Research about nontraditional Coriolis terms

Zonal temporal dispersion relations of the equatorially confined wave solutions with (black) and without (red) the nontraditional Coriolis terms (NCTs). Except the last frame of the animation, sound of piano is played at a sound frequency proportional to the effective buoyancy frequency used to plot every frame. (From Ong and Roundy 2020)

NCTs represent components of Coriolis force that turn eastward motion upward and upward motion westward, and vice versa.

Motivation

Most of the current global atmospheric models use the hydrostatic approximation to predict atmospheric motion, omitting NCTs and the vertical acceleration term. To justify the hydrostatic approximation, according to a scale analysis, the buoyancy term is much larger than the vertical NCT for midlatitude atmospheric flow, and the vertical NCT is much larger than the vertical acceleration term for atmospheric flow whose width is larger than depth. However, on the pathway toward relaxing the hydrostatic approximation, many models restore the vertical acceleration term but not NCTs. Moreover, for tropical large-scale flow, the buoyancy term may be small enough to consider NCTs. Is there any consequence of these inconsistencies?

Summary

Using idealized models without cloud physics, I found that omitting this effect directly yields a westerly wind bias in the convective rainy region that is about 10% of the westerly jet stream. Also, I found that it directly yields errors in the height of radiosonde-observed pressure levels that is about 5% of the height variability in the tropical atmosphere. Furthermore, using an idealized model with cloud physics, my study suggests that the inclusion of the nontraditional Coriolis effect speeds up eastward moving rainy systems and slows down westward moving ones. The speed change agrees with my theory without cloud physics. This study encourages restoring the nontraditional Coriolis effect to the atmospheric models to improve the accuracy of weather and short-term climate prediction. As a technical side product of this study, I developed a tool to test whether the nontraditional Coriolis effect is correctly included into an atmospheric model. The inclusion of NCTs also enables an atmospheric model to simulate deep (vertical extent of at least 80 km) atmosphere.

Broader Impact

This project shows the weather and climate modeling community that including the nontraditional Coriolis terms (NCTs) into their models can improve weather and climate prediction, which improves our lives. This project is important because it is critical to understand various potential pathways through which the omission of NCTs bias model simulations of tropical weather and climate. This issue is very important to address because tropical atmosphere has a global impact.