Chun-Chieh Wu

Typhoons Affecting Taiwan: Current Understanding and Future Challenges

Of all the natural disasters occurring in Taiwan, tropical cyclones are the most serious. Over a 20-yr period, Taiwan was hit by an average of 3.7 typhoons per year. These storms can produce heavy rainfall and strong winds, leading to severe damage to agriculture and industry, and serious loss of human life. An outstanding example is Typhoon Herb, which made landfall in Taiwan on 31 July 1996. Typhoon Herb took 70 lives and caused an estimated

The Interaction of Supertyphoon Maemi (2003) with a Warm Ocean Eddy

5 billion of damage to agriculture and property. Accurate prediction of the track, intensity, precipitation, and strong winds for typhoons affecting Taiwan is not an easy task. The lack of meteorological data over the vast Pacific Ocean and the strong interaction between typhoon circulation and Taiwans mesoscale Central Mountain range are two major factors that make the forecasting of typhoons in the vicinity of Taiwan highly challenging. Improved understanding of the dynamics of typhoon circulation and their interaction with the Taiwan terrain is needed for more ...

Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR): An Overview

Understanding the interaction of ocean eddies with tropical cyclones is critical for improving the understanding and prediction of the tropical cyclone intensity change. Here an investigation is presented of the interaction between Supertyphoon Maemi, the most intense tropical cyclone in 2003, and a warm ocean eddy in the western North Pacific. In September 2003, Maemi passed directly over a prominent (700 km 500 km) warm ocean eddy when passing over the 22°N eddy-rich zone in the northwest Pacific Ocean. Analyses of satellite altimetry and the best-track data from the Joint Typhoon Warning Center show that during the 36 h of the Maemi–eddy encounter, Maemi’s intensity (in 1-min sustained wind) shot up from 41 ms 1 to its peak of 77 m s 1 . Maemi subsequently devastated the southern Korean peninsula. Based on results from the Coupled Hurricane Intensity Prediction System and satellite microwave sea surface temperature observations, it is suggested that the warm eddies act as an effective insulator between typhoons and the deeper ocean cold water. The typhoon’s self-induced sea surface temperature cooling is suppressed owing to the presence of the thicker upper-ocean mixed layer in the warm eddy, which prevents the deeper cold water from being entrained into the upper-ocean mixed layer. As simulated using the Coupled Hurricane Intensity Prediction System, the incorporation of the eddy information yields an evident improvement on Maemi’s intensity evolution, with its peak intensity increased by one category and maintained at category-5 strength for a longer period (36 h) of time. Without the presence of the warm ocean eddy, the intensification is less rapid. This study can serve as a starting point in the largely speculative and unexplored field of typhoon–warm ocean eddy interaction in the western North Pacific. Given the abundance of ocean eddies and intense typhoons in the western North Pacific, these results highlight the importance of a systematic and in-depth investigation of the interaction between typhoons and western North Pacific eddies.

Upper-Ocean Thermal Structure and the Western North Pacific Category 5 Typhoons. Part II: Dependence on Translation Speed

DOTSTAR (Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region) is an international research program conducted by meteorologists in Taiwan partnered with scientists at the Hurricane Research Division (HRD) and the National Centers for Environmental Prediction (NCEP) of the National Oceanic and Atmospheric Administration (NOAA). The experiment is based on successful surveillance missions conducted in the Atlantic with NOAAs Gulfstream-IV jet aircraft. During the experiment, GPS dropwindsondes are released from a jet aircraft flying above 42000 ft in and around tropical cyclones approaching Taiwan to collect critical meteorological data for improving the analysis and the prediction of typhoons. After one-year of training, development and installation of all the needed software and hardware in the aircraft, the DOTSTAR research team initiated typhoon surveillance in 2003. Two missions (in Typhoons Dujuan and Melor) were conducted successfully,and seven or eight missions are expected to be conducted annually during the 2004 and 2005 typhoon seasons. The current manuscript provides an overview of the scientific objectives of DOTSTAR including operational plans, organization, data management, and data archiving. Preliminary results of the two missions in the first season in 2003 are presented. The experiment marks the beginning of typhoon surveillance in the western North Pacific and is expected to yield impressive improvements in typhoon research, observations and forecasting.

The Effect of the Ocean Eddy on Tropical Cyclone Intensity

Abstract Using new in situ ocean subsurface observations from the Argo floats, best-track typhoon data from the U.S. Joint Typhoon Warning Center, an ocean mixed layer model, and other supporting datasets, this work systematically explores the interrelationships between translation speed, the ocean’s subsurface condition [characterized by the depth of the 26°C isotherm (D26) and upper-ocean heat content (UOHC)], a cyclone’s self-induced ocean cooling negative feedback, and air–sea enthalpy fluxes for the intensification of the western North Pacific category 5 typhoons. Based on a 10-yr analysis, it is found that for intensification to category 5, in addition to the warm sea surface temperature generally around 29°C, the required subsurface D26 and UOHC depend greatly on a cyclone’s translation speed. It is observed that even over a relatively shallow subsurface warm layer of D26 ∼ 60–70 m and UOHC ∼ 65–70 kJ cm−2, it is still possible to have a sufficient enthalpy flux to intensify the storm to category 5...

