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At least 1,423 records · Page 79
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NASA NTRS1977

The clouds of Venus. II - An investigation of the influence of coagulation on the observed droplet size distribution

An approximate numerical technique is used to investigate the influence of coagulation, sedimentation and turbulent motions on the observed droplet size distribution in the upper layers of the Venus clouds. If the cloud mass mixing ratio is less than 0.000001 at 250 K or the eddy diffusivity throughout the cloud is greater than 1,000,000 sq cm per sec, then coagulation is unimportant. In this case, the observed droplet size distribution is the initial size distribution produced by the condensation of the droplets. It is found that all cloud models with droplet formation near the cloud top (e.g., a photochemical model) must produce the observed droplet size distribution by condensation without subsequent modification by coagulation. However, neither meteoritic or surface dust can supply sufficient nucleating particles to account for the observed droplet number density. If the cloud droplets are formed near the cloud bottom, the observed droplet size distribution can be produced solely by the interaction of coagulation and dynamics; all information about the initial size distribution is lost. If droplet formation occurs near the cloud bottom, the lower atmosphere of Venus is oxidizing rather than reducing.

Rossow, W. B.↗
NASA NTRS1978

Properties of the clouds of Venus, as inferred from airborne observations of its near-infrared reflectivity spectrum

The shape and absolute value of Venus' reflectivity spectrum is measured in the 1.2- to 4.0 micrometer spectral region with a circular variable filter wheel spectrometer having a spectral resolution of 1.5%. Comparing these spectra with synthetic spectra generated with a multiple-scattering computer code, a number of properties of the Venus clouds are inferred. Evidence is obtained indicating that the clouds are made of a water solution of sulfuric acid in their top unit optical depth, and that the clouds are made of this material down to an optical depth of at least 25. In addition, the acid concentration is 84 plus or minus 2% H2SO4 by weight in the top unit optical depth, the total optical depth of the clouds is 37.5 plus or minus 12.5, and the cross-sectional weighted mean particle radius lies between 0.5 and 1.4 micrometers in the top unit optical depth of the clouds. It is found that the average volume mixing ratio of H2SO4 and H2O contained in the cloud material both equal approximately 2 x 10 to the -6. Employing vapor pressure arguments, the acid concentration is shown to equal 84 plus or minus 6% at the cloud bottom and the water vapor mixing ratio beneath the clouds lies between 6 x 10 to the -4 and 10 to the -2.

Pollack, J. B.↗
NASA NTRS1979

Thermal infrared properties of the Martian atmosphere. III - Local dust clouds

Prior to and during the Martian global dust storms in 1977, local dust clouds were identified from thermal observations. Local dust clouds appear up to 40 K cooler than the surrounding region in the daytime and appear warm at night. In contrast to their well-developed and consistent thermal signatures, local dust clouds exhibited little albedo contrast. Multiple observations of the same areas show that local dust clouds form and dissipate in a few days or less, and clouds were observed to move with velocities of from 14 to at least 32 m/sec. Local dust clouds are common along the receding edge of the south polar cap (observed primarily for orbiter imaging) and in the restricted area -10 deg to -30 deg, 80 deg to 125 deg W (observed primarily by the infrared thermal mapper); several of the latter occurred over a ridge in Claritas Fossae, indicating strong topographic control. In their active growing stage, local dust clouds are probably about 8 km high and have relatively abrupt tops. The accumulation of dust from local clouds reached similar levels immediately prior to both global storms, supporting the hypothesis that such dust accumulations are responsible for triggering the global dust storms.

Peterfreund, A. R.↗
NASA NTRS1982

Motions of the cloud medium behind large scale galactic shocks

Mechanisms of decelerating the cloud medium in the large-scale galactic shock are studied. It is shown that the process of cloud-cloud collisions, which results in diffusive momentum transport and hence gives rise to the turbulent viscosity, is very effective in slowing down the cloud medium so that the postshock velocity of the intercloud medium can be matched within a short distance behind the shock front. The drag force exerted by the slow-moving intercloud medium alone is simply not enough to effectively decelerate the cloud medium in the shock. By the use of the results of Shu et al (1972), the internal structure of the shock of the cloud medium is analyzed by including turbulent viscous effects. The thickness of the shock is found to be on the order of 100 pc if the turbulent viscosity is taken proportional to the mean free path of the cloud-cloud collisions. The phase transition takes place in an even thinner layer on the order of 10 pc immediately after the viscous shock front of the intercloud medium.

