Biographical Research Sketch
Over the past 20 years, Prof. Chun-Chieh Wu has dedicated his full efforts to typhoon-related scientific research. Prof. Wu’s thesis work at MIT (Wu and Emanuel 1993, 1994, 1995a, b) involved understanding of hurricane movement from the perspective of potential vorticity, which was a pioneering work in proposing and identifying the baroclinic effect on hurricane motion, and in quantitatively evaluating the typhoon steering flow and its connection to the large-scale dynamical systems. During Prof. Wu’s post-doctoral tenure at Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton University, his attention was drawn to the development of the GFDL hurricane model, especially its initialization. Based on this model, a hurricane-environment interaction problem was demonstrated (Wu and Kurihara 1996).
After joining the faculty of Department of Atmospheric Sciences, National Taiwan University (NTU) in 1994, Prof. Wu started to put together a research laboratory, the Typhoon Dynamics Research Center (see http://typhoon.as.ntu.edu.tw) within the department. With a strong will to conduct top-notch typhoon research, he has always had high hopes for the Center to advance understanding of the dynamics, the physics, as well as the forecast of typhoons.? Prof. Wu headed the “Priority Typhoon Research Project”, specially funded by the Division of Natural Science, National Science Council (NSC) of Taiwan from 2002-2008.?
Prof. Wu headed the “Priority Typhoon Research Project”, specially funded by the Division of Natural Science, National Science Council (NSC) of Taiwan from 2002-2008.? The research team made distinguished contribution in the observation (DOTSTAR), modeling and theoretical aspects of typhoon research, and was recognized as one of the “50 Scientific Achievements” by the National Science Council (NSC) in commemoration of its 50th Anniversary in 2009, while also receiving recognition from the World Meteorological Organization (WMO) WWRP/THORPEX (In recognition of outstanding contribution to the WMO THORPEX Programme for the years 2005- 2014) in 2014.? With research advances made on Typhoon Dynamics, Targeted Observation, Typhoon-Ocean Interaction, Typhoon-Terrain Interaction, Typhoon Rainfall Processes, and Typhoon and Climate Variability, Prof. Wu received outstanding research awards from NSC three times (in 2007, 2009 and 2012), as well as the Academic Award of Ministry of Education in 2013.? Prof. Wu is a prominent atmospheric scientist not only in Taiwan but also in Asia and globally.? Starting in 2013, Prof. Wu has been serving as the Editor of “Journal of Atmospheric Sciences”, the leading journal for theoretical researches under American Meteorological Society.? Prof. Wu also received the Editor’s Award of American Meteorological Society in 2014.
The following is a sketch of Prof. Wu’s research foci, primarily funded by the National Science Council (NSC) (renamed as Ministry of Science and Technology, MOST, since 2013) of Taiwan, Central Weather Bureau (CWB) of Taiwan, and the Office of Naval Research (ONR) of the U.S. Navy.

DOTSTAR (Dropwindsonde Observations for Typhoon Surveillance near the TAiwan Region) and targeted observation research
The DOTSTAR (Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region) program was successfully carried out during 2003-2013 (Wu et al., 2004, MWR; Wu et al., 2005, BAMS; Wu et al., 2006, JAS; Wu et al., 2007a, Wea. Forecasting, b, JAS; Chou and Wu, 2008, MWR; Chen et al., 2009, JAS; Yamaguchi et al., 2009, MWR; Wu et al., 2009a, b, c, MWR; Chou et al., 2010, JGR; Wu et al., 2010, JAS; Chen et al., 2011, MWR; Chou et al., 2011, MWR; Liang et al., 2011, JAS; Weissmann et al., 2011, MWR; Yen et al., 2011, TAO; Huang et al., 2012, JAS; Jung et al., 2012, Tellus A.; Wu et al., 2012a, b, JAS, MWR).? A special collection (issue) on “Targeted Observation and Data Assimilation for Improving Tropical Cyclone Predictability” headed by Prof. Wu was published in the Monthly Weather Review in 2009 and 2010.
In total, 69 surveillance flight missions were conducted for 54 typhoons, with 363 flight hours and 1141 dropwindsondes released in the DOTSTAR project.? These typhoons affected not only Taiwan, but aslo Phillipines, Mainland China, Korea and Jaoan.? The result is a robust 20% improvement in numerical models (such as NCEP GFS) that represents significant contribution to the study of typhoons (Chou et al. 2011 MWR).
Multiple techniques were proposed to help design the flight path for the targeted observations in DOTSTAR.? Wu et al. (2007b JAS) developed a new theory to identify sensitive areas for tropical cyclone (TC) targeted observations based on the adjoint model.? By appropriately defining the response functions to represent a typhoon’s steering flow at the verifying time, a unique new parameter, the Adjoint-Derived Sensitivity Steering Vector (ADSSV) was designed to clearly demonstrate the sensitivity locations at the observing time.? The ADSSV was examined to demonstrate the precise sensitive locations for the binary interaction (Fujiwhara effect) of two typhoons.? The ADSSV was implemented and further examined in the case of Typhoon Shanshan (2006) (Wu et al. 2009b MWR) where the recurvature of the typhoon caused by the approaching mid-latitude trough was precisely captured by the signal of ADSSV and effectively verified by the potential vorticity diagnosis.? This is the first paper in which targeted methods had been interpreted dynamically from the potential vorticity perspective.? Validation and interpretation of the ADSSV and the ensemble transform Kalman filter (ETKF) as guidance for targeted CT observations was further examined in Chen et al. (2011, MWR) and Majumdar et al. (2011, QJRMS).? A new sensitivity analysis method was also developed based on the Ensemble Kalman Filter (EnKF) prediction system (Wu et al. 2010 JAS) in which a TC-position is taken as a metric (Ito and Wu 2013, JAS).
An inter-comparison study (Wu 2009c MWR) was conducted to examine the common features and differences among all the targeting techniques, such as the Singular Vectors of JMA, NOGAPS and ECMWF, ADSSV, and ETKF.? This work involved tremendous international collaborations, headed and coordinated by Prof. Wu, who integrated inputs from 11 co-authors from NTU, NRL, JMA/MRI, NCEP, ECMWF, NOAA/HRD.? The results provided valuable insights into the dynamic features of each targeted technique, and their potential applications in real-time targeted observations.? This paper was published simultaneously as an ECMWF Technical Memorandum (No. 582).? This work was well recognized in WMO’s third THORPEX Science Workshop in Monterey in September 2009, in which Drs. Istvan Szunyogh and Rolf Langland described and commented on its contribution.? In 2010, Prof. Wu was invited to give a talk on targeted observation and to serve as the rapporteur for the Seventh WMO International Workshop on Tropical Cyclones (IWTC-VII), La Reunion, France.?? In 2014, Prof. Wu was invited to Co-chair the Topics 2 for the Eighth WMO International Workshop on Tropical Cyclones (IWTC-VIII) at Jeju.? During the conference, Prof. Wu received recognition from WMO WWRP (World Weather Research Programme) for “outstanding contribution to the WMO THORPEX Programme for the years 2005-2014”.
The impact of targeted observations from DOTSTAR data during T-PARC was further evaluated in Harnisch and Weissmann (2011), Weissmann et al. (2011), Chou et al. (2011), Jung et al. (2012), and Wu et al. (2012b).? The positive impacts of targeted observations by DOTSTAR were demonstrated and recognized in these studies.? DOTSTAR had played a pivotal role in several international field experiments, including T-PARC (THORPEX/PARC; The Observation System Research and Predictability Experiment Pacific-Asian Regional Campaign, T-PARC) in 2008 and ITOP (Impact of Typhoons on the Ocean in the Pacific) in 2010.
Widely recommended as a fully-developed program, DOTSATR was included in the international THORPEX/PARC initiative under the World Meteorological Organization [especially in collaboration with the Japanese program, Typhoon Hunting 2008, TH08, led by Dr. Tetsuo Nakazawa of JMA/MRI; and the US program, “Tropical Cyclone Structure 2008, TCS-08, led by Dr. Patrick Harr of Naval Postgraduate School (currently serving as the Section Head for the Atmosphere Section within the Division of Atmospheric and Geospace Sciences, National Science Foundation, USA)].? This is the first program in which four airplanes (two jets for surveillance, and a P-3 and a C-130 for reconnaissance) were used to observe typhoons in the western North Pacific.? The unprecedented data collected are valuable for understanding the physics and dynamics of the genesis, structure change, recurvature, extra-tropical transition, targeting observation, and predictability of tropical cyclones.? National Geographic made a one-hour documentary featuring the DOTSTAR and T-PARC programs in 2009 which had been aired over 135 countries.
In ITOP, abundant data had been collected by ASTRA (DOTSTAR), two C130 aircrafts (US Air Force), in addition to numerous buoy and ship observations during the lifetime of Typhoons Fanapi, Malakus, and Megi.? The EnKF data assimilation system developed in Wu et al. (2010 JAS) provided a comprehensive high-resolution atmospheric model dataset for further study, especially for physical oceanographers to drive their ocean models in ITOP (D’Asaro et al. 2014, BAMS; Ko et al. 2014, JGR)
The continuing work in DOTSTAR has shed light on typhoon dynamics, improved the understanding and predictability of typhoon track through the targeted observations, placed the DOTSTAR team at the forefront of international typhoon research, and has made a significant contribution to the study of typhoons in the northwestern Pacific and East Asia region.
Starting from 2013, Prof. Wu successfully transferred the standard operation procedure of DOTSTAR to CWB (Central Weather Bureau) and TTFRI (Taiwan Typhoon and Flood Research Institute), with detailed information available at http://typhoon.as.ntu.edu.tw/ DOTSTAR/en/.? This is a paradigm shift for transferring the know-how from scientific research to operation.

