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Ding-Yi Wang and David C. Fritts

Abstract

An analysis of gravity wave-tidal interaction observed near the mesopause by the MST radar at Poker Flat in July of 1986 is presented. Our observations revealed daily mean wind maxima of ∼60 m s−1 westward and 20 m s−1 southward with daily mean momentum fluxes, contributed by gravity waves with periods less than 1 hour, of ∼4–5 m2 s−2 eastward and ∼1–2 m2 s−2 northward. Considerable hourly height variability was found to exist for both winds and momentum fluxes. A significant modulation of the fluxes by tidal winds was observed, characterized by out-of-phase correlations over a number of heights. Diurnal and semidiurnal winds and momentum fluxes exhibited very similar height variations, with amplitudes of ∼5–30 m s−1 and ∼1–5 m2 s−2, meridional winds leading zonal winds by ∼90°, and maximum fluctuating winds and momentum fluxes at lower levels. The observed variations in the amplitude and phase of winds and momentum fluxes provide some support to the gravity wave-tidal interaction model proposed by Fritts and Vincent and developed in this paper, and largely conform to theoretical expectations and other observations.

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Ding-Yi Wang and David C. Fritts

Abstract

An analysis of the wave motions observed with the Poker Flat MST radar during the winter, summer, and fall of 1986 is presented in this paper. Monthly and daily mean winds, momentum fluxes, and velocity variances are investigated in detail. While several features are in agreement with previous measurements, some significant differences also are found to exist in our observations. Monthly mean horizontal winds between 82 and 89 km have amplitudes of ∼20–40 m s−1 westward and ∼10–25 m s−1 southward in July and August. In fall and winter, the horizontal winds between 58 and 75 km are weaker and essentially eastward. Monthly mean vertical velocities are predominantly downward at ∼0.3 m s−1, which implies a vertical wave energy flux of ∼0.1 W m−2, higher than that estimated at low latitudes. Monthly mean horizontal variances are ∼1600 m2 s−2 in summer, and 600 m2 S−2 in fall and winter with vertical variances ∼3 m2 s−2. Monthly mean zonal and meridional momentum fluxes, contributed predominantly by waves of periods less than 1 hour, are of the order of 6–10 and 4 m2 s−2 in summer, and ∼−1 to −2 and +0.1 m2 s−2 in fall and winter. These momentum fluxes are significantly larger than those observed at lower latitudes, but are comparable with other measurements at high latitudes. Also observed is large daily variability of the velocities, momentum fluxes, and variances, suggestive of a highly active and variable wave environment and a strong influence of gravity waves on the mean mesospheric structures at high latitudes.

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David C. Fritts and Ding-Yi Wang

Abstract

Frequency spectra of horizontal and vertical velocities were inferred from MST radar observations near the summer mesopause at Poker Flat, Alaska. Height variations of the frequency spectra revealed only slight growth of wave amplitudes with increasing height. However, the observed vertical velocity spectra were influenced strongly by Doppler-shifting effects. Whereas horizontal velocity spectra were found to be relatively insensitive to horizontal wind speed, vertical velocity spectra acquired more negative slopes and larger energy densities at lower frequencies as the horizontal wind speed increased.

Observed spectra were compared with several Doppler-shifted model spectra, consisting of separable intrinsic frequency and wavenumber spectra. The observed spectrum of horizontal velocity provides a measure of p, the negative slope of the intrinsic frequency spectrum, while the observed vertical velocity spectrum is more sensitive to the form of the vertical wavenumber spectrum, wavefield anisotropy, and the degree of Doppler shifting. These observations suggest consistency with a gravity wave interpretation of atmospheric motions, an intrinsic frequency spectrum of horizontal velocity with p ∼ 4/3, a vertical wavenumber spectrum with variance concentrated near a characteristic vertical scale, and a high degree of anisotropy with gravity waves propagating predominantly against the mean flow.

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David C. Fritts, Ding-Yi Wang, and Robert C. Blanchard

Abstract

This study presents an analysis of density measurements made using high-resolution accelerometers aboard several space shuttles at altitudes from 60 to 140 km during reentry into the earth's atmosphere. The observed density fluctuations are interpreted in terms of gravity waves and tides and provide evidence of the importance of such motions well into the thermosphere. Height profiles of fractional density variance reveal that wave amplitudes increase at a rate consistent with observations at lower levels up to ∼90 km. The rate of amplitude growth decreases at greater heights, however, and appears to cease above ∼110 km. Wave amplitudes are nevertheless large at these heights and suggest that gravity waves may play an important role in forcing of the lower thermosphere.

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Qiong Wu, Hong-Qing Wang, Yi-Zhou Zhuang, Yin-Jing Lin, Yan Zhang, and Sai-Sai Ding

Abstract

Three infrared (IR) indicators were included in this study: the 10.8-μm brightness temperature (BT10.8), the BT difference between 12.0 and 10.8 μm (BTD12.0–10.8), and the BT difference between 6.7 and 10.8 μm (BTD6.7–10.8). Correlations among these IR indicators were investigated using MTSAT-1R images for summer 2007 over East Asia. Temporal, spatial, and numerical frequency distributions were used to represent the correlations. The results showed that large BTD12.0–10.8 values can be observed in the growth of cumulus congestus and associated with the boundary of different terrain where convection was more likely to generate and develop. The results also showed that numerical correlation between any two IR indicators could be expressed by two-dimensional histograms (HT2D). Because of differences in the tropopause heights and in the temperature and water vapor fields, the shapes of the HT2Ds varied with latitude and the type of underlying surface. After carefully analyzing the correlations among the IR indicators, a conceptual model of the convection life cycle was constructed according to these HT2Ds. A new cloud convection index (CCI) was defined with the combination of BTD12.0–10.8 and BTD6.7–10.8 on the basis of the conceptual model. The preliminary test results demonstrated that CCI could effectively identify convective clouds. CCI value and its time trend could reflect the growth or decline of convective clouds.

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Greg M. McFarquhar, Christopher S. Bretherton, Roger Marchand, Alain Protat, Paul J. DeMott, Simon P. Alexander, Greg C. Roberts, Cynthia H. Twohy, Darin Toohey, Steve Siems, Yi Huang, Robert Wood, Robert M. Rauber, Sonia Lasher-Trapp, Jorgen Jensen, Jeffrey L. Stith, Jay Mace, Junshik Um, Emma Järvinen, Martin Schnaiter, Andrew Gettelman, Kevin J. Sanchez, Christina S. McCluskey, Lynn M. Russell, Isabel L. McCoy, Rachel L. Atlas, Charles G. Bardeen, Kathryn A. Moore, Thomas C. J. Hill, Ruhi S. Humphries, Melita D. Keywood, Zoran Ristovski, Luke Cravigan, Robyn Schofield, Chris Fairall, Marc D. Mallet, Sonia M. Kreidenweis, Bryan Rainwater, John D’Alessandro, Yang Wang, Wei Wu, Georges Saliba, Ezra J. T. Levin, Saisai Ding, Francisco Lang, Son C. H. Truong, Cory Wolff, Julie Haggerty, Mike J. Harvey, Andrew R. Klekociuk, and Adrian McDonald

Abstract

Weather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation, and radiative processes, and their interactions. Projects between 2016 and 2018 used in situ probes, radar, lidar, and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN), and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF–NCAR G-V aircraft flying north–south gradients south of Tasmania, at Macquarie Island, and on the R/V Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons. Results show largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multilayered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of dynamics and turbulence that likely drive heterogeneity of cloud phase. Satellite retrievals confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.

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