Sensitivity of tropical cyclone forecasts as revealed by singular vectors

Singular vector (SV) sensitivity, based on the adjoint model of the U S. Navy Operation Global Atmosphere Prediction System (NOGAPS), is used to study the dynamics associated with tropical cyclone evolutions. For each model-predicted tropical cyclone, singular vectors are constructed that optimize perturbation energy within a 20 by 20 degree latitude/longitude box centered on the 48 hour forecast position of the cyclone. The initial singular vectors indicate regions where the 2-day forecast of the storm is very sensitive to changes in the analysis. Composites of the singular vectors for straight-moving cyclones and non-straight-moving cyclones occurred in the Northern Hemisphere summer season in 2003 are examined. For both groups, the initial-time singular vector sensitivity exhibits a maximum within an annulus approximately 500 km from the center of the storms, in the region where the potential vorticity gradient of the vortex first changes sign. In the azimuthal direction, the composite initial-time singular vector maximum for the straight-moving group is located in the rear right quadrant with respect to the storm motion. The composite based on the non-straight-moving cyclones does not have a preferred quadrant in the vicinity of the storms and has larger amplitude away from the cyclones in comparing with the straight-moving cyclones, indicating more environmental influence on these storms. For both groups, the maximum initial sensitive areas are collocated with regions of flows moving toward the storm. While the initial SV maximum is located where the potential vorticity gradient in the basic flow changes sign, the final SV maximum is located where the potential vorticity gradient is a maximum. Examinations of individual cases demonstrate how SV sensitivity can be used to identify different environmental influences on the storms. The relationship between the SV sensitivity and the potential vorticity will be discussed.