Subseasonal Forecasting with an Icosahedral, Vertically Quasi-Lagrangian Coupled Model. Part II: Probabilistic and Deterministic Forecast Skill Monthly Weather Review

Subseasonal forecast skill of the global hydrostatic atmospheric Flow-Following Icosahedral Model (FIM) coupled to an icosahedral-grid version of the Hybrid Coordinate Ocean Model (iHYCOM) is evaluated through 32-day predictions initialized weekly using a four-member time-lagged ensemble over the 16-yr period 1999-2014. Systematic biases in forecasts by the coupled system, referred to as FIM-iHYCOM, are described in a companion paper (Part I). This present study (Part II) assesses probabilistic and deterministic model skill for predictions of surface temperature, precipitation, and 500-hPa geopotential height in different seasons at different lead times ranging from 1 to 4 weeks. The coupled model appears to have reasonable agreement with reanalysis in terms of simulated weekly variability in sea surface temperatures, except in extratropical regions because the ocean model cannot explicitly resolve eddies there. This study also describes the ability of the model to simulate midlatitude tropospheric blocking frequency, Madden-Julian oscillation patterns, and sudden stratospheric warming events-all of which have been shown to be relevant on subseasonal time scales. The metrics used here indicate that the subseasonal forecast skill of the model is comparable to that of several operational models, including the National Oceanic and Atmospheric Administration's (NOAA's) operational Climate Forecast System version 2 and the European Centre for Medium-Range Weather Forecasts model. Therefore, FIM-iHYCOM-as a participant in NOAA's Subseasonal Experiment-is expected to add value to multimodel ensemble forecasts produced through this effort. Grant no. NA17OAR4320101

本研究针对耦合二十面体格点版本混合坐标海洋模式（Hybrid Coordinate Ocean Model, iHYCOM）的全球静力大气流跟随式二十面体格点模式（Flow-Following Icosahedral Model, FIM）的次季节预报技巧展开评估。评估基于1999至2014年共16年的预报数据完成，该数据集采用4成员时滞集合方案，每周开展一次初始条件初始化，预报时效为32天。配套论文（第一部分）已阐述该耦合系统（下称FIM-iHYCOM）预报中的系统偏差。本研究（第二部分）则针对不同季节、提前时效为1至4周的地表温度、降水及500百帕位势高度预报，评估模式的概率与确定性预报技巧。该耦合模式模拟的海表温度周际变率与再分析资料吻合良好，但中高纬度区域除外——这是由于海洋模式无法在该区域显式解析涡旋活动。本研究还分析了模式模拟中纬度对流层阻塞频数、马登-朱利安振荡（Madden-Julian oscillation, MJO）型以及平流层突然增温事件的能力，上述特征均被证实与次季节时间尺度密切相关。本次采用的评估指标显示，该模式的次季节预报技巧可与多款业务数值模式相媲美，包括美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration, NOAA）业务气候预报系统第2版以及欧洲中期天气预报中心（European Centre for Medium-Range Weather Forecasts, ECMWF）数值模式。因此，作为美国国家海洋和大气管理局次季节试验（Subseasonal Experiment）的参与模式，FIM-iHYCOM有望为该项目产出的多模式集合预报提供额外价值。本研究受项目编号NA17OAR4320101资助。