Targeting observations using singular vectors

Singular vectors with maximum energy at final time inside a verification ar ea are used to identify the target area where extra observations should be taken, at an initial time, to reduce the forecast error inside the verifica tion area itself. This technique is applied to five cases of cyclone develo pment in the Atlantic Ocean, with cyclones reaching the British Isles at th e final time. Three verification areas centered around this region are cons idered. First, the sensitivity of the target area to the choice of the forecast tra jectory along which the singular vectors are evolved, to the choice of the verification area where singular vector energy is maximized, and to the num ber of singular vectors used to define the target area is investigated. Res ults show little sensitivity to the choice of the verification area. but hi gh sensitivity to the choice of the trajectory. Regarding the number of sin gular vectors used, results based on the first 4 or the first 10 singular v ectors are shown to be very similar. Second, the potential forecast error reduction that could be achieved by ta king extra observations inside the target area is estimated by contrasting the error of a forecast started from the unperturbed analysis with the erro r of a forecast started by subtracting so-called pseudo-inverse perturbatio ns (estimated using the leading singular vectors) to the unperturbed analys is. Results indicate that root-mean-square errors in the verification regio n could be reduced by up to 13% by adding targeted observations. Overall, results suggest that linear models can be used to define the targe t area where adaptive observations should be taken.