Impact of the Low Wavenumber Structure in the Initial Vortex Wind Analyses on the Prediction of the Intensification of Hurricane Patricia (2015)

Ensemble clustering analysis was performed to explore the role of the initial hurricane vortex-scale wind structure in the prediction of the intensification of Hurricane Patricia (2015). Convection-allowing ensemble forecasts were classified into spin-down (SPD) and spin-up (SPU) groups. Specifically, 10 members with an intensification rate >0 m/s and 10 members with an intensification rate <0 m/s for the first 6 hr were defined as the SPD and SPU members. The result showed that the tangential winds outside the inner-core region in the SPD members were weaker compared to the SPU members. Additionally, the SPD members had a weaker inflow near the surface and a weaker outflow between the heights of 8 and 12 km than the SPU members. The SPU members showed more significant azimuthal asymmetry than the SPD members in the surface, tangential and radial winds. Wavenumber analysis showed that the low wavenumber components dominated the differences between the SPD and SPU members. Numerical experiments were conducted to test the hypothesis generated by the clustering analysis. It was found that the storm's maximum wind speed (MWS) intensified during the first 6 hr of the model forecast if only the low wavenumber structure in the SPU members was included in the initial conditions, whereas it decayed during the first 6 hr if only the low wavenumber structure in the SPD members was included. This result confirms that the low wavenumber structure of the initial wind analyses was important in predicting the intensity changes of Hurricane Patricia (2015). Plain Language Summary Accurate forecasts of the intensity of tropical cyclones (TCs) are important in both early warning systems and impact assessments. However, the spin-down (SPD) issue remains a challenge in the operational HWRF model and occurs in many TC cases, especially in the prediction of intense TCs, which can lead to substantial degradation of the short-term intensity forecast. In this paper, ensemble clustering analysis was performed to explore the role of the initial hurricane vortex-scale wind structure in the prediction of the intensification of Hurricane Patricia (2015). Convection-allowing ensemble forecasts were classified into spin-down (SPD) and spin-up (SPU) groups. Several statistically significant differences were found in the structure of the initial vortex wind analyses between the SPD and SPU members. Particularly, the low wavenumber components dominated the differences between the SPD and SPU members. Both the wavenumber analysis and numerical experiments suggested that the low wavenumber structure of the initial wind analyses in ensemble members was important in predicting the intensity changes of Hurricane Patricia (2015). These results would be helpful as a way for alleviating the SPD issue in the operational HWRF model system.