The Application Of Conditional Nonlinear Optimal Perturbation To Targeted Observations For Tropical Cyclone Prediction

This paper briefly but systematically introduces the application of Conditional Nonlinear Optimal Perturbation (CNOP) to targeted observations for tropical cyclone predictions. CNOP is a natural extension of the singular vector (SV) method into the nonlinear regime, and it has recently been used to identify the sensitive areas in typhoon targeted observations.First, it was demonstrated that CNOP is different from the internationally used method (SV) for targeted observation and can take nonlinear processes into account. Then, it was revealed that the locations of initial errors have a great effect on their nonlinear growth. For a given norm of initial errors and basic state, the CNOP-type initial errors introduced into their corresponding sensitive areas cause the largest changes to the final verification forecasts. These results suggest that the application of CNOP to targeted observation has a theoretical basis.Then, the properties of the CNOP-sensitive areas were examined, and the results indicated that the CNOP-sensitive areas have favorable features for targeted observations. Next, observing system simulation experiments (OSSEs) and the observation system experiments (OSEs) were conducted to evaluate the efficiency of the CNOP-sensitive areas. The impacts on typhoon forecasting skills achieved by assimilating simulated or real observation data in the CNOP-and SV-sensitive areas were compared. The results indicate that greater benefits were obtained by assimilating data in CNOP-sensitive areas averagely. Finally, the conditions under which the CNOP-sensitive areas would be more effective were summarized, and the execution of the CNOP method with a more advanced model and more reasonable initial constraint were studied.All the above results demonstrated that the CNOP is a useful tool for identifying the sensitive areas for targeted observations. The method can take the nonlinear processes of physical issues into account and possesses a solid theoretical foundation.