WRF with age-weighted water tracers: implementation, application, and new insights into the regionally accelerated atmospheric hydrological cycle under global warming

Atmospheric water residence time, here defined as time between the original evaporation and the returning of its respective water masses to the land surface as precipitation, is a measure of the speed of the atmospheric hydrological cycle. Traditional analytical methods are generally limited by crude assumptions in the coupling between the land surface and the atmosphere, and hence are not applicable to regions with complex monsoon systems under a changing climate. To this end, we have implemented the age-weighted water tracers into the Weather Research and Forecasting WRF model, namely, WRF-age, to follow the atmospheric water pathways and to derive atmospheric water residence times accordingly. The newly developed, physics-based WRF-age is used to regionally downscale the reanalysis of ERA-Interim and the MPI-ESM Representative Concentration Pathway 8.5 scenario (RCP8.5) simulation for an East Asian monsoon region, i.e., the Poyang Lake basin, for two 10-year slices of historical (1980-1989) and future (2040-2049) times. In comparison to the historical WRF-age simulation, the future 2-meter air temperature rises by 1.3 °C and precipitation decreases by 38% under RCP8.5 on average. In this context, global warming leads to decreased atmospheric residence times of the column-integrated water vapor (from 22 to 13 hours) and column-integrated condensed moisture (from 26 to 14 hours) in the atmosphere over the basin, but slightly increased atmospheric residence times of surface precipitation (from 12 to 15 hours) in agreement with reduced the precipitation amounts. Our findings demonstrate that global warming increases the complexity of regional atmospheric water cycle, especially the associated changes in the residence times of atmospheric water states of matter.