Land Surface Processes Create Patterns in Atmospheric Residence Time of Water

Recent studies determined the residence time of moisture in the atmosphere to be 8–10 days but with large spatial and temporal differences. An unexplained daily cycle in the probability density function (PDF) of the residence time of land evaporation was observed, which was not present for oceanic evaporation. Moreover, the PDF of atmospheric residence time of oceanic evaporation was found to be a monotonically decreasing, while for land evaporation, this function had increasing probabilities during the first few days and monotonically decreasing probabilities thereafter. This research determines the causes of (I) the daily cycle in this PDF and (II) the shape of the atmospheric residence time PDF. The strong daily cycle in the residence time PDF using ERA-Interim is attributed to the fact that the evaporation and precipitation have the same diurnal cycle in ERA-Interim. Therefore, evaporation entering the atmosphere has the highest probability of returning to the land surface as precipitation at the same moment during the day but possibly a number of days later. Interestingly, this diurnal cycle was almost absent in the simulations forced with GLDAS surface fluxes. Therefore, we conclude that the previously found daily cycle in the atmospheric residence time PDF is due to differences in the diurnal cycles of precipitation in the underlying data sets. Transpiration causes the increasing probabilities during the first days, while bare soil evaporation and canopy interception typically have a monotonically decreasing PDF. Therefore, we conclude that transpiration causes the largest differences in atmospheric residence time between ocean and land evaporation.