A new vortex initialization method based on the ensemble Kalman filter (EnKF) data assimilation system
A new TC vortex initialization method was developed in Wu et al. 2010 (JAS) based on the EnKF data assimilation system, which effectively provides well-balanced initial TC vortex structure dynamically consistent with the model.?? Three special observational parameters of TCs, including TC center position, storm motion vector and a single-level (either surface of flight level) axisymmetric wind profile, were innovatively adopted and assimilated via the EnKF methodology.?? This newly developed vortex initialization method had been deployed to numerical simulations of different typhoons, such as Typhoons Fung-wong (2008; Wu et al, 2010, JAS), Sinlaku (2008; Wu et al. 2011 MWR), Morakot (2009; Yen et al. 2011, TAO) and other typhoons, in particular, those with aircraft surveillance observations from DOTSTAR (2003-present), T-PARC (2008), and ITOP (2010).? Results of these studies showed improvement in typhoon simulations and forecasts when this vortex initialization method is applied.? Meanwhile, the ensemble members created from the EnKF data assimilation system provides information for predicting typhoon evolution, including the movement, intensity, structure and the associated precipitation.
This newly proposed vortex initialization had facilitated advancement in the dynamical research on typhoons in many aspects, such as in the formation and evolution of the concentric eyewall structure (Wu et al. 2012a MWR; Huang et al. 2012, JAS), and the impact of typhoons’ translation speed on the associated precipitation (Yen et al. 2011, TAO).? This EnKF vortex initialization methodology had been applied to studies of different issues on various typhoons [e.g., Typhoon-Ocean interaction in Typhoon Fanapi (2010) using ITOP data], and can also be used to help the design of idealized numerical experiments.? This method would continue to shed light on the scientific understanding of typhoons, and most importantly to improve typhoon forecasting.

Dynamics of typhoon (concentric) eyewall evolution:
A new study on the role of the diabatic process in affecting eyewall evolution has been carried out in Wu et al. 2009a (MWR), which highlights how the moist processes enhance the potential vorticity structure and support eyewall evolution.? This study points out the deficiency of the dry barotropic model in describing detailed eyewall dynamical processes, and provides new insights into the eyewall physics.? Idealized numerical experiments have been conducted (Wei and Wu 2012) to highlight the role of moist heating in affecting the eyewall dynamics.? It is shown that when the diabatic heating and 3-D flows are taken into account, the resultant vortex evolution paths are very different from those in the 2-D barotropic model. The role of convective heating on the maintenance of barotropically unstable eyewall PV ring was investigated in Wu et al. (2016 JAS), indicating the role of diabatic heating in the PV generation in the eyewall region.
In Wu et al. (2012a MWR) and Huang et al. (2012 JAS), a new pathway of the dynamics controlling the secondary eyewall formation (SEF) in TCs was presented.? A deeper understanding of the underlying dynamics of SEF had been obtained based on recently developed insights on the axisymmetric dynamics of tropical cyclone intensification.? This is an attractive paradigm on the physical grounds because of its simplicity and consistency with the 3-D numerical simulations presented.? Application of the two spin-up mechanisms set the scene for a progressive boundary layer control pathway to SEF.? The unbalanced boundary layer response to an expanding swirling wind field is an important mechanism for concentrating and sustaining deep convection in a narrow supergradient-wind zone in the outer-core region of a mature TC.? The findings point to a sequence of structural changes in the outer-core region of a mature TC, which culminates in the formation of a secondary eyewall.
A series of follow-up works had been proposed to provide complete dynamical analyses of SEF.? We believe this series of studies could further bring considerable dynamical insights into SEF, and thus would reveal the critical physical processes that need to be adequately represented in a numerical model, which could in turn facilitate further understanding of TC dynamics and improvement in typhoon forecasting.? Based on this series of research, a new and generalized theoretical framework/model for SEF is slated to be constructed, interpreting the axisymmetric/asymmetric and balanced/unbalanced vortex dynamics involved. This work is expected to improve the forecast of SEF (the timing and preferred radial intervals) and the evolution of a concentric eyewall cycle (including the associated structure and intensity changes, and the cycle’s duration), as well as the general forecast of a typhoon.? In all, the works of Wu et al. (2012a MWR) and Huang et al. (2012 JAS) had received high attention from the TC community, and a number of research groups (e.g., UCLA、SUNY Albany、Univ. of Washington、Univ. of Miami、Pennsylvania State Univ.、Naval Postgraduate school、Melbourne Univ.、Nanjing Univ.) have been investigating this new pathway for interpretation of the SEF dynamics. ?An invited review of this issue has also been published in the Encyclopedia of Atmospheric Science (Wu and Huang 2015).??

Dynamics of typhoon-terrain interactions:
Understanding how the Taiwan terrain affects the track, intensity, wind structure, and precipitation distribution is one of Prof. Wu’s key research thrusts. Both observational and numerical studies have been conducted to address this issue (Wu and Kuo 1999, BAMS; Wu 2001, MWR; Wu et al. 2002, Wea. & Forecating; Jian and Wu 2007, MWR; Galewsky et al. 2006, JGR).?
A paper studying the effects of the terrain on the eyewall dynamics and Vortex-Rossby waves of landfalling typhoons (Wu et al. 2003, GRL) was introduced in the “news and views in brief” column of Nature Magazine in September 2003.? The role of the adiabatic process in affecting the eyewall evolution was also examined in details in another paper (Wu et al. 2009a, MWR), which highlighted how the moist processes enhance the potential vorticity structure and support the eyewall evolution.? This study pointed out the deficiency of the dry barotropic model in describing the detailed eyewall dynamical processes, and provides new insights into the eyewall physics that is consistent with the new theories as described in Montgomery et al. (2008, 2009).? The role of terrain in affecting the looping motion of typhoons (channel effect) near the terrain was demonstrated in Jain and Wu (2008, MWR).? This study indicated how the terrain-induced channel effect leads to the unusual looping motion of Typhoon Haitang.? The looping motion of typhoons was further investigated in Huang et al. (2011, MWR).? The numerical simulations of Typhoon Krosa’s looping (2007) and an idealized set of numerical experiments were carried out to study the terrain-induced typhoon track deflections.? The study showed consistent results with Jian and Wu (2008, MWR) that the distinct southward track deflection prior to landfall can be attributed to the northerly jet enhanced by the channel effect at the narrow pathway between the high topography of Taiwan and the eyewall with high inertial stability of Krosa. Such findings in Huang et al. (2011) was recognized in the 2011 UCAR magazine.? This series of research provided clear insights into the physics of typhoon-terrain interactions, which were also observed in many similar typhoon events near Taiwan.?
A further study (Wu et al. 2016 JAS) with idealized model experiments under a wider spectrum of flow regimes was conducted to more thoroughly investigate the dynamics of such processes.? All the presented simulated storms experience southward track deflection prior to landfall.? Different from the mechanism related to the channeling-effect-induced low-level northerly jet as suggested in previous studies, (Wu et al. 2015 JAS) indicated the leading role of the northerly asymmetric flow in the mid-troposphere in causing the southward deflection of the simulated TC tracks.? The mid-tropospheric northerly asymmetric flow forms due to the wind speeds restrained east to the storm center and winds enhanced/maintained west to the storm center.? In all, the study highlights a new mechanism that contributes to the terrain-induced southward TC deflection in addition to the traditional channeling effect.

