The Path Toward Vertical Grid Options for the Community Atmosphere Model Version 7: The Impact of Vertical Resolution on the QBO and Tropical Waves

The Community Earth System Model currently contains two primary atmospheric configurations: the Community Atmosphere Model 6 (CAM6, 32 levels, $\sim$40-km top); and the Whole Atmosphere Community Climate Model 6 (WACCM6, 70 levels, $\sim$140-km top). For CAM7, a number of factors motivate a raising of the model top and enhancement of the vertical resolution and this study documents the decision making process toward this next generation vertical grid. As vertical resolution in the troposphere/lower stratosphere is increased, the role of the resolved waves in driving the Quasi-Biennial Oscillation (QBO) is enhanced, becoming more similar in magnitude to ERA5 reanalysis. This can be traced to improved equatorial Kelvin waves and their vertical momentum fluxes. It is further shown that a model lid at $\sim$80-km does not have detrimental impacts on the representation of the QBO compared to a 140-km top. Based on this analysis, the vertical grid for CAM7 will have an $\sim$80-km top with 93 levels, 500-m grid spacing in the troposphere and lower stratosphere, and 10 additional levels in the boundary layer compared to CAM6. A 58-level/$\sim$40-km low-top option will also be available. We further introduce new coupled simulations using CAM6 but with CAM7's vertical grid above the boundary layer and use these to demonstrate that basic features of the stratospheric circulation are similar to WACCM6, despite the lower model top. These simulations further show that despite the higher fidelity of the QBO, the observed connection between the QBO and the Madden-Julian Oscillation is absent.