Yuan, C.↗
NASA NTRS1982

Collapse models for dark interstellar clouds

Properties of self-consistent numerical hydrodynamic models are compared with observed properties of several dark clouds. The results are consistent with the view that these clouds are undergoing gravitational collapse. The clouds appear to have evolved from similar initial states and to have ages comparable to their free-fall times. Derived cloud masses range between 10 and 1000 solar masses, correlating with cloud size. The models reproduce observed cloud-to-cloud variations in the (C-13)O abundance, and they offer additional evidence that the (C-13)O abundance varies within individual clouds.

Villere, K. R.↗
NASA NTRS1982

Measurements of cloud nuclei in the effluents from launches of liquid- and solid-fueled rockets

Airborne measurements of two types of cloud nuclei, cloud condensation nuclei (CCN) and ice nuclei (IN), were conducted in the stabilized ground clouds which resulted from the launches of a liquid-fueled ATLAS/Centaur rocket and a solid-fueled TITAN III rocket. Results show that the concentrations of CCN in both types of clouds were greater than ambient values for the 2 hours duration of the measurements. The initial production of CCN active at 0.5% supersaturation in the ATLAS and TITAN clouds was found to be equivalent to a 20 and 700 sec emission, respectively, by the city of Denver, Colorado. After the initial production, the clouds continued to generate CCN at a rate of about 1/cu cm sec. However, concentrations of IN in the ATLAS cloud were greater than ambient values for only a short period after launch, and it appears that the nuclei resulted from entrained launch pad and ground debris. The concentrations of IN in the TITAN cloud were found to be at or below ambient levels until about 2 hours after launch when they increased substantially above ambient values.

Hindman, E. E.↗
NASA NTRS1982

Satellite measurements of cloud reflectance and optical thickness

The cloud reflectance and the optical thickness have been computed with the aid of atmospheric models from the first eleven months (April 1970-February 1971) of backscattered ultraviolet (BUV) data over the Pacific Ocean. Both the cloud and the optical thickness are derived from the 380 nm channel by assuming that the entire IFOV (200 km) is filled by stratiform clouds. The analysis shows a large variability in the cloud reflectance in both the intertropic conversion zone (ITCZ) and the high latitudes. It is also found that for 90 percent of the time in the tropics, the clouds have an optical thickness of less than 10. An analysis of clouds with optical thickness between 10 and 20 shows that in the tropics the minimum frequency coincides with the dry zone at 2.5 deg S and the maximum frequency for clouds of optical thickness 10-20 is greater in summer than in winter and irrespective of the season; 50 percent of the time the clouds have an optical thickness less than 13.

Ahmad, Z.↗
NASA NTRS1983

Cloud-particle galactic gas dynamics and star formation

Galactic gas dynamics, spiral structure, and star formation are discussed in relation to N-body computational studies based on a cloud-particle model of the interstellar medium. On the small scale, the interstellar medium is seen as cloud-dominated and supernova-perturbed. It is noted that the cloud-particle model simulates cloud-cloud collisions, the formation of stellar associations, and supernova explosions as dominant local processes. On the large scale, in response to a spiral galactic gravitational field, global density waves and galactic shocks develop having large-scale characteristics similar to those found in continuum gas dynamical studies. Both the system of gas clouds and the system of young stellar associations forming from the clouds figure in the global spiral structure. However, with the attributes of neither assuming a continuum of gas (as in continuum gas dynamical studies) or requiring a prescribed equation of state (such as the isothermal condition), the cloud-particle picture retains much of the detail lost in earlier work. By detail is meant the small-scale features and structures so important in understanding the local, turbulent state of the interstellar medium as well as the degree of raggedness often seen to be superposed on the global spiral structure.

Roberts, W. W., Jr.↗
NASA NTRS1984

Are interplanetary magnetic clouds manifestations of coronal transients at 1 AU?

Using proxy data for the occurrence of those mass ejections from the solar corona which are directed earthward, the association between the post-1970 interplanetary magnetic clouds of Klein and Burlaga (1982) and coronal mass ejections is investigated. The evidence linking magnetic clouds following shocks with coronal mass ejections is striking; six of nine clouds observed at earth were preceded an appropriate time earlier by meter-wave type II radio bursts indicative of coronal shock waves and coronal mass ejections occurring near central meridian. During the selected control periods when no clouds were detected near earth, the only type II bursts reported were associated with solar activity near the limbs. Where the proxy solar data to be sought are not so clearly suggested, that is, for clouds preceding interaction regions and clouds within cold magnetic enhancements, the evidence linking the clouds and coronal mass ejections is not as clear; proxy data usually suggest many candidate mass-ejection events for each cloud.

