The impact of verification area design on tropical cyclone targeted observations based on the CNOP method

This study investigated the impact of different verification-area designs on the sensitive areas identified using the conditional nonlinear optimal perturbation (CNOP) method for tropical cyclone targeted observations. The sensitive areas identified using the first singular vector (FSV) method, which is the linear approximation of CNOP, were also investigated for comparison. By analyzing the validity of the sensitive areas, the proper design of a verification area was developed.Tropical cyclone Rananim, which occurred in August 2004 in the northwest Pacific Ocean, was studied. Two sets of verification areas were designed; one changed position, and the other changed both size and position. The CNOP and its identified sensitive areas were found to be less sensitive to small variations of the verification areas than those of the FSV and its sensitive areas. With larger variations of the verification area, the CNOP and the FSV as well as their identified sensitive areas changed substantially. In terms of reducing forecast errors in the verification area, the CNOP-identified sensitive areas were more beneficial than those identified using FSV. The design of the verification area is important for cyclone prediction. The verification area should be designed with a proper size according to the possible locations of the cyclone obtained from the ensemble forecast results. In addition, the development trend of the cyclone analyzed from its dynamic mechanisms was another reference. When the general position of the verification area was determined, a small variation in size or position had little influence on the results of CNOP.

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