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

The operational Tropical Regional Atmospheric Model System (TRAMS) often underestimates the initial typhoon intensity when using the global analysis field as the initial condition. The TRAMS tropical cyclone (TC) initialization scheme, developed based on the incremental analysis updates (IAU) technique, effectively reduces initial bias. However, the original IAU-based TC initialization scheme only adjusted the wind field at the analysis moment, with other variables adjusted implicitly under the model's constraints according to the gradually inserted wind increment (named “univariate adjustment scheme” hereafter). The univariate adjustment scheme required approximately 3 h to reach a dynamic equilibrium state, which constrained the assimilation of hourly TC observations and caused excessive dissipation of meaningful short-wave information in adjustment increments. To address this limitation, this study developed a multivariate adjustment IAU-based TC initialization scheme incorporating gradient wind balance and hydrostatic balance as large-scale constraints. Numerical experiments with TC Hato (1713) demonstrated that the multivariate adjustment scheme reduces the IAU relaxation time to 1 h while marginally improving forecast skill. These findings were consistently replicated across 12 additional TC cases. The development of the IAU-based multivariate adjustment initialization scheme establishes a foundation for 4-D initialization with hourly TC observations.