006) 23 65 90 90 1-2 1-2 1-2 No PEIPAH(2007) 9 60 55 75 2 3-4* 2** Yes MITAG(2007) 5 95 65 65 2-3 2-3 2-3 No HALONG(2008) 15 65 45 55 1-2 2-3* 2-3 No FENGSHEN(2008) 53 70 60 60 2 3-4* 3-4 No NURI(2008) 7 95 90 90 2 4* 2-3** No CHAN-HOM(2009) 10 90 45 45 1-2 2* 2-3 No PARMA(2009) 12 135 65 65 2 3-4* 2-3** No CONSON(2010) 19 70 55 75 1-2 2-3* 2** Yes MEGI(2010) 9 155 90 115 1-2 3-4* 2-3** Yes
29
Same as in Table 1, but for typhoons that crossed Taiwan.. 518
TC NAME DT INTi INTo INTm REi REo REr OEC
KAITAK(2000) 5 65 55 55 2-3 --- --- --- BILIS(2000) 5 140 120 120 1-2 4* --- --- TORAJI(2001) 11 95 75 75 1-2 2* --- --- NARI(2001) 46 85 30 55 1-2 2* --- --- LEKIMA(2001) 28 90 25 35 2 4* --- --- MORAKOT(2003) 8 65 60 60 2 2-3* --- --- MINDULLE(2004) 11 65 45 45 3-4 2-3 --- --- HAITANG(2005) 11 95 75 75 2 2-3* --- --- TALIM(2005) 6 110 70 70 2 4* --- --- LONGWANG(2005) 5 115 90 90 1-2 2-3* --- --- PABUK(2007) 1 70 65 65 1-2 1-2* --- --- SEPAT(2007) 6 105 85 85 2 3-4* --- --- KROSA(2007) 3 115 90 90 1-2 4* --- --- KALMAEGI(2008) 10 90 55 55 1-2 3-4* --- --- FUNGWONG(2008) 17 95 70 70 2 3* --- --- SINLAKU(2008) 26 100 65 65 2 2-3* --- --- JANGMI(2008) 21 115 60 60 2-3 3-4* --- --- MORAKOT(2009) 11 80 45 45 2 4* --- --- FANAPI(2010) 10 105 65 70 2 3-4* --- ---
519
(a) (b)
Fig. 1. The terrain height of 30‐minute USGS (United States Geological Survey ) digital elevation model data (unit: m) of the Philippines (a) and Taiwan (b), respectively. The triangle and cross symbols in (b) represent the CWB radar and automatic rain gauge datastations in Taiwan.
35
40TYI TYO TSI TSO(a)
15
20
25
30
Number
0
5
10
15
Case
35
40TYI TYO TSI TSO
1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010Year
(b)
15
20
25
30
e Number
0
5
10
1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
Cas
1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
Fig. 2. Histogram of numbers of tropical storms (TS) and typhoons (TY) from JTWC best track from 1945 to 2010. TSI and TYI (TSO, TYO) indicate the tracks of TS and TY (not) crossing the Philippines (a) and Taiwan (b), respectively.
Year
(a) 00Xangsane (b) 00Xangsane (c) 00Xangsane
(d) 06Cimaron (e) 06Cimaron (f) 06Cimaron(d) 06Cimaron (e) 06Cimaron (f) 06Cimaron
(g) 10Megi (h) 10Megi (i) 10Megi
(j) 07Mitage (k) 07Mitage (l) 07Mitage
Fig. 3. The microwave images of 85‐91‐GHz brightness temperature for three typhoons (Xangsane in 2000, g g g p yp ( g ,Cimaron on 2006, and Megi in 2010) with the reorganization of the outer eyewall and one without (Mitagin 2007). Each case contains three individual images, showing the inner‐core structure when typhoons moved in, moved away from the Philippines and the reorganization of the larger outer eyewall when the storms reentered the ocean. Four rings showed in (a) indicate the distance from the storm center with radii of 0‐0.5, 0.5‐1, 1‐1.5, 1.5‐2.0 degrees, respectively.
