Uncertainty in Tropical Cyclone Intensity Predictions due to Uncertainty in Initial Conditions

Abstract

Focusing on the role of initial condition uncertainty, we use WRF initial perturbation ensemble forecasts to investigate the uncertainty in intensity forecasts of Tropical Cyclone (TC) Rammasun (1409), which is the strongest TC to have made landfall in China during the past 50 years. Forecast results indicate that initial condition uncertainty leads to TC forecast uncertainty, particularly for TC intensity. This uncertainty increases with forecast time, with a more rapid and significant increase after 24 h. The predicted TC develops slowly before 24 h, and at this stage the TC in the member forecasting the strongest final TC is not the strongest among all members. However, after 24 h, the TC in this member strengthens much more than that the TC in other members. The variations in convective instability, precipitation, surface upward heat flux, and surface upward water vapor flux show similar characteristics to the variation in TC intensity, and there is a strong correlation between TC intensity and both the surface upward heat flux and the surface upward water vapor flux. The initial condition differences that result in the maximum intensity difference are smaller than the errors in the analysis system. Differences in initial humidity, and to a lesser extent initial temperature differences, at the surface and at lower heights are the key factors leading to differences in the forecasted TC intensity. These differences in initial humidity and temperature relate to both the overall values and distribution of these parameters.

摘 要

关注初始条件的不确定性, 利用WRF模式和初值扰动的集合预报, 针对过去50年间登陆中国的最强热带气旋 “威马逊” (1409), 开展了热带气旋强度预报的不确定性研究. 结果显示, 初始条件的不确定性使得热带气旋的预报存在着不确定性, 这种不确定性主要反映在热带气旋强度的预报上, 并随时间而增大, 尤其在预报24h后迅速增大, 第48h时, 强度预报的最大差异可以达到37hPa (最低海平面气压) 和14 m s⁻¹ (10m最大风速); 预报的前24h, 热带气旋发展缓慢, 所有成员中在第48h时强度最强的成员在这一时期的强度要弱于其他一些成员, 但预报24h后, 该成员快速发展, 其强度的增强比其他成员更为迅速和显著; 热带气旋对流不稳定的发展、 降水的演变以及表面向上的热通量和水汽通量的变化都与热带气旋强度的变化相似, 热带气旋的强度与其表面向上的热通量、 水汽通量之间存在着强正相关; 不同成员间初始场的差异导致其强度预报的差异, 但初始场的这些差异总体上要比观测误差小得多, 初始场的最大差异也都在观测误差的范围内; 导致热带气旋强度预报的不确定性的关键因子为初始场上表面和低层的湿度场以及温度场的不确定性, 尤其是不同成员间低层湿度场的差异, 这种差异不仅在于总体大小的差异, 还在于其分布的差异.