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Hung-Chi Kuo
,
Chih-Pei Chang
, and
Ching-Hwang Liu

Abstract

This study examines the convection and rapid filamentation in Typhoon Sinlaku (2008) using the Naval Research Laboratory (NRL) P-3 aircraft data collected during the Tropical Cyclone Structure 2008 (TCS-08) and The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) field experiments. The high-resolution aircraft radar and wind data are used to directly compute the filamentation time, to allow an investigation into the effect of filamentation on convection. During the reintensification stage, some regions of deep convection near the eyewall are found in the vorticity-dominated area where there is little filamentation. In some other parts of the eyewall and the outer spiral rainband region, including areas of upward motion, the filamentation process appears to suppress deep convection. However, the magnitude of the suppression differs greatly in the two regions. In the outer spiral band region, which is about 200 km from the center, the suppression is much more effective, such that the ratio of the deep convective regime occurrence over the stratiform regime varies from around 50% (200%) for filamentation time shorter (longer) than 24 min. In the eyewall cloud region where the conditions are conducive to deep convection, the filamentation effect may be quite limited. While effect of filamentation suppression is only about 10%, it is still systematic and conspicuous for filamentation times shorter than 19 min. The results suggest the possible importance of vortex-scale filamentation dynamics in suppressing deep convection and organizing spiral bands, which may affect the development and evolution of tropical cyclones.

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Yun-Lan Chen
,
Chung-Hsiung Sui
,
Chih-Pei Chang
, and
Kai-Chih Tseng

Abstract

This paper studies the influences of the Madden–Julian oscillation (MJO) on East Asian (EA) winter rainfall using the singular value decomposition (SVD) approach. This method uses two-dimensional instead of latitudinally averaged variables in the commonly used real-time multivariate MJO (RMM) index. A comparison of the two approaches is made using the same OLR and zonal wind data over 37 boreal winter seasons of December–March. The SVD composite reveals a more conspicuous and coherent variation throughout the MJO cycle, while the RMM composite is more ambiguous. In particular, the SVD analysis identifies the convection anomalies over the Maritime Continent and the subtropical western Pacific (MCWP) as a major cause of enhanced rainfall in EA at RMM phases 8 and 1. This is at least one-eighth of a cycle earlier than the phases of convection development over the Indian Ocean (IO) that were emphasized by previous studies. A linearized global baroclinic model is used to demonstrate the mechanism of MJO forcing on EA rainfall during various phases, with a focus on the MCWP cooling. The result shows that the anomalous MCWP cooling and the resultant low-level anticyclonic flow interact with the East Asian jet, leading to an overall weakened EA winter monsoon circulation. The associated anomalous overturning circulation, with ascending motion and low-level horizontal moisture convergence in EA, contributes to the enhanced rainfall. This model result supports the interpretation of the SVD analysis, in that the MCWP cooling induced anomalous meridional circulation is a more direct cause of enhanced EA rainfall than the IO heating (or the IO–MCWP heating dipole) induced Rossby wave teleconnection.

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Hung-Chi Kuo
,
Chih-Pei Chang
,
Yi-Ting Yang
, and
Hau-Jang Jiang

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 filamentation moat width parameter. This parameter can be computed from the best-track typhoon intensity and the passive microwave satellite-estimated inner eyewall radius for each typhoon with concentric eyewalls. The filamentation moat width explains 40% of the variance of the satellite-observed moat width in the group with concentric eyewall formation intensity greater than 130 kt.

The typhoon intensity time series in both the concentric and nonconcentric composites are studied. The time series of intensity is classified according to the 24-h intensity change before and after the concentric eyewalls formation. The averaged concentric eyewall formation latitudes in the groups with negative intensity change before concentric eyewall formation are at higher latitudes than that of the positive intensity change groups. Intensity of the concentric typhoons tends to peak at the time of secondary eyewall formation, but the standard model of intensification followed by weakening is valid for only half of the cases. Approximately 74% of the cases intensify 24 h before secondary eyewall formation and approximately 72% of the cases weaken 24 h after formation. The concentric composites have a much slower intensification rate 12 h before the peak intensity (time of concentric formation) than that of the nonconcentric composites. For categories 4 and 5, the peak intensity of the concentric typhoons is comparable to that of the nonconcentric typhoons. However, 60 h before reaching the peak the concentric composites are 25% more intense than the nonconcentric composites. So a key feature of concentric eyewall formation appears to be the maintenance of a relatively high intensity for a longer duration, rather than a rapid intensification process that can reach a higher intensity.