10
12
14
16
18
20
22
nd
sh
ear
(m/s
)
Shear
30
40
50
60
n t
ime
(h
r)
Duration time
(a) (b)Mean = 15.9 Mean = 17.6
Mean = 10.0 Mean = 7.0
0
2
4
6
8
10
00be
binc
a
03kr
ovan
h
03m
elor
04na
nmad
ol
05ro
ke
06ch
ebi
06du
rian
06ut
or
07m
itag
08ha
long
08fe
ngsh
en
08nu
ri
09ch
anho
m
09pa
rma
00xa
ngsa
ne
03im
budo
04m
uifa
06ch
anch
u
06xa
ngsa
ne
06ci
mar
on
07pe
ipah
10co
nson
10m
egi
Ve
rtic
al w
in
Case
0
10
20
00
bebi
nca
03kr
ova
nh
03
mel
or
04n
anm
ado
l
05ro
ke
06c
heb
i
06d
uria
n
06u
tor
07
mita
g
08h
alon
g
08
fen
gsh
en
08nu
ri
09
cha
nhom
09
parm
a
00x
ang
san
e
03im
bu
do
04
mu
ifa
06
chan
chu
06x
ang
san
e
06ci
ma
ron
07p
eipa
h
10co
nso
n
10
me
gi
Du
rati
on
Case
24
25
26
27
28
29
30
31
32
urf
ac
e T
em
pe
ratu
re(℃
)
SST
60
65
70
75
80
85
90
95
100
Hu
mid
ity
(%)
RH
(c) (d)Mean = 75.0 Mean = 81.8 Mean = 27.5 Mean = 28.7
22
23
00b
ebin
ca
03k
rova
nh
03m
elo
r
04
nan
mad
ol
05ro
ke
06
che
bi
06
duri
an
06
utor
07m
itag
08h
alo
ng
08fe
ngsh
en
08nu
ri
09ch
an
hom
09p
arm
a
00
xan
gsa
ne
03i
mbu
do
04m
uifa
06ch
an
chu
06
xan
gsa
ne
06ci
ma
ron
07p
eip
ah
10c
ons
on
10m
egiS
ea
S
Case
50
55
00b
ebin
ca
03k
rova
nh
03m
elo
r
04n
anm
adol
05r
oke
06
che
bi
06du
rian
06
uto
r
07m
itag
08h
alo
ng
08f
en
gshe
n
08n
uri
09c
han
hom
09pa
rma
00xa
ngs
ane
03i
mbu
do
04m
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anc
hu
06xa
ngs
ane
06c
ima
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ah
10c
ons
on
10m
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Case
Fig. 4. The duration (a), environmental vertical wind shear (b), low‐level relative humidity (c), and sea surface temperature (d) for TY cases which crossed the Philippines in 2000‐2010. The vertical wind shear is averaged from 2 to 8 degree radius of the storm center and averaged within 48h after the storm left the islands. The low level relative humidity is averaged in a 5o x 5o box centered at the storm center and averaged within 48h after the storm left the islands. The sea surface temperature is also calculated as the mean in a 5o x 5o box centered at the storm center at the time before the typhoon made landfalltyphoon made landfall.
(a) 05Talim (b) 05Talim (c) 07Sepat (d) 07Sepat
(e) 07Krosa (f) 07Krosa (g) 08Jangmi (h) 08Jangmi
(i) 09Morakot (j) 09Morakot (k) 10Fanapi (l) 10Fanapi
Fig. 5. Same as in Fig. 2, but for six typhoons (Talim in 2005, Sepat and Krosa in 2007, Jangmi in 2008, Morakot in 2009, and Fanapi in 2010) that crossed Taiwan. Each case contains two individual images, showing the inner‐core structure changes when typhoons moved in, moved away from Taiwan.
