Line Clouds Research Articles

Trade-Wind Clouds and Hawaiian Rainbands

Abstract Two case studies of rainband evolution off the windward coast of the Big Island of Hawaii are presented along with an overview of the complete radar and satellite dataset from the Hawaiian Rainband Project conducted in the summer of 1990. These studies reveal that radar-observed rainbands and cells offshore of the windward coastline are nearly always embedded within larger-scale stratocumulus cloud patches and/or cloud lines moving in from the northeastern Pacific Ocean in the easterly trade winds. These cloud patches develop in the trade-wind flow downstream of the large shallow stratocumulus cloud mass that covers much of the northeastern Pacific Ocean between Hawaii and California. Tropical cyclones originating in the intertropical convergence zone sometimes move north through this stratocumulus cloud mass, producing, in their wake, cloud-free regions that then advect toward the Hawaiian Islands. These regions contribute to the variability of the organization of cloud patterns and rainband occ...

Line Emission in Active Galactic Nuclei: Effects of Local Delays upon Line Variability

view Abstract Citations (2) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Line Emission in Active Galactic Nuclei: Effects of Local Delays upon Line Variability Taylor, Jason A. Abstract Previous works concerning active galactic nuclei (AGNs) variability (e.g., Blandford & McKee 1982) have assumed that the emission characteristics of illuminated clouds are purely a function of the instant continuum flux to which they are exposed. This paper shows that this assumption is not necessarily justified and that the history of exposure accounting for "local delays" due to finite cloud equilibrium times can also be relevant. For this reason, a new formalism is developed in this paper for computing the observational properties of models that have local delays. The nature of the nonlinear behavior that results is calculated for some very simple nonlinear cloud line emission models. It is found that the mean response time is a function of the recent average value of the continuum. Linear models fitted to these nonlinear systems respond too slowly when there are low-energy (and generally rapid) changes in the continuum, yet respond too rapidly when there are high- energy (and generally slow) changes in the continuum. As with systems without local delays, the expression for the time-dependent line flux contains an integration over history of the "spatial" response function, which has structure at lags of the light travel times of the emission region. However, the kernel of this integral itself is a function of additional integrations over individual "cloud" response functions that have structure at lags of the equilibrium times of the cloud properties relevant to line emission. In the linear regime, the response can be approximated using a single response function. The integral of this function over lag is not generally equal to the mean flux in the line. Rather, it differs by a factor that is the strength of response for low- frequency continuum excitations or simply the "asymptotic gain," which is unity only in fully linear models. If instantaneous or linear response is incorrectly assumed, local delays and nonlinear response can make a system appear larger than it actually is. These effects are similar to those that beaming can cause. Local delays can also be a source of asymmetry about the peak of the cross-correlation function. Publication: The Astrophysical Journal Pub Date: October 1996 DOI: 10.1086/177866 arXiv: arXiv:astro-ph/9606009 Bibcode: 1996ApJ...470..269T Keywords: GALAXIES: ACTIVE; GALAXIES: NUCLEI; LINE: FORMATION; Astrophysics E-Print: To appear in ApJ, Vol. 470, October 10, 1996 full text sources arXiv | ADS |

On the Dust-to-Gas Ratio and Large Particles in the Interstellar Medium

view Abstract Citations (96) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Dust-to-Gas Ratio and Large Particles in the Interstellar Medium Kim, Sang-Hee ; Martin, P. G. Abstract A comparison of mass (or size) distributions of interstellar dust particles in various environments (from diffuse to dense), as determined from interstellar extinction and linear polarization curves, shows that for dense (dark) clouds with higher RV and λmax than typical of the diffuse interstellar medium the relative number of small particles is greatly diminished, whereas the shape of the mass distribution at the large size end (where much of the mass is) is much the same. This appears to be consistent with evolution of interstellar dust particles in dense clouds by aggregation involving small particles; there is only modest evolution of the larger particles. There is some evidence for dark cloud lines of sight with lower than average extinction per unit mass of gas (e.g., AV/NH is one measure). With specific application to the ρ Oph line of sight, we show quantitatively that this demands the existence of particles much larger than usually contemplated (10 μm). Large particles have low extinction efficiencies per unit volume in the optical and fairly neutral extinction with wavelength and so can lock up a lot of mass fairly inconspicuously. Producing such particles requires much more aggressive aggregation, with large particles sticking to one another. While our analysis is based on specific grain models, an instructive Kramers-Kronig treatment illustrated by these models suggests that our conclusions are robust. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177151 Bibcode: 1996ApJ...462..296K Keywords: ISM: DUST; EXTINCTION; ISM: ABUNDANCES full text sources ADS | data products SIMBAD (9)

