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.

Tropical Storm Bopha (2000) showed a very unusual southward course parallel to the east coast of Taiwan, mainly steered by the circulation associated with Supertyphoon Saomai (2000) to Bopha's east. The binary interaction between the two typhoons is well demonstrated by the potential vorticity (PV) diagnosis. With the use of the piecewise PV inversion, this paper quantitatively evaluates how Bopha moved southward due to the binary interaction with Saomai. A newly proposed centroid-relative track, with the position weighting based on the steering flow induced by the PV anomaly associated with the other storm, is plotted to highlight such binary interaction processes. Note that the above analysis can be well used to understand the more complicated vortex merging and interacting processes between tropical cyclones either from observational data or numerical experiments. The results also shed some light on the prediction of nearby tropical cyclones.

The typhoon surveillance program Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) has been conducted since 2003 to obtain dropwindsonde observations around tropical cyclones near Taiwan. In addition, an international field project The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) in which dropwindsonde observations were obtained by both surveillance and reconnaissance flights was conducted in summer 2008 in the same region. In this study, the impact of the dropwindsonde data on track forecasts is investigated for DOTSTAR and T-PARC (2008) experiments. Two operational global models from NCEP and ECMWF are used to evaluate the impact of dropwindsonde data. In addition, the impact on the two-model mean is assessed.The impact of dropwindsonde data on track forecasts is different in the NCEP and ECMWF model systems. Using the NCEP system, the assimilation of dropwindsonde data leads to improvements in 1-to 5-day track forecasts in about 60% of the cases. The differences between track forecasts with and without the dropwindsonde data are generally larger for cases in which the data improved the forecasts than in cases in which the forecasts were degraded. Overall, the mean 1-to 5-day track forecast error is reduced by about 10%-20% for both DOTSTAR and T-PARC cases in the NCEP system. In the ECMWF system, the impact is not as beneficial as in the NCEP system, likely because of more extensive use of satellite data and more complex data assimilation used in the former, leading to better performance even without dropwindsonde data. The stronger impacts of the dropwindsonde data are revealed for the 3-to 5-day forecast in the two-model mean of the NCEP and ECMWF systems than for each individual model.

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 track error reduction in the Weather Research and Forecasting (WRF) model, in which the initial conditions are directly interpolated from the operational GFS forecast, is 16%. However, the mean track improvement in the GFDL model is a statistically insignificant 3%. The 72-h-average track error reduction from the ensemble mean of the above three global models is 22%, which is consistent with the track forecast improvement in Atlantic tropical cyclones from surveillance missions. In all, despite the fact that the impact of the dropwindsonde data is not statistically significant due to the limited number of DOTSTAR cases in 2004, the overall added value of the dropwindsonde data in improving typhoon track forecasts over the western North Pacific is encouraging. Further progress in the targeted observations of the dropwindsonde surveillances and satellite data, and in the modeling and data assimilation system, is expected to lead to even greater improvement in tropical cyclone track forecasts.

An interesting eyewall evolution occurred in Typhoon Zeb (1998) when it devastated Luzon. The eyewall of Zeb contracted before landfall and broke down and weakened after landfall; then a much larger new eyewall formed and strengthened as it left Luzon and reentered the ocean. The fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) with four nested domains was used to perform numerical experiments to understand the effects of terrain and land surface variation on the observed eyewall evolution. Results show that the presence of Luzon plays a critical role in the observed eyewall evolution. Quite different from the conventional concentric eyewall replacement, the eyewall replacement that occurred in Typhoon Zeb was triggered by the mesoscale landmass and terrain variation with a horizontal scale similar to the core of the typhoon. In Typhoon Zeb, the original eyewall contracted and broke down because of enhanced surface friction after landfall. The outer eyewall was triggered by convective rainbands near the western coastal region of Luzon and formed as a result of axisymmetrization well after the dissipation of the inner eyewall convection.Several sensitivity experiments were conducted to elucidate the roles of both condensation heating and planetary boundary layer processes in the evolution of the typhoon eyewall. It is found that although condensational heating is the key to the maintenance of the annular potential vorticity (PV) structure, surface friction plays dual roles. Although friction is a sink to PV and thus dissipates PV in the eyewall, it helps keep the PV annulus narrow by enhancing the stretching deformation in the lower troposphere when condensational heating is present. In the absence of condensational heating, however, surface friction enhances the inward PV mixing by boundary layer frictional inflow and thus destroys the PV annulus. FIG. 2. (a) The four nested domains in the three major experiments. (b) The model's terrain height (contour interval of 500 m) of Luzon within the third mesh in CTL. JANUARY 2009 W U E T A L .