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The sensitive regions of conditional nonlinear optimal perturbations(CNOPs)and the first singular vector(FSV)for a northwest Pacific typhoon case are reported in this paper.A large number of probes have been designed in the above regions and the ensemble transform Kalman filter(ETKF)techniques are utilized to examine which approach can locate more appropriate regions for typhoon adaptive observations.The results show that,in general,the majority of the probes in the sensitive regions of CNOPs can reduce more forecast error variance than the probes in the sensitive regions of FSV.This implies that adaptive observations in the sensitive regions of CNOPs are more effective than in the sensitive regions of FSV.Furthermore,the reduction of the forecast error variance obtained by the best probe identified by CNOPs is twice the reduction of the forecast error variance obtained by FSV.This implies that dropping sondes,which is the best probe identified by CNOPs,can improve the forecast more than the best probe identified by FSV.These results indicate that the sensitive regions identified by CNOPs are more appropriate for adaptive observations than those identified by FSV.