Yi-Ting Yang

Western north Pacific typhoons with concentric eyewalls

Abstract This study examines the intensity change and moat dynamics of typhoons with concentric eyewalls using passive microwave data and best-track data in the western North Pacific between 1997 and 2006. Of the 225 typhoons examined, 55 typhoons and 62 cases with concentric eyewalls have been identified. The data indicate that approximately 57% of category 4 and 72% of category 5 typhoons possessed concentric eyewalls at some point during their lifetime. While major typhoons are most likely to form concentric eyewalls, the formation of the concentric structure may not be necessarily at the lifetime maximum intensity. Approximately one-third of concentric eyewall cases are formed at the time of maximum intensity. The moat is known to be heavily influenced by the subsidence forced by the two eyewalls. Rozoff et al. proposed that the rapid filamentation dynamics may also contribute to the organization of the moat. This paper examines the possibility of rapid filamentation dynamics by devising a filamentati...

Structural and Intensity Changes of Concentric Eyewall Typhoons in the Western North Pacific Basin

AbstractAn objective method is developed to identify concentric eyewalls (CEs) for typhoons using passive microwave satellite imagery from 1997 to 2011 in the western North Pacific basin. Three CE types are identified: a CE with an eyewall replacement cycle (ERC; 37 cases), a CE with no replacement cycle (NRC; 17 cases), and a CE that is maintained for an extended period (CEM; 16 cases). The inner eyewall (outer eyewall) of the ERC (NRC) type dissipates within 20 h after CE formation. The CEM type has its CE structure maintained for more than 20 h (mean duration time is 31 h). Structural and intensity changes of CE typhoons are demonstrated using a T–Vmax diagram (where T is the brightness temperature and Vmax is the best-track estimated intensity) for a time sequence of the intensity and convective activity (CA) relationship. While the intensity of typhoons in the ERC and CEM cases weakens after CE formation, the CA is maintained or increases. In contrast, the CA weakens in the NRC cases. The NRC (CEM) c...

Large Increasing Trend of Tropical Cyclone Rainfall in Taiwan and the Roles of Terrain

AbstractTaiwan, which is in the middle of one of the most active of the western North Pacific Ocean’s tropical cyclone (TC) zones, experienced a dramatic increase in typhoon-related rainfall in the beginning of the twenty-first century. This record-breaking increase has led to suggestions that it is the manifestation of the effects of global warming. With rainfall significantly influenced by its steep terrain, Taiwan offers a natural laboratory to study the role that terrain effects may play in the climate change of TC rainfall. Here, it is shown that most of the recently observed large increases in typhoon-related rainfall are the result of slow-moving TCs and the location of their tracks relative to the meso-α-scale terrain. In addition, stronger interaction between the typhoon circulation and southwest monsoon wind surges after the typhoon center moves into the Taiwan Strait may cause a long-term trend of increasing typhoon rainfall intensity, which is not observed before the typhoon center exits Taiwa...

Long-Lived Concentric Eyewalls in Typhoon Soulik (2013)

AbstractThe authors report on western North Pacific Typhoon Soulik (2013), which had two anomalously long-lived concentric eyewall (CE) episodes, as identified from microwave satellite data, radar data, and total precipitable water data. The first period was 25 h long and occurred while Soulik was at category 4 intensity. The second period was 34 h long and occurred when Soulik was at category 2 intensity. A large moat and outer eyewall width were present in both CE periods, and there was a significant contraction of the inner eyewall radius from the first period to the second period. The typhoon intensity decrease was partially due to encountering unfavorable environmental conditions of low ocean heat content and dry air, even though inner eyewall contraction would generally support intensification. The T–Vmax diagram (where T is the brightness temperature and Vmax is the best track–estimated intensity) is used to analyze the time sequence of the intensity and convective activity. The convective activity...

