40 years of continental moisture tracking using WAM-2layers

The WAM-2layers is an Eulerian moisture tracking numerical model. We used the Python version available on the GitHub repository (https://github.com/ruudvdent/WAM2layersPython). For a detailed description of the WAM-2layers, we refer the reader to van der Ent et al. (2013, 2014). Moisture tracking was conducted forward in time, focusing on the fate of evapotranspiration contributing to Terrestrial Moisture Recycling to calculate the amount of precipitation with continental origin. Our tracking experiment spans from 1979 to 2018. The model domain covers a global grid from 79.5ºN to 79.5ºS latitude. Calculations use data of specific humidity, zonal and meridional wind speeds at 24 pressure levels, surface pressure at 6-h intervals, and 3-h accumulated precipitation and evaporation, from the ERA-Interim reanalysis (Dee et al., 2011) on a 1.5º grid with 25,680 cells total (nx=240, ny=107). This set-up is based on previous studies indicating that it is adequate to track moisture spatially, both forward and backward in time and at regional and global scales.WAM-2layers是一套欧拉式水汽追踪数值模型（Eulerian moisture tracking numerical model）。我们使用了GitHub仓库（https://github.com/ruudvdent/WAM2layersPython）中提供的Python版本实现。关于WAM-2layers的详细技术说明，读者可参考van der Ent等（2013、2014）的相关研究。 本次水汽追踪实验采用正向时间追踪的方式，聚焦于贡献至陆地水汽循环（Terrestrial Moisture Recycling）的蒸散发（evapotranspiration）去向，以此计算源自大陆的降水量。本实验的时间跨度为1979年至2018年。 该模型的模拟区域覆盖全球格网，纬度范围介于北纬79.5°至南纬79.5°之间。计算所需的输入数据包括：24个气压层的比湿（specific humidity）、纬向风速（zonal wind speeds）与经向风速（meridional wind speeds），6小时间隔的地表气压（surface pressure），以及3小时累积降水量（accumulated precipitation）与蒸散发量，数据来源于ERA-Interim再分析资料（ERA-Interim reanalysis，Dee等，2011），其格网分辨率为1.5°，总格点数达25680个（nx=240，ny=107）。 本配置方案参考了既往研究成果，相关研究表明该方案可在时空双向、区域及全球尺度下实现可靠的水汽追踪。