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At least 1,171 records · Page 65

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NASA NTRS1979

Theoretical modelling and experimental data matching for active and passive microwave remote sensing of Earth terrain

Two theoretical models were developed to characterize terrain media: a random medium with a variance, a horizontal correlation length; and a homogeneous dielectric containing discrete scatterers. The earth terrain is modelled as layers of such scattering media bounded by air above and half-space below. Matching the theoretical results with experimental data collected from vegetation and snow-ice fields shows that: (1) for observation angles near nadir, rough surface effects are important; (2) for snow-ice field the horizontal correlation length is greater than the vertical correlation length whereas for vegetation field their relative sizes depend on the types of vegetation; (3) the vertically polarized backscattering cross-section is always larger than the horizontally polarized backscattering cross-section for half-space scattering media; (4) for snow field displaying diurnal change, a three-layer model including a thin top layer caused by sunlight illumination must be used; and (5) for a random medium with equal horizontal and vertical correlation lengths, the measured data can also be matched with a corresponding discrete scatterer model.

Kong, J. A.↗

NASA NTRS1980

Theoretical models for microwave snow response and applications to remote sensing

Two theoretical models were developed to characterize snowpacks: (1) a random medium with a variance, a horizontal correlation length, and a vertical correlation length; and (2) a homogeneous dielectric containing discrete scatters. Theoretical results were matched with experimental data collected from snow ice fields. For snow ice fields the horizontal correlation length is no less than the vertical correlation length signifying a more laminar structure. The correspondence between the continuum random medium and the discrete spherical scatter model can be verified when the vertical correlation length is equal to the horizontal correlation length. The vertically polarized backscattering cross section sigma (sub vv) is always greater than the horizontally polarized backscattering cross section sigma (sub hh) for half space scattering media and may become smaller for a two layer model. In order to account for diurnal change exhibited by snow fields in both the active and passive remote sensing cases, a three layer model with a thin top layer caused by solar illumination must be used.

Kong, J. A.↗

NASA NTRS1980

The applicability of the piecewise linear current profile in the baroclinic instability problem

The applicability of the piecewise linear function in place of a similar smoothly-varying current profile is examined in the baroclinic context. Within the framework of small-perturbation linearization, the behavior of the vertical velocity and the horizontal divergence is analyzed at the discontinuity of the current shear. In the conventional geostrophic-type instability regime, the discontinuity in the horizontal divergence at the shear discontinuity is suppressed, and, therefore, the piecewise linear profile leads to a useful approximation to the true solution. In the symmetric-type instability regime, however, due to the magnified discontinuity in the horizontal divergence at the shear discontinuity, the solution thus obtained will show a major distortion, rendering the piecewise linear profile inadequate for modeling the smoothly-varying current profile. Using exemplary current profiles, numerical results are presented to demonstrate the behavior of the horizontal divergence near the discontinuity of current shear.

Hyun, J. M.↗

NASA NTRS1982

The dynamics of fibril magnetic fields. III - Fibril configurations in steady flows. IV - Trapping in closed convective rolls

The present investigation has the objective to provide an illustration of the equilibrium form of a hypothetical fibril field beneath the surface of the sun. Equilibrium fibril paths in static and moving atmospheres are considered. The provided examples are related to horizontal flows in an atmosphere in which the two anchor points of the arched fibril lie along the flow direction relative to each other. A brief introduction to the subject of the motion of horizontal fibrils in convective rolls is also presented, taking into account some implications for the sun. The equations of motion for a slender, buoyant flux tube extending horizontally along a closed convective roll are solved to illustrate the motion of the flux tube relative to the fluid, and attention is given to theoretical possibilities for the escape of flux tubes from a large horizontal convective roll in the sun.

