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Tropical Cyclone Research and Review  
  Tropical Cyclone Research and Review--2012, 1 (3)   Published: 2012-09-15
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The Long Term Variations of Tropical Cyclone Activity in the South China Sea and the Vicinity of Hong Kong

Tsz-cheung Lee, Christy Yan-yu Leung, Mang-hin Kok, Ho-sun Chan
Tropical Cyclone Research and Review. 2012, 1 (3): 277;  doi: 10.6057/2012TCRR02.01
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This study examines the long-term variations of tropical cyclone (TC) frequency and intensity in the South China Sea (SCS) and the vicinity of Hong Kong from 1961 to 2010 based on the best track data of four main weather agencies in the western North Pacific, namely the Hong Kong Observatory, China Meteorological Administration, Regional Specialized Meteorological Center Tokyo and Joint Typhoon Warning Center of the United States. To account for the discrepancy in the best track data between agencies, the maximum sustained wind speeds are standardized to the 10-minute average before data analysis. Moreover, a sensitivity assessment based on three different data scenarios is also conducted to study the uncertainty in trend analysis due to the discrepancy in the datasets from various agencies. The results show that, likely modulated by the El Niño Southern Oscillation and the Pacific Decadal Oscillation, there exist strong inter-annual and inter-decadal variations in the TC frequency in the SCS and the vicinity of Hong Kong.   For the long-term trend, all dataset/scenario combinations depict a decrease in the TC frequency in the SCS and the vicinity of Hong Kong during the study period, but the trend is not statistically significant at 5% level for most of the datasets. As for the TC intensity, the discrepancy between weather agencies remains very substantial even when the difference in the wind speed averaging period is accounted for. The large differences in the available datasets do not allow for a reliable detection of the long-term trend of the TC intensity in the SCS. The study of the TC impacts on Hong Kong reveals that there is no significant trend on the TC-induced extreme rainfall in Hong Kong. The extreme high winds associated with TCs within 500 km range of Hong Kong have no significant trend at Waglan Island (offshore island) while those of the urban station at Kai Tak have a significant decreasing trend. 

Potential Indirect Effects of Aerosol on Tropical Cyclone Intensity: Convective Fluxes and Cold-Pool Activity

W. R. Cotton, G. M. Krall, G. G. Carrió
Tropical Cyclone Research and Review. 2012, 1 (3): 293;  doi: 10.6057/2012TCRR03.05
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We investigated the impacts of elevated aerosol concentrations on 2008 Western Pacific typhoon (NURI). Based on satellite and global aerosol model output, pollution levels were estimated. The typhoon was simulated for 96 hours beginning 17 August, 2008. During the final 60 hours CCN concentrations were enhanced as it neared the Philippines and coastal China.
Early during the ingestion of enhanced CCN, precipitation was reduced due to suppressed collision and coalescence, and storm winds strengthened. Subsequently, greater amounts of condensate were thrust into supercooled levels where the drops froze resulting in greater latent heating. Convection thereby intensified which resulted in enhanced rainfall and more vigorous convectively-produced downdrafts. As the convection intensified in the outer rainbands the storm drifted over the developing cold-pools, and low level wind speeds diminished. Very high amounts of pollution aerosols resulted in large amounts of condensate being thrust into the storm anvil which weakened convective downdrafts and cold-pools, yet reductions in wind speed (although weaker) occurred compared with the clean control run.
While the simulated storm was weaker than the observed storm, making it a borderline tropical storm or typhoon, the simulated response to aerosols was much different than anticipated based on previous simulations of aerosol impacts on tropical cyclones. In particular, the storm strengthened during the first period of aerosol ingestion followed by an extended period of storm weakening. Nonetheless, this study suggests that ingestion of elevated amounts of CCN into a tropical cyclone (TC) can appreciably alter the intensity of the storm. However, the pollution aerosols have very little impact on the storm track.

Seasonal Forecasts of Tropical Cyclone Activity over the Western North Pacific: A Review

Ruifen Zhan, Yuqing Wang, Ming Ying
Tropical Cyclone Research and Review. 2012, 1 (3): 307;  doi: 10.6057/2012TCRR03.07
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The attempt of this article is to provide a literature review on recent development and progress in seasonal forecasts for tropical cyclone (TC) activity over the western North Pacific (WNP). Since the predictability of seasonal TC activity mainly comes from the slowly-evolving sea surface temperature (SST) conditions and the large-scale atmospheric circulation teleconnection patterns, our current understanding on the relationships between the interannual TC variability and tropical SST forcing and variations of various climate modes is first reviewed. It serves as the scientific basis and gives us ideas how predictable the seasonal TC activity is.
The main body of the article focuses on an overview of the forecast approaches and methodologies, including statistical and dynamical models and their combination, currently used in seasonal forecasts for TCs over the WNP, and an initial assessment of their prediction skills in the past decade or so. Some outstanding issues, including the intrinsic limitation of predictability due to various uncertainties and the areas for future developments, are also briefly discussed. It is expected that the quality of the scientifically based seasonal TC forecasts would be steadily improved with the advancement in the forecast techniques and the driving of society needs.

