August 4, 2022
Research Highlight

How Model Simplifications Affect the Simulated Water Cycle

Relaxing the simplifying assumptions in model energy budgets can affect simulated regional water cycles

Satellite image of clouds over a technicolor version of the Pacific Ocean

The assumptions that go into Earth system models noticeably affect simulated regional water cycles.

(Image: NASA Worldview | NASA)

The Science                                

To represent the complex processes that make up the Earth system, models include numerous simplifying assumptions. A common simplifying assumption, that water carries heat proportional to its mass produces errors in the regional energy conservation. Researchers examined how these errors affected a climate model. They discovered that relaxing this simplifying assumption can have a large effect on simulations of the tropical water cycle.

The Impact

Assessing the sensitivity of models to simplifying assumptions can help guide future development efforts, pointing at areas that could have the greatest impacts. The simulated regional water cycle could be improved by relaxing the simplifications made in the model’s energy formulation, specifically in how moist enthalpy is defined. This could result in more accurate projections of rainfall, which is important for infrastructure planning.


Earth system models often assume that the heat carried by water as it moves between the atmosphere and other components of the Earth system is proportional to the mass of the water. This assumption breaks local energy conservation, making it difficult to interpret results from regional energy budget analyses and limiting our ability to understand future water cycle responses to warming. This study outlines these challenges, their physical basis, and a series of potential pathways forward to better model regional water cycles. Researchers used one of the simpler pathways forward, adding back the enthalpy of water to the atmosphere to force local energy conservation, as an example to assess its potential influence on understanding the water cycle. The Community Atmosphere Model version 6 has substantial water cycle sensitivity to reconciling the heat budget of water for the atmosphere. These findings suggest that future work dedicated to a harder, more physically rigorous treatment of water for use in the Community Atmosphere Model and other similar models is worthwhile.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory,


This study is supported by the U.S. Department of Energy, Office of Science Biological and Environmental Research program as part of the Regional and Global Model Analysis program area. MSP acknowledges co-funding from the National Science Foundation and computational support from the Extreme Science and Engineering Discovery Environment supported by the National Science Foundation. The data were produced using resources of the National Energy Research Scientific Computing Center, a Department of Energy Office of Science User Facility.

Published: August 4, 2022

B.E. Harrop, M.S. Pritchard, H. Parishani, A. Gettelman, S. Hagos, P.H. Lauritzen, L.R. Leung, J. Lu, K.G. Pressel, and K. Sakaguchi,(2022) “Conservation of dry air, water, and energy in CAM and its potential impact on tropical rainfall.” Journal of Climate, 35. [DOI: 10.1175/JCLI-D-21-0512.1].