Parker, E. N.↗

NASA NTRS1982

Mount St. Helens plume dispersion based on trajectory analyses

The major eruption of Mount St. Helens on 18 May 1980, had sufficient energy to traverse the troposphere (9 km above the mountain top) and to penetrate an additional 10 km into the stratosphere. This plume, initially quasi-vertical, rapidly acquired the horizontal momentum of the environmental winds and suffered differential rotation due to a positive speed shear in the troposphere and a negative shear in the stratosphere. Advected rapidly eastward by the undulating jet stream, the lower stratospheric portion of the plume circled the globe at an average speed of approx. 25 m s/l, reentering North America over California in early June. During the same period, the uppermost portion slowly looped over the northwestern United States and then moved westward over the northern Pacific Ocean. Thus, plume dispersion was initiated by the vertical shears of the horizontal winds which converted a nearly vertical plume to a thin, quasi-horizontal, quasi-zonal lamina. Horizontal shears then dispersed the lamina meridionally while small-scale, wave turbulent motions spread it slowly vertically.

Danielsen, E. F.↗

NASA NTRS1983

A comparison of coronal and interplanetary current sheet inclinations

The HAO white light K-coronameter observations show that the inclination of the heliospheric current sheet at the base of the corona can be both large (nearly vertical with respect to the solar equator) or small during Cararington rotations 1660 - 1666 and even on a single solar rotation. Voyager 1 and 2 magnetic field observations of crossing of the heliospheric current sheet at distances from the Sun of 1.4 and 2.8 AU. Two cases are considered, one in which the corresponding coronameter data indicate a nearly vertical (north-south) current sheet and another in which a nearly horizontal, near equatorial current sheet is indicated. For the crossings of the vertical current sheet, a variance analysis based on hour averages of the magnetic field data gave a minimum variance direction consistent with a steep inclination. The horizontal current sheet was observed by Voyager as a region of mixed polarity and low speeds lasting several days, consistent with multiple crossings of a horizontal but irregular and fluctuating current sheet at 1.4 AU. However, variance analysis of individual current sheet crossings in this interval using 1.92 see averages did not give minimum variance directions consistent with a horizontal current sheet.

Behannon, K. W.↗

NASA NTRS1983

A comparison of coronal and interplanetary current sheet inclinations

The HAO white light K-coronameter observations show that the inclination of the heliospheric current sheet at the base of the corona can be both large (nearly vertical with respect to the solar equator) or small during Cararington rotations 1660 - 1666 and even on a single solar rotation. Voyager 1 and 2 magnetic field observations of crossing of the heliospheric current sheet at distances from the sun of 1.4 and 2.8 AU. Two cases are considered, one in which the corresponding coronameter data indicate a nearly vertical (north-south) current sheet and another in which a nearly horizontal, near equatorial current sheet is indicated. For the crossings of the vertical current sheet, a variance analysis based on hour averages of the magnetic field data gave a minimum variance direction consistent with a steep inclination. The horizontal current sheet was observed by Voyager as a region of mixed polarity and low speeds lasting several days, consistent with multiple crossings of a horizontal but irregular and fluctuating current sheet at 1.4 AU. However, variance analysis of individual current sheet crossings in this interval using 1.92 see averages did not give minimum variance directions consistent with a horizontal current sheet. Previously announced in STAR as N83-31566

Behannon, K. W.↗

NASA NTRS1985

Flutter clearance of the Schweizer 1-36 deep-stall sailplane

A Schweizer 1-36 sailplane was modified for a controlled, deep-stall flight program. This modification allowed the horizontal stabilizer to pivot as much as 70 deg leading edge down. Ground vibration and flutter testing were accomplished on the sailplane with the horizontal stabilizer in the normal flight and deep-stall flight positions. Test results indicated satisfactory damping levels and trends for the structural modes of the sailplane. The modified sailplane was demonstrated to be free of aeroelastic instabilities to 83 KEAS with the horizontal stabilizer in the normal flight position and to 39 KEAS with the horizontal stabilizer in the deep-stall flight position. This flight envelope was adequate for the controlled, deep-stall flight experiments.

