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为欧洲中期天气预报中心（ECMWF）对20世纪首次进行的大气再分析，时间跨度为1900年至2010年。该数据集仅整合了地表气压和地表海洋风速的观测数据。它是ERA-CLIM项目的成果之一。

ERA-20C基于IFS版本CY38r1生成，与大气模型集成ERA-20CM的最终版本使用相同的地面和大气强迫力。通过整合地表观测数据，采用耦合的大气、陆地表面和海洋波浪模型对天气进行再分析。ERA-20C产品描述了大气（在91个垂直层次上，从地表到0.01 hPa之间）、陆地表面（在4个土壤层中）和海洋波浪（在25个频率和12个方向上）的空间-时间演变。

水平分辨率为约125公里（谱截断T159）。请注意，大气数据不仅可在原生的91个模型层次上获取，还可获得37个压力层次（如ERA-Interim所示）、16个潜在温度层次以及2个潜在涡度单位层次。美国国家大气研究中心（NCAR）的数据支持部门（DSS）正在提供ERA-20C的网格转换版本，其中原本以谱系数表示或存档在降低高斯网格上的变量已转换为320经度×160纬度的N80高斯网格。此外，DSS还在可用的地方从谱涡度和散度计算水平风（u分量，v分量）。数据正被组织成单一参数的时间序列。

数据同化方法为24小时4D-Var分析，对地表气压观测进行变分偏差校正。分析增量在T95水平分辨率（约210公里）上。分析结果提供了后续预报的初始条件，这些预报作为后续分析的背景。背景误差的空间-时间演变由事先生成的10成员集合提供。

ERA-20C中同化的观测数据包括来自ISPD v3.2.6和ICOADS v2.5.1的地表和平均海平面气压，以及来自ICOADS v2.5.1的地表海洋风速。ERA-20C的观测反馈，包括例如同化前后的偏差以及使用标记等，将在稍后阶段发布。

ERA-20C于2014年生成，耗时约6周，在ECMWF的IBM POWER7高性能计算机上完成。此次生产使用了22个并行流：1899年1月-1904年12月，1904年1月-1909年12月等。每个流的头一年仅用于启动并丢弃；最终产品由每个流的最后几年组成。

每日产品的时空分辨率为通常的3小时。在分析产品中，以下地表参数仅在每6小时可用：海冰覆盖率、雪密度、海表面温度、土壤水分层1的体积、土壤水分层2的体积、土壤水分层3的体积、土壤水分层4的体积、土壤温度层1、雪深度、2米温度、2米露点温度、土壤温度层2、土壤温度层3、雪层温度。