Typhoon Track,Intensity,and Structure:From Theory to Prediction

To improve understanding of essential aspects that influence forecasting of tropical cyclones(TCs),the National Key Research and Development Program,Ministry of Science and Technology of the People's Republic of China conducted a five-year project titled“Key Dynamic and Thermodynamic Processes and Prediction for the Evolution of Typhoon Intensity and Structure”(KPPT).Through this project,new understandings of TC intensification,including outer rainbanddriven secondary eyewall formation and the roles of boundary layer dynamics and vertical wind shear,and improvements to TC data assimilation with integrated algorithms and adaptive localizations are achieved.To promote a breakthrough in TC intensity and structure forecasting,a new paradigm for TC evolution dynamics(i.e.,the correlations,interactions,and error propagation among the triangle of TC track,intensity,and structure)is proposed;and an era of dynamic-constrained,big-data driven,and strongly coupled data assimilation at the subkilometer scale and seamless prediction is expected.

Tropical Cyclone Monitoring and Analysis Techniques: A Review

Tropical cyclones(TCs),including tropical depressions and different categories of typhoons,hurricanes,and cyclonic storms,mostly originate over the oceans in the absence of direct observations.Thus,detailed monitoring and analysis of TCs has always been an unsolved problem.In the recent 20 years,great changes have taken place in domestic and foreign TC monitoring techniques,imposing a significant impact on TC operations and research.Some new technologies and products gradually emerge to support operations,with improved monitoring accuracy.In this paper,the progress on TC monitoring and analysis via meteorological satellites,radars,and airplanes in China and the world is reviewed,compared,and summarized,with special focuses on multisatellite fusion observations,in situ aircraft measurements,and some unconventional observation equipment such as rockets,saildrones,and underwater gliders.On this basis,the paper points out future directions for improving TC monitoring and analysis in aid of better TC forecast and early warning.

Unusual tracks:Statistical,controlling factors and model prediction

The progress of research and forecast techniques for tropical cyclone(TC)unusual tracks(UTs)in recent years is reviewed.A major research focus has been understanding which processes contribute to the evolution of the TC and steering flow over time,especially the reasons for the sharp changes in TC motion over a short period of time.When TCs are located in the vicinity of monsoon gyres,TC track forecast become more difficult to forecast due to the complex interaction between the TCs and the gyres.Moreover,the convection and latent heat can also feed back into the synoptic-scale features and in turn modify the steering flow.In this report,two cases with UTs are examined,along with an assessment of numerical model forecasts.Advances in numerical modelling and in particular the development of ensemble forecasting systems have proved beneficial in the prediction of such TCs.There are still great challenges in operational track forecasts and warnings,such as the initial TC track forecast,which is based on a poor pre-genesis analysis,TC track forecasts during interaction between two or more TCs and track predictions after landfall.Recently,artificial intelligence(AI)methods such as machine learning or deep learning have been widely applied in the field of TC forecasting.For TC track forecasting,a more effective method of center location is obtained by combining data from various sources and fully exploring the potential of AI,which provides more possibilities for improving TC prediction.

OPERATIONAL TROPICAL CYCLONE FORECAST VERIFICATION PRACTICE IN THE WESTERN NORTH PACIFIC REGION

This paper summarizes results of a survey of the operational status of tropical cyclone(TC) forecast verifi-cation 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.

NUMERICAL EXPERIMENTAL STUDY FOR THE EFFECT OF A BINARY TYPHOON SYSTEM ON THE EXTREME RAINFALL OF TYPHOON MORAKOT(0908)

A high-resolution numerical simulation of the extreme rainfall caused by typhoon Morakot(0908)over Taiwan Province,China,was made using the WRF-ARW/NCAR model(Version 3.2),ERA Interim reanalysis data(resolution:1.5°×1.5°)from the European Centre for Medium-Range Weather Forecasts(ECMWF),and global real-time sea surface temperature analysis data(RTG_SST,0.5°×0.5°)from NCEP/NOAA.The numerical simulation results showed that the extreme rainfall caused by Morakot over Taiwan was closely related to water vapor transport of the southwesterly flow.However,the effect of a binary typhoon system between Morakot and Severe Tropical Storm Goni(0907)was also an important factor.Goni strengthened the intensity of the southwesterly flow and the water vapor transport to Morakot and resulted in the heavy rainfall increasing over central and southern mountainous areas of Taiwan.Furthermore,the effect of the binary typhoon system increasing the northward component of the track of Morakot,and the typhoon’s slow translation to Taiwan,caused the longtime and persistent severe rainfall over southern Taiwan.After removing Goni’s circulation in the model initial field,the cumulative precipitation was greatly reduced by 35.78%,33.03%and 31.5%in the 18-,6-and 2-km resolution model results,respectively.

FORECAST AND SERVICE PERFORMANCE ON RAPIDLY INTENSIFICATION PROCESS OF TYPHOONS RAMMASUN(2014) AND HATO(2017)

Super typhoons Rammasun(No.1409) and Hato(No.1713) both underwent rapidly intensification(RI) in the northern part of South China Sea before they made landfall. Forecast skills and service performance of tropical cyclones' RI process in the real-time operation is analyzed in this study. TCs are prone to intensify rapidly in the South China Sea, which is a complex process concluding complicated interaction between large scale environmental systems and tropical cyclone inner-core structure. The forecast performance of Rammasun and Hato shown that the subjective forecast of CMA has defect in the intensity forecast especially for the long-rang more than 48-hr. However, forecasters have chance to capture the signal of RI besides numerical operational models, which contribute to gain precious time for disaster reduction affairs. The role of local sea surface temperature and the warm core structure revealed by the numerical simulations are highlighted in doing comprehensive analysis in real-time forecast.