Wilson, R. M.↗
NASA NTRS1984

Solar reflection from interacting and shadowing cloud elements

An attempt is made to relate the reflection from a broken cloud field to that from a plane parallel cloud of the same optical properties by using Monte Carlo simulations for a very simple model of an array of regularly spaced, infinitely long bar clouds. Reflection from the array is represented by an effective cloud fraction, and it is shown that for many cases this may be obtained by treating the cloud elements to be noninteracting azimuthally symmetric reflectors. The effective cloud fraction is a function of solar zenith angle, the aspect ratio of the individual cloud elements, and the normal or zenith cloud fraction of the array. Model computations for reflection from an array of randomly distributed identical cylinders are also presented as an example of a possible application of this technique.

HARSHVARDHAN↗
NASA NTRS1985

Water-ice Clouds on Mars: Location and Seasonal Variation

Water-ice clouds were located on Mars using Viking infrared thermal mapper (IRTM) broadband spectral observations. The IRTM instrument had 5 thermal bands centered at 7, 9, 11, 15, and 20 microns. Clouds and hazes were consistently observed in four northern hemisphere regions centered over Tharsis, Arabia, Elysium, and along the boundary between the crater uplands and the northern plains. During the northern spring and summer when the atmosphere is relatively free of dust, there is a distinct difference between the cloud abundance in the Northern and Southern Hemispheres, with clouds and hazes being rare in the south. A second important class of water-ice clouds are those observed along the boundary of the retreating north polar cap. These clouds occur at all longitudes around the cap and are generally confined to within +/- 5 deg of the cap boundary. The cloud opacities can be estimated using a delta-Eddington radiative transfer model which incorporates Mie scattering and the electrical properties of water-ice. Assuming realistic, but non-unique, values for the ice particle size and cloud temperature, the derived opacities range from near-zero to 1.

Christensen, P. R.↗
NASA NTRS1984

GASP cloud encounter statistics - Implications for laminar flow control flight

The cloud observation archive from the NASA Global Atmospheric Sampling Program (GASP) is analyzed in order to derive the probability of cloud encounter at altitudes normally flown by commercial airliners, for application to a determination of the feasability of Laminar Flow Control (LFC) on long-range routes. The probability of cloud encounter is found to vary significantly with season. Several meteorological circulation features are apparent in the latitudinal distribution of cloud cover. The cloud encounter data are shown to be consistent with the classical midlatitude cyclone model with more clouds encountered in highs than in lows. Aircraft measurements of route-averaged time-in-clouds fit a gamma probability distribution model which is applied to estimate the probability of extended cloud encounter, and the associated loss of LFC effectiveness along seven high-density routes. The probability is demonstrated to be low.

Jasperson, W. H.↗
NASA NTRS1984

Diagnosing the severe thunderstorm environment by mesoscale cloud tracking - A new approach and new information

A new method for cloud tracking and observing storm development using satellite imagery in a cloud relative mode is presented. This method changes the analyst's function to one of monitoring cloud growth and changes over a finite time period and identifying portions of the cloud that are suitable for use as trackers. It is shown how many problems previously encountered by meteorologists in mesoscale cloud tracking can be overcome by using the cloud relative technique to determine mesoscale flow fields whose heights are determined using stereographic techniques. By separating growth and decay from cloud motion in a cumulus field using the method and accurately defining cloud heights, vertical wind shears within mesoscale domains may be defined. When this information is used to study the relative velocity of flows at different levels with respect to a developing or mature storm, improved insight into storm behavior can be achieved.

Purdom, J. F. W.↗
NASA NTRS1985

Remote sensing of cloud-top pressure using reflected solar radiation in the oxygen A-band

An algorithm has been developed for using the reflection of solar radiation in the oxygen A-band to determine cloud-top altitude. Because of multiple scattering and molecular absorption inside the cloud, the reflection of clouds is substantially modified in comparison with a mirror cloud, which is assumed to have a 100 percent reflection. To infer true cloud-top altitude, therefore, it is necessary to accurately estimate the amount of 'photon penetration'. Theoretical calculations indicate that the amount of photon penetration depends on the altitude, the scaled volume scattering coefficient, and the scaled optical thickness of the cloud. Algorithms using the reflection in the oxygen A-band to determine the cloud-top pressure have been applied to an aircraft field experiment in conjunction with CCOPE, 1981. Results of this study are very encouraging, especially for extended clouds.

