William H. Hooke

Ionospheric Irregularities Produced by Internal Atmospheric Gravity Waves

Abstract A perturbation treatment is used to determine the nature and magnitude of the effects of internal atmospheric gravity waves on the ambient rates of production, chemical loss, and motion of the ionization. The relative and absolute importance of these effects in the creation of ionospheric irregularities are assessed. This assessment yields several conclusions of particular interest. Firstly, in the F 2-region the dominant effect of the gravity waves is that of imparting the motion of the neutral gas parallel to the magnetic field to the ionization through collisional interaction. Secondly, at heights at or below the height of the F1 -ledge, chemical effects, in particular the effect of gravity waves on the rate of photoionization, are quite important. Thirdly, gravity waves affect the rate of photoionization at a given point by changing both the neutral gas number density and the ionizing radiation flux at that point, and this latter effect, hitherto ignored, is in some respects the more important of the two. Fourthly, as a result of the interplay of a number of factors, certain Fourier components of that portion of the gravity-wave spectrum permitted at ionospheric heights are more successful than others in creating observable disturbances. Finally, gravity waves creating neutral gas velocities of the order of 20 m sec −1 seem capable under the right conditions of creating TIDs of the largest magnitudes observed.

Estimating the Depth of the Daytime Convective Boundary Layer

Abstract Three in-situ and five remote sensing techniques for measuring the height of the daytime convective boundary layer were compared. There was, as a rule, good agreement between the different systems when the capping inversion was steep and well defined, and some variability when the stratification was not so sharply defined. Two indirect methods for estimating boundary-layer heights from the length scales of convective motions in the layer are also discussed.

Observed generation of an atmospheric gravity wave by shear instability in the mean flow of the planetary boundary layer

Observations of a single boundary-layer event — the generation of an atmospheric gravity wave by an unstable shear flow at Haswell, Colorado on November 12, 1971 — are briefly described and discussed. The observations were made using: (a) an acoustic echo sounder, (b) anemometers mounted at two fixed levels on a 150-m tower, (c) an anemometer and a thermometer mounted on a movable carriage on the tower, and (d) a microbarograph array, including one microbarograph mounted atop the tower. The wave phase velocity (−3.5–4.0 m s−1) was found to equal the wind velocity in the middle of the shear flow, as assumed by other authors. The wave-associated vertical fluxes of momentum and energy measured just above the wave critical layer were estimated to be −5 dyn cm−2 and −800 erg cm−2 s−1, respectively. These are large values. The annual average vertical flux of momentum at temperate and high latitudes is −0.25 dyn cm−2, while the average kinetic energy dissipation rate in a unit column of atmosphere is −5 × 103 erg cm−2 s−1. If the region of wave generation was itself propagating horizontally, its propagation velocity was large compared with the horizontal phase speed of the small-scale waves generated. Wave generation appeared to occur over an area large compared with the size of the microbarograph array (i.e., ≫ 2 km).

Acoustic echo-sounding techniques and their application to gravity-wave, turbulence, and stability studies

This paper is a brief summary of recent experimental studies conducted by the WPL staff in order to: (a) compare acoustic echo strengths with those predicted from measured turbulence intensities and scatter theory; (b) develop optimum experimental geometries for Doppler work, and (c) use the acoustic echo-sounder as a quantitative tool in studies of gravity wave dynamics in, and immediately above, the stable planetary boundary layer. We find that the observed acoustic echo strengths are roughly an order of magnitude greater than those predicted theoretically. This discrepancy might be in part due to partial reflection although the comparison is somewhat clouded by uncertainties in our knowledge of the equipment characteristics, propagation losses, etc. Comparisons between Doppler and in situ wind measurements give confidence in the Doppler results, but further experimentation and comparisons are needed. Preliminary use of acoustic Doppler data in a case study of gravity-wave dynamics in the planetary boundary layer has yielded boundary-layer wind speed and direction profiles which give insight into the mechanisms responsible for the wave generation. The Doppler data yield estimates of the wave associated momentum fluxes (∼a few dyn cm−2) as well. The results derived from the acoustic techniques are quite encouraging, but thus far remain unsubstantiated by independent wind and flux measurements.

