Influence of Future Tropical Cyclone Track Changes on Their Basin-Wide Intensity over the Western North Pacific: Downscaled CMIP5 Projections

The possible changes of tropical cyclone（TC） tracks and their influence on the future basin-wide intensity of TCs over the western North Pacific（WNP） are examined based on the projected large-scale environments derived from a selection of CMIP5（Coupled Model Intercomparison Project Phase 5） models. Specific attention is paid to the performance of the CMIP5 climate models in simulating the large-scale environment for TC development over the WNP. A downscaling system including individual models for simulating the TC track and intensity is used to select the CMIP5 models and to simulate the TC activity in the future.The assessment of the future track and intensity changes of TCs is based on the projected large-scale environment in the21 st century from a selection of nine CMIP5 climate models under the Representative Concentration Pathway 4.5（RCP4.5）scenario. Due to changes in mean steering flows, the influence of TCs over the South China Sea area is projected to decrease,with an increasing number of TCs taking a northwestward track. Changes in prevailing tracks and their contribution to basin-wide intensity change show considerable inter-model variability. The influences of changes in prevailing track make a marked contribution to TC intensity change in some models, tending to counteract the effect of SST warming. This study suggests that attention should be paid to the simulated large-scale environment when assessing the future changes in regional TC activity based on climate models. In addition, the change in prevailing tracks should be considered when assessing future TC intensity change.

STOCHASTIC SIMULATION OF TROPICAL CYCLONE TRACKS IN THE NORTHWEST PACIFIC REGION WITH CLASSIFICATION MODEL

Accurate simulation of tropical cyclone tracks is a prerequisite for tropical cyclone risk assessment.Against the spatial characteristics of tropical cyclone tracks in the Northwest Pacific region,stochastic simulation method based on classification model is used to simulate tropical cyclone tracks in this region.Such simulation includes the classification method,the genesis model,the traveling model,and the lysis model.Tropical cyclone tracks in the Northwest Pacific region are classified into five categories on the basis of its movement characteristics and steering positions.In the genesis model,Gaussian kernel probability density functions with the biased cross validation method are used to simulate the annual occurrence number and genesis positions.The traveling model is established on the basis of the mean and mean square error of the historical 6 h latitude and longitude displacements.The termination probability is used as the discrimination standard in the lysis model.Then,this stochastic simulation method of tropical cyclone tracks is applied and qualitatively evaluated with different diagnostics.Results show that the tropical cyclone tracks in Northwest Pacific can be satisfactorily simulated with this classification model.

Seasonal Variation Features of Western North Pacific Tropical Cyclone Tracks with Their Predictability

Analysis is done of monthly and seasonal variations as climatic features of the tracks from 1196 tropical cyclones originating in the western North Pacific over the period 1949 to 1980, followed by the investigation of 301 onland cyclone tracks over China mainland in terms of methodology for nonlinear system. Obtained by computing the accumulated distance distribution function of the tracks Cm (l) is the characteristic chaos quantity for the related dynamic systems and then the fractual dimensionality d = 4.86 and Kolmogorov entropy approximation K2 = 0.0164, thereby leading to the predictability time scale = 2.54 days. It is found that the reference path among the onland typhoon No.23 of 1971, or Bess in the international nomenclature. Our results could be of operational use as a kind of reference.

Impact of Initial Storm Intensity and Size on the Simulation of Tropical Cyclone Track and Western Pacific Subtropical High Extent

Typhoon Megi, the 13th typhoon of the 2010 typhoon season, was selected for case study by utilizing the Weather Research and Forecasting （WRF） model. Twelve sensitivity experiments with various initial tropical cyclone （TC） in- tensities and sizes were conducted to investigate their impacts on the simulation of typhoon track. Interaction between TC and the western Pacific subtropical high （WPSH） was also analyzed to explore the mechanism for the impact on TC track of the initial TC intensity and size. Numerical results indicate that the simulated TC size and TC track are sensitive to initial TC intensity and size. Stronger initial TC intensity and larger initial TC size often lead to larger simulated TC size and make TC turn northward earlier. Further analysis suggests that, with the increase of ini- tial TC intensity and size, more air mass enters into the TC region, which subsequently reduces the extent of WPSH. As a result, the steering flow changes significantly and eventually causes the TC to turn northward earlier. The present study confirms that the initial TC intensity and size have certain influences on the TC track simulation, which demonstrates the importance of accurate initial condition for successful simulation of the TC intensity and TC track. Moreover, it also deepens our understanding of the interaction between TC and WPSH, provides helpful clues for the TC track change study, and discusses the future directions for improvement of TC track forecast.