Kehoe, M. W.↗

NASA NTRS1986

Observations of vertical winds and the origin of thermospheric gravity waves launched by auroral substorms and westward travelling surges

Several sequences of observations of strong vertical winds in the upper thermosphere are discussed, in conjunction with models of the generation of such winds. In the auroral oval, the strongest upward winds are observed in or close to regions of intense auroral precipitation and strong ionospheric currents. The strongest winds, of the order of 100 to 200 m/sec are usually upward, and are both localized and of relatively short duration (10 to 20 min). In regions adjacent to those displaying strong upward winds, and following periods of upward winds, downward winds of rather lower magnitude (40 to about 80 m/sec) may be observed. Strong and rapid changes of horizontal winds are correlated with these rapid vertical wind variations. Considered from a large scale viewpoint, this class of strongly time dependent winds propagate globally, and may be considered to be gravity waves launched from an auroral source. During periods of very disturbed geomagnetic activity, there may be regions within and close to the auroral oval where systematic vertical winds of the order of 50 m/sec will occur for periods of several hours. Such persistent winds are part of a very strong large scale horizontal wind circulation set up in the polar regions during a major geomagnetic disturbance. This second class of strong horizontal and vertical winds corresponds more to a standing wave than to a gravity wave, and it is not as effective as the first class in generating large scale propagating gravity waves and correlated horizontal and vertical oscillations. A third class of significant (10 to 30 m/sec) vertical winds can be associated with systematic features of the average geomagnetic energy and momentum input to the polar thermosphere, and appear in statistical studies of the average vertical wind as a function of Universal Time at a given location.

Rees, D.↗

NASA NTRS1985

Optimal landing of a helicopter in autorotation

Gliding descent in autorotation is a maneuver used by helicopter pilots in case of engine failure. The landing of a helicopter in autorotation is formulated as a nonlinear optimal control problem. The OH-58A helicopter was used. Helicopter vertical and horizontal velocities, vertical and horizontal displacement, and the rotor angle speed were modeled. An empirical approximation for the induced veloctiy in the vortex-ring state were provided. The cost function of the optimal control problem is a weighted sum of the squared horizontal and vertical components of the helicopter velocity at touchdown. Optimal trajectories are calculated for entry conditions well within the horizontal-vertical restriction curve, with the helicopter initially in hover or forwared flight. The resultant two-point boundary value problem with path equality constraints was successfully solved using the Sequential Gradient Restoration Technique.

Lee, A. Y. N.↗

NASA NTRS1985

Effect of enclosure shape on natural convection velocities in microgravity

Natural convection velocities within two-dimensional enclosures of various shapes are estimated with reasonable accuracy, for Rayleigh numbers up to at least 5000, by considering the area to be equivalent to a circle of equal area. For the class of figures including the square, upright half-circle, and upright 2 x 1 aspect ratio rectangle, excellent agreement is obtained by considering the temperature difference across the maximum horizontal dimension to be equal to that across the equal area circle horizontal diameter. The agreement of two upright oblong shapes indicates probable agreement for any similar upright oblong shape of roughly the same aspect ratio. The horizontal oblong shapes have natural convection velocities approximately one-half that estimated based on the equal area circle, with the temperature difference across the maximum horizontal dimension taken to be equal to that across the equal area circle diameter.

Robertson, S. J.↗

NASA NTRS1988

Wavelength dependence of eddy dissipation and Coriolis force in the dynamics of gravity wave driven fluctuations in the OH nightglow

This paper examines the effect of inclusion of Coriolis force and eddy dissipation in the gravity wave dynamics theory of Walterscheid et al. (1987). It was found that the values of the ratio 'eta' (where eta is a complex quantity describing the ralationship between the intensity oscillation about the time-averaged intensity, and the temperature oscillation about the time-averaged temperature) strongly depend on the wave period and the horizontal wavelength; thus, if comparisons are to be made between observations and theory, horizontal wavelengths will need to be measured in conjunction with the OH nightglow measurements. For the waves with horizontal wavelengths up to 1000 km, the eddy dissipation was found to dominate over the Coriolis force in the gravity wave dynamics and also in the associated values of eta. However, for waves with horizontal wavelengths of 10,000 km or more, the Coriolis force cannot be neglected; it has to be taken into account along with the eddy dissipation.

