USE OF A NEW STEERING FLOW METHOD TO PREDICT TROPICAL CYCLONE MOTION

A tropical cyclone is a kind of violent weather system that takes place in warmer tropical oceans and spins rapidly around its center and at the same time moves along surrounding flows. It is generally recognized that the large-scale circulation plays a major role in determining the movement of tropical cyclones and the effects of steering flows are the highest priority in the forecasting of tropical cyclone motion and track. This article adopts a new method to derive the steering flow and select a typical swerving track case (typhoon Dan, coded 9914) to illustrate the validity of the method. The general approach is to modify the vorticity, geostropical vorticity and divergence, investigate the change in the non-divergent stream function, geoptential and velocity potential, respectively, and compute a modified velocity field to determine the steering flow. Unlike other methods in regular use such as weighted average of wind fields or geopoential height, this method has the least adverse effects on the environmental field and could derive a proper steering flow which fits well with storm motion. Combined with other internal and external forcings, this method could have wide application in the prediction of tropical cyclone track.

Impact of 10-60-Day Low-Frequency Steering Flows on Straight Northward-Moving Typhoon Tracks over the Western North Pacific

This study investigates the impact of low-frequency(intraseasonal and interannual) steering flows on straight northward-moving(defined as a meridional displacement two times greater than the zonal displacement) typhoons over the western North Pacific using observational data. The year-to-year change in the northward-moving tracks is affected by the interannual change in the location and intensity of the subtropical high. A strengthened northward steering flow east of 120°E and a weakened easterly steering flow south of the subtropical high favor more frequent straight northward tracks. Examining each of the individual northward-moving typhoons shows that they interact with three types of intraseasonal(10-60-day) background flows during their northward journey. The first type is the monsoon gyre pattern, in which the northward-moving typhoon is embedded in a closed cyclonic monsoon gyre circulation. The second type is the wave train pattern, where a cyclonic(anticyclonic) vorticity circulation is located to the west(east) of the northward-moving typhoon center. The third type is the mid-latitude trough pattern, in which the northward-moving typhoon center is located in the maximum vorticity region of the trough.

Impacts of Steering Flows with Different Timescales on the Track of Typhoon Sanba(2012)

Typhoon Sanba(2012),the strongest tropical cyclone(TC)of the year worldwide,moved northward almost along130°longitude during its lifetime and passed through different background flows from low to high latitudes.The steering flows with different timescales for Sanba are retrieved by using the NCEP reanalysis data with the total wind field separated into:a mean state,an interannual component,an intraseasonal component,and a synoptic component.Our analysis indicates that the intraseasonal timescale wave train(WT)with east–west oriented circulations made the largest contribution to the movement of Sanba.The effects of the environmental steering with different timescales on Sanba’s movement are investigated with numerical simulations using the Weather Research and Forecasting(WRF)model.In the control simulation,total fields from the NCEP reanalysis are used as initial and boundary conditions,and the northward motion of Sanba is well captured.In sensitivity experiments,each of the intraseasonal and interannual components is removed one at a time.The steering vectors associated with these timescales can explain their influences on the movement of Sanba in the experiments.Vorticity budget analyses indicate that the horizontal vorticity advection made the largest contribution to the movement of the storm.

An Analysis of the Low Moving Speed of Landfalling Typhoon In-Fa in 2021

The movement speed of Typhoon In-Fa(2021)was notably slow,at 10 km h-1or less,for over 20 hours following its landfall in Zhejiang,China,in contrast to other typhoons that have made landfall.This study examines the factors contributing to the slow movement of Typhoon In-Fa,including the steering flow,diabatic heating,vertical wind shear(VWS),and surface synoptic situation,by comparing it with Typhoons Yagi(2018)and Rumbia(2018)which followed similar tracks.The findings reveal that the movement speed of Typhoons Yagi and Rumbia is most closely associated with their respective 500 h Pa environmental winds,with a steering flow of 10^(-12)m s^(-1).In contrast,Typhoon InFa’s movement speed is most strongly correlated with the 850 h Pa environmental wind field,with a steering flow speed of only 2 m s^(-1).Furthermore,as Typhoon In-Fa moves northwest after landfall,its intensity is slightly greater than that of Typhoons Yagi and Rumbia,and the pressure gradient in front of Typhoon In-Fa is notably smaller,leading to its slow movement.Additionally,the precipitation distribution of Typhoon In-Fa differs from that of the other two typhoons,resulting in a weak asymmetry of wavenumber-1 diabatic heating,which indirectly affects its movement speed.Further analysis indicates that VWS can alter the typhoon’s structure,weaken its intensity,and ultimately impact its movement.

