ERA-20C Project (ECMWF Atmospheric Reanalysis of the 20th Century)

ERA-20C is ECMWF's first atmospheric reanalysis of the 20th century, from 1900-2010. It assimilates observations of surface pressure and surface marine winds only. It is an outcome of the ERA-CLIM project.ERA-20C was produced with IFS version CY38r1 and the same surface and atmospheric forcings as the final version of the atmospheric model integration ERA-20CM. A coupled atmosphere land surface and ocean wave model is used to reanalyze the weather, by assimilating surface observations. The ERA-20C products describe the spatial-temporal evolution of the atmosphere (on 91 vertical levels, between the surface and 0.01 hPa), the land-surface (on 4 soil layers), and the ocean waves (on 25 frequencies and 12 directions).The horizontal resolution is approximately 125 km (spectral truncation T159). Note that atmospheric data are not only available on the native 91 model levels, but also on 37 pressure levels (as in ERA-Interim), 16 potential temperature levels, and the 2 potential vorticity unit level. NCAR's Data Support Section (DSS) is performing and supplying a grid transformed version of ERA-20C, in which variables originally represented as spectral coefficients or archived on a reduced Gaussian grid are transformed to a regular 320 longitude by 160 latitude N80 Gaussian grid. In addition, DSS is also computing horizontal winds (u-component, v-component) from spectral vorticity and divergence where these are available. The data is being organized into single parameter time series.The assimilation methodology is 24-hour 4D-Var analysis, with variational bias correction of surface pressure observations. Analysis increments are at T95 horizontal resolution (approximately 210 km). The analyses provide the initial conditions for subsequent forecasts that serve as backgrounds to the next analyses. The spatial temporal evolution of background errors was provided by a 10-member ensemble produced a priori.The observations assimilated in ERA-20C include surface and mean sea level pressures from ISPD v3.2.6 and ICOADS v2.5.1, and surface marine winds from ICOADS v2.5.1. The observation feedback from ERA-20C, including, for example, departures before and after assimilation and usage flags, will be released at a later stage.ERA-20C was produced in 2014 in about 6 weeks on ECMWF's IBM POWER7 high-performance computer. This production employed 22 parallel streams: January 1899-December 1904, January 1904-December 1909, etc. The first year of each stream was only used for spin-up and discarded; the final product is made of the last years of each stream.The temporal resolution of the daily products is usually 3-hourly. Exceptions are in the analysis products, where the following surface parameters are only available 6-hourly: Sea-ice cover, Snow density, Sea surface temperature, Volumetric soil water layer 1, Volumetric soil water layer 2, Volumetric soil water layer 3, Volumetric soil water layer 4, Soil temperature level 1, Snow depth, 2 meter temperature, 2 meter dew point temperature, Soil temperature level 2, Soil temperature level 3, Temperature of snow layer.

ERA-20C是欧洲中期天气预报中心（ECMWF）首款针对20世纪（1900-2010年）的大气再分析数据集，仅同化地面气压与海洋表面风场观测数据，属于ERA-CLIM项目的研究成果。该数据集依托集成预报系统（Integrated Forecasting System，IFS）CY38r1版本生成，所用的地表与大气强迫场与大气模式集成产物ERA-20CM的最终版本完全一致。研究采用耦合的大气-陆面-海浪模式开展天气再分析，仅同化地面观测数据。ERA-20C的产品涵盖大气（在地表至0.01百帕的91个垂直层上）、陆面（4层土壤）以及海浪（25个频率、12个方向）的时空演变特征，水平分辨率约为125千米（谱截断T159）。需注意，大气数据不仅可在原生的91个模式层获取，还可参考ERA-Interim的设置，提供37个气压层、16个位温层以及2个位涡单位层的数据。美国国家大气研究中心（NCAR）的数据支持部门（Data Support Section，DSS）正在处理并提供ERA-20C的网格转换版本：将原本以谱系数表示或存储于缩减高斯网格的变量，转换为规则的320经度×160纬度的N80高斯网格。此外，当谱涡度与散度数据可用时，DSS还将基于二者计算水平风场的u分量与v分量，相关数据将整理为单参数时间序列形式。本次同化采用24小时四维变分（4D-Var）分析方法，并对地面气压观测实施变分偏差校正，分析增量的水平分辨率为T95（约210千米）。上述分析结果为后续预报提供初始条件，而这些预报又作为下一阶段分析的背景场；背景场误差的时空演变由先验生成的10成员集合提供。ERA-20C所同化的观测数据包括：来自ISPD v3.2.6与ICOADS v2.5.1的地面及平均海平面气压，以及来自ICOADS v2.5.1的海洋表面风场。ERA-20C的观测反馈数据（例如同化前后的偏差量、观测使用标记等）将在后续阶段发布。ERA-20C于2014年在ECMWF的IBM POWER7高性能计算机上完成生产，耗时约6周。本次生产采用22个并行流任务，时间范围依次为1899年1月—1904年12月、1904年1月—1909年12月等；每个并行流的第一年仅用于模式自旋调整并最终舍弃，最终产品由每个并行流的最后几年数据拼接而成。每日产品的时间分辨率通常为3小时间隔，但分析产品存在例外：以下地表参数仅提供6小时间隔的数据：海冰覆盖率、雪密度、海表温度、第1层土壤体积含水量、第2层土壤体积含水量、第3层土壤体积含水量、第4层土壤体积含水量、第1层土壤温度、积雪深度、2米气温、2米露点温度、第2层土壤温度、第3层土壤温度、积雪层温度。

搜集汇总

背景与挑战

背景概述

ERA-20C是欧洲中期天气预报中心（ECMWF）对20世纪（1900-2010年）的首次大气再分析数据集，仅同化地表压力和海洋表面风观测，提供全球天气、陆面和海洋波浪的高分辨率时空演变信息。数据集具有约125公里的水平分辨率、91个垂直层次，覆盖多种气象变量，数据格式为GRIB1和GRIB2，总容量达80.24 TB，适用于气候研究和历史天气分析。