ON THE MEASUREMENT OF THE DEUTERIUM ABUNDANCE AND THE INTERPRETATION OF THE LYMAN EDGE IN QSO ABSORPTION-LINE SYSTEMS

ABSTRACT A detailed examination of the detectability of the Lyman series of deuterium in QSO absorption-line systems shows that observations of the high-order Lyman lines in clouds of very high column density and low velocity dispersion (such as "damped Ly-alpha systems") might produce efficient and precise measures of the deuterium to hydrogen ratio, although these systems typically also display heavy element absorption and therefore are not ideal for specifying the primordial [D/H] ratio. This work is complementary to the earlier work of Webb et al. (1991), who explored the measurement of the deuterium in systems with higher velocity dispersions. The dependence of the shape of the Lyman continuum cut-off on deuterium abundance, velocity dispersion and column denisty is also examined in detail, and low to moderate resolution synthetic spectra are presented. For clouds of Doppler dispersion sigma, the "effective" wavelength of the Lyman edge is shifted to ~911.76(1 + (sigma/c)2/3)A. for absorbing clouds of high column density, DI can make a significant contribution to the opacity close to the "effective" Lyman cut-off, where the Doppler broadening is comparable to the spacing between the high-order lines.

Numerical modeling of ship tracks produced by injections of cloud condensation nuclei into marine stratiform clouds

Ship tracks are long‐lived, linear regions of enhanced reflectivity in low‐lying marine clouds that appear in satellite imagery downwind of ships. Ship tracks were first observed as cloud lines in visible satellite imagery (type 1). A second (and more common) type of ship track (type 2), which is masked at visible wavelengths by natural variability in cloud reflectivity, is seen at near‐infrared wavelengths in satellite imagery. A one‐dimensional numerical model is used to simulate measurements of both types of ship tracks and to investigate interactions between aerosol and cloud microphysics, radiative transfer, and turbulent mixing in the cloud‐topped marine boundary layer that lead to the formation and provide for the persistence of ship tracks. We find that cloud condensation nuclei (CCN) injections can account for many of the observed properties of ship tracks. Higher CCN concentrations produce increased droplet concentrations, which enhance cloud reflectivity by reducing droplet radius and increasing droplet cross‐sectional area. The smaller droplets also reduce the drizzle rate, which can allow cloud water to increase under some conditions, thereby leading to higher cloud reflectivity. However, smaller droplets also evaporate more readily below cloud base. Increased evaporation reduces mixing between the cloud and the subcloud layers during daytime, which causes a decrease in cloud water. The distinction between the two types of ship tracks is suggested to be due to differences in ambient concentrations of CCN that cause variations in turbulent mixing in the boundary layer, through the effect of cloud droplet concentrations on cloud‐top longwave radiative cooling. The model predicts lifetimes of > 1 day and > 2 days for the simulated type 1 and type 2 ship tracks, respectively. In the atmosphere, processes not treated in the model, such as horizontal dispersion and changes in large‐scale atmospheric conditions, may limit ship track lifetimes.

Ship-produced cloud lines of 13 July 1991

On 13 July 1991, a well-defined cloud line produced by an unidentified steaming ship was detected in satellite imagery and was simultaneously photographed from the R/V EGABRAG III . The EGABRAG produced a much less well-defined cloud line. Measurements made from the EGABRAG revealed that the cloud lines formed in a shallow boundary layer which was nearly saturated, unstable, drizzling and nearly free of cloud condensation nuclei (CCN). The EGABRAG passed through the plume of the ship as indicated by elevated CCN concentrations coincident with the cloud line. Thereafter, both ships passed under a shallow stratus layer where background CCN concentrations increased significantly. Only the cloud line produced by the ship extended into the stratus layer, the EGABRAG did not affect the layer. The CCN and updraft from the ship were involved in the formation of the cloud line. In contrast, the CCN and updraft from the EGABRAG were insufficient to produce a well-defined cloud line. Production of the cloud lines appeared dependent on a combination of environmental conditions and ship-produced CCN and updrafts.

