Perturbation structure and evolution in tropical cyclones Noul and Nepartak based on singular vectors

This study examined tropical cyclones (TCs) Noul and Nepartak to demonstrate the evolutions of singular vector perturbations. Perturbations comprise eigen-solutions for dry total energy from two domains with different focuses, namely the TC itself and the background atmospheric flow in East Asia. These perturbations are usually used to construct ensemble members for the probabilistic forecasts of the TC trajectory. The horizontal shapes of the TC singular vectors were consistent with asymmetric flows with the wave number of 1 or 2, with the exception of the azimuthal circulation. Most of their vertical structures had higher amplitudes in upper layers mainly concentrated at approximately 200-300hPa. Thus, the upper part of the perturbations developed first, followed by the lower part over time. This process was the opposite of that noted in background singular vector perturbations in mid-latitude East Asia. Calculations of the transient eddy total energy based on analysis data for the two storms revealed that growth occurred earlier in the upper layer than in the lower layer. The cross-sectional eddy evolution in TCs exhibited a similar process to that of singular vectors. In addition, the cross-section of the three-dimensional E-vector convergence and the zonal profiles of the E-vector component also supported the downward process in TC areas. Mid-latitude E-vector analysis indicated that upward energy propagation was consistent with previous findings.