Typhoon rainfall:
Typhoon-induced rainfall has been an important research theme especially in Taiwan, considering the mountainous nature of its typography and the disastrous impact the heavy rainfall can have on people’s lives and property. Wu et al. (2002, WF) conducted a series of numerical experiments to examine the ability of a high-resolution mesoscale model to simulate the track, intensity change, and detailed mesoscale precipitation distributions associated with Typhoon Herb (1996), which made landfall and resulted in serious damage in Taiwan.? It was shown that, with an accurate track simulation, the ability of the model to simulate successfully the observed rainfall depends on two key factors: the model’s horizontal grid spacing and its ability to describe the Taiwan terrain.?
The existence of the Central Mountain Range has only a minor impact on the storm track, but it plays a key role in substantially increasing the total rainfall amounts over Taiwan.? The analysis presented showed that the model and terrain resolutions play a nearly equivalent role in the heavy precipitation over Mount Ali.? The presence of maximum vertical motion and heating rate in the lower troposphere, above the upslope mountainous region, is a significant feature of forced lifting associated with the interaction of the typhoon’s circulation and Taiwan’s mountainous terrain.? Overall, Typhoon Herb is a case in point to indicate the intimate relation between Taiwan’s topography and the rainfall distribution associated with typhoons at landfall.? Wu et al. (2002, WF) was a milestone work on the rainfall simulation issue in Taiwan, and had been cited by SCI-journal publication for 115 times.
Wu et al. (2009 MWR), which examines a heavy rainfall event in the Taiwan area associated with the interaction between Typhoon Babs (1998) and the East Asia winter monsoon is another important study in the area of rainfall associated with TC-monsoon-terrain interaction, orremote rainfall.??? Typhoon Babs is a case in point demonstrating the often-observed phenomenon that heavy rainfall can be induced in the eastern and/or northeastern region of Taiwan in late typhoon season.? Such heavy rainfall was caused by the joint convergent flow associated with the outer circulation of typhoons and the strengthening northeasterly monsoon in late typhoon season, even though Babs remained distant from Taiwan when it moved through the island of Luzon in the Philippines and stayed over the south.? It was shown that the terrain played a key role in changing the low-level convergence pattern between typhoon circulation and monsoonal northeasterlies. This is the first paper published in an SCI journal that discusses the rainfall mechanism associated with the TC-winter monsoon-terrain interaction (also called as remote rainfall), which is well illustrated in the schematic diagram of Wu et al. (2009, MWR).
Based on the EnKF data assimilation (Wu et al. 2010, 2011), Yen et al. (2011) showed in a simulation with nearly-doubled translation speed of Typhoon Morakot that the 55% increase of the translation speed (12->19 km/h; 36 % less duration time) leads to a 33% reduction in the maximum accumulated rainfall (1800->1207 mm), while the rainfall distribution over Taiwan remains similar.? Furthermore, the 28 ensemble members provide abundant information on their spread and other statistics, which reveal the usefulness of the ensemble simulation for the quantitative precipitation forecast.? It was also suggested that the ensemble simulations with coherent high model and terrain resolutions are valuable in assessing the issue of terrain-induced heavy rainfall, one of the most critical forecast issues in Taiwan.? The paper was awarded “The Dr. Shiah-Shen Huang Outstanding Paper Award” in 2012 by the Meteorological Society of ROC (Taiwan).
Typhoon Morakot (2009) was one of the deadliest typhoons that have impacted Taiwan in the past 50 years.? Since this extreme rainfall event, there had been extensive studies focusing on its record-breaking amount of rainfall from various scientific and forecast perspectives. To communicate and discuss various aspects of this deadly typhoon, a conference named “The International Workshop on Typhoon Morakot (2009),” co-organized by Prof. Wu was held from March 25-26, 2010, in Taipei, Taiwan.? The conference specifically aimed to identify gaps in our understanding of TCs, and to discuss advanced forecast guidance tools required to improve warnings of these extreme precipitation and flooding events.? The community (headed by Prof. Wu, as the Editor in Chief of TAO Journal) went a step further to propose a special issue to the journal Terrestrial, Atmospheric and Oceanic Sciences (TAO) in order to provide a comprehensive summary of Morakot and other extreme rainfall events associated with landfalling TCs.? The special issue, “Typhoon Morakot (2009): Observation, Modeling, and Forecasting Applications,” was published in December 2011 and covered observation analyses of circulations and structures, mesoscale model simulations, data assimilation techniques, and practical forecast verification and guidance.? Another paper highlighting the significance of this special issue was published in Wu (2013, BAMS).

Typhoon-Climate Study:?
In recent 5 years, Prof. Wu also broadened his research field to the study of TC-climate problems, one emerging important issue in our research community.? A post-doctoral research fellow (Dr. Zhan) from Shanghai Typhoon Research Institute visited Prof. Wu at NTU in 2010, and since then they worked together on this research topic.? Zhan et al. (2011 J. Climate) showed that the EIO SSTA affects TC genesis frequency in the entire genesis region over the western North Pacific (WNP) by significantly modulating both the western Pacific summer monsoon and the equatorial Kelvin wave activity over the western Pacific, two major large-scale dynamical controls of TC genesis over the WNP. Additional sensitivity experiments were performed for two extreme years: one (1994) with the highest and another (1998) with the lowest TC annual frequencies in the studied period.
The effect of ENSO on landfalling TCs over the Korean Peninsula was examined by another post-doctoral fellow researcher from Korea (Choi et al. 2011 Asia-Pac J. Atmos. Sci.).? It was found that although difference in landfalling frequency is not statistically significant between different ENSO phases, the landfalling tracks are shifted northward in response to the decrease in Nino-3.4 index.? In the neutral ENSO phase, many TCs pass through (mainland) China before making landfall on the Korean Peninsula due to the westward expansion of the western North Pacific subtropical high.
As another visiting scientist to Prof. Wu’s group, Kim et al. (2011 TAO) investigated the contribution of TC rainfall (PTC) to the inter-decadal change in summer (June, July and August) rainfall (PTotal) over southern China between 1981 - 1992 (ID1) and 1993 - 2002 (ID2).? In an area-averaged sense, the inter-decadal change in PTotal was largely attributed to non-TC rainfall for the summer total and the months of June and July, while PTC became comparable in August. When the month-to-month spatial variability was considered, noticeable nega?tive PTC contributions appeared over the southeastern coast of China, Hainan Island, and Taiwan in June and over the southern coastal regions in July, where less TC activity was observed.? In June, the condition was attributed to reduced basin-wide TC activity due to unfavorable large-scale environments in ID2, whereas in July, an enhanced cyclonic circulation centered at Taiwan in ID2 limited the number of TCs from the Philippine Sea.
Choi et al. (2013 Theor. Appl Climatol.) used teleconnection patterns to make seasonal predictions for tropical cyclone frequency around Taiwan, and further stated that the frequency of summer TCs in the areas of Japan, Korea, and Taiwan (JKT) has a positive correlation with the Arctic Oscillation (AO) in the preceding spring, while summer TC frequency in the Philippines (PH), located in the low latitudes, has a negative correlation with the AO of the preceding spring (Choi et al., 2012 Climate Dynamics).? During a positive AO phase, when the anomalous anticyclone forms over the mid-latitudes of East Asia, other anomalous cyclones develop not only in the high latitudes but also in the low latitudes from the preceding spring to the summer months.? With such a difference, while the southeasterly in the JKT area derived from the mid-latitude anticyclone plays a role in steering TCs toward this area, the northwesterly strengthened in the PH area by the low-latitude cyclone prevents TC movement toward this area.? Also because of this pressure systems developed during this AO phase, TCs occur, move, and recurve in further northeastern part of the western North Pacific than they do during a negative AO phase.
Prof. Wu implemented the International Pacific Research Center (IPRC) Regional Climate Model (iRAM) and examined the internal variability of dynamically downscaled TCs over the WNP based on four simulations of 20 typhoon seasons (1982?2001) initialized on four successive days using iRAM (Wu et al. 2012d J. Climate). The results showed that on both seasonal and interannual timescales, the initial conditions significantly affect the downscaled TC activity, with the largest internal variability occurring in August on the seasonal timescale. The spreads between any of the individual simulations and the ensemble mean are comparable to and in some circumstances greater than the interannual variation of the observed TC frequency.? These works have established solid foundation for my approach to study the TC-climate problems.
Meanwhile, Wu et al. (2015, BAMS) illustrates the importance of the increase in the number of available stations in assessing the long-term rainfall characteristic of typhoon-associated heavy rainfall in Taiwan.