A high-resolution survey of interstellar NA I D1 lines

We present high-resolution (0.5 km/s) spectra, obtained with the McDonald Observatory 2.7 m coude echelle spectrograph, of interstellar Na I D1 absorption toward 38 bright stars. Numerous narrow, closely blended absorption components, showing resolved Na I hyperfine structure, are evident in these spectra; such narrow components appear in both low halo and quite local gas, as well as in gas toward more distant disk stars. We have used the method of profile fitting in an attempt to determine column densities, line widths, and velocities for the individual interstellar clouds contributing to the observed absorption lines. The resulting sample of 276 clouds is significantly larger, and likely more complete, than several previous samples of 'individual' interstellar clouds, and allows more precise investigation of various statistical properties. We find that the cloud column density (N) and line width parameter (b) are not correlated, for 0.3 km/s approximately less than b approximately less than 1.5 km/s and 10.0/sq cm approximately less than log (N(Na I)) approximately less than 11.6/sq cm. The median b is about 0.73 km/s, the median log N is about 11.09/sq cm, and the median separation between adjacent components is about 2.0 km/s. All these are overestimates of the true median values, however, due to our inability to completely resolve all the component structure present in some cases; even at a resolution of 0.5 km/s, we may have discerned only 60% of the full number of individual components actually present. The one-dimensional dispersion of component velocities, in the local standard of rest, is approximately 8.6 km/s; the distribution of velocities is broader and displaced to more negative velocities for the weaker components. If 80 K is a representative temperature for the interstellar clouds seen in Na I absorption, then at least 38% (and probably the majority) of the clouds have subsonic internal turbulent motions. The range in N(H I) observed at a given N(Na I) increases as N(Na I) decreases below about 10(exp 11)/sq cm, so that N(Na I) becomes a less reliable predictor of N(H I) at low column densities. These spectra will be quite useful in future detailed studies of these lines of sight with the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) echelle -- to determine accurate velocities and to reveal the detailed interstellar component structures that cannot be discerned at the 3.5 km/s resolution available with the GHRS.

Magnetohydrodynamic wave propagation in one-dimensional nonhomogeneous, self-gravitating clouds

We study the propagation of magnetohydrodynamic (MHD) waves through nonhomogeneous, selfgravitating, magnetic media representative of molecular cloud environments and focus on the issues of cloud support and line profiles. Since the general treatment of this topic is burdened by a complex mathematical formalism, we consider simplifying geometries which yield analytical solutions in the linear wave limit. In particular, we study both magnetoacoustic and Alfven wave propagation along the density gradient in a one-imensional slab. Of specific relevance to molecular clouds, we find that the back reaction of the Alfven waves can provide a pressure along the direction of the magnetic field lines; this pressure can help support a density enhancement against gravitational collapse. Furthermore, we find that the velocity amplitudes of these waves increase as the density decreases, in rough agreement with observational estimates of line width versus density relations.

A new look at the dust toward Omicron Persei and Sigma Scorpii with the International Ultraviolet Explorer

The first IUE low dispersion spectra for Omicron Persei and Sigma Scorpii have been obtained and extinction curves from 1150 to 3250 A are presented. It is of interest to consider Omicron Persei and Sigma Scorpii dust in the context of the average extinction law of Cardelli et al (1989), here referred to as 'CCM', which depends on the R(v) parameter; this is defined as the ratio of total to selective extinction. Omicron Persei lies behind the edge of a dark cloud, and its dust shows a broad bump shifted to shorter wavelengths, similar to other dark cloud lines of sight, but shows no deviation from the CCM average curve in the far UV. Sigma Scorpii lies within the H II region near a dark cloud, and its dust shows a marked deviation from the CCM law in the far UV. These deviations are more typical of dark clouds or diffuse lines of sight, rather than of bright nebula lines of sight. The Omicron Persei and Sigma Scorpii dust characteristics exemplify a continuum of characteristics observed along various lines of sight, depending on the local dust environemnts.

