Determining Tropical Cyclone Intensity Change through Balanced Vortex Model Applications

Forecasting intensity changes and rapid intensification (RI) of tropical cyclones (TC) is challenging as evidenced by the National Hurricane Center's (NHC) intensity forecast skill in recent years. The NHC is reliant on the skill of available TC guidance. Statistical models, like SHIPS and LGEM, tend to outperform the global and regional dynamical models. However, improvements in intensity forecast skill in the statistical models have been somewhat slow. In addition, gaining skill in predictions for RI is an even greater challenge. In order to see further improvement in TC guidance, new methods for forecasting intensity change must be applied to current techniques.

The balanced vortex model and the associated transverse circulation and geopotential tendency equations show that the placement of the diabatic heating in terms of the inertial stability of the regions around the radius of maximum wind (RMW) accounts for observed TC development. Through the use of idealized TC tangential velocity profiles, diabatic heating within the low inertial stability region outside of the RMW does not affect TC intensity. However, diabatic heating in a high inertial stability region inside of the RMW causes the TC intensity to increase rapidly. The goal of this project is to determine if the simplified balanced vortex model can be used to provide short-term intensity forecasts. The method lies between the empirically based statistical models and full three-dimensional prediction systems.

The balanced vortex model requires the low-level symmetric tangential wind and diabatic heating radial profiles. These can be estimated from aircraft flight-level data and either microwave satellite imagery or radar composites. As a first test of the method, the balanced vortex model is applied to the tangential velocity and diabatic heating generated from the Hurricane Weather Research and Forecasting model (HWRF) for TCs in the Atlantic and Eastern Pacific basins during the 2011 Hurricane Season. Cases will be considered to demonstrate intensity change diagnosed from the balanced vortex model in comparison to the model intensity change. Cases will be presented reflecting different stages of TC development and placement of diabatic heating in relation to the inertial stability region.

DISCLAIMER: The views, opinions, and findings in this report are those of the authors and should not be construed as an official NOAA and/or U.S. Government position, policy, or decision.