Dynamics of typhoon intensity change
One of the most difficult problems which remain unsolved to date in typhoon research is identifying the physical mechanisms that determine changes in typhoon intensity. We conducted an observational analysis (Wu and Cheng 1999, MWR) to show the roles of eddy momentum flux and vertical shear in affecting the intensity change of two different types of typhoons. Both idealized and real-case numerical simulations were set up to address this critical issue, with a review paper published in MAP (Wang and Wu 2004), while an observational study has also been conducted to assess the influence of the environmental factors on typhoon intensity (Zeng et al. 2006, MWR).

Dynamics of typhoon-ocean interaction
The cooling of the ocean due to the passage of typhoons has been documented from satellite-retrieved SST data, while response to the wind change has also been demonstrated (Lin et al. 2003a, GRL).? Meanwhile, a striking interdisciplinary issue on the dramatic bio-response and ocean primary production due to typhoons has also been raised (Lin et al. 2003b, GRL).? The above two papers were introduced in the “news and views in brief” column of Nature Magazine in the 2003 March and August issues, respectively.? We also combined the Sea Surface Height Anomaly data with a simple coupled model (CHIPS) to investigate the role of warm ocean eddies in the intensity change of Typhoon Maemi (2003) (Lin. et al. 2005, MWR).? It was shown that the warm eddy plays a critical role as an efficient insulator that prevents the storm-induced SST cooling, thus enabling Maemi to maintain its intensity as a super typhoon.? This research project had received notable attention in the typhoon research community. The intensification of Hurricane Katrina (2005) is a case in point to highlight the role of warm ocean eddies and the warm Loop Current as depicted in our paper.?
Inspired by recent observations, Wu et al. (2007, JAS) used a simple yet comprehensive, typhoon-ocean coupled model to study the influence of the ocean mixed-layer structure and the warm ocean eddy on such feedback problems, and to study the influence of the typhoon-induced SST cooling on typhoon intensity. Numerical experiments with different oceanic thermal structures were designed to elucidate the responses of tropical cyclones to the ocean eddy and the effects of tropical cyclones on the ocean.? This simple model showed that rapid intensification occurs as a storm encounters the ocean eddy due to enhanced heat flux.? While strong winds usually cause strong mixing in the mixed layer and thus cool down the sea surface, negative feedback to the storm intensity of this kind is limited by the presence of a warm ocean eddy which provides insulating effect against the storm-induced mixing and cooling.? Two new eddy factors were defined to evaluate the effect of the eddy on tropical cyclone intensity.? The efficiency of the eddy feedback effect depends on both the oceanic structure and other environment parameters, including properties of the tropical cyclone.? Analysis of the functionality of the eddy factor showed that the mixed-layer depth either associated with the large-scale ocean or with the eddy is the most important factor in determining the magnitude of eddy feedback effect.? Next to them are the storm’s translation speed and the ambient relative humidity.? This work provided useful new insight into the understanding of typhoon-ocean interaction and the role of the warm eddy.
Further work had been carried out to understand the role of warm and deep ocean gyre and warm eddies as “Super-typhoon Boosters” in the NW Pacific (Lin et al. 2008a, b, MWR; 2009, GRL; 2011, TAO).
Based on detailed in situ air-deployed ocean and atmospheric measurement pairs collected during the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign (D’Asaro et al. 2014), Lin et al. (2013, GRL) modified the widely used Sea Surface Temperature Potential Intensity (SST_PI) index by including information from the subsurface ocean temperature profile to form a new Ocean coupling Potential Intensity (OC_PI) index.
In the most recent work (Wu et al. 2015 JGR), a mesoscale model coupling the Weather Research and Forecasting model and the three-dimensional Price-Weller-Pinkel ocean model was used to investigate the dynamical ocean response to Megi (2010).? It was found that Megi induces sea surface temperature (SST) cooling very differently in the Philippine Sea (PS) and the South China Sea (SCS).? The results are compared to the in situ measurements from ITOP, satellite observations, as well as ocean analysis field from Eastern Asian Seas Ocean Nowcast/Forecast System of the U.S. Naval Research Laboratory.? The uncoupled and coupled experiments simulate relatively accurately the track and intensity of Megi over PS; however, the simulated intensity of Megi over SCS varies significantly among the experiments.? Only the experiment coupled with three-dimensional ocean processes, which generates rational SST cooling, reasonably simulates the storm intensity in SCS.? The results suggest that storm translation speed and upper ocean thermal structure are two main factors responsible for Megi’s distinct different impact over PS and over SCS.? In addition, it was shown that coupling with one-dimensional ocean process (i.e. only vertical mixing process) is not enough to provide sufficient ocean response, especially under slow translation speed (~2-3 m s-1), during which vertical advection (or upwelling) is significant.? Therefore, coupling with three-dimensional ocean processes is necessary and crucial for TC forecasting.? Finally, the simulation results showed that the stable boundary layer forms on top of the Megi-induced cold SST area and increases the inflow angle of the surface wind.

Numerical simulation and data assimilation of typhoons
As described in Wu and Kuo (1999, BAMS), our understanding of typhoon dynamics and typhoon forecasting in the Taiwan area hinges very much on our ability to incorporate available data into high-resolution numerical models through advanced data assimilation techniques. Our team has thus made considerable efforts on data assimilation research.? Prof. Wu completed simulation experiments based on the 4-dimensional variational data assimilation to help understand the key variables affecting the initialization and simulation of typhoons (Wu et al. 2006, JAS).? The follow-up adjoint sensitivity study can play an important role in identifying important areas and parameters, which should help construct strategies for adaptive observations.? Work had been carried out to identify the best approach to incorporate dropwindsonde data and the bogused vortex based on 3D-VAR and 4D-VAR methods in order to improve the track and intensity simulations of typhoons (Chou and Wu, 2008, MWR).? This work gave rise to a new method to optimally combine the bogused vortex and dropwindsonde data for improving the track and intensity forecast of typhoons.? A new scheme to improve typhoon initialization has been developed based on the Ensemble Kalman Filter (EnKF) (Wu et al. 2010).

Potential vorticity diagnostics of typhoons
Wu and Emanuel (1993, 1994, 1995a, b) and Wu and Kurihara (1996) improved the understanding of hurricane movement from the perspective of potential vorticity, which was a pioneering work in proposing and identifying the baroclinic effect on hurricane motion, and in quantitatively evaluating the typhoon steering flow and its connection to the large-scale dynamical systems. ?The potential vorticity diagnostics was designed to understand the controlling factors affecting typhoon movements.? To highlight the binary interaction between two typhoons, the track of one typhoon is plotted as centroid-relative, and with its position weighting based on the steering flow induced by the PV anomaly associated with the other typhoon (Wu et al. 2003, MWR; Yang et al. 2008, MWR).? Further research was conducted to evaluate and quantify the physical factors leading to the uncertainty of typhoon movements, such as for Typhoon Sinlaku (2002) (Wu et al. 2004, MWR).? This methodology had been adopted by the Central Weather Bureau (CWB) both for research and analysis, and for diagnosing biases in the Bureau’s model forecasts.? Further work had been proposed to gain more insight into the physics of the statistical behavior of typhoon tracks in the entire north-western Pacific region. The impacts of the ITCZ and other large scale circulations on the typhoon tracks were also quantified.?
A quantitative analysis of the steering flow based on the PV diagnosis indicates that the Pacific subtropical high to the east of Sinlaku is a primary factor that advects Sinlaku northwestward, while the monsoon trough plays a secondary role (Wu et al. 2012, MWR).? The evaluation provides quantitative analysis on how the DOTSTAR data during T-PARC field program improved the track prediction of Sinlaku.

