Finding the Leaks That Could Sink a Great Ship
For computer models that simulate the evolution of Earth’s climate, conserving the total amount of atmospheric water is an important feature. Even small inaccuracies in the water budget can lead to sizable errors in projecting sea level change in century-long simulations. A study led by researchers at the U.S. Department of Energy’s (DOE) Pacific Northwest National Laboratory quantified and reduced various sources of water conservation errors in the atmosphere component of DOE’s Energy Exascale Earth System Model (E3SM).
Reduction of systematic water conservation errors in atmosphere models is important for accurately simulating components of Earth’s water cycle, such as global mean precipitation and sea level. In an earlier version of the E3SM Atmosphere Model (EAM), which is E3SM’s atmosphere component, water conservation contained errors that could have produced spurious sea level changes comparable to those observed in the 20th century. After researchers identified and addressed several sources of water conservation errors in EAM, such errors became negligible and insensitive to model resolution in EAM version 1, which improved the overall water conservation property of E3SM.
Global Earth system simulations require massive numerical calculations. Because of limited computing resources to perform these detailed simulations, models use mathematical approximations to improve computational efficiency. Although the inaccuracies associated with such approximations can seem small in short tests, they can add up to significant amounts in longer simulations. For example, in a prototype version of E3SM, researchers diagnosed a spurious source of atmospheric water arising from approximations. If all of that extra water had condensed and rained out to the surface, it would lead to an artificial sea level rise of more than 10 centimeters (cm) per simulated century in the low-resolution model, and over 30 cm per century at higher resolutions. The actual sea level rise in the 20th century was estimated to be 17−20 cm, so these offsets, or biases, were significant.
In this study, researchers identified, quantified, and corrected several sources of water conservation error within EAM. They found the largest error sources in the equations for specific interactions between different physical phenomena in the Earth system. After correction, the artifact affecting sea level rise was negligible (less than 0.002 cm per century). This makes the updated version of E3SM a much more accurate tool for predictions related to Earth’s water cycle.