The Kamishak Gap wind as depicted in DMSP OLS and SSM/I data

Abstract DMSP visual and infrared Operational Line Scanner (OLS) data reveal several effects of a mountain gap wind generated by wind flow through the Kamishak Gap on the west side of Cook Inlet, Alaska. The existence of the wind can be inferred through one of three separate effects apparent in the data: (1) a roughened sea effect, appearing as a dark grey shade swath within a sunglint pattern; (2) an anomalous grey shade effect induced by sea spray and aerosols, and, (3) a multiple cloud line effect originating as a result of a moisture flux from the sea to the air in a high speed, cold air, coastal outbreak. The Special Sensor Microwave Imager (SSM/I) of the DMSP satellite provides another means to detect the Kamishak Gap wind due to the sensitivity of this sensor to changes in microwave emission of the sea surface as a result of sea spray and foam in high wind speed areas and to associated changes in integrated water vapour content (oceanic total precipitable water). Developed algorithms provide a methodology to quantify from a satellite perspective some of the characterisitics of such events.

Studies of the ISM in the LMC using SN1987A

The exceptional brightness of SN1987A provided a wealth of opportunities for probing not only the interstellar medium in our Galaxy and in the Large Magellanic Cloud (LMC), but also any intergalactic matter between the two. Spectroscopic work has been directed both towards searches for very weak absorption lines, which require data of exceptionally high signal-to-noise ratio, and towards recording spectra of known features at unprecedentedly high resolution. Both approaches have yielded exciting and unexpected results. The first detection of [FeX] absorption has revealed the presence of million-degree gas in the interstellar medium of the LMC, possibly resulting from the explosions of previous supernovae in the 30-Doradus HII region. The ultra-high-resolution observations have been successful in resolving the hyperfine structure of the sodium D lines in several interstellar clouds along the line of sight to the supernova. This implies that the clouds are at temperatures of, at most, 170 K and have internal turbulent velocities of not more than 0.2 km s−1; large-scale motions thus appear to be mainly subsonic in these clouds. Radio observations of HI emission at 21-cm with the Parkes telescope have been combined with measurements of a variety of ultraviolet absorption lines, obtained with the International Ultraviolet Explorer satellite, to give the most detailed picture yet of the chemical composition of the gas between the Galaxy and the LMC. Finally, photographic monitoring of the light echo of SN 1987A over the last two years has provided a three-dimensional view of the interstellar environment in which SN 1987A exploded, complementing vividly the information deduced from the spectroscopic results.

On formation of convective roll vortices by internal gravity waves: A theoretical study

The linearized atmospheric equations system is solved analytically in a two layer model. The solutions show that the thermal disturbance located at the interface can induce internal gravity wave, which propagates downstream in the stable layer and brings about flow disturbances in the lower unstable layer. Motion of roll vortices with flow pattern similar to that found in the convective cloud street forms in the lower part of the upper layer and the upper part of the lower layer. If proper content of water vapor exists the cloud lines presenting small angle with the mean wind appear at the top of the lower layer. The effects of the wind speed and the temperature structures of the atmosphere in the lower convective layer and the overlying stable layer on the characteristics of the roll vortices are also discussed in this study.

The giant molecular outflow in MON R2

A large, massive outflow associated with the Mon R2 infrared cluster has been identified and mapped. The outflow line components are found to be well separated in velocity from the ambient cloud line component. These detached wings suggest high optical depths and filling factors that may be characteristic of old, evolved flows. The blueshifted lobe shows evidence of shell structure similar to that seen in the L1551 outflow. The redshifted lobe is spatially more confined in the plane of the sky and its structure is uncertain. A dense, flattened, rotating core is found near the origin of the bipolar outflow. The physical properties of this core are consistent with magnetized collapsing cloud models. The derived dynamical mass of the core, roughly 1013 solar masses, is supercritical by a factor of about 10. The large-scale velocity structure of the cloud do not appear to be consistent with such angular momentum conservation. 37 refs.

