Evaluation of Operational Model Cyclone Structure Forecasts during Extratropical Transition

Abstract

Cyclone structure is known to be directly linked to the sensible weather effects produced by the weather system. The extratropical transition (ET) process leads to immense changes in cyclone structure and therefore to changes in the associated weather experienced. Although structure is clearly an important cyclone characteristic, validation of cyclone structure forecasts in operational numerical models has not been previously performed. In this study, short-term (12?36 h) forecasts of cyclone structure from tropical genesis to the completion of ET are validated using fields from the Navy Operational Global Atmospheric Prediction System and the NCEP Aviation model. The cyclone phase space (CPS) is used to quantify differences between forecast and analyzed storm structure, both on a point-by-point basis and through a cyclone-type-based comparison. This cyclone-type comparison exploits a previously defined breakdown of cyclone structure regimes in the CPS. The impacts of synthetic vortex insertion on the ensuing agreement between forecast and analyzed storm structure are explored. While the results show reasonable forecast skill for well-defined (i.e., nonhybrid) systems, cyclones in the process of ET are found to be poorly forecast, emphasizing the need for improved understanding and simulation of the structural changes experienced by ET cyclones.