(a) 05Talim (c) 07Sepat (d) 07Sepat
(b) 05Talim
(g) 08Jangmi (h) 08Jangmi
(e) 07Krosa (f) 07Krosa
(i) 09Morakot (k) 10Fanapi (l) 10Fanapi
( ) ( )
Fig. 6. The radar reflectively evolutions for six particular typhoons (Talim in 2005, Sepat and Krosa in 2007,
(j) 09Morakot
Jangmi in 2008, Morakot in 2009, and Fanapi in 2010) that crossed Taiwan. Each case contains three individual images, showing the inner‐core structure changes when typhoons moved in, moved away from Taiwan. Three solid rings with 0.5 degree radius are shown for identifying the maximum convection regions near the core of the typhoons. The locations of typhoons are interpolated from 3‐h best track of CWB.
Typhoon Hiatang (2005)
19/12Z
Typhoon Talim (2005) Typhoon Longwang (2005)(a) (b) (c) (d) Typhoon Pabuk (2007)
17/12Z
17/23Z
18/15Z
31/06Z
31/18Z
01/00Z
01/12Z
01/12Z
01/21Z
02/02Z
02/18Z
07/09Z
07/17Z07/18Z
08/03Z
Typhoon Sepat (2007)
17/21Z
18/03Z
19/03Z
Typhoon Krosa (2007)
06/17Z
07/09Z
Typhoon Kalmaegi (2008)
17/13Z
17/23Z
18/15Z
Typhoon Fungwong (2008)
27/22Z
28/06Z
29/00Z
(e) (f) (g) (h)
17/03Z 05/18Z
06/06Z
16/15Z 27/06Z
27/22Z
Typhoon Sinlaku (2008)
15/00Z
Typhoon Jangmi (2008)
29/09Z
Typhoon Morakot (2009)
08/18Z
(i) (j) (k) (l) Typhoon Fanapi (2010)
20/00Z
13/00Z
13/17Z
14/10Z
27/15Z
28/08Z
28/20Z
06/18Z
07/14Z
08/05Z
18/06Z
19/00Z
19/09Z
Fig. 7. Radius‐time Hovmöller diagram of the azimuthally‐averaged radar reflectively (dBZ) for 12 typhoons that crossed Taiwan. The dotted (dashed) horizontal line indicates the time when typhoons made landfall at (exited from) Taiwan.
13/00Z 27/15Z 06/18Z 18/06Z
(a) (b) (c) (d)Typhoon Hiatang (2005)
19/12Z
Typhoon Talim (2005) Typhoon Longwang (2005) Typhoon Pabuk (2007)
17/12Z
17/23Z
18/15Z
31/06Z
31/18Z
01/00Z
01/12Z
01/12Z
01/21Z
02/02Z
02/18Z
07/09Z
07/18Z
08/16Z
07/17Z
Typhoon Fungwong (2008)
29/00Z
Typhoon Kalmaegi (2008)
17/23Z
18/15Z
Typhoon Krosa (2007)
07/09Z
(e) (f) (g) (h)
17/12Z 31/06Z 01/12Z 07/09Z
Typhoon Sepat (2007)
19/03Z
Typhoon Sinlaku (2008) Typhoon Jangmi (2008) Typhoon Morakot (2009) Typhoon Fanapi (2010)
27/06Z
27/22Z
28/06Z
16/15Z
17/13Z
05/18Z
06/06Z
06/17Z
(i) (j) (k) (l)
17/03Z
17/21Z
18/03Z
13/17Z
14/10Z
15/00Z
28/08Z
28/20Z
29/09Z
07/14Z
08/05Z
08/18Z
19/00Z
19/09Z
20/00Z
(i) (j) (k) (l)
Fig. 8. Radius‐time Hovmöller diagram of the azimuthally‐averaged (relative to the storm center) observed rainfall rate (mm h‐1) for 12 typhoons that crossed Taiwan. The dotted (dashed) horizontal line indicates the time when typhoons made landfall at (exited from) Taiwan The observed rainfall rates are
13/00Z 27/15Z 06/18Z 18/06Z
indicates the time when typhoons made landfall at (exited from) Taiwan. The observed rainfall rates are calculated from the CWB automatic rain gauge.
