Showing papers by "David C. Fritts published in 1992"

Sources of Mesoscale Variability of Gravity Waves. Part II: Frontal, Convective, and Jet Stream Excitation

Abstract: We present studies of four cases of mesoscale variance enhancements of horizontal velocity and temperature due to frontal activity, nonfrontal convection, and wind shear. These data were obtained aboard commercial aircraft during the Global Atmospheric Sampling Program (GASP) in 1978 and 1979 and from the corresponding meteorological analyses and satellite imagery. Additional GASP data were used to permit a statistical assessment of the importance of various sources of enhanced variances. Our results, and those in a companion paper addressing the variance enhancements associated with topography, represent refinements of previous source analyses using the GASP dataset. Significant findings include mean variance enhancements of velocity and temperature due to convection and jet-stream flow ranging from ∼2 to 8 for 64-km and 256-km data segments, and enhancements for individual segments as high as ∼20 to 100. The mean 64-km variance enhancement for all variables and source types, relative to a quies...

Sources of Mesoscale Variability of Gravity Waves. Part I: Topographic Excitation

Abstract: Aircraft measurements of winds and temperatures collected during the GASP program are used to study the effects of topography as a source of mesoscale variability. Variances of fluctuations at the mesoscale over rough terrain are enhanced up to nearly two orders of magnitude compared to nonsource regions in some cases and are frequently enhanced by an order of magnitude. The implications of these episodic enhancements of variances for the vertical transports of energy and momentum are considered in the framework of gravity wave theory. The observed flight data are used to estimate the momentum flux u′w′ on several flight segments. Results show that the flux is generally negative with mean value −0.26 m2 s−2 and with magnitudes ranging up to −1.5 m2 s−2. Spectral analysis shows that the largest contributions to the net flux come from horizontal scales of ∼25 < λx <60 km. Finally, the observed momentum fluxes are used to infer the anisotropy factor of gravity waves over rough terrain, which is foun...

Gravity Wave Excitation by Geostrophic Adjustment of the Jet Stream. Part I: Two-Dimensional Forcing

Abstract: A previous study of inertio–gravity wave motions radiating from a two-dimensional ageostrophic Gaussian jet is extended here to a three-dimensional jet source. Fourier integral solutions of the initial-value problem are obtained in analytic form to facilitate comparison with the previous two-dimensional results. For an initial disturbance elongated along the jet axis, the wave solutions near the midpoint are nearly indistinguishable from those obtained in two dimensions and approach those solutions as the jet increases in length. At locations not symmetric with respect to the longitudinal jet axis, inertio–gravity wave structure departs increasingly from the two-dimensional results. In such cases, the early response is determined by the nearby jet structure and exhibits propagation primarily normal to the jet axis. At later times, however, the response is due to the initial disturbance at other locations along the jet axis and reveals a tendency for propagation parallel to the jet. The mean motio...

Sources of mesoscale variability of gravity waves. I - Topographic excitation. II - Frontal, convective, and jet stream excitation

Abstract: The effect of topography as a source of mesoscale variability was investigated using aircraft measurements of winds and temperature collected during the Global Atmospheric Sampling Program, with results showing marked increases in the variance of zonal and meridional wind speeds and of potential temperature over rough terrain. In addition, four cases of mesoscale variance enhancements of horizontal velocity and temperature due to frontal activity, nonfrontal convection, and wind shear were studied. The implications of these episodic enhancements of variances for the vertical transports of energy and momentum are considered in the framework of the gravity wave theory.

Dynamics of the Equatorial Mesosphere Observed Using the Jicamarca MST Radar during June and August 1987.

Abstract: The Jicamarca MST radar was used in two campaigns during June and August 1987 to measure wave influences, flow variability, and mean structure in the equatorial stratosphere and mesosphere. This paper presents observations of motions and momentum fluxes in the mesosphere during each campaign. A companion paper by Hitchman et al. addresses the mean structure and fluxes as well as comparisons with other datasets. Results presented here indicate that the equatorial mesosphere is dynamically very active, with considerable gravity-wave and tidal motions and persuasive evidence of inertial instability and wave-filtering processes. Vertical velocities at high frequencies are comparable to those observed at other locations. Hourly mean horizontal motions and momentum fluxes are likewise large and variable, exhibiting enormous vertical shears and strong modulation of the wave spectrum and momentum fluxes at higher frequencies. Daily mean profiles reveal persistent structures with vertical scales of ∼6–10 ...

First observations of mesospheric dynamics with a partial reflection radar in Hawaii (22°N, 160°W)

Abstract: A partial-reflection MF radar was installed at the Pacific Missile Range Facility on Kanai, Hawaii in September 1990. The wind measurements obtained with this radar system in Hawaii during the first year of operation are reviewed. The data reveal a broad spectrum of motions ranging from a mix of the equatorial mesopause semiannual oscillation (MSAO) and the annual mean cycle at middle and high latitudes in the zonal mean structure to low-frequency, tidal, and gravity wave motions at higher frequencies. The zonal mean wind structure is characterized by a downward progression of strong eastward and westward phases of the MSAO from approximately January to July. An eastward maximum of about 60/ms near 80 km during January and February is found, which descends rapidly, and a westward maximum of about 50 ms near 85 km during March and April is found, which descends much more slowly. The second MSAO cycle is greatly suppressed relative to the first due to the reversal of the correlation between this and the annual cycle at higher latitudes from July to December.

Mean winds and momentum fluxes over Jicamarca, Peru, during June and August 1987

Abstract: Data from the mesosphere-stratosphere-troposphere radar at Jicamarca, Peru, together with other available data, are used to diagnose the mean structure of winds and gravity-wave momentum fluxes from the surface to 90 km during two-ten-day campaigns in June and August of 1987. In the stratosphere a layer of maximum eastward flow associated with the quasi-biennial oscillation (QBO) was seen to strengthen and descend rapidly from June to August, overlying persistent westward flow. A layer of enhanced signal return, suggestive of a turbulent layer, was observed just above the descending QBO eastward maximum. Notable zonal asymmetries were present during this transition and the local meridional circulation departed from zonal-mean QBO theory. A substantial northeastward momentum flux was found below 25 km, which may be related to topographic gravity waves excited by southeastward flow across the Andes. In the lower mesosphere a relatively weak 'second' mesopause semiannual oscillation is confirmed. In both the lower stratosphere and mesosphere, body forces were comparable in magnitude to inferred Coriolis torques.

Wave-wave interactions in a compressible atmosphere: 1. A general formulation including rotation and wind shear

Abstract: We present here a formulation of the resonant and nonresonant interactions among gravity waves, acoustic waves, and vortical modes in the atmosphere under the weak interaction approximation. This work generalizes previous studies by including the effects of rotation and wind shear. Rotation influences both wave and vortical mode structures, while wind shear is shown to influence the relative frequencies of gravity waves and vortical modes and thus may alter the conditions for gravity wave-vortical mode interactions occurring in its absence. The theory is applied in the absence of rotation and is shown to permit interactions that depend in a complex manner on the gravity wave and vortical mode characteristics and relative orientation and on the mean wind shear. In a companion paper we apply this formalism in a study of the influences of variable stratification and inhomogeneity on resonant and nonresonant interactions.