Diagnostics of tropical cyclogenesis in four operational global NWP models: case studies for typhoons Megi (2010) and Jelawat (2012)

Abstract

This study aims to elucidate why some operational global models can predict TC genesis successfully, while others cannot by conducting two case studies for Typhoons Megi (2010) and Jelawat (2012). The archived data of the global model forecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF), Japan Meteorological Agency (JMA), National Centers for Environmental Prediction (NCEP) and United Kingdom Met O ce (UKMO) are analyzed and compared. For Megi (2010), the ECMWF model showed the best skill in predicting the genesis with the longest leading time. For Jelawat (2012), the NCEP model was the best among the four models. In order to differentiate the developing and non-developing systems in the forecast fields, a new index, called GPIOWZ is introduced in this study. It is based on the original Genesis Potential Index (GPI) proposed by Emanuel and Nolan (2004), and modified using the Okubo-Weiss-Zeta (OWZ) parameter (Tory et al. 2013) instead of the absolute vorticity in order to detect the rotational component of vorticity associated with a disturbance. The results show that the GPIOWZ is a good indicator of the developing and non-developing systems. The failed forecasts tend to show a smaller OWZ parameter, lower relative humidity (RH) in the mid-troposphere, stronger vertical wind shear (VWS) and/or weaker maximum potential intensity (MPI). The advantage of using the new GPIOWZ index is that sensitivity experiments can be conducted to identify which parameter(s) contribute negatively to the index and thus produce a negative e ect on the failed genesis forecasts. Results from our limited sensitivity experiments indicate that the parameters with negative effects vary from model to model and with different forecasts. In addition to the GPIOWZ, the comparison of convective available potential energy (CAPE) and the vertical overlapping of the OWZ parameter at 850 hPa and 500 hPa were conducted. The failed forecasts tend to show higher CAPE than the successful forecasts for Megi, suggesting that the convective scheme cannot release the CAPE well in the failed forecasts (although this is less systematic for Jelawat). The successful forecasts tend to show larger vertical overlapping regions of the OWZ parameter, but some failed forecasts also show large overlapping area, indicating that the environmental conditions are also important. The difference in the vertical overlapping grid numbers of the OWZ parameter between the failed and successful forecasts is model dependent. Insights into the model improvements and the potential extension of the GPIOWZ are briefly discussed.