Further study of the atmospheric gravity waves over the eastern seaboard on 18 March 1969

Abstract Washington, D. C., microbarograph records for 18 March 1969 reveal gravity-wave-associated pressure oscillations which appear to be directly related to upper tropospheric wave structure observed at the same time with a Wallops Island 10-cm wavelength radar. The consistency between the two sets of data provides new observational support for a hypothesis of long standing in the microbarograph community; namely, that shear instability in the upper tropospheric flow is indeed the mechanism responsible for the generation of such waves. The comparison presented here suggests that microbarograph arrays might be useful adjuncts to future radar studies of upper tropospheric wave dynamics, supplying such wave parameters as phase velocity and wavelength in favorable cases. A closer examination of the radar data pertinent to this event reveals an apparent vertical wave phase variation, permitting a very approximate and somewhat uncertain estimate of the wave-associated vertical flux of horizontal momentum, w...

Atmospheric waves observed in the planetary boundary layer using an acoustic sounder and a microbarograph array

Acoustic sounder and microbarograph records of atmospheric waves propagating in the planetary boundary layer over Table Mountain, Colorado, are presented and compared. The two observing techniques are complementary in that the array provides wave amplitude, horizontal phase speed, direction, and wavelength, while the sounder provides a detailed picture of temporal changes in the structure of the lowermost kilometer or so of the Earths atmosphere.

Dissipative Waves Excited by Gravity-Wave Encounters with the Stably Stratified Planetary Boundary Layer

Abstract We suggest that the strata of strong echo returns frequently revealed by remote-sensor records of the stably stratified planetary bound layer (PBL) represent the wavefronts of dissipative waves (viscous and thermal-conduction waves) excited by gravity-wave encounters with the PBL and the earths surface. The viscous waves appear to be more strongly forced and should therefore dominate the observations. This simple picture accounts for the following observed properties of the strata: 1) their nearly ubiquitous presence within the stably stratified PBL, 2) their nearly horizontal orientation, 3) the small spacing (some tens of meters, typically) separating the strata, 4) variability in that spacing in both height and time, and 5) the high shears and temperature gradients associated with the strata. Preliminary calculations of the energy fluxes and stresses associated with the wave motions, also presented here, suggest strongly that such waves are not mere curiosities of the PBL but reveal important...

E-REGION IONOSPHERIC IRREGULARITIES PRODUCED BY INTERNAL ATMOSPHERIC GRAVITY WAVES.

Abstract A linearized perturbation treatment is developed for study of the roles of both photochemical and dynamical effects in the gravity-wave production of E -region ionospheric irregularities. It is found that the two effects may be comparable in magnitude. It is also found that the small wave-associated vertical motions of the neutral gas may play a significant role in producing ion convergence. Results of numerical calculations are presented to illustrate these statements.

Waves Observed in the Planetary Boundary Layer using an Array of Acoustic Sounders

Abstract Three acoustic sounders, with antennas having vertically-pointed beams located at the vertices of a triangle about 300 m on a side, have been used successfully to calculate horizontal phase velocities of gravity waves in the lowest 1000 m of the atmosphere. The wave parameters obtained were compared with those obtained by an array of micrographs. The results from the two independent techniques agreed well in all cases where the waves were detected by both methods. However, the acoustic-sounder array was also able to detect gravity waves propagating in an inversion overlying the convective boundary layer that were undetected by the microbarograph array because of wave attenuation in the convective region. Wave-associated vertical-velocity fluctuations inside the undulating structure were measured using Doppler techniques to an accuracy of about ±0.1 m sec−1. Vertical-velocity fluctuations of about ±0.5 m sec−1 were measured inside one wave with displacement amplitude of 120 m and wavelength of 5.5...

Ionospheric response to an isotropic spectrum of internal gravity waves

Abstract This paper presents computer-produced tonal-value plots in which darkness or intensity are used to display the ionospheric response to an isotropic spectrum of internal gravity waves. Each plot shows at a glance those portions of the wave spectrum, i.e. those wave periods and azimuths of propagation, producing the ionospheric irregularities of greatest magnitude. Arrays of the plots illustrate the variations of this response with time of day, season, latitude, geomagnetic dip, and height.