Research Foci / 研究重點

研究歷程:
吳教授過去二十年持續專注於颱風科學研究。吳教授於美國麻省理工學院完成的博士論文(Wu and Emanuel 1993, 1994, JAS; 1995a, b, MWR)探討如何從位渦觀點瞭解颱風運動,創先提出斜壓對颱風運動的影響,以位渦量化颱風駛流與大尺度動力系統的關係,此方法對了解影響颱風運動的關鍵動力有重要幫助。在普林斯頓大學地球物理流體動力實驗室(GFDL; Geophysical Fluid Dynamics Laboratory)的博士後研究期間,吳教授參與發展及使用著名的GFDL 颱風模式,特別是其初始化與參數化方法改進模擬結果。另外吳教授也利用GFDL颱風模式模擬探討颱風與環境的互動關係與回饋機制(Wu and Kurihara 1996, JAS; Wu et al. 2000, JMSJ; Wu 2001, MWR)。
有鑑於颱風資料的匱乏,為求突破,吳教授自2002年起擔任國科會(現為科技部)「颱風重點研究」總主持人職務,自2008年8月起繼續擔任「颱風整合研究」總主持人職務,成功領導國際颱風飛機觀測實驗(DOTSTAR;追風計畫),同時使用及發展高解析度的數值模擬及同化模式,利用蒐集的飛機觀測資料與所發展的資料同化方法,改善數值模擬的結果。透過數值實驗設計與深入的動力分析,針對有關颱風移動、強度演變、降雨及颱風-環境、地形/颱風-海洋交互作用的動力原理提出開創性的詮釋。吳教授帶領其團隊以ASTRA型飛機,過去十年在颱風侵臺期間觀測64個颱風個案。這些珍貴數據及後續的突破研究不但提昇了颱風分析與預報的準確度,且已經成了中央氣象局的標準作業程序,也在聯合國世界氣象組織的颱風研究上佔有一席之地。此外,這項研究工作涉及標靶觀測理論開展,甚至轉移了一些典範,成功領導國內相關颱風整合研究工作,且DOTSTAR團隊的研究成果在國際上備受肯定,在西北太平洋地區(東亞)扮演颱風標靶觀測研究的領導角色。
吳教授建置並領導台大「颱風研究中心」進行前瞻性颱風研究(2009~2014年),等實質之專業行政與學術服務職務,竭盡熱忱奉獻於學術工作崗位。在2007、2009及2012年三次獲得國科會傑出研究獎後,吳教授持續研發颱風模擬同化系統(EnKF),將衛星遙測、dropwindsonde等特殊大氣資料同化至颱風數值預報系統中,透過對於模式中颱風的深入分析及特別設計的理想數值實驗,增進對於颱風移動、發展、強度、眼牆演變、降雨及與地形/海洋交互作用等動力過程的瞭解,並有效評估觀測策略及各式特殊資料對於颱風預報改進的影響。吳教授近年於研究與學術創作上更具能量,在颱風策略性(標靶)觀測、颱風資料同化、颱風(眼牆)動力、颱風與地形交互作用、降雨機制、颱風與海洋交互作用及颱風與氣候研究上有具體突破研究成果,持續發表深具影響力的研究論文,領導國際研究合作,在國際相關領域發揮重要影響,深具能見度與影響力。
1995年起,吳教授在臺灣大學大氣科學系著手建構颱風動力研究室(請見http://typhoon.as.ntu.edu.tw),目標是要進行一流的颱風研究,研究方向是以颱風之動力探討,模擬預報改進及四維同化研究為主軸,希望透過一系列的研究工作,一方面深入探索颱風學理,一方面則結合學術成果加以應用,有效改善颱風預報,對於科學本質及社會民生有具體貢獻與回饋。
吳教授返台服務二十年,於教學場域積極與學生互動,引領未來的科學人才登堂入室;於研究工作孜孜不倦,用心投入各項基礎科學研究計畫,特別是於2002年起主持了國科會自然處的「颱風重點研究」:侵臺颱風飛機策略性(標靶)觀測的大型研究「追風計畫」,從規劃、籌組團隊、跨國合作到實際執行,十年有成。追風計畫不僅提升了颱風分析與預報的準確度、對國家社會有實質的貢獻,也為大氣科學累積了質量兼具的突破性學術研究成果,在國際科學舞臺上佔有一席之地。吳教授並在2014年11月獲聯合國世界氣象組織(WMO)頒發對THORPEX(2005-2014)十年計畫貢獻卓著表揚狀肯定(In recognition of outstanding contribution to the WMO THORPEX Programme for the years 2005-2014 2005-2014),吳教授為獲表揚之唯一臺灣科學家。追風計畫的成功,歷經許多挑戰,可以說是吳教授過去工作粹練的進展與驗證。
過去十年吳教授所率領之追風團隊與國內外各學術、作業單位完美合作,並在國科會及中央氣象局的支持與經費支援下,成功開創並完成許許多多的觀測任務與重大科學進展論文發表,目前追風研究團隊已完成在台灣追風任務的開創、技術研發及理論應用等階段性使命。2013年起,已完整將追風計畫相關標準作業流程、技術與理論移轉給中央氣象局及其它相關單位(如國家實驗研究院之台灣颱風洪水研究中心),此為國科會所支持之創新研究成功移轉至實務作業之典範。此計畫成果於2009年獲肯定為國科會50週年「50科學成就」。

研究重點及重要貢獻:
自1993年於國外發表第一篇期刊論文至今二十年,總計已發表80篇SCI期刊論文,另外參與國際會議發表論文193篇,質與量兼具,無論是個人研究、領導國內研究、甚至是領導國際研究與實驗合作皆有突出成果、在國際相關領域發揮重要影響、在國際學術及預報作業領域相當活躍,並有實質具體貢獻。上述論文皆為大氣科學領域之主要期刊,並多數為美國氣象學會 (AMS) 及美國地球科學學會 (AGU) 最具代表性發表相關研究內涵的主要期刊。按Web of Science (Thompson Scientific) 統計,已被SCI期刊論文引用2546次,H number為28。

以下為重要學術研究成果:

颱風路徑動力:
Wu and Emanuel(1993, 1994, JAS; 1995a, b, MWR)探討如何從位渦觀點瞭解颱風運動,不僅創先提出斜壓對颱風運動的影響,更首度以位渦度量化評估颱風駛流與大尺度動力系統的關係。另外以位渦診斷創新建立雙颱風交互作用之物理架構,以瞭解雙颱風互動的過程(Wu et al. 2003, MWR; Yang et al. 2008, MWR)。客觀及量化分析影響颱風路徑之主要大氣系統特性,透過位渦診斷分析得以瞭解影響颱風路徑及移動速度變化的物理機制,同時診斷各數值模式無法掌握颱風路徑的原因(即數值模式之預測偏差)。此研究對於即時颱風路徑分析與預測,以及颱風觀測策略提供有用的思路(Wu et al. 2004, 2009b, 2012b, MWR)。此系統亦為中央氣象局科技研究中心與預報中心之研發與分析團隊使用,可應用於修正其颱風路徑預報。Wu et al.(2015, JAS)探討台灣地形對颱風路徑之影響,提出颱風登陸後路徑往南偏折作用、機制,以及地形導致狹道效應的新見解。

颱風強度:
控制颱風強度變化的主要物理機制為何,乃是目前颱風研究最重要的議題之一。針對臺灣地形如何影響颱風路徑、強度、眼牆結構及風雨分布進行觀測分析與高解析度數值模擬研究(Wu and Kuo 1999, BAMS; Wu 2001, MWR; Wu et al. 2002, Wea. & Forecasting; Jain and Wu 2007, MWR;Wu 2009a, MWR)。Wu and Cheng(1999, MWR)透過資料分析以瞭解環境風切、角動量通量、海表面溫度、外流層及位渦等因素了解影響颱風強度的重要因子。目前正在進行更多的理想與真實個案模擬,以進一步解開颱風強度研究的難題。Wang and Wu(2004, MAP)已發表一篇相關的回顧論文,並被廣為引用。Zeng et al.(2006, MWR)則透過觀測上的研究來了解環境參數對於颱風強度所扮演的角色。

