Search Results

You are looking at 1 - 3 of 3 items for :

  • Author or Editor: Tetsuo Nakazawa x
  • Targeted Observations, Data Assimilation, and Tropical Cyclone Predictability x
  • Refine by Access: Content accessible to me x
Clear All Modify Search
Munehiko Yamaguchi
,
Takeshi Iriguchi
,
Tetsuo Nakazawa
, and
Chun-Chieh Wu

Abstract

An Observing System Experiment (OSE) has been performed to investigate the effectiveness of dropwindsonde observations and a sensitivity analysis technique on a typhoon track forecast. Using dropwindsonde observations for Typhoon Conson at 1200 UTC 8 June 2004, which are derived from Dropwindsonde Observation for Typhoon Surveillance near the Taiwan Region (DOTSTAR), four numerical experiments are conducted, which are different only in terms of the number of dropwindsonde observations used in a data assimilation system: (i) no observation is assimilated; (ii) all observations are assimilated; (iii) observations within a sensitive region as revealed by a singular vector method at the Japan Meteorological Agency (JMA) are assimilated; and (iv) observations outside the sensitive region are assimilated. In the comparison of the four track forecasts, Conson’s northeastward movement is expressed in the second and third simulations while in the first and fourth experiments Conson stays at almost the same position as its initial position. Through the OSE, it is found that DOTSTAR observations had a positive impact on the track forecast for Conson, and that observations within the sensitive region are enough to predict the northeastward movement of Conson, indicating that the JMA singular vector method would be useful for the sampling strategy of targeted observations like DOTSTAR.

Full access
Kun-Hsuan Chou
,
Chun-Chieh Wu
,
Po-Hsiung Lin
,
Sim D. Aberson
,
Martin Weissmann
,
Florian Harnisch
, and
Tetsuo Nakazawa

Abstract

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 (2003–09) 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.

Full access
Chun-Chieh Wu
,
Jan-Huey Chen
,
Sharanya J. Majumdar
,
Melinda S. Peng
,
Carolyn A. Reynolds
,
Sim D. Aberson
,
Roberto Buizza
,
Munehiko Yamaguchi
,
Shin-Gan Chen
,
Tetsuo Nakazawa
, and
Kun-Hsuan Chou

Abstract

This study compares six different guidance products for targeted observations over the northwest Pacific Ocean for 84 cases of 2-day forecasts in 2006 and highlights the unique dynamical features affecting the tropical cyclone (TC) tracks in this basin. The six products include three types of guidance based on total-energy singular vectors (TESVs) from different global models, the ensemble transform Kalman filter (ETKF) based on a multimodel ensemble, the deep-layer mean (DLM) wind variance, and the adjoint-derived sensitivity steering vector (ADSSV). The similarities among the six products are evaluated using two objective statistical techniques to show the diversity of the sensitivity regions in large, synoptic-scale domains and in smaller domains local to the TC. It is shown that the three TESVs are relatively similar to one another in both the large and the small domains while the comparisons of the DLM wind variance with other methods show rather low similarities. The ETKF and the ADSSV usually show high similarity because their optimal sensitivity usually lies close to the TC. The ADSSV, relative to the ETKF, reveals more similar sensitivity patterns to those associated with TESVs. Three special cases are also selected to highlight the similarities and differences among the six guidance products and to interpret the dynamical systems affecting the TC motion in the northwestern Pacific. Among the three storms studied, Typhoon Chanchu was associated with the subtropical high, Typhoon Shanshan was associated with the midlatitude trough, and Typhoon Durian was associated with the subtropical jet. The adjoint methods are found to be more capable of capturing the signal of the dynamic system that may affect the TC movement or evolution than are the ensemble methods.

Full access