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Bin Wang
,
Zhiwei Wu
,
Chih-Pei Chang
,
Jian Liu
,
Jianping Li
, and
Tianjun Zhou

Abstract

This study investigates the causes of interannual-to-interdecadal variability of the East Asian (EA; 0°–60°N, 100°–140°E) winter monsoon (EAWM) over the past 50 yr (1957–2006). The winter mean surface air temperature variations are dominated by two distinct principal modes that together account for 74% of the total temperature variance. The two modes have notably different circulation structures and sources of variability. The northern mode, characterized by a westward shift of the EA major trough and enhanced surface pressure over central Siberia, represents a cold winter in the northern EA resulting from cold-air intrusion from central Siberia. The southern mode, on the other hand, features a deepening EA trough and increased surface pressure over Mongolia, representing a cold winter south of 40°N resulting from cold-air intrusion from western Mongolia. The cold northern mode is preceded by excessive autumn snow covers over southern Siberia–Mongolia, whereas the cold southern mode is preceded by development of La Niña episodes and reduced snow covers over northeast Siberia. These remarkably different spatiotemporal structures and origins are primarily associated with interannual variations. On the decadal or longer time scale their structures are somewhat similar and are preceded by similar autumn sea surface temperature anomalies over the North Atlantic and tropical Indian Ocean. The two modes found for the EA region also represent the winter temperature variability over the entire Asian continent. Thus, study of the predictability of the two modes may shed light on understanding the predictable dynamics of the Asian winter monsoon.

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Pei-ken Kao
,
Chi-Cherng Hong
,
An-Yi Huang
, and
Chih-Chun Chang

Abstract

The cross-basin interaction of the second EOFs of the interannual SST in the North Atlantic and North Pacific—the North Atlantic tripole (NAT) SST and Pacific meridional mode (PMM)—is discussed. Observations revealed that the total variances of the NAT and PMM have simultaneously experienced interdecadal enhancement since the 1990s. Wavelet analysis indicated that this enhancement was associated with the interdecadal variations (8–16 years) of the NAT and PMM, which have become significantly and positively coherent since the 1990s. This interdecadal variation also changed the interannual NAT–PMM relationship from negative to positive. The regression analysis indicated that the NAT forced a Matsuno–Gill circulation anomaly, which had a substantial lag impact on the PMM SST through wind–evaporation–SST feedback. Additionally, the NAT induced oceanic temperature advection, which also partially contributed to the PMM SST. On the other hand, the PMM-associated middle–upper atmospheric teleconnection, a North Atlantic Oscillation (NAO)-like circulation anomaly in the North Atlantic, gave positive feedback to the NAT. The numerical experiments suggest that the enhancement of the NAT–PMM interaction since the 1990s was associated with the eastward shift of PMM-associated convection, which was further enhanced by eastward extension of the upper-level extratropical jet in the North Pacific.

Significance Statement

This study aimed at a better understanding of the cross-basin interaction between the North Atlantic and North Pacific. Our study indicates that the cross-basin interaction in the interannual sea surface temperature between the Pacific meridional mode (PMM) and North Atlantic tripole (NAT) became stronger since the 1990s. The observation yields that this enhancement was associated with the interdecadal variations of the NAT and PMM, which have become significantly and positively coherent since the 1990s. The observation yields that the NAT-forced atmospheric large-scale circulation anomaly had a substantial lag impact on the PMM. On the other hand, the PMM-induced middle–upper atmospheric teleconnection, a North Atlantic Oscillation (NAO)-like circulation anomaly, gave positive feedback to the NAT. The numerical experiments suggest that the enhancement of the NAT–PMM interaction since the 1990s primarily resulted from the eastward shift of PMM-associated convection.

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Bin Wang
,
Zhiwei Wu
,
Jianping Li
,
Jian Liu
,
Chih-Pei Chang
,
Yihui Ding
, and
Guoxiong Wu

Abstract

Defining the intensity of the East Asian summer monsoon (EASM) has been extremely controversial. This paper elaborates on the meanings of 25 existing EASM indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979–2006 period. The existing indices can be classified into five categories: the east–west thermal contrast, north–south thermal contrast, shear vorticity of zonal winds, southwesterly monsoon, and South China Sea monsoon. The last four types of indices reflect various aspects of the leading mode of interannual variability of the EASM rainfall and circulations, which correspond to the decaying El Niño, while the first category reflects the second mode that corresponds to the developing El Niño.

The authors recommend that the EASM strength can be represented by the principal component of the leading mode of the interannual variability, which provides a unified index for the majority of the existing indices. This new index is extremely robust, captures a large portion (50%) of the total variance of the precipitation and three-dimensional circulation, and has unique advantages over all the existing indices. The authors also recommend a simple index, the reversed Wang and Fan index, which is nearly identical to the leading principal component of the EASM and greatly facilitates real-time monitoring.

