垂直分层加密和预报区域扩大对GRAPES_TYM台风预报的影响

为提升GRAPES_TYM对西北太平洋和中国南海热带气旋路径及强度的预报能力、增加对北印度洋热带气旋的预报，2019年8月GRAPES_TYM 3.0版投入业务运行。GRAPES_TYM 3.0版的模式垂直分层由GRAPES_TYM 2.2版的50层增加到68层；预报区域由覆盖西北太平洋、中国南海扩展到覆盖北印度洋。试验结果显示:模式垂直分层增加可以改进模式对强台风及超强台风的预报能力，减小平均路径预报误差、显著减小平均强度预报误差以及强度预报负偏差；模式预报区域扩大到覆盖北印度洋对平均路径误差和平均强度误差影响不显著，但长时效预报比较敏感，如20°N以北热带气旋120 h预报路径。2016—2018年的回算结果与NCEP-GFS和ECMWF的预报结果对比分析表明:GRAPES_TYM 3.0版的平均路径误差与NCEP-GFS接近，同ECMWF相比误差较大；但24—96 h强度预报误差明显小于NCEP-GFS和ECMWF，NCEP-GFS和ECMWF对热带气旋强度预报存在明显的负偏差。综上所述，模式垂直分层由50层增加到68层对热带气旋强度预报至关重要，而长时效路径预报对模式预报区域扩大到覆盖北印度洋更为敏感。 

Impacts to tropical cyclone prediction of GRAPES_TYM from increasing of model vertical levels and enlargement of model forecast domain

In order to improve the model performance for the forecast of tropical cyclone(TC)intensity and track in the western North Pacific and South China Sea and provide TC forecasts in the northern Indian Ocean,the GRAPES_TYM has been upgraded to version 3.0 in August 2019.The vertical levels are increased from 50 to 68,and the model domain is enlarged to cover the northern Indian Ocean.The results show that the increase in vertical resolution can reduce the mean track error of 72—120 h TC forecasts and remarkably reduce the mean intensity error of 24—96 h forecasts by decreasing the negative bias in the forecast of severe typhoons and super typhoons.There is no significant difference in the mean track and intensity errors when the model domain is expended to the northern Indian Ocean,except that the simulated TC track to the north of 20°N is more sensitive to the model domain.The comparative analysis of the retrospective results from 2016 to 2018 and the predictions of NCEP-GFS and ECMWF shows that the average track error simulated by the GRAPES_TYM-v3.0 is close to that by NCEP-GFS,and the error is larger than that simulated by ECMWF.The intensity error is obviously smaller than that by NCEP-GFS and ECMWF.NCEP-GFS and ECMWF both have larger negative biases.In summary,the intensity error in the forecast of GRAPES_TYM is reduced remarkably through increasing the vertical levels from 50 to 68,and long term track forecast is more sensitive to the enlarged domain which covers the northern Indian Ocean.