Observational and Numerical Study of a Microburst Line-Producing Storm

An analysis is performed on a microburst line-producing cloud that occurred near Denver, Colorado on 13 July 1982. The cloud line developed in an environment conducive to the production of low-reflectivity microbursts. Doppler radar analysis revealed strong convergence above cloud base into the region of downdraft 3.5 to 4.5 km above ground. Aircraft measurements detected light rain with graupel aloft in microburst downdrafts. A two-dimensional cloud model simulation captured many of the observed features of the cloud line structure and wind fields. In particular, both the development of multiple microbursts and the convergence aloft were well simulated. The formation of graupel/hail was important to the precipitation process in the model. The loading of rain and graupel and the cooling effect of rain evaporation and graupel melting were all important in microburst production—the graupel in the formative stages of the downdraft, and the rain in the further intensification of the downdraft and enhancement of the microburst outflow.

Numerical simulations of hailstorm feeder‐cloud initiation

Abstract A simple, moist two‐dimensional numerical model is developed and used to study the mechanisms for the formation of hailstorm feeder clouds. By using high spatial resolution, the model is able to simulate low‐level storm outflows and reproduce the observed separation between hailstorm feeder clouds. The cold outflow emanating from the storm is induced by evaporation of precipitation. The simulations show that feeder clouds develop in response to the gravity waves generated by wind shear that is inherent in the ambient environment and/or created by the interaction between the low‐level inflow and the storm outflow. The subcloud moisture also plays an important role in the formation of a wave‐like line of feeder clouds. It contributes in the cloud development as well as in the enhancement of gravity waves.

Structure and Evolution of North Australian Cloud Lines Observed during AMEX Phase I

North Australian Clouds Lines are distinctive, squall-line phenomena that occur in easterly flow across northern Australia. Three basic types have been identified, ranging from a long, narrow line of convective clouds (Type 1) to a severe squall line (Type 3). In this paper we examine a group of Type 1 lines, which occurred during the first phase of the Australian Monsoon Experiment (AMEX). The lines occurred in an ambient easterly flow with a distinct maximum near 850 hPa. Most of the lines developed on the western side of deep convective cells along the sea-breeze front in a manner that had substantial similarities to the African squall-line development described by Bolton. The resolvable structure resembled a shallow version of the Moncrieff–Miller squall line.

The Relationship between Satellite-Inferred Frontogenesis and Squall Line Formation

A study of three years of GOES satellite imagery has been conducted to determine whether synthesis of the imagery with surface diagnostic analyses may prove useful for predicting the precise location and time of formation of squall lines generated by a particular type of frontal circulation transverse to surface cold fronts. Existence of this circulation is inferred from the development of a thin Line of shallow Convection clouds (LC) along the front simultaneously with that of a mesoscale (<100 km wide) Clear Zone (CZ) immediately behind the front and at the leading edge of a large area of stratus clouds. The observations suggest that a thermally direct circulation transverse to the surface cold front generated the line convection and clear zone (in the upward and downward branches of the circulation, respectively) in all 15 cases which met the strict criteria for an LC/CZ. Squall lines were observed to form from the LC in 10 of the 15 cases examined, and nearly always within 90 min following the time when the CZ reached its maximum width. In addition, initial cumulonimbus development always occurred within 100 km of the diagnosed frontogenesis center at the LC. Therefore, this study suggests that both the timing and location of such squall lines should be predictable with very high accuracy. It is also shown that thermodynamic instability was insufficient for the formation of deep convection in the five non-thunderstorm cases. Our results also strongly support the hypothesis of Koch (1984) that this mesoscale circulation was generated by differential sensible heating acting in conjunction with geostrophic deformation effects. The contrast of cloudy skies behind the front (prior to CZ formation) with nearly clear skies ahead of the front is largely responsible for creation of the differential heating pattern. This suggests that forecasters should watch for such cloud patterns near cold fronts. Synoptic climatological conditions favoring the occurrence of this relatively rare phenomenon are also identified. The LC/CZ appears during the afternoon almost solely over the Great Plains states during spring and autumn. The line convection was found in all but one case to be parallel to, and either along or on the cyclonic side of, a prefrontal 850 mb jet. Although the LC/CZ is usually found on the anticyclonic side of upper-level jet streaks, it does not seem to prefer any particular jet quadrant. Diagnosis of the Sawyer-Eliassen equation for one case suggested that the mesoscale circulation was linked to a thermally direct circulation cell associated with the upper-level frontal zone. The information provided in this paper should be valuable to the operational forecaster concerned with having some guidance about specific mesoscale trigger mechanisms for squall lines. This phenomenon can be isolated with conventional surface and satellite data in real time to provide accurate and timely forecasts of the formation of squall line activity.