Relationship between Typhoons with Concentric Eyewalls and ENSO in the Western North Pacific Basin

AbstractThe typhoons with concentric eyewalls (CE) over the western North Pacific in different phases of the El Nino–Southern Oscillation (ENSO) between 1997 and 2012 are studied. They find a good correlation (0.72) between the annual CE typhoon number and the oceanic Nino index (ONI), with most of the CE typhoons occurring in the warm and neutral episodes. In the warm (neutral) episode, 55% (50%) of the typhoons possessed a CE structure. In contrast, only 25% of the typhoons possessed a CE structure in the cold episode. The CE formation frequency is also significantly different with 0.9 (0.2) CEs per month in the warm (cold) episode. There are more long-lived CE cases (CE structure maintained more than 20 h) and typhoons with multiple CE formations in the warm episodes. There are no typhoons with multiple CE formations in the cold episode. The warm episode CE typhoons generally have a larger size, stronger intensity, and smaller variation in convective activity and intensity. This may be due to the fact ...

Deep convection in elliptical and polygonal eyewalls of tropical cyclones

In observations, tropical cyclones with cyclonically rotating elliptical eyewalls are often characterized by wave number 2 (WN2) deep convection located at the edge of the major axis. A simple modeling framework is used to understand this phenomenon, where a nondivergent barotropic model (NBM) is employed to represent the elliptical vortex in the free atmosphere, and an asymmetric slab boundary layer (SBL) model is used to simulate the frictional boundary layer (BL) underneath the free atmosphere. The interaction is one way in that the overlying cyclonic flow drives the BL, but the BL pumping does not feed back to the overlying flow. The nonlinear-balanced pressure field from the NBM drives the winds in the SBL model, which then causes BL convergence and pumping near the eyewall. The strong updrafts at the edge of the major axis for the elliptic vortex in the BL are induced by the larger convergent radial wind from the asymmetric distribution of the pressure fields of the free atmosphere with noncircular vortex. The large radial inflow maintains the supergradient wind at the edge of the elliptical vortex. The results emphasize the cyclonic rotation of the WN2 feature of strong updrafts at the top of the BL from the local shock-like BL radial wind structure. Similar radial profiles and strong BL top updrafts occur at the edges of higher-order polygonal eyewalls with the magnitude of the peak updraft decreasing as the wave number structure of the vortex increases.

Large Increasing Trend of Tropical Cyclone Rainfall in Taiwan and the Roles of Terrain and Southwest Monsoon

Large Increasing Trend of Tropical Cyclone Rainfall in Taiwan and the Roles of Terrain and Southwest Monsoon Hung-Chi Kuo∗, Chih-Pei Chang∗,†,¶, Yi-Ting Yang∗, Yu-Han Chen∗, Shih-Hao Su‡ and Lee-Yaw Lin§ ∗National Taiwan University, Taipei, Taiwan †Naval Postgraduate School, Monterey, California, U.S.A. ‡Chinese Culture University, Taipei, Taiwan §National Center for Disaster Reduction, Taipei, Taiwan ¶cpchang@nps.edu

Satellite Climatology of Tropical Cyclone with Concentric Eyewalls

An objective method is developed to identify concentric eyewalls (CEs) for tropical cyclones (TCs) using passive microwave satellite imagery from 1997 to 2014 in the western North Pacific (WNP) and Atlantic (ATL) basin. There are 91 (33) TCs and 113 (50) cases with CE identified in the WNP (ATL). Three CE structural change types are classified as follows: a CE with the inner eyewall dissipated in an eyewall replacement cycle (ERC, 51 and 56% in the WNP and ATL), a CE with the outer eyewall dissipated first and the no eyewall replacement cycle (NRC, 27 and 29% in the WNP and ATL), and a CE structure that is maintained for an extended period (CEM, 23 and 15% in the WNP and ATL). The moat size and outer eyewall width in the WNP (ATL) basin are approximately 20–50% (15–25%) larger in the CEM cases than that in the ERC and NRC cases. Our analysis suggests that the ERC cases are more likely dominated by the internal dynamics, whereas the NRC cases are heavily influenced by the environment condition, and both the internal and environmental conditions are important in the CEM cases. A good correlation of the annual CE TC number and the Oceanic Niño index is found (0.77) in WNP basin, with most of the CE TCs occurring in the warm episodes. In contrast, the El Niño/Southern Oscillation (ENSO) may not influence on the CE formation in the ATL basin. After the CE formation, however, the unfavorable environment that is created by ENSO may reduce the TC intensity quickly during warm episode. The variabilities of structural changes in the WNP basin are larger than that in the ATL basin.