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At least 307 records · Page 17

The energetics of molecular clouds. V - The S37 molecular cloud

The molecular clouds of S37, which coincides with a region of visual obscuration, is observed at several molecular transition frequencies and at IR wavelengths in the 2-125 micron range. It is found that the temperature and the column density peak near the reflection nebulae vdB 118 and 119, with line splitting and possible self-absorption being observed near the latter. Although many 2-micron sources are observed in the direction of the cloud, no bright, 10-micron sources are seen, and far-IR emission is found over an extensive region in the cloud. An energetics analysis leads to the prediction of a dust cooling rate consistent with the far-IR observations, which indicate not less than 15,000-35,000 solar luminosities, and more detailed energetics examinations show that the conditions necessary for collision heating of the gas by warm dust exist.

Evans, N. J., II↗

Venus: Cloud level circulation during 1982 as determined from Pioneer cloud photopolarimeter images. II - Solar longitude dependent circulation

Pioneer Venus Orbiter images obtained in 1982 indicate a marked solar-locked dependence of cloud level circulation in both averaged cloud motions and cloud layer UV reflectivity. An apparent relationship is noted between horizontal divergence and UV reflectivity: the highest reflectivities are associated with regions of convergence at high latitudes, while lower values are associated with equatorial latitude regions where the motions are divergent. In solar-locked coordinates, the rms deviation of normalized UV brightness is higher at 45-deg latitudes than in equatorial regions.

Limaye, Sanjay S.↗

Oxygen chemistry of shocked interstellar clouds. III - Sulfur and oxygen species in dense clouds

The chemical evolution of oxygen and sulfur species in shocked dense clouds is studied. Reaction rate constants for several important neutral reactions are examined, and revised values are suggested. The one-fluid magnetohydrodynamic shock structure and postshock chemical evolution are calculated for shocks of velocity v(s) = 10 km/s through clouds of initial number density n(0) = 100,000/cu cm and of molecule/atom ratios H2/H = 10, 1000, and 100,000 with most sulfur contained initially in molecules SO2 and SO. Abundances of SO2, SO, CS, and OCS remain near their preshock values, except in clouds containing substantial amounts of atomic hydrogen, where significant destruction of sulfur-oxygen species occurs. Abundances of shock-enhanced molecules HS and H2O are sensitive to the molecule/atom ratio. Nonthermal oxygen-hydrogen chemistry has a minor effect on oxygen-sulfur molecules in the case H2/H = 10.

Leen, T. M.↗

Satellite observation and mesoscale cloud modeling of St. Anthony, Minnesota storm clouds

Both rawinsonde data and geosynchronous satellite imagery were used to study the life cycles of St. Anthony, Minnesota's severe convective storms. Cloud modeling, with input sounding data from Saint Cloud, Minnesota, and rapid-scan imagery from GOES were used to investigate storm cloud formation, development and dissipation. Comparisons were made among the seven groups of severe storms which produced an outbreak of 30 tornadoes across the United States.

Hung, R. J.↗

Cloud optical parameters as derived from the multispectral cloud radiometer

Here, researchers analyze the data taken in the marine stratocumulus intensive field observation component of the First ISCCP Regional Experiment (FIRE), conducted off the California coast in July, 1987 using the Multispectral Cloud Radiometer (MCR). Researchers installed an efficient retrieval scheme for deriving the cloud optical thickness and droplet mode radius, and have described the behavior of the retrieval error. Generally the scheme can retrieve the optical thickness and mode radius adequately unless they are too small; the use of the forward scattering region is more sound than the use of the backscattering portion. Applying the method to real data, researchers derived the two-dimensional distribution of optical thickness and mode radius for a portion of one of the FIRE marine stratocumulus missions. In this case study, the droplet size showed a more uniform distribution than optical thickness with some correlation between large droplet size and small optical thickness. Although researchers can find microphysical reasons for these tendencies, they suspect that the remotely sensed droplet size may be overestimated. As a future problem, they will compare results with in situ data of the droplet size distribution. Also it will be very important to check several reasons why the droplet radius might be overestimated, e.g., soot contamination, effect of escaping photons from the lateral sides of broken clouds, and so on.

