A diagnostic study on a southward southwest vortex track forecasted by CMA-GFS: the role of initial field

This study presents a comprehensive diagnostic analysis of a southward deviation in the forecasted track of a southwest vortex (SWV) by the China Meteorological Administration's Global Assimilation and Prediction System (CMA-GFS). The SWV is a significant weather system in China, often responsible for severe weather events such as rainstorms. The study aims to identify the sources of forecast errors and propose improvements for more accurate tracking of SWVs. A severe convective weather process occurring on 11–13 April 2022 is selected for case study, where the CMA-GFS forecast exhibits a noticeable southward deviation in the SWV track, leading to incorrect rainfall area predictions. The analysis traces the deviation to the steering flow at 500 hPa, finding that the SWV shift is directly attributed to the deviation caused by the weak forecast of the upper-level trough. The underestimation of the initial vorticity advection by CMA-GFS is identified as the dominant factor contributing to the weak development of the trough. This underestimation is linked to the weak geostrophic wind resulting from the weak initial meridional and zonal gradients of the midlevel height in front of the trough. The study further examines the assimilation process of CMA-GFS, revealing that it has a negative effect on the SWV track forecast. The assimilation process is found to weaken the π field at mid-model level, leading to a weak midlevel height gradient in front of the trough. A verified numerical experiment using a more reasonable initial field significantly modifies the southward shift of the SWV, underscoring the importance of accurate initial analysis for precise SWV track forecasting.The research also explores the impact of initial field deviation in key regions on late-period forecasts, emphasizing the need for systematic diagnosis and improvement of SWV track forecasts. The study suggests that the initial height field, particularly at midlevels, can significantly influence the SWV track in the late forecast period. The use of ensemble prediction with initial condition perturbation strengthens the conclusions drawn from the diagnosis.

The important impact of initial field deviation in key regions on the forecast in the late period is highlighted and a systematic diagnosis process for identifying and addressing forecast issues on SWV track is proposed in this study. This research could provide a comprehensive approach for diagnosing the forecast deviation associated with SWV track.