Subseasonal Forecasting with an Icosahedral, Vertically Quasi-Lagrangian Coupled Model. Part I: Model Overview and Evaluation of Systematic Errors Monthly Weather Review

The atmospheric hydrostatic Flow-Following Icosahedral Model (FIM), developed for medium-range weather prediction, provides a unique three-dimensional grid structure-a quasi-uniform icosahedral horizontal grid and an adaptive quasi-Lagrangian vertical coordinate. To extend the FIM framework to subseasonal time scales, an icosahedral-grid rendition of the Hybrid Coordinate Ocean Model (iHYCOM) was developed and coupled to FIM. By sharing a common horizontal mesh, air-sea fluxes between the two models are conserved locally and globally. Both models use similar adaptive hybrid vertical coordinates. Another unique aspect of the coupled model (referred to as FIM-iHYCOM) is the use of the Grell-Freitas scale-aware convective scheme in the atmosphere. A multiyear retrospective study is necessary to demonstrate the potential usefulness and allow for immediate bias correction of a subseasonal prediction model. In these two articles, results are shown based on a 16-yr period of hindcasts from FIM-iHYCOM, which has been providing real-time forecasts out to a lead time of 4 weeks for NOAA's Subseasonal Experiment (SubX) starting July 2017. Part I provides an overview of FIM-iHYCOM and compares its systematic errors at subseasonal time scales to those of NOAA's operational Climate Forecast System version 2 (CFSv2). Part II uses bias-corrected hindcasts to assess both deterministic and probabilistic subseasonal skill of FIM-iHYCOM. FIM-iHYCOM has smaller biases than CFSv2 for some fields (including precipitation) and comparable biases for other fields (including sea surface temperature). FIM-iHYCOM also has less drift in bias between weeks 1 and 4 than CFSv2. The unique grid structure and physics suite of FIM-iHYCOM is expected to add diversity to multimodel ensemble forecasts at subseasonal time scales in SubX. Grant no. NA17OAR4320101

专为中期天气预报研发的大气静力追随流二十面体模型（Flow-Following Icosahedral Model，FIM）拥有独特的三维网格架构：准均匀二十面体水平网格与自适应准拉格朗日垂直坐标系。为将FIM框架拓展至次季节时间尺度，研究人员开发了混合坐标海洋模型的二十面体网格变体（Hybrid Coordinate Ocean Model，iHYCOM）并将其与FIM耦合。由于二者共享同一水平网格，两模型间的气海通量可在局地与全球尺度上严格守恒，且两者均采用类似的自适应混合垂直坐标系。
该耦合模型（下文简称FIM-iHYCOM）的另一独特之处在于其大气模块采用了Grell-Freitas尺度感知对流参数化方案。要验证次季节预测模型的潜在应用价值并开展即时偏差校正，需开展多年的回顾性后报研究。本文两篇论文均基于FIM-iHYCOM的16年时长后报结果展开分析——该模型自2017年7月起，为美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration，NOAA）的次季节实验（Subseasonal Experiment，SubX）提供最长提前4周的实时预报。
第一篇论文概述了FIM-iHYCOM的整体架构，并将其在次季节时间尺度下的系统误差与NOAA业务化第二代气候预报系统（Climate Forecast System version 2，CFSv2）进行了对比。第二篇论文则利用经过偏差校正的后报数据，评估了FIM-iHYCOM的确定性与概率性次季节预报技巧。相较于CFSv2，FIM-iHYCOM在部分要素（包括降水）上的偏差更小，在另一些要素（如海表温度）上的偏差则相当；同时其偏差在第1周至第4周间的漂移幅度也小于CFSv2。
FIM-iHYCOM独特的网格结构与物理过程套件有望为SubX计划下的次季节多模式集合预报增添多样性。资助编号：NA17OAR4320101