Northward Shift in Landfall Locations of Tropical Cyclones over the Western North Pacific during the Last Four Decades

This study analyzes landfall locations of tropical cyclones(TCs)over the western North Pacific during 1979–2018.Results demonstrate that the landfall locations of TCs over this region have shifted northward during the last four decades,primarily due to the shift of landfalling TC tracks,with the decreasing/increasing proportion of westward/northward TC tracks.In particular,the northward shift of the landfalling TCs was not related to their formation locations,which have not markedly changed,whereas"no-landed"TCs have significantly shifted northward.TC movement was significantly and positively correlated to the zonal component of the steering flow,while the correlation between TC movement and the meridional component of the steering flow was relatively unobvious.The westward steering flow in the tropical central Pacific that occurred around the formation and early development of the westward TCs was significantly weakened,which was unfavorable for their westward movement,thereby,causing the higher proportions of northward moving tracks.This weakened westward flow was related to the northward shift of the subtropical high ridge,which was caused by significant weakening of the southern part of the subtropical high.The vertical wind shear,sea surface temperature,and convective available potential energy also showed that the northern region of the western North Pacific became more favorable for TC development,whereas the upper divergence,low-layer relative vorticity,and accumulated water vapor content were not obviously related to the northward shift of TCs.

STATISTICAL CLASSIFICATION AND CHARACTERISTICS ANALYSIS OF BINARY TROPICAL CYCLONES OVER THE WESTERN NORTH PACIFIC OCEAN

Using the 1949-2007 western North Pacific tropical cyclones (TCs) best-track data archived at the Shanghai Typhoon Institute of China Meteorological Administration for the western North Pacific from 1949 to 2007,both the characteristics of binary and multiple TCs and samples of interactions among TCs and multi-TCs are identified and statistically analyzed.According to the various features of individual TC tracks and interacting tracks,seven distinct types are proposed to describe the binary system of TCs and their interaction samples.The mean trajectories of the west and east component of binary TCs in each type are obtained using a new cluster analysis technique.These types are then analyzed in terms of landfall process,occurrence seasonality,coexistent lifetime,especially the large-scale patterns of atmospheric circulation.Finally,typical steering flows and conceptual models of the binary TCs at different phases are established based on six-hourly flow maps of the binary system and the averages are determined of the mean steering flow of ten representative binary TCs.Then,typical steering flows and conceptual models at the beginning,middle and final phase in each type are established to describe the large-scale circulation patterns of the binary system interaction types.

Statistical Characteristics and Mechanistic Analysis of Suddenly Reversed Tropical Cyclones over the Western North Pacific Ocean

Based on best track data of tropical cyclones（TCs） from the Japan Meteorological Agency, the characteristics of suddenly reversed TCs（SRTCs）, which have turning angles usually approaching 180°, are statistically analyzed from 1949 to 2011 over the western North Pacific Ocean. The typical large-scale circulation patterns of SRTCs are investigated using reanalysis data and dynamical composite analysis. Results show that turnings mainly occur in low latitudes between 10°N and 20°N,and mainly west of 135°E. The majority of SRTCs reach their peak intensity at, or slightly before, the turning time and subsequently decrease at some variable rate. Specifically, SRTCs are divided into four types, each containing two groups（i.e.eight groups in total） in terms of the moving-direction changes. The moving speed of all SRTC types except the south–north type decreases to its lowest during the 24 h, corresponding to a significant reduction in the primary steering components.According to the analysis of the 13 typical flow patterns found in this study, we suggest that sudden track changes are caused by the reversal steering flow. The original balance of the background flow patterns are broken up by new systems, e.g. binary TCs or dispersion-induced anticyclones. Additionally, sudden track changes are often due to double ridge variations of the subtropical high or weakened/strengthened high pressure in the east and west, respectively.

