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  1. 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.

     
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  2. 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 perturbationsTi'(z) and the wave varianceVar(T′(z)) and GW potential energyEpm(z) between 85 and 100 km. Seasonal averages of GWVar(T′(z)) andEpm(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,Epm(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 forEpmare (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.

     
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  3. Abstract

    We report the first lidar observations of vertical fluxes of sensible heat and meteoric Na from 78 to 110 km in late May 2020 at McMurdo, Antarctica. The measurements include contributions from the complete temporal spectrum of gravity waves and demonstrate that wave‐induced vertical transport associated with atmospheric mixing by non‐breaking gravity waves, Stokes drift imparted by the wave spectrum, and perturbed chemistry of reactive species, can make significant contributions to constituent and heat transport in the mesosphere and lower thermosphere (MLT). The measured sensible heat and Na fluxes exhibit downward peaks at 84 km (−3.0 Kms−1and −5.5 × 104 cm−2s−1) that are ∼4 km lower than the flux peak altitudes observed at midlatitudes. This is likely caused by the strong downwelling over McMurdo in late May. The Na flux magnitude is double the maximum at midlatitudes, which we believe is related to strong persistent gravity waves in the MLT at McMurdo. To achieve good agreement between the measured Na flux and theory, it was necessary to infer that a large fraction of gravity wave energy was propagating downward, especially between 80 and 95 km where the Na flux and wave dissipation were largest. These downward propagating waves are likely secondary waves generated in‐situ by the dissipation of primary waves that originate from lower altitudes. The sensible heat flux transitions from downward below 90 km to upward from 97 to 106 km. The observations are explained with the fully compressible solutions for polarization relations of primary and secondary gravity waves withλz > 10 km.

     
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  4. 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 (KH) and constituent (KWave) diffusivities. Both show strong annual oscillations with maxima in winter, which increase with increasing altitude.KHandKWaveexhibit summer minima of ∼40 and ∼70 m2s−1, respectively, that are approximately constant with altitude. In winter,KHvaries from ∼50 at 85 to ∼180 m2s−1at 100 km, whileKWavevaries from ∼110 at 85 to ∼340 m2s−1at 100 km. These values are much larger than the eddy diffusivity (Kzz∼ 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.

     
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  5. Abstract

    Random‐noise‐induced biases are inherent issues to the accurate derivation of second‐order statistical parameters (e.g., variances, fluxes, energy densities, and power spectra) from lidar and radar measurements. We demonstrate here for the first time an altitude‐interleaved method for eliminating such biases, following the original proposals by Gardner and Chu (2020,https://doi.org/10.1364/ao.400375) who demonstrated a time‐interleaved method. Interleaving in altitude bins provides two statistically independent samples over the same time period and nearly the same altitude range, thus enabling the replacement of variances that include the noise‐induced biases with covariances that are intrinsically free of such biases. Comparing the interleaved method with previous variance subtraction (VS) and spectral proportion (SP) methods using gravity wave potential energy density calculated from Antarctic lidar data and from a forward model, this study finds the accuracy and precision of each method differing in various conditions, each with its own strengths and weakness. VS performs well in high‐SNR, yet its accuracy fails at lower‐SNR as it often yields negative values. SP is accurate and precise under high‐SNR, remaining accurate in worse conditions than VS would, yet develops a positive bias under low‐SNR. The interleaved method is accurate in all SNRs but requires a large number of samples to drive random‐noise terms in covariances toward zero and to compensate for the reduced precision due to the splitting of return signals. Therefore, selecting the proper bias removal/elimination method for actual signal and sample conditions is crucial in utilizing lidar/radar data, as neglecting this can conceal trends or overstate atmospheric variability.

     
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  6. Abstract

    We report the first lidar observations of regular occurrence of mid‐latitude thermosphere‐ionosphere Na (TINa) layers over Boulder (40.13°N, 105.24°W), Colorado. Detection of tenuous Na layers (∼0.1–1 cm−3from 150 to 130 km) was enabled by high‐sensitivity Na Doppler lidar. TINa layers occur regularly in various months and years, descending from ∼125 km after dusk and from ∼150 km before dawn. The downward‐progression phase speeds are ∼3 m/s above 120 km and ∼1 m/s below 115 km, consistent with semidiurnal tidal phase speeds. One or more layers sometimes occur across local midnight. Elevated volume mixing ratios above the turning point (∼105–110 km) of Na density slope suggest in situ production of the dawn/dusk layers via neutralization of converged Na+layers. Vertical drift velocity of TINa+calculated with the Ionospheric Connection Explorer Hough Mode Extension tidal winds shows convergent ion flow phases aligned well with TINa, supporting this formation hypothesis.

     
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  7. The precision of lidar measurements is limited by noise associated with the optical detection process. Photon noise also introduces biases in the second-order statistics of the data, such as the variances and fluxes of the measured temperature, wind, and species variations, and establishes noise floors in the computed fluctuation spectra. When the signal-to-noise ratio is low, these biases and noise floors can completely obscure the atmospheric processes being observed. We describe a novel data processing technique for eliminating the biases and noise floors. The technique involves acquiring two statistically independent datasets, covering the same altitude range and time period, from which the various second-order statistics are computed. The efficacy of the technique is demonstrated using Na Doppler lidar observations of temperature in the upper mesosphere and lower thermosphere acquired recently at McMurdo Station, Antarctica. The results show that this new technique enables observations of key atmospheric parameters in regions where the signal-to-noise ratio is far too low to apply conventional processing approaches.

     
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