Study on the Sensitivity of Initial Perturbations to the Development of a Vortex Observed in Southwest China

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AbstractIn this study, numerical experiments on the sensitivity of initial perturbations to the development of southwest vortices (SWVs), which are frequently observed in Southwest China, are performed with weather research and forecasting model. Regarding the development of SWVs as a strong nonlinear process, conditional nonlinear optimal perturbation (CNOP) with an adjoint‐free algorithm is used. The sensitivity denotes how and where the perturbations of model variables at the initial time can influence non‐moving SWV enhancement. The CNOPs leading to the strengthening of the SWV observed at 5UTC July 17, 2010 are obtained. Three sensitive areas are identified based on the distribution of the most significant CNOP norms. Two of them are located on the Eastern Tibetan Plateau (TP), and the third one is found in the eastern Sichuan Basin (SCB) near the SWV center. Sensitivity experiments indicate that the strengthening of the SWV is mainly associated with perturbations at the middle level at approximately 500–450 hPa in the eastern part of the TP and below 450–400 hPa in the eastern SCB near the center of the SWV. Furthermore, the potential vorticity (PV) budget, divergence, vorticity, vertical velocity, diabatic heating, and southerly wind are also analyzed in different simulations. The results show that at the initial stage of SWV formation, the diabatic heating term is not the main cause of PV budget differences between simulations; it is the dynamical effect that is the main contributing factor. However, when it comes to the intensification stage, the diabatic heating caused by the release of latent heat of condensation becomes important.