Westward Migration of Tropical Cyclone Activity in the Western North Pacific during 1982–2020:Features and Possible Causes

The westward migration of tropical cyclone(TC)activity has been identified in the western North Pacific(WNP),but the related features and causes remain elusive.Here,based on the best track data from China,Japan,and the US,and the NCEP–NCAR reanalysis data in 1982–2020,we investigate characteristics of the westward migration of the WNP TC activity with various metrics,and reveal possible causes for the migration of TC tracks through analyzing its seasonality and dependence on environmental conditions.The results show that the WNP TCs show significant westward migrations in a number of metrics,including location of tracks,genesis,the first track point at which TC reaches its lifetime-maximum intensity,and the last track point in the TC lifetime.It is found that TC tracks exhibit more significant westward migrations in the easterly steering flow than in the westerly steering flow.Meanwhile,the TC longitude shift shows notable seasonal variations,for which the TCs in the easterlies move further west than those in the westerlies during July–September,vice versa during October–December.The dependence of the westward migration of TC tracks on background steering flow is associated with the different environmental conditions.The westward shift in the westerly steering is mainly due to the reduced vertical wind shear(VWS),while the weakened zonal easterly steering and reduced VWS are both closely related to the westward migration in the easterly steering.These results have important implications for understanding current and future variations in TC longitude shift.

Factors controlling northward and north-eastward moving tropical cyclones near the coast of East Asia

The impacts of multi-time-scale flows on northward and north-eastward moving tropical cyclones(TCs)near the east coast of China in August and September are investigated using reanalysis data from 1982 to 2012.TCs of interest are under the influence of the subtropical high-pressure system in the western North Pacific(WNP).In August when the subtropical highpressure system is strong and close to the coast line,most TCs in the region move northward,while more TCs move north-eastward in September when the subtropical highpressure system retreats to the east.To investigate the influence from different time-scales,the environmental flow is divided intofour components,the synoptic flow,the intraseasonal flow,the interannual flow and the climatological background field.Analysis of steering flows between 25°N and 30°N indicates that the meridional steering vectors from all time-scales point to the north,dominated by the intraseasonal component.The deciding factor on whether a TC moves to the north or north-east between 25°N and 30°N is the zonal steering vector.For the northward moving group,the sum of the zonal steering from all time-scales is very small.On the other hand,the north-east moving group has a net eastward zonal component mainly contributed by the climatological mean flow.Several individual cases that stood out from the majority of the group are analyzed.For those cases,the intraseasonal flow plays an important role in affecting the movement of the TCs mainly through the wave train,in which a cyclonic circulation is located to the north-west(north)and an anticyclonic circulation to the south-east(east)of TCs.The analysis of the steering vectors indicates the importance of all components with different timescales to the movement of TCs.

Analysing the atmospheric-oceanic conditions driving the sustained long track and intensity of Tropical Cyclone Freddy

During February–March 2023,the record-breaking tropical cyclone(TC)Freddy caused widespreadflooding and damages across southeastern Africa.While<5%of TCs make landfall into southern Africa,TC Freddy made landfall twice and is the only TC in the past two decades that has tracked over 8000 km across the entire southern Indian Ocean.To understand why TC Freddy was so unique,this study investigated the evolution,track and atmospheric-oceanic mechanisms driving TC Freddy using the ERA5,CFSv2,OSTIA,NCEP-NCAR datasets and track data from various sources.It was found that SSTs were>27◦C during TC Freddy’s lifetime,while TC Dingani and a split Mascarene High played a role in steering TC Freddy across the southern Indian Ocean.Leading up to the development of TC Freddy,conditions were favourable for TC genesis,as indicated by the levels of the Genesis Potential Parameter(GPP)and its modified version(GPPI),the tropical cyclone heat potential levels,and elevated SSTs.Ridging subtropical anticyclones and the Mascarene High alongside favourable steeringflow and GPP(and GPPI)conditions resulted in Freddy’s double landfall in Mozambique.In assessing the tracks,it was found that there are discrepancies in the track of the commonly used IBTrACS when compared to ERA5 and RSMC tracks,which has implications for impact studies due to the underestimation of landfall considerations.This study reveals the unique characteristics and atmospheric-oceanic mechanisms driving TC Freddy,emphasising the impor-tance of accurate representation of favourable conditions and track data for enhancing TC forecasting and impact assessments.