Seasonal Variations of Gravity Wave Induced Thermal and Constituent Diffusivities in the Mesopause Region Above Ft. Collins, CO (40.6°N, 105.1°W)

Quasi-random vertical displacement fluctuations, caused by the spectrum of non-breaking gravity waves, mix the atmosphere, similar to turbulence, which induces significant vertical transport of heat and constituents in the upper atmosphere. Multi-decade observations of temperature, made between 85 and 100 km with a Na lidar at Colorado State University (CSU, 40.6°N, 105.1°W), are used to derive the seasonal variations of the wave-induced thermal (K_H) and constituent (K_Wave) diffusivities. Both show strong annual oscillations with maxima in winter, which increase with increasing altitude. K_H and K_Wave exhibit summer minima of ∼40 and ∼70 m²s⁻¹, respectively, that are approximately constant with altitude. In winter, K_H varies from ∼50 at 85 to ∼180 m²s⁻¹ at 100 km, while K_Wave varies from ∼110 at 85 to ∼340 m²s⁻¹ at 100 km. These values are much larger than the eddy diffusivity (K_zz ∼ 35 m²s⁻¹) predicted for this site by the Whole Atmosphere Community Climate Model. The CSU diffusivities are comparable to similar measurements made at other mid-latitude mountain sites in both hemispheres, and derived from global observations of atomic O. However, the seasonal variations differ from the O observations, which may reflect differences in wave sources at these sites and the different approaches employed to derive the wave diffusivities. Even so, the CSU results demonstrate that heat and constituent transport by unresolved, non-breaking gravity waves are important processes that need to be incorporated in global chemistry models to properly characterize the thermal and constituent structure of the upper atmosphere.