Exploring the Effects of Nonhydrostatic Formulation of Atmospheric Motions in High-Resolution All-Ocean Simulations
Researchers found that in a humid environment such as the tropics, moist processes can drastically increase the differences between simulations using two formulations of atmospheric motions.
Global climate models typically use a hydrostatic formulation that assumes a much larger horizontal than vertical scale of motion to determine the atmospheric motions. Theory suggests that when the grid size of an atmospheric model is smaller than approximately 10 km, the hydrostatic formulation is no longer valid as the model resolves stronger vertical motion. Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory found that in a humid environment, moist processes can push the hydrostatic limit to a larger value. This finding suggests the need for a nonhydrostatic formulation even for models at 12 km to 36 km resolution.
Most global climate models use the hydrostatic formulation to represent the dynamical state (motions) of the atmosphere. This is generally valid in coarse-resolution models because the vertical air movement is negligible when averaging over a model grid cell exceeding 10 km x 10 km in size. The finding that the hydrostatic formulation might not be valid at 12 km to 36 km resolution cautions the use of climate models with a hydrostatic formulation at high resolution even if it is computationally feasible.
This study explored the impact of a nonhydrostatic dynamical formulation in high-resolution regional climate simulations using an aquaplanet (all-ocean) configuration. Researchers conducted simulations with hydrostatic and nonhydrostatic formulations at horizontal grid spacings of 36 km, 12 km, and 4 km using the Weather Research and Forecasting model. Researchers found notable differences between the hydrostatic and nonhydrostatic simulated precipitation even at horizontal grid spacings of 12 km in the tropical rain belt. At horizontal grid spacings of 12 km and 4 km over the tropics, the precipitation difference was sensitive to whether a cumulus representation scheme was used. Scientists found that, with a land mountain of an idealized shape in the tropics, differences in the precipitation and vertical velocity between the hydrostatic and nonhydrostatic simulations were significant even at horizontal grid spacings of 36 km due to the moist processes. This study found that latent heat released from water vapor condensation to clouds intensified the differences between the nonhydrostatic and hydrostatic simulations over the mountain and downstream in the tropics. Overall, differences between the hydrostatic and nonhydrostatic simulations existed even at resolutions between 12 km and 36 km, and the differences were sensitive to the representations of moist physics. Future studies are needed to compare simulations with hydrostatic and nonhydrostatic solvers in climate models at resolution between 12 km and 36 km in more realistic configurations.
Pacific Northwest National Laboratory
- Regional & Global Climate Modeling
- Water Cycle and Climate Extremes Modeling