Case study on validation and interpretation of adjoint-derived sensitivity steering vector as targeted observation guidance of tropical cyclones

The adjoint-derived sensitivity steering vector (ADSSV) which identifies the sensitive areas at the observation time to the steering flow at the verification time through the adjoint calculation has been proposed and applied as one of the targeted observation guidances in the field programs for improving tropical cyclone predictability, such as THORPEX–Pacific Asian Regional Campaign (T-PARC). In addition, the ADSSV signals of influence from the synoptic systems such as the midlatitude trough and the subtropical high prior to the recurvature of Typhoon Shanshan (2006) have also been demonstrated.

As a follow-up study, the procedure in systematically perturbing the initial field is employed to examine the impact of perturbations collocated with the high and low ADSSV sensitivity on the model simulations. Results show that experiments with the perturbed initial conditions located at the high ADSSV signal (i.e., the midlatitude trough and the subtropical high) lead to more track deflection relative to the control run than experiments with perturbations in the nonsensitive area. The evolution of deep-layer mean steering flow and the comparison with the direction of ADSSV are also shown to interpret and validate its physical meaning. Furthermore, the effect of perturbations only in the sensitive vertical layers is investigated. It is indicated that perturbations within the sensitive layers grow as comparably as those in the thick-layer troposphere.