A Subjective Method for Assessing Tropical Cyclogenesis at the Joint Typhoon Warning Center

Matthew E. Kucas, James W. E. Darlow
Tropical Cyclone Research and Review. 2012, 1 (3): 325;  doi: 10.6057/2012TCRR03.09
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Evaluation of Cone of Uncertainty in Tropical Cyclone Track Forecast over North Indian Ocean Issued by India Meteorological Department

M. Mohapatra, D. P. Nayak, B. K. Bandyopadhyay
Tropical Cyclone Research and Review. 2012, 1 (3): 331;  doi: 10.6057/2012TCRR03.02
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India Meteorological Department (IMD) introduced cone of uncertainty (COU) in cyclonic disturbances (CDs) alongwith the 72 hr track forecast over the north India Ocean (NIO) in 2009. The track forecast for CDs is issued for +6, +12, +18, +24, +36, +48, +60 and +72 hrs time period from the stage of deep depression onwards. An attempt is made to evaluate COU forecast issued by IMD during 2009-2011(3 years). The size of the cone is deduced from climatological track forecast errors. The accuracy of COU forecast has been analysed with respect to basin of formation, season of formation, intensity and type of track (climatological/straight moving and recurving/looping type) of CDs by calculating percentage of total number of forecasts in each category lying within COU. The observed track lies within the forecast COU in about 60% of the cases over the NIO. The accuracy of COU forecast is about 66% in post monsoon season and about 50% in pre-monsoon season. The observed track lies within the forecast COU in 90% cases of climatological/straight moving CDs and 39% cases of recurving/ looping CDs. The observed track lies within COU forecast in about 71% cases of severe cyclonic storm and 37% cases of cyclonic storm/deep depression.

Evaluating the Performance of Western North Pacific Tropical Cyclone Intensity Guidance. Part I: Basic Characteristics

Caizhu Chen, Qingqing Li, Jinhua Yu, Lin Chen
Tropical Cyclone Research and Review. 2012, 1 (3): 340;  doi: 10.6057/2012TCRR03.10
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The overall statistics of the accuracy of eight statistical and dynamical tropical cyclone intensity guidance techniques used in the East China Regional Meteorological Center during the 2008 and 2009 western North Pacific typhoon seasons were evaluated herein. Skill scores which were calculated based on contingency tables show that the forecast skill of the selected techniques did not persistently decline with the increasing forecast time. The probabilities of the forecast intensity inaccuracy being within 6 m s-1 and greater than 15 m s-1 were examined in the study. Additionally, the majority of the forecasts were featured by underintensity forecasts in 2008, while some guidance showed pronounced overforecasts at relatively long forecast intervals in 2009. The forecast biases and overintensity forecast errors at typhoon dissipation were verified as well.

Comparison of Performance of Various Multiple-model Ensemble Techniques in Forecasting Intensity of Tropical Cyclone

Nursalleh K. Chang, L.S. Lee, Y.S. Li
Tropical Cyclone Research and Review. 2012, 1 (3): 353;  doi: 10.6057/2012TCRR03.03
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The objective of this study was to explore an optimal multiple-model ensemble technique to aid the forecasting of tropical cyclone (TC) intensity. The maximum winds of TCs as forecast by the models of the European Centre for Medium-Range Weather Forecasts, the Japan Meteorological Agency and the National Centers for Environmental Prediction for the period from July 2010 to October 2011 were studied. Performance of various multiple-model ensemble techniques, including equally weighted ensemble, weighted ensemble based on initial forecast error, weighted ensemble based on 12-hour forecast error, bias-corrected equally weighted ensemble and bias-corrected weighted ensemble based on initial forecast error, was verified against the TC intensities post-analysed by the Hong Kong Observatory. Results showed that the equally weighted ensemble technique generally outperformed the best of the individual models and other multiple-model ensemble techniques. The mean absolute errors of the equally weighted ensemble technique were the lowest at 12, 24 and 36-hour forecasts, and the error spreads were generally the smallest from 12 to 72-hour forecasts.

Operational Tropical Cyclone Forecast Verification Practice in the Western North Pacific Region

Hui Yu, Sai Tick Chan, Barbara Brown, Masashi Kunitsugu, Edward Fukada
Tropical Cyclone Research and Review. 2012, 1 (3): 361;  doi: 10.6057/2012TCRR03.06
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This paper summarizes results of a survey of the operational status of tropical cyclone (TC) forecast verification in the western North Pacific region, covering the Members of the ESCAP/WMO Typhoon Committee, RSMC Tokyo - Typhoon Center, and JTWC. Results of the survey indicate that significant efforts have been focused on the verification of TC forecast guidance by operational forecast agencies in the region, either individually or through international cooperation. A variety of TC forecast verification products are available to the forecasters in either real time or in post-season through multiple means, with the main focus on the deterministic track and intensity forecasts. Only a few verification products are available for probabilistic forecasts. Verification of TC precipitation and high wind forecasts are also lacking sufficient attention in the region. It would be helpful in the future to focus more attention toward these aspects and incorporate methods that will provide a greater depth of information regarding the TC forecast performance.