Ensemble forecast of tropical cyclone tracks based on deep neural networks

A nonlinear artificial intelligence ensemble forecast model has been developed in this paper for predicting tropical cyclone(TC)tracks based on the deep neural network(DNN)by using the 24-h forecast data from the China Meteorological Administration(CMA),Japan Meteorological Agency(JMA)and Joint Typhoon Warning Center(JTWC).Data from a total of 287 TC cases over the Northwest Pacific Ocean from 2004 to 2015 were used to train and validate the DNN based ensemble forecast(DNNEF)model.The comparison of model results with Best Track data of TCs shows that the DNNEF model has a higher accuracy than any individual forecast center or the traditional ensemble forecast model.The average 24-h forecast error of 82 TCs from 2016 to 2018 is 63 km,which has been reduced by 17.1%,16.0%,20.3%,and 4.6%,respectively,compared with that of CMA,JMA,JTWC,and the error-estimation based ensemble method.The results indicate that the nonlinear DNNEF model has the capability of adjusting the model parameter dynamically and automatically,thus improving the accuracy and stability of TC prediction.

Determination of the Effect of Initial Inner-Core Structure on Tropical Cyclone Intensification and Track on a Beta Plane

The sensitivity of TC intensification and track to the initial inner-core structure on a β plane is investigated using a numerical model. The results show that the vortex with large inner-core winds（CVEX-EXP） experiences an earlier intensification than that with small inner-core winds（CCAVE-EXP）, but they have nearly the same intensification rate after spin-up. In the early stage, the convective cells associated with surface heat flux are mainly confined within the inner-core region in CVEXEXP, whereas the vortex in CCAVE-EXP exhibits a considerably asymmetric structure with most of the convective vortices being initiated to the northeast in the outer-core region due to the β effect. The large inner-core inertial stability in CVEX-EXP can prompt a high efficiency in the conversion from convective heating to kinetic energy. In addition, much stronger straining deformation and PBL imbalance in the inner-core region outside the primary eyewall ensue during the initial development stage in CVEX-EXP than in CCAVE-EXP, which is conducive to the rapid axisymmetrization and early intensification in CVEX-EXP. The TC track in CVEX-EXP sustains a northwestward displacement throughout the integration, whereas the TC in CCAVE-EXP undergoes a northeastward recurvature when the asymmetric structure is dominant. Due to the enhanced asymmetric convection to the northeast of the TC center in CCAVE-EXP, a pair of secondary gyres embedded within the large-scale primary β gyres forms, which modulates the ventilation flow and thus steers the TC to move northeastward.

IMPACT OF CONVECTION OVER THE SOUTH CHINA SEA ON TROPICAL CYCLONE MOTION OVER THE WESTERN NORTH PACIFIC DURING SUMMER MONSOON

The intraseasonal oscillation(ISO) of the South China Sea(SCS, 105-120°E, 5-20°N) convection and its influences on the genesis and track of the western North Pacific(WNP) tropical cyclones(TCs) were explored, based on the daily average of NCEP/NCAR reanalysis data, the OLR data and the western North Pacific tropical cyclone best-track data from 1979 to 2008. The mechanism of the influences of ISO on TC movement and the corresponding large-scale circulation were discussed by a trajectory model. It was found as follows.(1) During the SCS summer monsoon, the SCS convection exhibits the ISO features with active phases alternating with inactive phases. The monsoon circulation patterns are significantly different during these two phases. When the SCS convection is active(inactive), the SCS-WNP monsoon trough stretches eastward(retreats westward) due to the activity(inactivity) of SCS monsoon, and the WNP subtropical high retreats eastward(stretches westward), which enhances(suppresses) the monsoon circulation.(2) The amount of TC genesis in the active phase is much more than that in the inactive phase. A majority of TCs form west of 135 °E during the active phases but east of 135 °E in the inactive phases.(3) The TCs entering the area west of 135 °E and south of 25 °N would move straight into the SCS in the active phase, or recurve northward in the inactive phase.(4) Simulation results show that the steering flow associated with the active(inactive)phases is in favor of straight-moving(recurving) TCs. Meanwhile, the impacts of the locations of TC genesis on the characteristics of TC track cannot be ignored. TCs that occurred father westward are more likely to move straight into the SCS region.

The Effect of Asymmetric Wind Structures of Tropical Cyclones on Their Tracks

A model is proposed relating a variety of factors of tropical cyclones (TCs)to their tracks, and attentions are paid to the influence of the asymmetric wind structures of TCs.Ideal numerical calculation shows that the asymmetric wind structures of TCs have conspicuousinfluence on their motion tracks. When moving due westward initially, an axisymmetric TC willdeflect right, and the overall trend is eastward. When it is asymmetric but the asymmetry is notsharp, wherever the area of maximum wind is, the TC deflects northwest first, and then, to variousareas depending on the positions of the areas of maximum wind, i.e. for the area of maximum wind inQuadrant Ⅰ, to the southeast; for in Quadrant Ⅱ, to the northeast; for in Quadrant Ⅲ, to thenorthwest; for in Quadrant IV, to the southwest. And in the above four cases, the TC tracks are allaccompanied by clockwise looping motions. With the asymmetry of the TC intensifiying, the track ofthe TC motion will be stretched further. For the area of maximum wind in Quadrants Ⅰ and Ⅱ, the TCtracks are still accompanied by clockwise looping motions; for the area of maximum wind inQuadrants Ⅲand Ⅳ, the TC tracks become oscillatory, without clockwise looping motions.

