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This study examines the impact of nonlinearity on the targeted observations for tropical cyclone prediction. The nonlinearity of the typhoon is determined by comparing the first singular vector (FSV) and the conditional nonlinear optimal perturbation (CNOP), which is the nonlinear extension of FSV. If the similarity between the CNOP and FSV is larger than 0.5, then the typhoon is categorized as weak nonlinearity, otherwise, the typhoon is categorized as strong nonlinearity. First, the impact of nonlinearity on the typhoon targeted observations due to different resolutions is studied. Two typhoons, Meari (2004) and Matsa (2005), with 24 h forecast length are chosen, with 120-, 60-, and 30-km resolutions, respectively. It is found that the nonlinearity of both cases becomes stronger as the resolution increases. However, the sensitive areas identified with lower resolutions are more similar to each other than those identified with finer resolutions. This means that when the motion of typhoon has been described as linear or weakly nonlinear, the sensitive area may be easier to determine. Then, the impact of nonlinearity on the typhoon targeted observations due to different forecast length is investigated. In this part, typhoons Meari (2004) and Matsa (2005) with 60 km resolution are considered with 12-, 24-, and 36-h forecast lengths. We further studied two issues. In the first the initial time is fixed, while in the second the forecast time is fixed. Results show that no matter which issue is considered, typhoon Matsa exhibits stronger nonlinearity than typhoon Meari. Accordingly, Meari is categorized as a linear case, while Matsa as a nonlinear case. In the linear case, the sensitive areas identified for special forecast times (when the initial time is fixed) resemble those identified for other forecast times. Targeted observations deployed to improve a specific time forecast would thus also benefit forecasts at other times. In the nonlinear case, the similarities among the sensitive areas identified for different forecast times were more limited. The deployment of targeted observations in the nonlinear case would therefore need to be adapted to achieve large improvements for different targeted forecasts. For both cases, the closer the forecast time, the higher the similarities of the sensitive areas. When the forecast time is fixed, the sensitive areas in the linear case diverge continuously from the verification area as the forecast period lengthens due to the determination of the subtropical high in the movement of the typhoon, while those in the nonlinear case are always located around the initial cyclone indicating that the main factors affecting the typhoon movements are located within the typhoon. The deployment of targeted observations to improve a special forecast depends strongly on the time of deployment. Generally, it seems that the sensitive areas are easy to be determined in the linear case and more beneficial for the forecast. In the nonlinear case, the identification of sensitive areas is more difficult, which results in harder deployments in targeted observations.
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- The Impact of Nonlinearity on the Targeted Observations for Tropical Cyclone Prediction
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