Hickey, M. P.↗

NASA NTRS1989

Tests of crustal divergence models for Aphrodite Terra, Venus

This paper discusses the characteristics of Aphrodite Terra, the highland region of Venus which is considered to be a likely site of mantle upwelling, active volcanism, and extensional tectonics, and examines the relation of these features to three alternative kinematic models for the interaction of mantle convection with the surface. These the 'vertical tectonics' model, in which little horizontal surface displacement results from mantle flow; the 'plate divergence' model, in which shear strain from large horizontal displacements is accommodated only in narrow zones of deformation; and the 'distributed deformation' model, in which strain from large horizontal motions is broadly accommodated. No convincing observational evidence was found to support the rigid-plate divergence, while the evidence of large-scale horizontal motions of Aphrodite argues against purely vertical tectonics. A model is proposed, involving a broad disruption of a thin lithosphere. In such a model, lineaments are considered to be surface manifestations of mantle convective flow.

Grimm, Robert E.↗

NASA NTRS1992

Simulations of surface winds at the Viking Lander sites using a one-level model

The one-level model developed by Mass and Dempsey for use in predicting surface flows in regions of complex terrain was adapted to simulate surface flows at the Viking lander sites on Mars. In the one-level model, prediction equations for surface winds and temperatures are formulated and solved. Surface temperatures change with time in response to diabatic heating, horizontal advection, adiabatic heating and cooling effects, and horizontal diffusion. Surface winds can change in response to horizontal advection, pressure gradient forces, Coriolis forces, surface drag, and horizontal diffusion. Surface pressures are determined by integration of the hydrostatic equation from the surface to some reference level. The model has successfully simulated surface flows under a variety of conditions in complex-terrain regions on Earth.

Bridger, Alison F. C.↗

NASA NTRS1992

Comparisons of the NASA ER-2 meteorological measurement system with radar tracking and radiosonde data

Measurements of aircraft longitude, latitude, and velocity, and measurements of atmospheric pressure, temperature, and horizontal wind from the meteorological measurement system (MMS) on board the NASA ER-2 aircraft were compared with independent measurements of these quantities from radiosondes and radar tracking of both the ER-2 and radiosonde balloons. In general, the comparisons were good and within the expected measurement accuracy and natural variability of the meteorological parameters. Radar tracking of the ER-2 resolved the velocity and position drift of the inertial navigation system (INS). The rms errors in the horizontal velocity components of the ER-2, due to INS errors, were found to be 0.5 m/s. The magnitude of the drift in longitude and latitude depends on the sign and magnitude of the corresponding component velocity drift and can be a few hundredths of a degree. The radar altitudes of the ER-2 and radiosondes were used as the basis for comparing measurements of atmospheric pressure, temperature, and horizontal wind from these two platforms. The uncertainty in the MMS horizontal wind measurement is estimated to be +/- 2.5 m/s. The accuracy of the MMS pressure and temperature measurements were inferred to be +/- 0.3 hPa and +/- 0.3 K.

Gaines, Steven E.↗

NASA NTRS1981

Interpreting plant responses to clinostating. I - Mechanical stresses and ethylene

The possibility that the clinostat mechanical stresses (leaf flopping) induces ethylene production and, thus, the development of epinasty was tested by stressing vertical plants by constant gentle horizontal or vertical shaking or by a quick back-and-forth rotation (twisting). Clinostat leaf flopping was closely approximated by turning plants so that their stems were horizontal, rotating them quickly about the stem axis, and returning them to the vertical, with the treatment repeated every four minutes. It was found that horizontal and vertical shaking, twisting, intermittent horizontal rotating, and gentle hand shaking failed to induce epinasties that approached those observed on the slow clinostat. Minor epinasties were generated by vigorous hand-shaking (120 sec/day) and by daily application of Ag(+). Reducing leaf displacements by inverting plants did not significantly reduce the minor epinasty generated by vigorous hand-shaking.