颱風與地形交互作用:
臺灣地形如何影響颱風路徑、強度、眼牆結構及風雨分布一直是吳教授主要研究專長與興趣,特別是利用觀測分析與數值模擬探討此議題 (Wu and Kuo 1999, BAMS; Wu 2001, MWR; Wu et al. 2002, Wea. & Forecasting; Galewsky et al. 2006, JGR; Jian and Wu 2008, MWR)。Wu and Kuo (1999, BAMS) 針對臺灣颱風研究的進展與挑戰發表具指標性的重要回顧論文,已獲146次SCI期刊論文引用。Wu et al. (2003, GRL)使用高解析度的數值模擬,以瞭解地形對眼牆重新發展的影響及登陸颱風中Vortex Rossby waves的演變情形,此成果亦為 Nature 雜誌的「news and views in brief」所報導。
Jian and Wu (2008, MWR)使用WRF模式探討2005年海棠颱風登陸臺灣前產生之特殊打轉移動路徑動力機制,特別是首次針對颱風與地形交互作用所引起的狹道效應(channel effect)提出完整的動力解釋。Huang et al. (2011, MWR) 研究除了探討柯羅莎颱風 (Krosa; 2007) 登陸北臺灣前打轉運動之動力機制,更利用考慮較複雜、完整物理過程的模式進行一系列的理想模擬實驗,發現了強颱在接近臺灣北部和中部時皆有顯著的南偏運動,而登陸不久後路徑又會迅速的向北偏轉,形成類似打轉的運動軌跡。不論是Krosa的個案分析或是理想實驗的結果,皆顯示颱風登陸前所發生的南偏運動與狹道效應有密切關係;此研究被2011年的UCAR magazine所引用報導。Wu et al. (2009a, MWR)則提出颱風在登陸前後眼牆之收縮、破壞及再生成的演變動力過程及其對於颱風結構與強度的影響理論,透過數值模擬探討地形與下表面變化對於颱風眼牆演變的效應,並進一步釐清非絕熱作用在眼牆維持上所扮演之角色,亦針對正壓動力於詮釋眼牆不完整之處提出新的見解,此概念與目前眼牆動力理論所強調對流擾動發展角色一致(如Montgomery et al. 2008, 2009; Moon et al. 2010)。吳教授利用理想模擬敏感性實驗探討地形對於颱風路徑的影響,透過位渦診斷及動量分析,瞭解颱風接近地形時,路徑偏折大小、方向之差異與原因;並探討造成狹道效應之條件及可能原因(Wu et al. 2015)。

颱風眼牆動力:
使用T-PARC實驗在辛樂克(Sinlaku)颱風期間所獲得前所未有的飛機觀測資料,進行EnKF資料同化與數值模擬研究分析,提出雙眼牆形成之新動力機制(Wu et al. 2012, MWR 與 Huang et al. 2012, JAS)。Wu et al. (2012, MWR) 使用Wu et al. (2010)發展之颱風初始化方法,並運用2008年T-PARC追風觀測資料(包括4趟C-130之完整穿越颱風中心觀測所得颱風內部的飛機觀測資料),進行辛樂克颱風之模擬。數值模擬結果有效掌握辛樂克的演變過程,包含其路徑、強度及結構的變化。其中我們特別受到矚目的研究議題為辛樂克之雙眼牆的形成及演變,此雙眼牆過程在此研究中被成功地模擬,並於第二部份研究中進行深入的動力分析,特別是發展出雙眼牆形成的關鍵新動力機制。
Huang et al. (2012, JAS;此申請案五篇代表著作之一)透過Wu et al. (2011)同化模擬辛樂克颱風的數值資料,此研究針對雙眼牆的形成進行一系列的動力分析,探討雙眼牆形成之關鍵動力機制。此研究檢驗了邊界層內及附近的環流變化,發現在雙眼牆形成的區域偏離梯度風平衡之情況特別顯著,伴隨而來的主、次環流變化過程會進一步增強此不平衡之狀態,此持續的正回饋過程與雙眼牆之形成有密切關係。此研究提出一個全新的雙眼牆形成動力機制,即探討邊界層內及附近的入流與環流變化,及超梯度風不平衡動力所扮演雙眼牆形成的關鍵角色。此研究乃是雙眼牆動力的全新架構與熱門議題,國際上已有多個研究團隊(如UCLA、SUNY Albany、Univ. of Washington、Univ. of Miami、Pennsylvania State Univ.、Naval Postgraduate school、Center for Australian Weather and Climate Research、Nanjing Univ.、Peking Univ.) 廣泛引用此理論於後續研究中。
根據「Web of Science」JCR (Journal Citation Reports)資料顯示,Wu et al. (2012) 及Huang et al. (2012)此兩篇皆為2013年高引用數論文(highly cited papers)。吳教授另與美國NPS(Naval Postgraduate School)Montgomery教授等人合作發表雙眼牆最新研究,探討雙眼牆的形成屬於線性邊界層過程或非線性邊界層模式的議題(Montgomery et al. 2014, JAS)。吳教授最新發表之研究工作(Wu et al. 2016, JAS)探討對流潛熱釋放對於眼牆維持扮演重要角色,全新解釋為何真實颱風的環狀眼牆結構如何不受二維正壓不穩定結構影響而被破壞,釐清颱風眼牆結構基本動力過程。
吳教授於颱風雙眼牆動力機制的研究成果,受邀於2015年大氣科學領域最新出版之大氣百科全書「Encyclopedia of Atmospheric Sciences. 2nd Edition」中撰寫其中有關雙眼牆形成的「Tropical Cyclones: Secondary Eyewall Formation」章節。且另在大氣科學領域重要最新專書「Dynamics and Predictability of Large-Scale High-Impact Weather and Climate Events」一書撰寫其中「Secondary Eyewall Formation in Tropical Cyclones」章節。

颱風與海氣交互作用(與林依依教授合作):
Lin et al. (2005, MWR)使用海表面高度距平 (SSHA) 與一個簡單的海洋耦合模式(CHIPS),探討海洋暖渦旋在颱風強度改變的議題中所扮演的角色。研究結果顯示一個新的詮釋觀點(與過去學者所強調之Ocean heat content概念有所不同),即海洋暖渦抑制颱風引起海表面溫度冷卻反應之負回饋作用,即暖渦旋所伴隨之較厚混合層可有效降低颱風引發之海表面溫度冷卻作用,使梅米颱風得以發展至超級強烈颱風。此理論亦在2005年侵襲美國紐奧良地區的卡崔娜颶風中得到充分印證,並已為相關研討及文獻所引用。
吳教授與林依依教授進一步探討海洋暖渦所扮演的強烈颱風加強作用角色(Wu et al. 2007a, JAS; Lin et al. 2008, MWR, 2009a, GRL, b, MWR, 2011, TAO)。Wu et al. (2007a, JAS;此申請案五篇代表著作之二) 設計使用理想的颱風海洋耦合模式來探討海洋暖渦對颱風強度影響的問題。研究中藉由設計不同的海洋熱力結構來探討颱風與海洋的相互影響情形,清楚釐清各物理量對於颱風與海洋交互作用的影響以及海洋暖渦結構所扮演的角色。為凸顯海洋熱力結構的角色,此研究創新提出一個有關ocean eddy feedback 的無因次參數,並藉由近一千五百組的數值實驗,界定出幾個重要物理參數(如颱風移速、海洋混合層厚度、海洋分層結構等)對於颱風與海洋交互作用的定量影響。
另外2010年夏天吳教授與物理海洋科學家、及美、日相關領域科學家合作,同步參與ITOP (Impact of Typhoons on the Ocean in the Pacific)的颱風海洋交互作用國際觀測實驗,結合臺灣追風團隊的ASTRA及美國的C130飛機共同進行颱風相關的大氣聯合觀測資料,加上臺灣海洋界、美、日等國許多船舶、浮標(buoy)等設備觀測颱風期間海洋方面的資料。此為針對海洋結構及海氣通量在颱風結構與強度扮演的角色所進行之國際觀測計畫,追風計畫亦為量測大氣環境資料重要的一環,透過豐富的資料蒐集,海洋與大氣的耦合作用,cold wake的形成與維持及其對颱風的反饋進行更深入的研究。D’Asaro et al ,2013 (BAMS)即運用ITOP實驗所獲得之珍貴海氣資料,探討並研究2010年梅姬颱風與海洋間的交互作用與機制,並且釐清在西北太平洋的海氣交互作用機制與大西洋的海氣交互作用異同。Wu et al. (2015, JGR)透過颱風-海洋耦合模式,綜整比較ITOP實測海洋資料,深入探討Megi颱風在南海較小OHC(Ocean Heat Content)區域的cold wake形成過程及對於Megi強度的影響。此也是颱風與海氣交互作用重要機制的最新科學研究探討議題。