Impacts of a Large-scale Adaptive Blending Scheme for CMA-MESO on Regional Forecasts-A Case Study of Typhoon Haima

Large-scale atmospheric information plays an important role in the regional model for the forecasts of weather such as tropical cyclone(TC).However,it is difficult to be fully represented in regional models due to domain size and a lack of observation data,particularly at sea used in regional data assimilation.Blending analysis has been developed and implemented in regional models to reintroduce large-scale information from global model to regional analysis.Research of the impact of this large-scale blending scheme for the Global/Regional Assimilation and PrEdiction System(CMA-MESO)regional model on TC forecasting is limited and this study attempts to further progress by examining the adaptivity of the blending scheme using the two-dimensional Discrete Cosine Transform(2D-DCT)filter on the model forecast of Typhoon Haima over Shenzhen,China in 2016 and considering various cut-off wavelengths.Results showed that the error of the 24-hour typhoon track forecast can be reduced to less than 25 km by applying the scale-dependent blending scheme,indicating that the blending analysis is effectively able to minimise the large-scale bias for the initial fields.The improvement of the wind forecast is more evident for u-wind component according to the reduced root mean square errors(RMSEs)by comparing the experiments with and without blending analysis.Furthermore,the higher equitable threat score(ETS)provided implications that the precipitation prediction skills were increased in the 24h forecast by improving the representation of the large-scale feature in the CMA-MESO analysis.Furthermore,significant differences of the track error forecast were found by applying the blending analysis with different cut-off wavelengths from 400 km to 1200 km and the track error can be reduced less than by 10 km with 400 km cut-off wavelength in the first 6h forecast.It highlighted that the blending scheme with dynamic cut-off wavelengths adapted to the development of different TC systems is necessary in order to optimally introduce and ingest the large-scale information from global model to the regional model for improving the TC forecast.In this paper,the methods and data applied in this study will be firstly introduced,before discussion of the results regarding the performance of the blending analysis and its impacts on the wind and precipitation forecast correspondingly,followed by the discussion of the effects of different blending scheme on TC forecasts and the conclusion section.

NUMERICAL EXPERIMENT OF THE EFFECT OF ENVIRONMENTAL FLOWS ON TROPICAL CYCLONE MOTION

A barotropic prirnitive equation model is used to simulate the tropical cyclone motion.Tropical cyclone movements under different environmental flow backgrounds were examined and sensitivity of tropical cyclone tracks were discussed.Conclusions of practical significance have been obtained in this paper.

Impacts of Randomly Distributed Small-Scale Vortices on Typhoon Tracks

Impacts of small-scale vortices on typhoon or tropical cyclone（TC） tracks in a system of three components （an idealized subtropical high ridge,a TC,and small-scale vortices） were examined numerically using a barotropic primitive equation model and idealized initial fields.Two small-scale vorticity fields were generated stochastically,where the number of initial small-scale vortices is 100.In the two fields,the number, size,structure as well as the total kinetic energy of the small-scale vortices were all the same,but their center coordinates were randomly distributed.Two experiments,named EXA and EXB,with the integration time of 56 h,were performed. Comparison of the integration outputs from the two experiments indicates that various small-scale vorticity fields involved could produce both various horizontal winds in the TC outer area and various intensities of the environmental steering for TC.The values of the environmental flow speed along the east-west direction are 7.8,8.2,and 8.7 m s^（-1） in EXA and 8.3,9.5,and 9.7 m s^（-1） in EXB at t = 24,36,and 48 h,respectively. The values along the south-north direction are 0.9,1.8,and 2.5 m s^（-1） in EXA and 2.3,2.3,and 5.9 m s^（-1） in EXB,respectively.The various intensities of the environmental steering could lead to various TC center positions.For example,the difference between the coordinates of TC centers in EXA and EXB reaches 120 km at t = 48 h,which is about 56%of the prediction error of TC tracks in the northwestern Pacific region derived from eight operational models. Dynamics of the interplays between a subtropical high ridge and a TC in previous works has shown that when the TC was located equatorwards of the subtropical ridge,the relationship between the propagation of TC and the local absolute vorticity gradient was characterized by points scattered around their mean values.After introducing the stochastic vorticity field into the model,the interplays among the subtropical high ridge,the TC,and small-scale vortices might make this relationship more complex,that is,besides the scattered points,there also exists a high correlation between the propagation of TC and the local absolute vorticity gradient.