WFDE5 over land merged with ERA5 over the ocean (W5E5)
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The W5E5 dataset was compiled to support the bias adjustment of climate input data for the impact assessments carried out in phase 3b of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP3b). Version 1.0 of the W5E5 dataset covers the entire globe at 0.5° horizontal and daily temporal resolution from 1979 to 2016. Data sources of W5E5 are version 1.0 of WATCH Forcing Data methodology applied to ERA5 data (WFDE5; Weedon et al., 2014; Cucchi et al., 2020), ERA5 reanalysis data (Hersbach et al., 2019), and precipitation data from version 2.3 of the Global Precipitation Climatology Project (GPCP; Adler et al., 2003). Variables (with short names and units in brackets) included in the W5E5 dataset are Near Surface Relative Humidity (hurs, %), Near Surface Specific Humidity (huss, kg kg-1), Precipitation (pr, kg m-2 s-1), Snowfall Flux (prsn, kg m-2 s-1), Surface Air Pressure (ps, Pa), Sea Level Pressure (psl, Pa), Surface Downwelling Longwave Radiation (rlds, W m-2), Surface Downwelling Shortwave Radiation (rsds, W m-2), Near Surface Wind Speed (sfcWind, m s-1), Near-Surface Air Temperature (tas, K), Daily Maximum Near Surface Air Temperature (tasmax, K), Daily Minimum Near Surface Air Temperature (tasmin, K), Surface Altitude (orog, m), and WFDE5-ERA5 Mask (mask, 1). W5E5 is a merged dataset. It combines WFDE5 data over land with ERA5 data over the ocean. The mask used for the merge is included in the dataset. The mask is equal to 1 over land and equal to 0 over the ocean. Over land, orog is the surface altitude used for elevation corrections in WFDE5. For all other variables already included in WFDE5 (huss, prsn, ps, rlds, rsds, sfcWind, tas), W5E5 data over land are equal to the daily mean values of the corresponding hourly WFDE5 data. W5E5 hurs over land is the daily mean of hourly hurs computed from hourly WFDE5 huss, ps, and tas using the equations of Buck (1981) as described in Weedon et al. (2010). W5E5 pr over land is the daily mean of the sum of hourly WFDE5 rainfall and snowfall. Note that W5E5 pr and prsn over land are based on WFDE5 rainfall and snowfall bias-adjusted using GPCC monthly precipitation totals. W5E5 psl over land is the daily mean of hourly psl computed from hourly WFDE5 orog, ps, and tas according to psl = ps * exp((g * orog) / (r * tas)), where g is gravity, and r is the specific gas constant of dry air. Lastly, W5E5 tasmax and tasmin over land are the daily maximum and minimum, respectively, of hourly WFDE5 tas. Over the ocean, W5E5 data are based on temporally (from hourly to daily resolution) and spatially (from 0.25° to 0.5° horizontal resolution) aggregated ERA5 data. The spatial aggregation using first-order conservative remapping was always done after the temporal aggregation. For tasmax and tasmin, hourly tas values were aggregated to daily maximum and minimum values, respectively. For all other variables, hourly values were aggregated to daily mean values. Variables unavailable in ERA5 (huss, hurs, sfcWind, orog) were first derived from available variables at hourly temporal and 0.25° horizontal resolution and then aggregated like all other variables. huss and hurs were derived from Near Surface Dewpoint Temperature, ps, and tas using the equations of Buck (1981) as described in Buck (2010). sfcWind was derived from Eastward Near-Surface Wind (uas) and Northward Near-Surface Wind (vas) according to sfcWind = sqrt(uas * uas + vas * vas). orog is equal to surface geopotential divided by gravity. Lastly, pr and prsn were bias-adjusted such that monthly W5E5 precipitation totals match GPCP version 2.3 values over the ocean. Monthly rescaling factors used for this purpose were computed following the scale-selective rescaling procedure described by Balsamo et al. (2010).