颱風飛機觀測(追風計畫)(領導國際團隊合作,包括林博雄教授、Dr. Tetsuo Nakazawa、Prof. Patrick Harr(現為美國國家科學基金會Section Head for the Atmosphere Section within the Division of Atmospheric and Geospace Sciences, National Science Foundation, USA)、Prof. Sharan Majumdar等):歷年來颱風屢屢造成臺灣地區重大災害,颱風研究的重要性不容小覷。
國科會(科技部)於2002年8月起提供相當經費(2008年起由中央氣象局後續支持經費),進行由本人所主持的「颱風重點研究」(National Priority Typhoon Research)。首要研究項目是以「全球衛星定位式投落送」(GPS Dropwindsonde)進行飛機觀測,名為「侵台颱風之飛機偵察及投落送觀測實驗(DOTSTAR) 」(Dropwindsonde Observation for Typhoon Surveillance near the TAiwan Region),又名追風計畫。成功規劃及執行西北太平洋地區之策略性(標靶)颱風飛機觀測重大國際實驗,從2003年至2013年,颱風投落送觀測計畫已針對杜鵑等54個颱風完成69航次之飛機偵察及投落送觀測任務,總計在颱風上空飛行363小時、並成功投擲1141枚投落送。在觀測的同時,這些寶貴的投落送資料皆即時進入中央氣象局及世界各國氣象單位之電腦預測系統中,協助預測颱風路徑及分析其周圍結構,如暴風半徑及雨帶結構等,並協助衛星資料之驗證。所獲得的飛機觀測資料對臺灣及世界主要氣象預報中心之電腦模式之颱風預報有具體改進。
此先驅實驗亦成功建置國內使用飛機進行其他特殊天氣/氣候/大氣環境之重要觀測平台,例如:高空閃電(追電計畫)、西南氣流(追雨計畫)及空氣污染觀測實驗(追雲計畫)之平台,並圓滿完成世界氣象組織2008年國際聯合颱風觀測實驗(THORPEX-PARC)。此T-PARC實驗共針對如麗、辛樂克、哈格比、薔蜜等四個侵台颱風完成超過25架次國際聯合的飛機觀測,而追風計畫(DOTSTAR, Wu et al. 2005)2008年10次的任務中有多達6次參與國際的聯合飛機觀測。2008年的T-PARC實驗期間,國內追風計畫以國內跨單位整合約50小時飛行時數,難能可貴地爭取到額外十倍(500小時)豐沛的國際合作飛機觀測資源。另外2010年8月至10月追風及海洋團隊與美、日科學家合作進行ITOP(Impact of Typhoons on the Ocean in the Pacific;颱風與海洋交互作用研究)國際實驗,取得颱風活動期間珍貴的大氣及海洋資料。因為這些前所未有的觀測資料的幫助,國、內外科學家得以在颱風路徑預報、颱風形成、結構演變、路徑偏轉及變性等相關研究有重大突破。(Wu et al. 2006, 2007b, c, JAS; Chou and Wu 2007, MWR; Wu et al. 2009b, d, MWR; Yamaguchi et al.2009, MWR; Chen et al. 2010, MWR; Chou et al. 2010, JGR; Wu et al. 2010, JAS, 2012a, b, MWR; Huang et al. 2011, JAS; Yen et al. 2011, TAO)。
Chou et al. (2011, MWR)亦探討DOTSTAR (2003-09) 及T-PARC (2008) 期間所獲得的投落送資料對颱風路徑預報的影響,結果凸顯T-PARC及DOTSTAR期間投落送資料對於NCEP模式模擬颱風路徑的重要助益。其中投落送資料改善NCEP模式-1到5天的路徑模擬結果,平均改善程度為10%-30%。Chou et al. (2010, JGR) 為第一篇以投落送資料系統性驗證颱風環境中QuikSCAT海面風場資料的論文,運用投落送資料高垂直解析度特性,此研究發展出全新的投落送海面風場估計值(W40),經由DOTSTAR超過400筆資料,得以找出針對不同風場大小流域、QuikSCAT海面風場的最新誤差統計特性。加上使用微波衛星資料,此研究提出QuikSCAT現有rain flag 不夠完整之修正建議。Weissmann et al. (2011, MWR)針對T-PARC 期間所獲得的投落送資料,探討此珍貴資料對不同模式(ECMWR、JMA、NCEP、及WRF)模擬颱風路徑預報的影響,結果顯示T-PARC期間所獲得的投落送資料對於上述所有模式之颱風模擬路徑均有相當程度的改善,其中對於NCEP及WRF模式之平均改善程度達20%-40%。根據“Web of Science”網站之JCR (Journal Citation Reports),Weissmann et al. (2011, MWR)為2011及2012年高引用數論文(highly cited papers)。
Jung et al. 2012採用T-PARC實驗所獲得之珍貴觀測資料,探討投落送資料及EnKF同化方法對辛樂克颱風模擬的影響。結果發現不但投落送資料可明顯改善颱風初始位置及後續颱風路徑預報之誤差,EnKF同化方法也表現甚佳,其ensemble spread較為集中,顯示EnKF同化方法可有效同化投落送資料,並改善系集預報之成果。

颱風策略性(標靶)觀測理論:
提出以共軛模式計算出颱風觀測之敏感區域的創新策略 (ADSSV, Adjoint-Derived Sensitivity Steering Vector; Wu et al. 2007c, JAS; 2009b, d, MWR; Chen et al. 2011, MWR; Majumdar et al. 2011, QJRMS)。Wu et al. (2007c, JAS;此申請案五篇代表著作之三) 所創建的ADSSV乃是現有各種策略性觀測理論中最能直接反應颱風移動駛流的創新概念。巧妙利用矩陣原理與共軛模式特性,計算出駛流向量對於初始渦度場的敏感度,並以一簡單向量(ADSSV)呈現、為兼具數學與動力理論,且有助於實質策略性颱風觀測的重要工具。ADSSV已被採用作為新一代國際(如美國國家海洋大氣總署所屬颶風研究中心)颱風飛機觀測之重要參考。並獲邀針對此颱風策略觀測專題於2006年聯合國世界氣象組織(WMO)於Costa Rica所舉辦的「第六屆國際颱風研討會」進行30分鐘的專題講演 (Wu 2006)。在分別由臺灣國科會、美國NSF及ONR經費支持下,領導國際相關研究團隊成員進行颱風觀測策略理論比較及資料同化研究(Wu et al. 2009b),此為世界氣象組織於第六屆及第七屆國際颱風研討會後所宣示之重點議題之一。並於2009年美國氣象學會所發行Monthly Weather Review國際著名學術期刊中發表相關十數篇由吳教授所主導並衍生之國際性論文專刊(Special Collection on “Targeted Observations, Data Assimilation, and Tropical Cyclone Predictability”)。
Wu et al. (2009b, MWR)以共軛模式敏感駛流向量(ADSSV)的觀點探討影響珊珊(2006) 颱風運動的敏感區域及大尺度系統,並進一步利用位渦診斷分析這些系統對於颱風駛流的貢獻,與ADSSV的敏感性結果作驗證。這是以位渦動力詮釋觀測策略理論的創新工作。提出以共軛模式計算出颱風觀測敏感區域之颱風觀測的創新策略理論(Wu et al. 2007c),以預先評估關鍵的敏感觀測位置,配合飛機航程及航管限制以決定投落送的最適當投落位置。目前已被採用作為新一代國際(如美國國家海洋大氣總署所屬颶風研究中心)颱風飛機觀測之參考。Wu et al. (2009c, MWR;此申請案五篇代表著作之四)為吳教授領導國際一流相關研究團隊成員進行颱風觀測策略理論比較之獨特研究,分別由臺灣國科會、美國NSF及ONR經費支持下所完成。此研究為國際合作,共有11作者,結合世界最先進作業中心與研究單位(NTU, NRL, JMA/MRI, NCEP, ECMWF, NOAA/HRD), Univ. of Miami)針對颱風之觀測策略理論進行系統性的分析與動力比較,已瞭解各式觀測策略理論方法之異同(包括JMA SV, NOGAPS SV, ECMWF SV, NTU ADSSV, ETKF, NCEP Variance)及其動力特徵,作為實質策略性觀測之重要指標。此論文於2009年9月的WMO 3rd THORPEX Science Workshop 的「Session on Targeted observation」為主持人兼引言人(Prof. Istvan Szunyogh and Dr. Rolf Langland)加以引述為有關觀測策略理論最新的指標成果。此論文同步於ECMWF(全世界最頂尖數值預報中心)Research Department以Technical Memoranda#582刊印。
吳教授並於2010年獲邀至法國位於南印度洋屬地的La Reunion參加四年一度的「Seventh WMO International Workshop on Tropical Cyclones」(IWTC-VII),針對此議題擔任「Targeted Observation專題報告」主講人及session chair。並與 University of Miami 的Majumdar 教授合作(Majumdar et al. 2011, QJRMS)以系集技術的ETKF為研究工具,較以往不同的是,此研究提出一套新的ETKF計算方式,為凸顯颱風不對稱結構與環境流場對於影響颱風運動的重要性,並降低因颱風系集路徑預報誤差造成風場變異的貢獻,因此以Kurihara et al. (1993)濾除渦旋的方式將每個系集成員之颱風分量去除,再重新計算ETKF敏感性,針對幾項議題探討ETKF在熱帶氣旋環境下所呈現的特徵。Ito and Wu (2013) 開創TyPOS(Typhoon-Position-Oriented sensitivityanalysis)颱風標靶敏感區分析法,而TyPOS訊號可定量反應出颱風初始場擾動的系集平均位置變化。
吳教授所發展最新的颱風EnKF資料同化方法 (Wu et al. 2010, JAS),有別於過去同化傳統的觀測資料、虛擬渦旋資料,或是直接做資料取代的颱風初始化方案,本研究創新針對颱風渦旋設計嶄新特殊觀測算符,包含颱風中心位置、渦旋移速與海表面軸對稱風速,直接以EnKF的技術同化這些特殊觀測量,此方法等同於直接將颱風的路徑與軸對稱平均結構同化至模式中,同時並能夠兼顧大氣質量場與運動場間近乎平衡之關係。本研究提供一套有效的方法,可用來進行短時段的颱風初始化也可進行長時段的同化分析,也有應用於作業模式預報上的重要潛力。此方法已成功用來探討辛樂克颱風(2008)的雙眼牆形成之關鍵動力機制,在此研究上已有重大突破 (Wu et al. 2012b, MWR; Huang et al. 2012, JAS)。Wu et al. (2012b, MWR)使用位渦診斷方法定量分析辛樂克颱風駛流場,結果顯示位於颱風東邊之太平洋高壓為導引辛樂克向西北移動的主要因子,另外也凸顯T-PARC期間DOTSTAR投落送資料對於NCEP GFS模式模擬颱風的重要助益。

