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Abstract The Southwestern North American megadrought began in 2000 and is now believed to be the driest 22‐year period in the region since 800 CE. The precipitation deficit during the megadrought (8.3% during 2000–2021) has been accompanied by a significant decrease in gravity waves observed in the upper atmosphere. Prior to the drought (1990–2000), the mean wave‐driven temperature fluctuation variances, between 85 and 100 km at Albuquerque and Ft. Collins, were comparable (62.2 ± 5.3 K²and 60.5 ± 1.8 K², respectively), with the largest variances occurring during winter and summer storm seasons. During the first decade of the drought (2001–2010), wave activity above Ft. Collins decreased by 28 ± 3%, mostly above 94 km, and changed from primarily semiannual to primarily annual variations. These changes may be related to reduced wave generation by tropospheric storms during the megadrought and to an altered geographic distribution of precipitation events in the western and mid‐western United States. 

Abstract 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_HandK_Waveexhibit summer minima of ∼40 and ∼70 m²s⁻¹, respectively, that are approximately constant with altitude. In winter,K_Hvaries from ∼50 at 85 to ∼180 m²s⁻¹at 100 km, whileK_Wavevaries from ∼110 at 85 to ∼340 m²s⁻¹at 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. 

Abstract Utilizing 956 nights of Na lidar nocturnal mesopause region temperature profiles acquired at Fort Collins, CO (40.6°N, 105.1°W) over a 20‐year period (March 1990–2010), we deduce background nightly mean temperatureand the square of the buoyancy frequencyN²(z) at 2‐km resolution between 83 and 105 km. The temperature climatology reveals the two‐level mesopause structure with clarity and sharp mesopause transitions, resulting in 102 days of summer from Days 121 to 222 of the year. The same data set analyzed at 10‐min and 1‐km resolution gives the gravity wave (GW) temperature perturbationsT_i'(z) and the wave varianceVar(T′(z)) and GW potential energyE_pm(z) between 85 and 100 km. Seasonal averages of GWVar(T′(z)) andE_pm(z) between 90 and 100 km, show thatVar(T′) for spring and autumn are comparable and lower than for summer and winter. Due mainly to the higher background stability, or largerN²(z) in summer,E_pm(z) between 85 and 100 km is comparable in spring, summer, and autumn seasons, but ∼30%–45% smaller than the winter values at the same altitude. The uncertainties are about 4% for winter and about 5% for the other three seasons. The values forE_pmare (156.0, 176.2, 145.6, and 186.2 J/kg) at 85 km for (spring, summer, autumn, and winter) respectively, (125.4, 120.2, 115.2, and 168.7 J/kg) at 93 km, and (207.5, 180.5, 213.1, and 278.6 J/kg) at 100 km. Going up in altitude, all profiles first decrease and then increase, suggesting that climatologically, GWs break below 85 km. 

Abstract We present midlatitude solar response and linear trend from Colorado State University/Utah State University Na lidar nocturnal temperature observations between 1990 and 2017. Along with the nightly mean temperatures (_Ngt), we also use the corresponding 2‐hr means centered at midnight (_2MN), resulting in vertical trend profiles similar in shapes as those previously published. The 28‐year trend from _Ngt (_2MN) data set starts from a small warming at 85 km, to cooling at 87 (88) km, reaching a maximum of 1.85 ± 0.53 (1.09 ± 0.74) at 92 (93) km and turns positive again at 102 (100) km. The 6‐month winter trend is much cooler than the 4‐month summer trend with comparable solar response varying around 5 ± 1 K/100 SFU throughout the profile (85–105 km) with higher summer values. We explore the observed summer/winter trend difference in terms of observed gravity wave heat flux heating rate at a nearby station and the long‐term trend of gravity wave variance at a midlatitude. Between 89 and 100 km, the lidar trends are within the error bars of the Leibniz Middle Atmosphere (LIMA) summer trends (1979–2013), which are nearly identical to the lidar‐Ngt trend. We address the need of long data set for reliable analysis on trend, the extent of trend uncertainty due to possible tidal bias, the effect of a Pinatubo/episodic function, and the impact of stratospheric ozone recovery.