The proposed index highlights the significance of the mei-yu/baiu/changma rainfall in gauging the strength of the EASM. The mei-yu, which is produced in the primary rain-bearing system, the East Asian (EA) subtropical front, better represents the variability of the EASM circulation system. This new index reverses the traditional Chinese meaning of a strong EASM, which corresponds to a deficient mei-yu that is associated with an abnormal northward extension of southerly over northern China. The new definition is consistent with the meaning used in other monsoon regions worldwide, where abundant rainfall within the major local rain-bearing monsoon system is considered to be a strong monsoon.

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See Yee Lim
,
Charline Marzin
,
Prince Xavier
,
Chih-Pei Chang
, and
Bertrand Timbal

Abstract

TRMM rainfall data from 1998–2012 are used to study the impacts and interactions of cold surges (CSs) and the Madden–Julian oscillation (MJO) on rainfall over Southeast Asia during the boreal winter season from November to February. CSs are identified using a new large-scale index. The frequencies of occurrences of these two large-scale events are comparable (about 20% of the days each), but the spatial pattern of impacts show differences resulting from the interactions of the general flow with the complex orography of the region. The largest impact of CSs occurs in and around the southern South China Sea as a result of increased low-level convergence on the windward side of the terrain and increased shear vorticity off Borneo that enhances the Borneo vortex. The largest impact of the MJO is in the eastern equatorial Indian Ocean, sheltered from CSs by Sumatra. In general CSs are significantly more likely to trigger extreme rainfall. When both systems are present, the rainfall pattern is mainly controlled by the CSs. However, the MJO makes the environment more favorable for convection by moistening the atmosphere and facilitating conditional instability, resulting in a significant increased rainfall response compared to CSs alone. In addition to the interactions of the two systems in convection, this study confirms a previously identified mechanism in which the MJO may reduce CS frequency through opposing dynamic structures.

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Chih-Pei Chang
,
Michael Ghil
,
Hung-Chi Kuo
,
Mojib Latif
,
Chung-Hsiung Sui
, and
John M. Wallace
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Bin Wang
,
Michela Biasutti
,
Michael P. Byrne
,
Christopher Castro
,
Chih-Pei Chang
,
Kerry Cook
,
Rong Fu
,
Alice M. Grimm
,
Kyung-Ja Ha
,
Harry Hendon
,
Akio Kitoh
,
R. Krishnan
,
June-Yi Lee
,
Jianping Li
,
Jian Liu
,
Aurel Moise
,
Salvatore Pascale
,
M. K. Roxy
,
Anji Seth
,
Chung-Hsiung Sui
,
Andrew Turner
,
Song Yang
,
Kyung-Sook Yun
,
Lixia Zhang
, and
Tianjun Zhou
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Yali Luo
,
Renhe Zhang
,
Qilin Wan
,
Bin Wang
,
Wai Kin Wong
,
Zhiqun Hu
,
Ben Jong-Dao Jou
,
Yanluan Lin
,
Richard H. Johnson
,
Chih-Pei Chang
,
Yuejian Zhu
,
Xubin Zhang
,
Hui Wang
,
Rudi Xia
,
Juhui Ma
,
Da-Lin Zhang
,
Mei Gao
,
Yijun Zhang
,
Xi Liu
,
Yangruixue Chen
,
Huijun Huang
,
Xinghua Bao
,
Zheng Ruan
,
Zhehu Cui
,
Zhiyong Meng
,
Jiaxiang Sun
,
Mengwen Wu
,
Hongyan Wang
,
Xindong Peng
,
Weimiao Qian
,
Kun Zhao
, and
Yanjiao Xiao

Abstract

During the presummer rainy season (April–June), southern China often experiences frequent occurrences of extreme rainfall, leading to severe flooding and inundations. To expedite the efforts in improving the quantitative precipitation forecast (QPF) of the presummer rainy season rainfall, the China Meteorological Administration (CMA) initiated a nationally coordinated research project, namely, the Southern China Monsoon Rainfall Experiment (SCMREX) that was endorsed by the World Meteorological Organization (WMO) as a research and development project (RDP) of the World Weather Research Programme (WWRP). The SCMREX RDP (2013–18) consists of four major components: field campaign, database management, studies on physical mechanisms of heavy rainfall events, and convection-permitting numerical experiments including impact of data assimilation, evaluation/improvement of model physics, and ensemble prediction. The pilot field campaigns were carried out from early May to mid-June of 2013–15. This paper: i) describes the scientific objectives, pilot field campaigns, and data sharing of SCMREX; ii) provides an overview of heavy rainfall events during the SCMREX-2014 intensive observing period; and iii) presents examples of preliminary research results and explains future research opportunities.

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