颱風降水機制探討:
對臺灣而言,由於時常受到颱風侵襲且地形複雜,降水機制與定量降水預報的探討仍是重大科學議題 (Wu et al. 2009b, MWR)。颱風所伴隨降水現象之機制與預報,是颱風研究之關鍵議題。以1996年賀伯颱風造成阿里山測站破紀錄之日降水量(1736 mm)為個案,Wu et al. (2002;此申請案五篇代表著作之五)乃是建構新的颱風初始化方法,以高解析度數值模式模擬颱風降雨及探討臺灣地形模式解析度角色的指標性研究論文。此研究工作開啟臺灣區域颱風降雨數值模擬議題,引領國內外更多後續研究工作,已達115次SCI期刊論文的引用。
Wu et al. (2009c, MWR) 研究則為首次於SCI國際期刊發表探討秋颱降雨機制之論文,即秋季時巴士海峽上颱風(Typhoon Babs)與東北季風之共伴環流效應所導致的劇烈降雨特徵。透過數值實驗此研究特別釐清颱風環流、東北季風及臺灣地形三者對於降雨所扮演的相對角色。文中所列之降雨機制示意圖及概念,已為學者多所引用。Wu et al. (2010, MWR) 藉由1999年之雙颱(Rachel及Paul颱風),探討去除Paul颱風環流及所處大尺度季風槽系統,對Rachel颱風路徑及降水現象所造成的影響。Yen et al. (2011, TAO) 則創新研究運用EnKF同化方法 (Wu et al. 2010, JAS) 控制颱風之移動速度,定量探討2009年莫拉克颱風 (Morakot) 移速對颱風累積降水量所造成的影響。結果發現當颱風移速增加近一倍時,即颱風滯留陸地時間減少36%時,颱風通過臺灣期間的累積降水量減少約33 %,此量化結果有效釐清莫拉克颱風移速對於颱風累積降水的角色,對於瞭解颱風降水機制有所助益,也有利於氣象實際作業單位之預報參考,並為颱風所帶來降雨總量與颱風移速之關係,提供清晰概念銓釋及啟發。
此篇論文獲得中華民國氣象學會2012年「黃廈千博士學術論文獎」。 Wu et al. 2013 (MWR)結果顯示不同颱風路徑群所造成台灣地區不同降雨結果與地形效應。顯示台灣地形與颱風路徑預報對台灣地區颱風定量降水預報之重要性。另外吳教授以TAO (Terrestrial, Atmospheric and Oceanic Sciences)總編輯身份規畫並推動於2011年發行 「Special issue on “Typhoon Morakot (2009): Observation, Modeling, and Forecasting”」,並在專刊中發表4篇研究成果。另外吳教授以TAO總編輯身份發表「Typhoon Morakot (2009): A special issue in Terrestrial, Atmospheric and Oceanic Science (TAO) Journal」於Bulletin of the American Meteorological Society (BAMS)期刊(Wu 2013)。

氣候與颱風研究:
Wu et al. (2012, J. Climate)使用區域大氣模式探討西北太平洋之熱帶氣旋特徵,顯示模擬中的熱帶氣旋不論在季節尺度或者年際變化上,皆存在相當大的變異度。也發現系集平均能夠提供較準確且合理的颱風個數之變化。另外也探討海溫距平及ENSO與颱風間的關係(Zhan et al,. 2011 J. Climate; Choi et al. 2011, A-P JAS; Choi et al. 2012, Clim. Dynam.)。有關颱風與氣候間之研究,乃是吳教授近年全新拓展的研究領域。Choi et al. (2011, Asia-Pac J. Atmos. Sci.)探討ENSO對登陸韓國颱風之影響,結果發現當Nino-3.4指數減少時,颱風登陸的路徑會略為偏北。而中性ENSO狀態時,許多颱風登陸韓國前,均因太平壓高壓西伸使得颱風先通過中國大陸陸地而強度減弱。
Zhan et al. (2011, J. Climate)探討西北太平洋颱風生成個數與東印度洋海溫距平之關係,結果發現當移除兩個東印度洋極端年的海溫距平 (1994年的最高值及1998年的最低值)之後,西北太平洋颱風生成的個數即回復正常氣候平均值,顯見兩者關係相當密切。此為呈現西北太平洋颱風與大尺度(印度洋)洋溫關聯之重要新貢獻論文。Wu et al. 2012c (J. Climate).為了探討颱風模擬在氣候模式中的掌握能力以及不確定性,使用國際太平洋研究中心(International Pacific Research Center)區域大氣模式模擬西北太平洋之熱帶氣旋特徵,利用初始擾動探討四組系集成員之間的模擬差異。結果顯示即使側邊界與下邊界條件在四個系集模擬中完全相同,給予不同的初始擾動後,模式中的熱帶氣旋不論在季節尺度或者年際變化上,皆表現出相當顯著的變異度。除此之外也發現系集平均的結果能夠提供較準確且合理的颱風個數之變化。此研究反映現今颱風模擬在氣候模式中所遭遇的困難,並指出模式的內部動力過程對於颱風的生成有著關鍵的影響,而系集模擬的技術可有效的降低此種變異度帶來的不確定性,為未來颱風氣候研究的課題提供一個有用的參考。
吳教授最近在氣候與颱風降水研究方面,也有新的進展。Wu et al. (2015)已經在大氣學門Impact Factor最高(Impact Factor=11.808)的Bulletin of the American Meteorological Society (BAMS) 期刊接受發表。此篇研究進行過去台灣長期(1993~ 2013年)侵台颱風降水之雨量資料分析與統計研究,釐清台灣地區颱風降水之長期趨勢,以及由測站數多寡與測站高度及其區域分佈對降水資料統計之影響程度,亦探討雨量資料代表性與長期統計結果可信度之科學議題,將對台灣地區探討相關颱風降水統計之方法產生主要影響,此研究成果亦對台灣極端劇烈降水與氣候變遷(climate change)間關係的瞭解有相當助益。