Linear polarization of millimeter-wave emission lines in clouds without large velocity gradients

view Abstract Citations (14) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Linear Polarization of Millimeter-Wave Emission Lines in Clouds without Large Velocity Gradients Lis, D. C. ; Goldsmith, P. F. ; Dickman, R. L. ; Predmore, C. R. ; Omont, A. ; Cernicharo, J. Abstract An equivalent model for linearly polarized emission of optically thin lines in dark clouds without embedded infrared sources which discards the large velocity gradient assumption is presented. Molecular clouds generally consist of a high-density core surrounded by an envelope with density decreasing with distance from the core. If the density contrast between the core and the outer portion of the envelope is large, the excitation temperature can drop rapidly with radius. Gradients in the excitation temperature through the cloud can be expected to produce an anisotropic radiation field away from the cloud center. If the excitation temperature is large enough, this anisotropy may produce significant differences in the population of different Zeeman sublevels of a given rotational level, resulting in the polarization of emergent radiation. Observations of (H-13)CO(+) and HCO(+) in four dark clouds show no linear polarizations. The significance of the achieved upper limits is discussed. Publication: The Astrophysical Journal Pub Date: May 1988 DOI: 10.1086/166293 Bibcode: 1988ApJ...328..304L Keywords: Emission Spectra; Interstellar Magnetic Fields; Line Spectra; Linear Polarization; Millimeter Waves; Molecular Clouds; Computational Astrophysics; Fine Structure; Formyl Ions; Molecular Ions; Velocity Distribution; Astrophysics; INTERSTELLAR: MAGNETIC FIELDS; INTERSTELLAR: MOLECULES; POLARIZATION full text sources ADS | data products SIMBAD (11)

AAT Observations of the Interstellar Medium towards SN 1987A

AbstractThe exceptional brightness of SN 1987A has provided a unique opportunity to probe intervening gas clouds in the disk and halo of our Galaxy and in the Large Magellanic Cloud, as well as intergalactic matter between the two. At the AAO we have exploited this opportunity in two ways: in searches for very weak interstellar features requiring exceptionally high signal-to-noise ratio spectra, and in recording known interstellar lines with unprecedentedly high spectral resolution. We are also monitoring photographically the evolution of the light-echoes to map the three-dimensional distribution of interstellar matter near the supernova. Surprisingly high column densities of million-degree gas have been found in the LMC through the first detection of [Fe X] in absorption. The hot gas may fill the interior of a ‘superbubble’, created by the combined effects of previous supernovae in this active region of star-formation; this cavity may be related to the shells of interstellar matter giving rise to the light-echoes. The ultra-high resolution observations, which required the rapid construction of a dedicated new spectrograph, were successful in resolving the hyperfine structure of the sodium D lines in several interstellar clouds. This implies that the clouds are at temperatures of at most 170 K and have internal turbulent velocities of no more than 0.3 km s−1, even though some are moving with high velocities relative to the Sun.

Simulation of Tropical Convective Systems. Part II: Simulations of Moving Cloud Lines

The new cumulns parameterization of Part I of this paper is used in a setes of simulations of tropical mesoscale convective lines. The structures and life cycles of the simulated systems are quite similar to those observed in many studies of tropical convection, supporting the use of the parametedzation in mesoscale model. The modeled cloud lines formed extensive upper-level nimbostratus clouds, primarily from air detrained below cloud top by the parameterized convection. These clouds produced copious rainfall and tended to shift the net diabafic heating predicted by the model to higher levels. When a cloud model lacking lateral detrainment was used, the simulated cloud lines grew rapidly, had short lifetimes, did not develop nimbostratus ions and concentrated the diabatic heating in the upper levels. Radiative processes, though crudely simulated, were shownto have potentially large effects on the intensity of the conyective system Decreasing the intercept, No, in the drop size distribution in the explicit moisture scheme had relatively small effects on the structure of the simulated systems, but it inctsed the rainfall and caused a small upward shir in the vertical heating probe.