W5E5数据集旨在支持跨部门影响模型比对项目第三阶段（Inter-Sectoral Impact Model Intercomparison Project Phase 3b, ISIMIP3b）开展的影响评估所需气候输入数据的偏差校正工作。W5E5数据集V1.0版本覆盖全球范围，水平分辨率为0.5°，时间分辨率为日尺度，时段涵盖1979年至2016年。W5E5数据集的数据源包括应用于ERA5再分析数据的WATCH强迫数据方法学V1.0（WFDE5; Weedon等, 2014; Cucchi等, 2020）、ERA5再分析数据（Hersbach等, 2019），以及全球降水气候学计划（Global Precipitation Climatology Project, GPCP）V2.3版本的降水数据（Adler等, 2003）。W5E5数据集包含以下变量（括号内为变量简称与单位）：近地面相对湿度（hurs, %）、近地面比湿（huss, kg·kg⁻¹）、降水量（pr, kg·m⁻²·s⁻¹）、降雪通量（prsn, kg·m⁻²·s⁻¹）、地面气压（ps, Pa）、海平面气压（psl, Pa）、地面向下长波辐射（rlds, W·m⁻²）、地面向下短波辐射（rsds, W·m⁻²）、近地面风速（sfcWind, m·s⁻¹）、近地面气温（tas, K）、日最高近地面气温（tasmax, K）、日最低近地面气温（tasmin, K）、地面高程（orog, m）以及WFDE5-ERA5掩膜（mask, 1）。W5E5为融合数据集，将陆地区域的WFDE5数据与海洋区域的ERA5数据进行合并，合并所用的掩膜文件已包含在数据集中，该掩膜在陆地区域取值为1，海洋区域取值为0。陆地区域的orog为WFDE5中用于高程校正的地面高程数据。对于WFDE5已涵盖的其余变量（huss、prsn、ps、rlds、rsds、sfcWind、tas），W5E5陆地区域数据为对应逐小时WFDE5数据的日均值。陆地区域的hurs为参照Weedon等（2010）所述的Buck（1981）公式，利用逐小时WFDE5的huss、ps与tas计算得到的逐小时hurs的日均值。陆地区域的pr为逐小时WFDE5降雨与降雪量之和的日均值。需注意，陆地区域的pr与prsn基于经过GPCC月降水总量偏差校正后的WFDE5降雨与降雪数据生成。陆地区域的psl为根据公式psl = ps·exp((g·orog)/(r·tas))（其中g为重力加速度，r为干空气比气体常数），由逐小时WFDE5的orog、ps与tas计算得到的逐小时psl的日均值。陆地区域的tasmax与tasmin分别为逐小时WFDE5 tas的日最高值与日最低值。海洋区域的W5E5数据基于对ERA5数据进行时间聚合（从逐小时转换为日尺度）与空间聚合（从0.25°水平分辨率转换为0.5°）得到。空间聚合采用一阶保守重映射方法，且始终在时间聚合之后执行。对于tasmax与tasmin，逐小时tas数据分别聚合为日最高与日最低值；其余变量的逐小时数据则聚合为日均值。对于ERA5中未包含的变量（huss、hurs、sfcWind、orog），首先从逐小时、0.25°分辨率的可用变量中推导得到，再按照其余变量的方式完成聚合。其中，huss与hurs参照Buck（2010）所述的Buck（1981）公式，利用近地面露点温度、ps与tas计算得到；sfcWind根据公式sfcWind = √(uas² + vas²)，由近地面东向风速（uas）与近地面北向风速（vas）计算得到；orog等于地面位势除以重力加速度。最后，海洋区域的pr与prsn经过偏差校正，使得W5E5月降水总量与GPCP V2.3版本的海洋区域降水数据一致，该校正所用的月尺度缩放因子参照Balsamo等（2010）所述的尺度选择性缩放程序计算得到。
