Binary interaction of typhoons Soulik and Cimaron in 2018—Part Ⅰ:Observational characteristics and forecast error

To understand structural changes and forecast error,a case study of binary typhoons in the western North Pacific(WNP)of 2018 was investigated using best track and reanalysis data.Soulik was generated on August 16 and Cimaron was generated on August 18,respectively.The 19 th typhoon Soulik and 20 th typhoon Cimaron co-existed from August 18 to 24 and approached each other.Soulik was located on the western side and Cimaron was located on the eastern side of the WNP.They were located approximately 1300 km from each other at 00 UTC August 22.The Soulik structure began changing around August 22 and became weak and slow,while Cimaron maintained its intensity,size,and moving speed.This observational evidence is likely caused by the binary interaction between two typhoons within a certain distance and environmental steering flow,such as the location of the North Pacific high and strong jet stream of the northern flank of the North Pacific high.Soulik was initially forecasted to make landfall and reach Seoul;however,its track changed from northward to northeastward from August 21 to 23 according to both official guidance and unified model(UM).Four global numerical weather prediction models forecasted different tracks of Soulik.UM and JGSM forecasted a northward track whereas ECMWF and GFS showed a northeastward track for 12 UTC August 21 through 12 UTC August 24.The latter models were similar to the best track.The track forecast error and spread of Soulik were larger than those of Cimaron.The mean absolute error of the maximum wind speed of Soulik was similar to the average of total typhoons in 2018.