Evaluation of the mesospheric polar vortices in WACCM

This work evaluates the wintertime mesospheric polar vortices in the Whole Atmosphere Community Climate Model. Mesospheric altitudes are where high-top models underestimate the concentration of nitrogen oxides produced by energetic particle precipitation. For this reason, we seek to determine the extent to which the observed mesospheric vortex size and frequency of occurrence is reproduced by the model. We compare 13 years (2005-2017) of "specified dynamics" Whole Atmosphere Community Climate Model (where meteorology in the troposphere and stratosphere is constrained by observations) to Sounding of the Atmosphere using Broadband Emission Radiometry and Mesospheric Limb Sounder observations. Near 75 km, the model reproduces observed winter mesospheric vortex frequency of occurrence rates and size reasonably well. The simulated Antarctic mesospheric vortex is displaced toward the Pacific longitude sector, and the Arctic mesospheric vortex is present 10-20% less often over the pole but similar to 5% more often in midlatitudes, particularly over the continents. These differences are attributed to larger planetary waves in the model. There are significant differences between the modeled and observed mean polar winter circulation near 90 km. Model-measurement differences suggest that the simulated Antarctic mesospheric vortex does not extend to high enough altitudes, and the Arctic mesospheric vortex is too displaced from the pole. Despite these differences, the model accurately captures the evolution of structural changes to the mesospheric vortex following prolonged sudden stratospheric warmings. Thus, we conclude that model underestimates of nitrogen oxides produced by energetic particle precipitation after these warmings are not due to a misrepresentation of the mesospheric vortex strength and size below 80 km.