Simulation of Ocean Responses to an Idealized Landfalling Tropical Cyclone Using a Coupled Atmosphere-Wave-Ocean Modeling System

Huiqing Liu, Bin Liu, Lian Xie, Keqi Zhang
Tropical Cyclone Research and Review. 2012, 1 (3): 373;  doi: 10.6057/2012TCRR03.01
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Oceanic responses to a hypothetical landfalling tropical cyclone (TC) are studied by using a coupled atmosphere-wave-ocean modeling system (CAWOMS). A set of experiments are conducted to compare the effects of atmosphere-wave-ocean interaction on ocean responses in coastal and deep waters. The results show that in a three-way coupled atmosphere-wave-ocean system, the resonse to a tropical cyclone is considerably different in coastal water and deep water. In a three-way coupled system, air-sea interactions tend to increase coastal storm surge, inundation, significant wave heights and ocean currents in shallow coastal areas as a result of waveenhanced air-sea heat and moisture fluxes. But the change is little in sea surface temperature and mixed-layer structure due to the well-mixed nature in the coastal zone. In contrast, in a three-way coupled system, air-sea interactions enhance sea surface cooling, increase mixed layer depth in deep waters largely due to the tendency of a wave-enhanced TC to induce strong mixing and entrainment in the upper ocean. A stronger TC also strengthens the surface currents and significant wave height in the offshore waters. The inclusion of waves in air-sea interactions fundamentally changes the dynamic and thermodynamic coupling between tropical cyclone and the underlying ocean. In the absence of TC-wave consideration, a negative feedback between the TC and the upper ocean mixed layer results in a weakening of the TC system and a cooling in the offshore upper ocean and therefore reduces coastal storm surge, flooding areas, significant wave height and ocean currents. Only in a TC-waveocean three-way coupled system, air-sea interaction may correspond to a stronger TC due to wave-induced airsea heat and moisture fluxes which compensate the effect of negative feedback between the TC and the upper ocean. In coastal waters, the negative feedback between the TC and the ocean mixed layer is fairly weak. Airsea interaction is dominated by the positive TC-wave feedback. As a result, air-sea interaction increases coastal storm surge, inundation, currents and significant wave height.

Trapped-Fetch Wave Model Application to Typhoon Case

Sangwook PARK, Jong-Suk PARK, Tae-Ryong KIM
Tropical Cyclone Research and Review. 2012, 1 (3): 390;  doi: 10.6057/2012TCRR03.08
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The ‘Trapped-Fetch Wave Model (TFWM)’, which is developed for wave prediction in north Atlantic hurricanes, is applied to typhoon cases in western North Pacific (WNP). The comparison with operational numerical ocean wind wave prediction system at Korea Meteorological Administration (KMA) is examined. In application to WNP typhoon, the TFWM has shown some advantage against typical operational spectral wave models. Even though the full spectral 3rd generation numerical wave model can provides a reliable wave field prediction, it has little value when the atmospheric model poorly predicts the location and intensity of concerned storms or tropical cyclone. The analysis of TFWM output should lead the forecaster back to a more in-depth examination of the full spectral wave model output, resulting in an improved forecast product. As a supporting guidance tool for marine forecaster, the TFWM has shown its own uniqueness and necessity.

Modeling Tropical Cyclone Induced Stream Flow in Tar Pamlico River of North Carolina

Qianhong Tang, Lian Xie, Bin Liu
Tropical Cyclone Research and Review. 2012, 1 (3): 402;  doi: 10.6057/2012TCRR03.04
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In order to better understand and simulate large coastal watershed hydrology and hydro-meteorological processes associated with tropical cyclone (TC) induced inland flooding, the Annualized Agricultural Nonpoint Source Pollution Model (AnnAGNPS) with Muskingum channel routing has been applied to the Tar-Pamlico river basin in North Carolina, USA. The study focuses on three major hydro-meteorological processes: 1) largescale atmospheric environment, 2) watershed hydrological processes and 3) groundwater response. The modeling results indicate that although the AnnAGNPS model can perform well in predicting the total amount of watershed runoff, channel routing is needed to improve the hydrographs of flow discharge during hurricane events. Sensitivity analysis of soil saturated hydrological conductivity (Ks) indicates that both base flow and event total runoff are sensitive to Ks. Base flow increases as Ks increases when 0 < Ks ≤ 15 m/day, but decreases slightly when Ks > 15 m/day. Peak runoff exponentially decreases as Ks increases. The results also show that without the preceding Hurricane Dennis, the outlet discharge as a result of Hurricane Floyd would have been as much as 37% lower than that caused by the combined Dennis-Floyd effect.

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