Salisbury, Frank B.↗

NASA NTRS1992

Changes in leg volume during microgravity simulation

Little published information exists regarding the magnitude and time course of cephalad fluid shift resulting from microgravity simulations. Six subjects were exposed to 150 min each at horizontal bed rest, 6-deg head-down tilt, and horizontal water immersion. Fluid shift was estimated by calculating leg volumes from eight serial girth measurements from groin to ankle before, during, and after exposure. Results were compared with data from the first 3 h of spacecraft. By the end of exposure, total leg volume for the six subjects decreased by 2.6 +/- 0.8 percent, 1.7 +/- 1.2 percent, and 4.0 +/- 1.6 percent for horizontal, head-down, and immersion, respectively. Changes had plateaued for horizontal and head-down and had slowed for immersion. Relatively more fluid was lost from the lower leg than the thigh for all three conditions, particularly head-down. During the first 3 h of spaceflight, total leg volume decreased by 8.6 percent, and relatively more fluid was lost from the thigh than the lower leg. The difference in volume changes in microgravity and simulated microgravity may be caused by the small transverse pressures still present in ground-based simulations and the extremely nonlinear compliance of tissue.

Thornton, William E.↗

NASA NTRS1992

Results of a zonally truncated three-dimensional model of the Venus middle atmosphere

Although the equatorial rotational speed of the solid surface of Venus is only 4 m s(exp-1), the atmospheric rotational speed reaches a maximum of approximately 100 m s(exp-1) near the equatorial cloud top level (65 to 70 km). This phenomenon, known as superrotation, is the central dynamical problem of the Venus atmosphere. We report here the results of numerical simulations aimed at clarifying the mechanism for maintaining the equatorial cloud top rotation. Maintenance of an equatorial rotational speed maximum above the surface requires waves or eddies that systematically transport angular momentum against its zonal mean gradient. The zonally symmetric Hadley circulation is driven thermally and acts to reduce the rotational speed at the equatorial cloud top level; thus wave or eddy transport must counter this tendency as well as friction. Planetary waves arising from horizontal shear instability of the zonal flow (barotropic instability) could maintain the equatorial rotation by transporting angular momentum horizontally from midlatitudes toward the equator. Alternatively, vertically propagating waves could provide the required momentum source. The relative motion between the rotating atmosphere and the pattern of solar heating, which as a maximum where solar radiation is absorbed near the cloud tops, drives diurnal and semidiurnal thermal tides that propagate vertically away from the cloud top level. The effect of this wave propagation is to transport momentum toward the cloud top level at low latitudes and accelerate the mean zonal flow there. We employ a semispectral primitive equation model with a zonal mean flow and zonal wavenumbers 1 and 2. These waves correspond to the diurnal and semidiurnal tides, but they can also be excited by barotropic or baroclinic instability. Waves of higher wavenumbers and interactions between the waves are neglected. Symmetry about the equator is assumed, so the model applies to one hemisphere and covers the altitude range 30 to 110 km. Horizontal resolution is 1.5 deg latitude, and vertical resolution is 1.5 km. Solar and thermal infrared heating, based on Venus observations and calculations drive the model flow. Dissipation is accomplished mainly by Rayleigh friction, chosen to produce strong dissipation above 85 km in order to absorb upward propagating waves and limit extreme flow velocities there, yet to give very weak Rayleigh friction below 70 km; results in the cloud layer do not appear to be sensitive to the Rayleigh friction. The model also has weak vertical diffusion, and very weak horizontal diffusion, which has a smoothing effect on the flow only at the two grid points nearest the pole.

Newman, M.↗