ERA-Interim Project

ERA-Interim represents a major undertaking by ECMWF (European Centre for Medium-Range Weather Forecasts) to produce a reanalysis with an improved atmospheric model and assimilation system which replaces those used in ERA-40, particularly for the data-rich 1990s and 2000s, and to be continued as an ECMWF Climate Data Assimilation System (ECDAS) until superseded by a new extended reanalysis.

Preliminary runs indicated that several of the inaccuracies exhibited by ERA-40 such as too-strong precipitation over oceans from the early 1990s onwards and a too-strong Brewer-Dobson circulation in the stratosphere, were eliminated or significantly reduced. Production of ERA-Interim, from 1989 onwards, began in summer of 2006. (The period 1979-1988 was prepended in 2011.)

Through systematic increases of computing power, 4-dimensional variational assimilation (4D-Var) became feasible and part of ECMWF operations since 1997, paving the way to base ERA-Interim on 4D-Var (rather than 3D-Var as in ERA-40). Enhanced computing power also allowed horizontal resolution to be increased from T159 (N80, nominally 1.125 degrees for ERA-40) to T255 (N128, nominally 0.703125 degrees), and the latest cycle of the atmospheric model (IFS CY31r1 and CY31r2) to be used, taking advantage of improved model physics. ERA-interim retains the same 60 model levels used for ERA-40 with the highest level being 0.1 hectopascal. In addition, data assimilation of ERA-Interim also benefits from quality control that draws on experience from ERA-40 and JRA-25, variational bias correction of satellite radiance data, and more extensive use of radiances with an improved fast radiative transfer model. ERA-Interim uses sets of observations and boundary forcing fields acquired for ERA-40 through 2001, and from ECMWF operations thereafter. Noteworthy exceptions include new ERS (European Remote Sensing Satellite) altimeter wave heights, EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites) reprocessed winds and clear-sky radiances, GOME (Global Ozone Monitoring Experiment) ozone data from the Rutherford Appleton Laboratory, and CHAMP (CHAllenging Minisatellite Payload), GRACE (Gravity Recovery and Climate Experiment), and COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) GPS radio occultation measurements processed and archived by UCAR (University Corporation for Atmospheric Research).

NCAR's Data Support Section (DSS) is performing and supplying a grid transformed version of ERA-Interim, in which variables originally represented as spectral coefficients or archived on a reduced Gaussian grid are transformed to a regular 512 longitude by 256 latitude N128 Gaussian grid. In addition, DSS is also computing horizontal winds (u-component, v-component) from spectral vorticity and divergence where these are available. Processing of analysis groups and the surface forecast has been completed for January 1979 through December 2014 (inclusive), or at least 36 years, and will continue as ERA-Interim becomes available thereafter. Data is currently available via NCAR's High Performance Storage System (HPSS), or by delayed mode request which transfers files from the HPSS to our web server for internet download, or via direct internet download, or NCAR's GLADE file system.

ERA-Interim是欧洲中期天气预报中心（European Centre for Medium-Range Weather Forecasts, ECMWF）开展的一项重大科研项目，旨在研发经改进的大气模式与同化系统，以替代ERA-40所使用的对应方案，尤其适配数据资源丰富的1990年代与2000年代；该数据集后续将以欧洲中期天气预报中心气候数据同化系统（ECMWF Climate Data Assimilation System, ECDAS）的形式持续产出，直至被新一代扩展再分析产品取代。
初步运行结果显示，ERA-40存在的若干缺陷已被消除或大幅改善，例如1990年代初以来海洋上空降水偏强的问题，以及平流层中布鲁尔-多布森环流（Brewer-Dobson circulation）过强的问题。ERA-Interim的生产工作于2006年夏季启动，起始时间为1989年；1979年至1988年的数据集则于2011年补充加入。
随着计算能力的系统性提升，四维变分同化（four-dimensional variational assimilation, 4D-Var）自1997年起成为ECMWF的业务化方案，为ERA-Interim采用4D-Var（而非ERA-40所使用的三维变分同化（three-dimensional variational assimilation, 3D-Var））奠定了基础。更强的计算能力还使得水平分辨率从ERA-40的T159（N80，标称分辨率1.125°）提升至T255（N128，标称分辨率0.703125°），同时可搭载最新版本的大气模式——综合预报系统CY31r1与CY31r2（Integrated Forecast System, IFS CY31r1、CY31r2），充分利用其改进后的模式物理过程。ERA-Interim沿用了ERA-40的60层模式垂直结构，最高层气压为0.1百帕。此外，ERA-Interim的数据同化还受益于融合ERA-40与JRA-25经验开发的质量控制方案、卫星辐射数据的变分偏差校正方法，以及借助改进的快速辐射传输模型更广泛地应用辐射观测数据。
ERA-Interim所使用的观测数据集与边界强迫场，2001年及之前的数据沿用自ERA-40，2001年之后的数据则取自ECMWF的业务观测系统。值得注意的例外包括：新的欧洲遥感卫星（European Remote Sensing Satellite, ERS）高度计海浪高度数据、欧洲气象卫星开发组织（European Organisation for the Exploitation of Meteorological Satellites, EUMETSAT）再处理后的风场与晴空辐射数据、拉瑟福德·阿普尔顿实验室提供的全球臭氧监测实验（Global Ozone Monitoring Experiment, GOME）臭氧数据，以及由大学大气研究公司（University Corporation for Atmospheric Research, UCAR）处理并归档的挑战微卫星载荷（CHAllenging Minisatellite Payload, CHAMP）、重力恢复与气候实验（Gravity Recovery and Climate Experiment, GRACE）以及气象、电离层与气候星座观测系统（Constellation Observing System for Meteorology, Ionosphere and Climate, COSMIC）的GPS无线电掩星测量数据。
美国国家大气研究中心（National Center for Atmospheric Research, NCAR）的数据支持科（Data Support Section, DSS）正在处理并提供ERA-Interim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512经度×256纬度的N128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（u分量、v分量）。1979年1月至2014年12月（含首尾月份）的分析组与地面预报数据的处理工作已完成，时长至少36年，后续随着ERA-Interim数据集的更新，该处理工作也将持续推进。目前，数据可通过NCAR的高性能存储系统（High Performance Storage System, HPSS）获取，或通过延迟模式请求（将文件从HPSS传输至网站服务器以供互联网下载）、直接互联网下载，以及NCAR的GLADE文件系统获取。