Showing papers by "David C. Fritts published in 1990"

Studies of Velocity Fluctuations in the Lower Atmosphere Using the MU Radar. Part II: Momentum Fluxes and Energy Densities

Abstract: This paper describes a study of the frequency spectra, the vertical profiles of energy density, and the momentum flux of the motion field observed during a six-day campaign in March 1986 using the MU Radar in Shigaraki, Japan. Our results reveal significant differences between the mean zonal and meridional frequency spectra as well as different profiles of mean energy density with height for different frequency bands and for zonal and meridional components. The vertically averaged momentum flux exhibited considerable temporal variability and good consistency between adjacent beam pairs. A mean vertically averaged flux toward 267° was inferred, suggesting an essentially zonal drag in regions of wave dissipation. A mean frequency spectrum and height profiles of momentum flux in four frequency bands revealed a westward flux at all frequencies. Ratios of the differenced to the mean variance of radial velocity in each vertical plane suggested an ∼20%–30% excess of westward over eastward propagating wa...

Mesospheric Momentum Fluxes Observed by the MST Radar at Poker Flat, Alaska

Abstract: An analysis of the wave motions observed with the Poker Flat MST radar during the winter, summer, and fall of 1986 is presented. 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 the observations. Monthly mean horizontal winds between 82 and 89 km have amplitudes of 20-40 m/s westward and 10-25 m/s southward in July and August. In fall and winter, the horizontal winds between 58 and 75 km are weaker and essentially eastward.

Further Study of Terrain Effects on the Mesoscale Spectrum of Atmospheric Motions

Abstract: Wind and temperature data collected on commercial airliners are used to investigate the effects of underlying terrain on mesoscale variability. These results expand upon those of Nastrom et al., by including all available data from the Global Atmospheric Sampling Program (GASP) and by more closely focusing on the coupling of variance with the roughness of the underlying terrain over mountainous regions. The earlier results, showing that variances are larger over mountains than over oceans or plains, with greatest increases at wavelengths below about 80 km, are confirmed. Statistical tests are used to confirm that these differences are highly significant. Over mountainous regions the roughness of the underlying terrain was parameterized from topographic data and it was found that variances are highly correlated with roughness and, in the troposphere, with background windspeed. Average variances over the roughest terrain areas range up to about ten times larger than those over the oceans. These results are found to follow the scaling with stability predicted in the framework of linenar gravity wave theory. The implications of these results for vertical transports of momentum and energy, assuming they are due to gravity waves and considering the effects of intermittency and anisotroy, are also discussed.

Studies of Velocity Fluctuations in the Lower Atmosphere Using the MU Radar. Part 1. Azimuthal Anisotropy

Abstract: We present in this paper a study of the azimuthal anisotropy of the motion field observed during a six-day campaign in March 1986 using the MU radar in Shigaraki, Japan. The radial wind velocity was observed at 20° zenith angle, at every 30° of azimuth during four days, and at every 45° during two days. A jet stream was present during the entire six days. The average radial velocity variance from 10.4 to 19.2 km was calculated every four minutes and then averaged over 20 minutes or one hour. The average variance was found to be a strong function of both azimuth and time. The azimuthal variations were analyzed in terms of the mean and the first and second harmonics. The mean is proportional to the kinetic energy per unit mass of the radial wind fluctuations, and the first harmonic is proportional to the vertical flux of horizontal momentum per unit mass. The strong azimuthal variation was usually dominated by the second harmonic; i.e., with two peaks, but was occasionally dominated by the first ha...

Studies of velocity fluctuations in the lower atmosphere using the MU radar. I - Azimuthal anisotropy. II - Momentum fluxes and energy densities

Abstract: Results are presented from a six-day campaign to observe velocity fluctuations in the lower atmosphere using the MU radar (Fukao et al, 1985) in Shigaraki, Japan in March, 1986 Consideration is given to the azimuthal anisotropy, the frequency spectra, the vertical profiles of energy density, and the momentum flux of the motion field It is found that all of the observed azimuthal variations are probably caused by a gravity wave field whose parameters vary with time The results show significant differences between the mean zonal and meridional frequency spectra and different profiles of mean energy density with height for different frequency bands and for zonal and meridional components

Gravity Waves in the Middle Atmosphere of the Southern Hemisphere

Abstract: Gravity waves in the southern hemisphere exhibit many of the same characteristics as those in the northern hemisphere in terms of their scales, amplitude growth with height, and influences on the lower and middle atmosphere. Yet there are likely to be significant differences in the wave spectra and their effects as well, due to differing source distributions and strengths in the two hemispheres. Presented here will be a brief review of our current understanding of gravity wave saturation processes and effects, and of the evidence for and likely sources of wave variability, in both hemispheres. We will then examine some of the evidence for hemispheric differences in gravity wave sources, energies, drag, and induced diffusion.