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Tropical Cyclone Research and Review  
  Tropical Cyclone Research and Review--2015, 4 (2)   Published: 2015-05-15
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Interannual Anomaly in Tropical Cyclone Frequency around Korea by Western North Pacific Monsoon

Jae-Won Choi, Seung-Wook Lee, Jeoung-Yun Kim
Tropical Cyclone Research and Review. 2015, 4 (2): 49;  doi: 10.6057/2015TCRR02.01
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  In this study, the correlation between the frequency of summer tropical cyclones (TC) affecting areas around Korea over the last 37 years and the western North Pacific monsoon index (WNPMI) was analyzed. A clear positive correlation existed between the two variables, and this high positive correlation remained unchanged even when excluding El Niño-Southern Oscillation (ENSO) years. To investigate the causes of the positive correlation between these two variables, ENSO years were excluded, after which the 8 years with the highest WNPMI (strong WNPM phase) and the 8 years with the lowest WNPMI (weak WNPM phase) were selected, and the average difference between the two phases was analyzed.
  In the strong WNPM phase, TCs usually occurred in the eastern waters of the tropical and subtropical western North Pacific, and tended to pass the East China Sea on their way north toward Korea and Japan. In the weak WNPM phase, TCs usually occurred in the western waters of the tropical and subtropical western North Pacific, and tended to pass the South China Sea on their way west toward the southeastern Chinese coast and the Indochina peninsula. Therefore, TC intensity was higher in the strong WNPM phase, during which TCs are able to gain sufficient energy from the sea while moving a long distance to areas nearby Korea. TCs also tended to occur more often in the strong WNPM phase.
  In the difference between the two phases regarding 850 hPa and 500 hPa streamline, anomalous cyclones were reinforced in the tropical and subtropical western North Pacific, while anomalous anticyclones were reinforced in mid-latitude East Asian areas. Due to these two anomalous pressure systems, anomalous southeasterlies developed in areas near Korea, with these anomalous southeasterlies playing the role of anomalous steering flows making the TCs head toward areas near Korea. Also, due to the anomalous cyclones developed in the tropical and subtropical western North Pacific, more TCs could occur in the strong WNPM phase.

Further Exploring the Potential for Assimilation of Unmanned Aircraft Observations to Benefit Hurricane Analyses and Forecasts

Jason A. Sippel, Fuqing Zhang, Yonghui Weng, Scott A. Braun, Daniel J. Cecil
Tropical Cyclone Research and Review. 2015, 4 (2): 64;  doi: 10.6057/2015TCRR02.02
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This study explores the potential of assimilating data from multiple instruments onboard high-altitude, longendurance unmanned aircraft to improve hurricane analyses and forecasts. A recent study found a significant positive impact on analyses and forecasts of Hurricane Karl when an ensemble Kalman filter was used to assimilate data from the High-altitude Imaging Wind and Rain Airborne Profiler (HIWRAP), a new Doppler radar onboard the NASA Global Hawk (GH) unmanned airborne system. The GH can also carry other useful instruments, including dropsondes and the Hurricane Imaging Radiometer (HIRAD), which is a new radiometer that estimates large swaths of wind speeds and rainfall at the ocean surface. The primary finding is that simultaneously
assimilating data from HIWRAP and the other GH-compatible instruments results in further analysis and forecast improvement for Karl. The greatest improvement comes when HIWRAP, HIRAD, and dropsonde data are simultaneously assimilated.

Selective Ensemble Mean Technique for Tropical Cyclone Track Forecasts Using Multi-Model Ensembles

Masato Nishimura, Munehiko Yamaguchi
Tropical Cyclone Research and Review. 2015, 4 (2): 71;  doi: 10.6057/2015TCRR02.03
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A selective ensemble mean technique for tropical cyclone (TC) track forecasts, which excludes from the ensemble those models that have large position errors at short lead times, was applied to a set of TC track forecasts produced by 11 operational global deterministic models. The position errors of the resulting selective ensemble mean TC track forecasts were verified for 91 TCs in the western North Pacific from 2010 to 2013 that reached an intensity classification of “tropical storm” or stronger. The TC position errors of the selective ensemble mean were smaller than those of a simple 11-member ensemble mean by 14.4%, 7.4% and 4.7% at forecast times of 24, 48 and 72 hours, respectively. However, the errors were larger than those of the best single-model-based deterministic forecasts, which were ECMWF forecasts. The correlation between TC position errors at short and long lead times was weak, which partially explains why the selective ensemble mean technique in this study had lesser skill than ECMWF forecasts. For operational forecasting, simple ensemble mean forecasts by ECMWF and NCEP generally provide the best forecast performance for verification samples from 2010 to 2013.

Verification of Ensemble Track Forecasts of Tropical Cyclones during 2014

Xiping Zhang, Guomin Chen, Hui Yu, Zhihua Zeng
Tropical Cyclone Research and Review. 2015, 4 (2): 79;  doi: 10.6057/2015TCRR02.04
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The track forecasts of five ensemble prediction systems (JMA-WEPS, CMA-GEFS, ECMWF-EPS, NCEPGEFS and MSC-CENS, respectively) in 2014 are evaluated in this paper. First, on the assumption of bivariate normal distribution, a probability ellipse based on ensemble spread is proposed to represent forecast uncertainties, and then the ellipse and ensemble mean are used to evaluate the five different systems. It is found that the probability ellipse of ECMWF-EPS is smaller, with a high hit ratio, and its mean track errors are just a little larger than those of NCEP-GEFS within 48 h, meaning overall it outperforms the other four systems. Meanwhile, the performance of CMA-GEFS is found to be the poorest.

Interdecadal Variation in the Activity of Tropical Cyclones Affecting Korea

Jae-Won Choi and Yumi Cha
Tropical Cyclone Research and Review. 2015, 4 (2): 88;  doi: 10.6057/2015TCRR02.05
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By performing a statistical change-point analysis of activities of the tropical cyclones (TCs) that have
affected Korea (K-TCs), it was found that there was a significant change between 1983 and 1984. During the
period of 1984-2004 (P2), more TCs migrated toward the west, recurved in the southwest, and affected Korea,
compared to the period of 1965-1983 (P1). These changes for P2 were related to the southwestward expansion
of the subtropical western Pacific high (SWPH) and, simultaneously, elongation of its elliptical shape toward
Korea. Because of these changes, the central pressure and lifetime of K-TC during P2 were deeper and longer,
respectively, than figures for P1. This stronger K-TC intensity for P2 was related to the more southwestward
genesis due to the southwestward expansion of the SWPH. The weaker vertical wind shear environment during
P2 was more favorable for K-TC to maintain a strong intensity in the mid-latitudes of East Asia.

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