Targeted Observations of Tropical Cyclone Movement Based on the Adjoint-Derived Sensitivity Steering Vector

Abstract The rapid intensification of Hurricane Katrina followed by the devastation of the U.S. Gulf States highlights the critical role played by an upper-oceanic thermal structure (such as the ocean eddy or Loop Current) in affecting the development of tropical cyclones. In this paper, the impact of the ocean eddy on tropical cyclone intensity is investigated using a simple hurricane–ocean coupled model. Numerical experiments with different oceanic thermal structures are designed to elucidate the responses of tropical cyclones to the ocean eddy and the effects of tropical cyclones on the ocean. This simple model shows that rapid intensification occurs as a storm encounters the ocean eddy because of 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 an insulating effect against the storm-induced mixing and cooling. Two...

Interaction of a Baroclinic Vortex with Background Shear: Application to Hurricane Movement

Since 2003, a field program has been conducted under the name of Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR). As the name DOTSTAR suggests, targeted observation is one of its key objectives. The prerequisite for designing the observing strategy is to identify the sensitive areas, which would exert great influence on the results of numerical forecast or the extent of the forecast error. In addition to various sensitivity products already adopted in DOTSTAR, a new way to identify the sensitive area for the targeted observation of tropical cyclones based on the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) is proposed in this paper. By appropriately defining the response functions to represent the steering flow at the verifying time, a simple vector, adjoint-derived sensitivity steering vector (ADSSV), has been designed to demonstrate the sensitivity locations and the critical direction of typhoon steering flow at the observing time. Typhoons Meari and Mindulle of 2004 have been selected to show the use of ADSSV. In general, unique sensitive areas 36 h after the observing time are obtained. The proposed ADSSV method is also used to demonstrate the signal of the binary interaction between Typhoons Fungwong and Fengshen (2002). The ADSSV is implemented and examined in the field project, DOTSTAR, in 2005 as well as in the surveillance mission for Atlantic hurricanes conducted by the Hurricane Research Division. Further analysis of the results will be vital to validate the use of ADSSV.

Concentric Eyewall Formation in Typhoon Sinlaku (2008) Part II: Axisymmetric Dynamical Processes

Abstract Most extant studies of tropical cyclone movement consider a barotropic vortex on a β plane. However, observations have shown that real tropical cyclones are strongly baroclinic, with broad anticyclones aloft. Also, the distribution of the large-scale potential vorticity gradient in the tropical atmosphere is very nonuniform. These properties may substantially influence the movement of such storms. Note that the anticyclone above a hurricane will interact with the lower hurricane vortex and induce storm motion. Such interaction can be caused by both the direct effect of ambient vertical shear and the effect of vertical variation of the background potential vorticity gradient. In this paper, an attempt to isolate the effect of background vertical shear is made. The hurricane is represented in a two-layer quasigeostrophic model as a point source of mass and zero potential vorticity air in the upper layer, collocated with a point cyclone in the lower layer. The model is integrated by the method of co...

The Impact of Dropwindsonde Data on Typhoon Track Forecasts in DOTSTAR

AbstractIn Part I of this study, the association between the secondary eyewall formation (SEF) and the broadening of the outer swirling wind in Typhoon Sinlaku (2008) was documented. The findings from Part I help lay the groundwork for the application of a newly proposed intensification paradigm to SEF. Part II presents a new model for SEF that utilizes this new paradigm and its axisymmetric view of the dynamics.The findings point to a sequence of structure changes that occur in the vortex’s outer-core region, culminating in SEF. The sequence begins with a broadening of the tangential winds, followed by an increase of the corresponding boundary layer (BL) inflow and an enhancement of convergence in the BL where the secondary eyewall forms. The narrow region of strong BL convergence is associated with the generation of supergradient winds in and just above the BL that acts to rapidly decelerate inflow there. The progressive strengthening of BL inflow and the generation of an effective adverse radial force ...

Environmental Dynamical Control of Tropical Cyclone Intensity—An Observational Study

Abstract Starting from 2003, a new typhoon surveillance program, Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR), was launched. During 2004, 10 missions for eight typhoons were conducted successfully with 155 dropwindsondes deployed. In this study, the impact of these dropwindsonde data on tropical cyclone track forecasts has been evaluated with five models (four operational and one research models). All models, except the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model, show the positive impact that the dropwindsonde data have on tropical cyclone track forecasts. During the first 72 h, the mean track error reductions in the National Centers for Environmental Prediction’s (NCEP) Global Forecast System (GFS), the Navy Operational Global Atmospheric Prediction System (NOGAPS) of the Fleet Numerical Meteorology and Oceanography Center (FNMOC), and the Japanese Meteorological Agency (JMA) Global Spectral Model (GSM) are 14%, 14%, and 19%, respectively. The tra...