热带气旋结构对其运动的影响

我们采用无辐散正压数值模式,在无基本气流的条件下研究了热带气旋结构对其运动的一些作用。正如 DeMaria 先前所指出的,初始最大风速对涡旋轨迹影响甚微。与β效应相联系的涡旋移动十分敏感地依赖于离中心300至1000km 范围内的气流强度。如果此圆环内的气流气旋性加强,则涡旋轨迹也会气旋性地转向,在北半球其移动更偏向于西。通过分解出对称和不对称环流,我们讨论了β漂移的动力学。对称气流经历了最大风速稍微减弱和在600km 以外激发出一反气旋环流的过程。不对称环流由方位1波环流所支配,中心东侧有一反气旋环流圈,西侧有一气旋性环流圈,两环流圈之间有一近于均匀的,尺度较宽的通风气流。涡旋移动的速度和方向几乎等同于该通风气流在涡旋内有显著气旋性环流区域上的平均值。模式流函数趋势方程的分析证实,线性β项导致初始不对称环流圈的形成。对称涡旋气流平流不对称环流,这一非线性项使两环流圈之间的内部区域发生扭曲,并使通风气流的方向由向北演变成向西北。由于该项几乎与线性β强迫相平衡,所以流函数时间趋势(及涡旋移动)主要由通风气流平流对称涡旋这一项决定。

ADVANCES IN UNDERSTANDING AND FORECASTING RAPIDLY CHANGING PHENOMENA IN TROPICAL CYCLONES

This review of new understanding and forecasting of tropical cyclones(TCs) is based on presentations at the International Top-level Forum on Rapid Change Phenomena in Tropical Cyclones in Haikou, China. The major topics are the sudden changes in tracks, rapid changes in structure and intensity, rapid changes in rainfall, and advances in forecasting and forecaster requirements. Although improved track forecast guidance has been achieved with the Australian ACCESS-TC model and in track forecasts to 120 h by the China Meteorological Administration, there is a continuing need for better understanding and improved track forecast guidance. Advances in understanding of processes related to rapid intensification(RI), secondary eyewall formation, mechanisms controlling inner-core structure and size changes, and structure and intensity changes at landfall have been achieved, but progress in prediction of rapid changes in structure and intensity has been slow. Taking into account complex interactions involved in TC-related rainfall, a prioritized list of physical processes that govern rainfall from landfalling TCs in China has been developed. While forecaster participants were generally encouraged by the progress being made, they expressed a strong desire for a transition of that new knowledge to timely and reliable forecast guidance products.

RECENT ADVANCES IN RESEARCH AND FORECASTING OF TROPICAL CYCLONE TRACK, INTENSITY, AND STRUCTURE AT LANDFALL

This review prepared for the fourth International Workshop on Tropical Cyclone Landfall Processes(IWTCLP-4) summarizes the most recent(2015-2017) theoretical and practical knowledge in the field of tropical cyclone(TC) track, intensity, and structure rapid changes at or near landfall. Although the focus of IWTCLPIV was on landfall, this summary necessarily embraces the characteristics of storms during their course over the ocean prior to and leading up to landfall. In the past few years, extremely valuable observational datasets have been collected for TC forecasting guidance and research studies using both aircraft reconnaissance and new geostationary or low-earth orbiting satellites at high temporal and spatial resolution. Track deflections for systems near complex topography such as that of Taiwan and La Réunion have been further investigated, and advanced numerical models with high spatial resolution necessary to predict the interaction of the TC circulation with steep island topography have been developed. An analog technique has been designed to meet the need for longer range landfall intensity forecast guidance that will provide more time for emergency preparedness. Probabilistic track and intensity forecasts have also been developed to better communicate on forecast uncertainty. Operational practices of several TC forecast centers are described herein and some challenges regarding forecasts and warnings for TCs making landfall are identified. This review concludes with insights from both researchers and forecasters regarding future directions to improve predictions of TC track, intensity, and structure at landfall.

TROPICAL CYCLONE PREDICTION ON SUBSEASONAL TIME-SCALES

Here we discuss recent progress in understanding tropical cyclone(TC)subseasonal variability and its prediction.There has been a concerted effort to understand the sources of predictability at subseasonal time-scales,and this effort has continued to make progress in recent years.Besides the Madden-Julian Oscillation(MJO),other modes of variability affect TCs at these time-scales,in particular various equatorial waves.Additionally,TC activity is also modulated by extratropical processes via Rossby wave breaking.There has also been progress in the ability of models to simulate the MJO and its modulation of TC activity.Community efforts have created multi-model ensemble datasets,which have made it possible to evaluate the forecast skill of the MJO and TCs on subseasonal time-scales in multiple forecasting systems.While there is positive skill in some cases,there is strong dependence on the ensemble system considered,the basin examined,and whether the storms have extratropical influences or not.Furthermore,the definition of skill differs among studies.Forecasting centers are currently issuing subseasonal TC forecasts using various techniques(statistical,statistical-dynamical and dynamical).There is also a strong interest in the private sector for forecasts with 3-4 weeks lead time.

