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for the data-rich 1990s and 2000s, and to be continued as an ECMWF Climate Data Assimilation System (ECDAS) until superseded by a new extended reanalysis.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代表欧洲中期天气预报中心（ECMWF）的一项重大努力，旨在通过改进大气模型和同化系统，生产一种再分析数据集，以替代ERA-40中使用的模型，特别是在数据丰富的1990年代和2000年代。该数据集旨在作为ECMWF气候数据同化系统（ECDAS）的延续，直至被新的扩展再分析所取代。初步运行表明，ERA-40所表现出的某些不准确性，如自1990年代初以来海洋上过强的降水和大气平流层中过强的布罗厄尔-道布森环流，已被消除或显著降低。ERA-Interim的生产始于2006年夏季，追溯至1989年。 （1979-1988年期间的数据于2011年补充。）

通过计算能力的系统性提升，四维变分同化（4D-Var）自1997年以来成为ECMWF操作的一部分，并为基于4D-Var的ERA-Interim奠定了基础（而不是ERA-40中的3D-Var）。增强的计算能力还使得水平分辨率从T159（N80，对于ERA-40而言，名义上为1.125度）提高到了T255（N128，名义上为0.703125度），并利用改进的模型物理特性使用了最新的大气模型周期（IFS CY31r1和CY31r2）。此外，ERA-Interim保留了与ERA-40相同的60个模型层级，最高层级为0.1百帕。此外，ERA-Interim的数据同化还受益于从ERA-40和JRA-25汲取的经验，对卫星辐射亮度数据进行变分偏差校正，以及更广泛地使用改进的快速辐射传输模型中的辐射。

ERA-Interim使用的是为ERA-40获取的观测集和边界强迫场，直至2001年，此后则使用ECMWF的操作数据。值得注意的例外包括新的ERS（欧洲遥感卫星）海浪高度、EUMETSAT（欧洲气象卫星组织）重新处理的风和晴空辐射亮度、Rutherford Appleton实验室的GOME（全球臭氧监测实验）臭氧数据，以及由UCAR（大气研究大学公司）处理和归档的CHAMP（挑战性微型卫星有效载荷）、GRACE（重力恢复和气候实验）和COSMIC（气象、电离层和气候星座观测系统）GPS无线电掩星测量。

美国国家大气研究中心（NCAR）的数据支持部门（DSS）正在提供ERA-Interim的网格转换版本，其中原本以谱系数表示或存档在降维高斯网格上的变量被转换为一个规则的512经度乘以256纬度的N128高斯网格。此外，DSS还在计算水平风（u分量、v分量），在这些变量可用的情况下，从谱涡度和发散中得出。分析组和地表预报的处理已完成，时间范围为1979年1月至2014年12月（含），或至少36年，并将随着ERA-Interim的可用性继续进行。数据目前可通过NCAR的高性能存储系统（HPSS）获取，或通过延迟模式请求，将文件从HPSS传输到我们的网络服务器进行互联网下载，或直接通过互联网下载，或通过NCAR的GLADE文件系统。