Estimates of the Global Water Budget and Its Annual Cycle Using Observational and Model Data

TLDR

The authors review NCAR Climate Analysis Section research on the water cycle to set the stage for estimating the global hydrological cycle. They compare three precipitation datasets, compute monthly estimates of precipitation, evapotranspiration, and atmospheric moisture convergence using the Community Land Model with realistic forcings, and contrast E–P with ECMWF ERA‑40 budgets to assess uncertainties. The analysis yields a new estimate of the global water budget for long‑term annual means, presents the mean annual cycle from 1979–2000, and shows that ERA‑40 data produce physically unrealistic evaporation‑precipitation balances, especially in the tropics and subtropics.

Abstract

Abstract A brief review is given of research in the Climate Analysis Section at NCAR on the water cycle. Results are used to provide a new estimate of the global hydrological cycle for long-term annual means that includes estimates of the main reservoirs of water as well as the flows of water among them. For precipitation P over land a comparison among three datasets enables uncertainties to be estimated. In addition, results are presented for the mean annual cycle of the atmospheric hydrological cycle based on 1979–2000 data. These include monthly estimates of P, evapotranspiration E, atmospheric moisture convergence over land, and changes in atmospheric storage, for the major continental landmasses, zonal means over land, hemispheric land means, and global land means. The evapotranspiration is computed from the Community Land Model run with realistic atmospheric forcings, including precipitation that is constrained by observations for monthly means but with high-frequency information taken from atmospheric reanalyses. Results for E − P are contrasted with those from atmospheric moisture budgets based on 40-yr ECMWF Re-Analysis (ERA-40) data. The latter show physically unrealistic results, because evaporation often exceeds precipitation over land, especially in the Tropics and subtropics.