An Ensemble Analysis on Abrupt Northward Turning of Typhoon Meranti (1010) under the Influence of an Upper-Tropospheric Cold Low

The sudden change of typhoon track is a challenging issue in typhoon forecast. Typhoon “Meranti” (2010) experienced an abrupt northward turning when it moved westward to the south of Taiwan Island. Ensemble forecast based on the European Medium Range Weather Forecasts (ECMWF) presented a large spread for the northward turning. The good group that consisted of eight members and predicted the abrupt northward turning is compared to the poor group that didn’t predict the unusual track to study the main influence factors on the northward turning of typhoon Meranti. Results show that: (1) An upper-tropospheric cold low (UTCL) cut off from the tropical upper-tropospheric trough (TUTT) was an important factor impacting the typhoon track change. The abrupt northward turning of typhoon Meranti was related to the coupling between Meranti and the nearby UTCL in the north-south direction. (2) The UTCL could change the upper-tropospheric environmental flow around the typhoon, and influence the typhoon steering flow. When the UTCL moved to the north of Meranti, the southerly component of the steering flow was enhanced, which was helpful for the northward movement of Meranti. (3) The northward turning of Meranti was also associated with the vertical structure of the UTCL. The UTCL with wide and deep cyclonic circulation in vertical direction was more favorable for the track change. (4) The diagnostic analysis based on the potential vorticity tendency equation shows that, under the coupling between Meranti and the UTCL in the north-south direction, horizontal advection of potential vorticity to the north of the typhoon was strengthened, which was helpful for the northward movement of Meranti. And the asymmetric advection of symmetric potential vorticity made a major contribution to the northward track under the influence of the UTCL.