Multivariate adjustment in the IAU-based tropical cyclone initialization scheme in TRAMS model

The operational Tropical Regional Atmospheric Model System (TRAMS) model often underestimates the initial typhoon intensity when using the global analysis field as the initial condition. The tropical cyclone (TC) initialization scheme developed based on incremental analysis updates (IAU) technique can help reduce the initial bias. However, the IAU-based TC initialization scheme only adjusted the wind field at the analysis moment, with other variables adjusted implicitly under the constraints of the model according to the gradually inserted wind increment (named “univariate adjustment scheme” hereafter). The univariate adjustment scheme required approximately 3  to reach a dynamic equilibrium state, limiting the use of hourly TC observation information and dissipating too much meaningful short-wave information of the adjustment increment. To reduce the equilibrium adjustment time, this study constructed a multivariate adjustment IAU-based TC initialization scheme by introducing the gradient wind balance and hydrostatic balance as large-scale constraints. The case sensitivity tests of TC Hato (1713) demonstrated that the multivariate adjustment scheme can reduce the IAU relaxation time to 1  and slightly improve TC forecasts. By incorporating the equilibrium assumption as a strong constraint for the TC axisymmetric component, the multivariate adjustment scheme achieved a faster convergence of the model to its equilibrium state, reducing the loss of useful observed information. This conclusion was further confirmed with 12 other TCs. The IAU-based multivariate adjustment initialization scheme developed in this study provides a foundation for 4-D initialization with hourly TC observations.