A hierarchy of synthetic vortex initialization methods for tropical cyclones

In order to provide realistic numerical predictions of tropical cyclone (TC) structure and intensity change, an accurate specification of the initial TC structure in a high-resolution model is necessary. The current capabilities of observational data (particularly when aircraft are not present) and data assimilation schemes preclude this from being achievable without considerable theoretical effort and computational expense. For many research and forecasting applications, a convenient but imperfect solution is the initialization of the model with a synthetic (or "bogus") vortex. In this study, we employ a systematic and hierarchical approach towards vortex initialization, beginning with a coarse Rankine vortex that is inserted into the Weather Research and Forecasting (WRF) model. The influence of modifications to the radial, vertical, and asymmetric structure of the initial vortex is then diagnosed in an OSSE framework. More specifically, a range of analytical vortices is investigated, along with their parameters that control important radial markers such as the radius of maximum wind (storm intensity) and the radius of gale force wind (storm size). A realistic outward slope of the radius of maximum winds and decay rate of the wind field with height are also incorporated. Divergent wind fields are also included to explore the benefits of specifying a pre-existing secondary circulation. We propose that a well-constructed initial vortex will serve as a convenient benchmark against which new, sophisticated data assimilation schemes can be evaluated.