Climatology and Seasonal Variations of Temperatures and Gravity Wave Activities in the Mesopause Region Above Ft. Collins, CO (40.6°N, 105.1°W)

Abstract

Based on observations at a polar site (69°N), Lübken and von Zahn (1991) reported a counter-intuitive bistable pattern of mesopause, high in winter (192K at 98 km) and low in summer (129K at 88 km). Though unexpected, Na lidar observations showed that the bistable pattern also exists in a midlatitude station (40.6°N) with winter mesopause at 101 km and summer mesopause at 86 km (She et al., 1993;. The two-level mesopause was further confirmed by shipborne K lidar observations (von Zahn & Höffner, 1996), which enabled observations between 71°S and 54°N from late April to early July 1996. This set of measurements which included southern and northern equatorial regions showed that the summer state does not exist in the equatorial region. Using 2 years of data (1996)(1997) from Fort Collins (40.6°N, 105°W) and Kühlungsborn, Germany (54°N, 12°E), along with older monthly mean data from ALOMAR, Norway (69°N, 16°E), the concept of the two-level mesopause was further elucidated (She & von Zahn, 1998) and idealized in Figure 1 of their paper. They asserted the two-level mesopause structure exists globally in midlatitude and polar regions but not in equatorial regions.In the two-level structure, there exists an altitude of minimum annual variation in temperature near 98 km and an altitude of maximum annual variation in temperature near 86 km. There also exists a high-altitude winter mesopause near 100 km and a low-altitude summer mesopause near 88 ± 3 km with a clear and abrupt jump between them, despite of the fact that unlike at 69°N and 54°N, the sharp summer/winter transitions were not observed at 40.6°N. The less clear summer/winter mesopause transition at 40.6°N is due to the relatively smaller difference between summer and winter mesopause temperatures at a lower-latitude station, which may be over-powered by superimposed day-to-day wave perturbations in a 2-year data set. However, when the true climatology of the temperature structure is revealed, the proposed two-level mesopause structure with sharp winter/summer transitions can be ascertained. One objective of this paper is to use the 20 years (956 nights) of nocturnal temperature observations to investigate the climatology of the two-level mesopause and to reveal the sharp mesopause transitions at midlatitude.

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

“…3 of 17 in the mesopause region above Ft. Collins where Var(T′) ∼ 60 K 2 and T ∼ 200 K (She et al, 2022). By combining (1) and ( 2) the wave-driven constituent flux can also be written as…”

“…The sample temperature and lapse rate fluctuation variances were computed for each night of observations and then fitted at each altitude to a harmonic model that included the annual mean plus the 12-, 6-, 4-, and 3-month oscillations. The fitted temperature variance is plotted in contour format in Figure 4a and the seasonal mean profiles are plotted in Figure 5a in She et al (2022), see Figures S4 and S5 in Supporting Information S1). The uncertainties are derived directly from the harmonic fits to the nightly observations.…”

“…Unfortunately, we are not aware of any measurements of the seasonal variations in α down near CSU and measurements of the seasonal variations of K zz are rare anywhere. However, when such information becomes available, it is straightforward to recompute K H and K Wave using the measured seasonal variations of Var( T ′), Var(∂ T ′/∂ z ), T , and N contained in this paper and in the companion paper (She et al., 2022).…”

“…Note, here we define the seasons as those time periods centered on the respective equinoxes and solstices. A more detailed description of this data set can be found in the CSU temperature and gravity wave trend and climatology papers (She et al, 2019(She et al, , 2022.…”

“…In this paper, we use the multi‐decade observations of the mesopause region temperature structure made by a Na Doppler lidar at Ft. Collins, CO (40.6°N, 105.1°W) to characterize the seasonal variations of the gravity wave‐induced transport of heat and constituents as quantified by K H and K Wave . This extensive data set is unique, consisting of Na lidar observations of the mesopause region temperature profile collected at relatively high vertical and temporal resolutions during the 20‐year period from March 1990 to 2010 (She et al., 2022). The data set includes 956 nights of observations that are roughly evenly distributed over the spring, summer, autumn, and winter seasons, which enables us to accurately characterize the seasonal variations of wave activity and heat and constituent transport at this mid‐latitude site.…”