VERTICAL WIND SHEAR AND OCEAN HEAT CONTENT AS ENVIRONMENTAL MODULATORS OF WESTERN NORTH PACIFIC TROPICAL CYCLONE INTENSIFICATION AND DECAY

A general framework in which Ocean Heat Content(OHC) may modulate tropical cyclone intensification and decay in conjunction with Vertical Wind Shear(VWS) as the primary environmental control is formulated in terms of a two-dimensional phase diagram.Case studies of the life cycles of three tropical cyclones during the Impact of Typhoons on the Ocean in the Pacific(ITOP) field experiment during August-October 2010 are selected as examples owing to numerous Air-deployed e Xpendable Bathy Thermographs(AXBTs) and special atmospheric observations.Vertical wind shears calculated from European Center for Medium-range Weather Forecasting analyses within a 3°-5° lat.annulus around the center may not properly represent the VWS in nonuniform,asymmetric,and time-varying conditions in the western North Pacific.As expected,VWS is the primary environmental control during the formation and early intensification stage over regions of large OHC,and during the decay phase over regions of small OHC.The challenging intensity forecasting problems are in the intermediate conditions of an intense tropical cyclone moving slowly over a region of low to moderate OHC when the negative feedback from the ocean may or may not lead to a decrease in intensity.

THE WORLD METEOROLOGICAL ORGANIZATION FOURTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONE LANDFALL PROCESSES(IWTCLP-IV): A SUMMARY

The Fourth International Workshop on Tropical Cyclone Landfall Processes(IWTCLP-4) was held in Macao, China from 5-7 December 2017. The workshop was organized by the World Meteorological Organization(WMO) Expert Team on Tropical Cyclone Landfall Processes in partnership with the WMO Tropical Cyclone Program. The workshop provided a forum for discussion between researchers and forecasters on the current status of tropical cyclone landfall processes and on priorities and opportunities for research. More than 60 leading research scientists and warning specialists working on topics related to tropical cyclone landfall examined current knowledge, forecasting and research trends from an integrated global perspective. The workshop offered a number of recommendations for future forecasting studies and research with special regard to the varying needs of different tropical cyclone af fected regions. The recommendations emanating from the workshop will be presented at the upcoming Ninth International Workshop on Tropical Cyclones(IWTC-9)(Hawaii, USA, 3-7 December 2018).

OUTER VORTEX WIND STRUCTURE CHANGES DURING AND FOLLOWING TROPICAL CYCLONE SECONDARY EYEWALL FORMATION

Some recent studies have utilized flight-level(700 mb) winds to document the maximum wind speeds(Vmax)and radius of Vmax(Rmax) of the original and secondary eyewalls during 24 Atlantic hurricane eyewall replacement cycles(ERC).In this study,Hurricane Wind(H*Wind) analyses of Atlantic hurricanes during 2003-2005 are utilized to document changes in the outer vortex surface wind profile beyond the secondary eyewall,with a focus on the radii of gale-force winds(R34) that are often defined operationally as size changes.In Mode 1,complete and partial ERCs in which the pre-,during-,and post-ERC outer wind profiles have approximately the same shape,the outward displacements of Rmax leads to size(R34) increases as much as 100 km.Mode 2 ERCs are characterized by sharpened wind profiles outside the secondary eyewall that offset the larger Rmax radii to produce only small R34 increases.While statistically significant results are not obtained,the differences in size changes for Mode 1 and Mode 2 SEF cases suggest practical significance for forecasts and warnings.

RECENT ADVANCES IN RESEARCH AND FORECASTING OF TROPICAL CYCLONE RAINFALL

In preparation for the Fourth International Workshop on Tropical Cyclone Landfall Processes(IWTCLP-IV), a summary of recent research studies and the forecasting challenges of tropical cyclone(TC) rainfall has been prepared. The extreme rainfall accumulations in Hurricane Harvey(2017) near Houston, Texas and Typhoon Damrey(2017) in southern Vietnam are examples of the TC rainfall forecasting challenges. Some progress is being made in understanding the internal rainfall dynamics via case studies. Environmental effects such as vertical wind shear and terrain-induced rainfall have been studied, as well as the rainfall relationships with TC intensity and structure. Numerical model predictions of TC-related rainfall have been improved via data assimilation, microphysics representation, improved resolution, and ensemble quantitative precipitation forecast techniques. Some attempts have been made to improve the verification techniques as well. A basic forecast challenge for TC-related rainfall is monitoring the existing rainfall distribution via satellite or coastal radars, or from over-land rain gauges. Forecasters also need assistance in understanding how seemingly similar landfall locations relative to the TC experience different rainfall distributions. In addition, forecasters must cope with anomalous TC activity and landfall distributions in response to various environmental effects.

OPPORTUNITIES FOR PROBABILISTIC FORECASTS OF TROPICAL CYCLONE FORMATIONS AND TRACKS ON INTRASEASONAL TIMESCALES

An opportunity that now exists for probabilistic forecasts of western North Pacific tropical cyclone formations and tracks on timescales of 7-30 days is demonstrated from the 51-member European Center for Medium-range Weather Forecasts 32-day ensemble predictions made once a week during the 2008 and 2009 seasons. A methodology has been developed to match the ensemble member vortices to form ensemble storms, and a weightedmean vector motion technique is applied to generate the track. An objective validation relative to the Joint Typhoon Warning Center best-tracks is supplemented with a subjective evaluation of the quality of the agreement.Success of the approach was first demonstrated with three intense typhoons during the 2008 season. Although some successful predictions were also achieved with some moderate typhoons and tropical storms, the early and late season typhoons and the tropical depressions were not predictable on 7-30 day timescales. Evaluation of the performance during the more active and typical 2009 season again indicated success in predicting typhoons and most of the weak tropical storms and tropical depressions. The exceptions were some multiple storm events and track bifurcation scenarios due to midlatitude interactions.Among the challenges to produce an operational product are the requirements to make the entire procedure more automated, more objective, and with forecaster-friendly displays. A documentation of the reliability and of the limitations of the predictions will be helpful in gaining forecaster acceptance.