Japanese 25-year Reanalysis Project, Monthly Means

The Japanese 25-year Reanalysis (JRA-25) represents the first long-term global atmospheric reanalysis undertaken in Asia. Covering the period 1979-2004, it was completed using the Japan Meteorological Agency (JMA) numerical assimilation and forecast system and specially collected and prepared observational and satellite data from many sources including the European Center for Medium-Range Weather Forecasts (ECMWF), the National Climatic Data Center (NCDC), and the Meteorological Research Institute (MRI) of JMA. A primary goal of JRA-25 is to provide a consistent and high-quality reanalysis dataset for climate research, monitoring, and operational forecasts, especially by improving the coverage and quality of analysis in the Asian region.

In JRA-25, three-dimensional variational (3D-Var) data assimilation and a global spectral model were employed to produce 6-hourly atmospheric analysis and forecast cycles. The global spectral model was based on a 320 by 160 (~1.125 degree) Gaussian grid with T106 truncation. Vertical discretization employed a hybrid sigma-pressure coordinate utilizing 40 levels where 0.4 hPa represents the model top level. A predictive mass-flux Arakawa-Schubert scheme was utilized for cumulus parameterization, and Simple Biosphere (SiB) parameterizations for land-surface processes. Assimilated variables include temperature, relative humidity, and surface pressure from conventional observations, and also winds retrieved from geostationary satellites, radiative brightness temperature from TIROS Operational Vertical Sounder (TOVS), and precipitable water from Special Sensor Microwave/Imager (SSM/I). Variables not directly assimilated include daily sea surface temperature (SST) and sea ice based on Centennial in-situ Observation-Based Estimates (COBE), and ozone profiles based on chemical transport models constrained by observations from Total Ozone Mapping Spectrometer (TOMS).

The JRA-25 shows marked improvement over previous reanalyses in several notable areas, especially predicted (both 6-hourly and long term) precipitation, with more realistic variability and fewer spurious trends due to contamination of satellite data by volcanic eruptions. JRA-25 is also the first reanalysis to assimilate wind profiles around tropical cyclones deduced from best-track data, resulting in improved tropical cyclone analysis in a global context. In addition, low-level (stratus) cloud decks along the western subtropical coasts of continents are also better simulated, improving radiation budgets in these regions.

In 2006, JMA started real-time operation of the JMA Climate Data Assimilation System (JCDAS). JCDAS employs the same system as JRA-25 and the data assimilation cycle is extended to the present time. JRA-25 and JCDAS products will enable users to conduct climate diagnostics with a long-term, and current, homogeneous reanalysis dataset. The JMA has also engaged in ongoing cooperation with ECMWF (European Center for Medium-Range Weather Forecasts) on reanalysis, including the ECMWF CDAS (ECDAS), more commonly known as ERA-Interim.

《日本25年再分析（JRA-25）》标志着亚洲地区首次进行的长期全球大气再分析研究。该数据集涵盖了1979年至2004年的时段，通过采用日本气象厅（JMA）的数值同化与预报系统，并结合了来自欧洲中期天气预报中心（ECMWF）、国家气候数据中心（NCDC）以及JMA气象研究所等多家机构收集和整理的观测和卫星数据而完成。JRA-25的主要目标是为气候研究、监测和业务预报提供一致且高质量的重分析数据集，特别是在提升亚洲区域分析覆盖面和质量方面发挥着至关重要的作用。
在JRA-25中，采用了三维变分（3D-Var）数据同化和全球谱模式，以生成每6小时一次的大气分析和预报循环。全球谱模式基于320×160（约1.125度）的高斯网格和T106截断，垂直方向采用混合的sigma-压力坐标，共40个水平层，其中0.4 hPa代表模型顶界。为了对流参数化，使用了Arakawa-Schubert方案预测质量通量，而对于陆地表面过程则采用了简单生物圈（SiB）参数化。同化的变量包括来自传统观测的温度、相对湿度和地表气压，以及从地球静止卫星获取的风速，来自泰罗斯业务垂直声雷达（TOVS）的辐射亮度温度，以及来自特殊传感器微波/成像仪（SSM/I）的降水量。未直接同化的变量包括基于世纪地面观测估计（COBE）的每日海表温度（SST）和海冰，以及基于化学传输模型且受总臭氧映射光谱仪（TOMS）观测约束的臭氧剖面。
与之前的重分析相比，JRA-25在多个显著领域表现出显著的改进，尤其是在预测（包括每6小时和长期）降水方面，具有更真实的变率，且由于卫星数据受火山喷发污染而导致的不真实趋势更少。JRA-25还是第一个将热带气旋周围的风剖面（由最佳轨迹数据推断得出）纳入同化的重分析，从而在全球范围内改善了热带气旋的分析。此外，大陆西海岸的低层（层积云）云层也得到了更好的模拟，从而改善了这些区域的辐射预算。
2006年，JMA开始对JMA气候数据同化系统（JCDAS）进行实时运行。JCDAS采用了与JRA-25相同的系统，并将数据同化周期扩展至现今。JRA-25和JCDAS产品将使用户能够使用长期和当前同质重分析数据集进行气候诊断。JMA还与ECMWF（欧洲中期天气预报中心）在重分析领域进行了持续的合作，包括ECMWF CDAS（ECDAS），更广为人知的是ERA-Interim。