Wu, M.-L. C.↗
NASA NTRS1986

Turbulent dispersion of the icing cloud from spray nozzles used in icing tunnels

To correctly simulate flight in natural icing conditions, the turbulence in an icing simulator must be as low as possible. But some turbulence is required to mix the droplets from the spray nozzles and achieve an icing cloud of uniform liquid water content. The goal for any spray system is to obtain the widest possible spray cloud with the lowest possible turbulence in the test section of a icing tunnel. This investigation reports the measurement of turbulence and the three-dimensional spread of the cloud from a single spray nozzle. The task was to determine how the air turbulence and cloud width are affected by spray bars of quite different drag coefficients, by changes in the turbulence upstream of the spray, the droplet size, and the atomizing air. An ice accretion grid, located 6.3 m downstream of the single spray nozzle, was used to measure cloud spread. Both the spray bar and the grid were located in the constant velocity test section. Three spray bar shapes were tested: the short blunt spray bar used in the NASA Lewis Icing Research Tunnel, a thin 14.6 cm chord airfoil, and a 53 cm chord NACA 0012 airfoil. At the low airspeed (56 km/hr) the ice accretion pattern was axisymmetric and was not affected by the shape of the spray bar. At the high airspeed (169 km/hr) the spread was 30 percent smaller than at the low airspeed. For the widest cloud the spray bars should be located as far upstream in the low velocity plenum of the icing tunnel. Good comparison is obtained between the cloud spread data and predicitons from a two-dimensional cloud mixing computer code using the two equation turbulence (k epsilon g) model.

Marek, C. J.↗
NASA NTRS1985

Dynamical evolution of the Oort cloud

New studies of the dynamical evolution of cometary orbits in the Oort cloud are made using a revised version of Weissman's (1982) Monte Carlo simulation model, which more accurately mimics the perturbation of comets by the giant planets. It is shown that perturbations by Saturn and Jupiter provide a substantial barrier to the diffusion of cometary perihelia into the inner solar system. Perturbations by Uranus and Neptune are rarely great enough to remove comets from the Oort cloud, but do serve to scatter the comets in the cloud in initial energy. The new model gives a population of 1.8 to 2.1 x 10 to the 12th comets for the present-day Oort cloud, and a mass of 7 to 8 earth masses. Perturbation of the Oort cloud by giant molecular clouds in the galaxy is discussed, as is evidence for a massive 'inner Oort cloud' internal to the observed one. The possibility of an unseen solar companion orbiting in the Oort cloud and causing periodic comet showers is shown to be dynamically plausible but unlikely, based on the observed cratering rate on the earth and moon.

Weissman, P. R.↗
NASA NTRS1985

Interstellar clouds - From a dynamical perspective on their chemistry

The possibility is examined that in the course of its dynamical evolution, a single mass of interstellar gas would exhibit properties of diffuse clouds, dense clouds and finally also of clouds perturbed by shocks or intense UV or X-ray radiation generated by a star of its own creation. This concept provides a common thread through the bewildering diversity of physical and chemical compositional properties shown by interstellar clouds. From this perspective, instead of being static objects, interstellar clouds are possibly incessantly evolving from initially diffuse to later dense state and then to star formation which ultimately restructures or disperses the remaining cloud material to begin the whole evolutionary process once again. Based on a simplified study of interstellar chemistry from a dynamical perspective, the ideas are presented as an heuristic: to encourage thought on the future direction of molecular astrophysics and the need to consider the chemical behavior of interstellar clouds in conjunction with, rather than in isolation from, their dynamical behavior. A physical basis must be sought for the semiempirical temperature formula which has been given a critical role in the collapse of diffuse clouds. Self-shielding effects in the chemistry of CO were neglected and this drawback should be removed; the ability of the model to explain the fractional abundances of more complex molecules, such as cyanopolyynes, should be examined.

Prasad, S. S.↗
NASA NTRS1986

On the association of magnetic clouds with disappearing filaments

Evidence is presented that an interplanetary magnetic cloud preceding an interaction region, observed at earth on January 24, 1974, is associated with the eruptive filament of disparition brusque (DB) near central meridian on January 18. The DB was also associated with a long-decay soft X ray transient and a long-duration gradual-rise-and-fall radio burst. To assess whether magnetic clouds are generally associated with DBs, results from statistical testing of the relation of 33 magnetic clouds (and 33 control samples without magnetic clouds) to disappearing filaments near central meridian (approximately less than 45 deg central meridian distance) are presented. The hypothesis that magnetic cloud are the 1-AU counterparts of either eruptive filaments or the coronal mass ejections which probably accompany them is supported. The major result is that disappearing filaments occur more frequently on the days when magnetic clouds are launched than on control days, a result obtained with greater than 99 pct confidence. There is a suggestion that clouds following shocks, probably launched at times of solar flares, are not as strongly associated with disappearing filaments as are clouds launched less violently.

Wilson, R. M.↗