Nakajima, Eruyuki↗

Modelling of IR emission of interstellar clouds. I - Emission of isolated clouds and dust abundance variations

A model for the IR emission and extinction properties of small dust particles is used here to compute the IR emissions in the IRAS photometric bands for a set of isolated, spherical, and nonhomogeneous clouds heated by the local interstellar radiation field. It is predicted that the IR limb brightening (LB) in the IRAS photometric bands caused by selective absorption of UV photons of different energies in the cloud generally happens for central extinctions over 4 mag at 12 microns and greater than 10 mag at 100 microns and intermediate extinctions for the other IRAS bands. The surface brightness in the four IRAS bands is limited to about 0.4, 0.6, 3, and 10 MJy/sr at 12, 25, 60, and 100 microns respectively when no strong density discontinuity is present at the cloud edge.

Bernard, J. P.↗

Warm neutral halos around molecular clouds. IV - H I and continuum: Aperture synthesis observations towards the molecular cloud B5

We present aperture synthesis observations of H I (21 cm) line radiation and continuum emission at 408 and 1420 MHz towards a field centered on the molecular cloud B5. The H I emission shows an extended atomic halo around the molecular cloud. The opacity of the halo is derived using H I absorption toward several background sources and a simple source model is presented. The model indicates that the halo is not gravitationally bound to the molecular cloud and that it is in fact expanding away from it. Approximately 350 solar masses are contained in the H I halo. Flux densities and spectral indices for the sources detected in both of the continuum bands are given.

Andersson, B.-G.↗

Pre-main sequence stars in the Chamaeleon cloud and pre-main sequence stars in the Rho Ophiuchi cloud core

This grant funds two closely related studies of young stars under the ROSAT Guest Observer Program. A major paper on ROSAT discovery of X-ray emitting young stars in the Chamaeleon star forming cloud was published in the Astrophysical Journal. Two soft X-ray images of the Chamaeleon I star forming cloud obtained with the ROSAT Position Sensitive Proportional Counter are presented. Seventy reliable, and 19 possible additional, X-ray sources are found. Most are confirmed to be new weak T Tauri stars. A study of X-ray emitting stars in a deep ROSAT image of the rho Ophiuchi star forming cloud cores has been completed, and is being submitted for publication in the Astrophysical Journal. A list of X-ray sources, fluxes, signal-to-noise ratios and boresight-corrected positions has been compiled, and optical identification down to 22(exp m) are obtained. Of the 55 reliable ROSAT sources, virtually all are associated with embedded infrared-detected T Tauri stars. T Tauri X-rays may provide a crucial link in the photoionization self-regulation of star formation in the galaxy.

Feigelson, Eric D.↗

The analysis of global cloud and radiation data for the study of cloud-climate interactions

During the past year, ISCCP C1 data has been analyzed for two regions, the stratocumulus area off California and the tropical Atlantic. The first part of the study dealing with stratocumulus clouds is complete and the results form the technical portion of this report. A manuscript that has recently been prepared covering this subject is attached as an Appendix. The abstract describes the key finding that cloudy mean liquid water path (LWP) is invariant with cloud fraction for cloud cover between 20% - 80%. Similar conclusions are reached with ISCCP and LANDSAT data. This has important implications for General Circulation Models. A possible application is described in the Discussion section of the Appendix. An idea that has been proposed for dealing with GCM hydrology can be used.

HARSHVARDHAN↗

The Use of a Two-Dimensional, Time-Dependent Cloud Model to Predict Convective and Stratiform Clouds and Precipitation

A two-dimensional, time-dependent cloud model has been used in two field projects to forecast the convective development during the day from the morning sounding. In effect, the cloud model gives a dynamic analysis of the sounding as affected by heating and evaporation at the earth's surface, divergence of the winds throughout the atmosphere, and cloud shadow effects. During the initial project, the Cooperative Huntsville Meteorological Experiment, the results were mixed. Model runs were easily made when soundings were available. but displaying the results in a meaningful and useful way was the limiting factor. In a later experiment, the North Dakota Thunderstorm Project, the problem of displaying results was overcome and soundings were available from the local weather service forecast office with a high degree of reliability. The experimental model correctly forecasts convective development about 80% of the time, and precipitation or no precipitation more than 70% of the time.

Kopp, Fred J.↗

WFPC2 Observations of Star Clusters in the Magellanic Clouds: The Oldest Star Clusters in the Small Magellanic Cloud - Report 2

We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) observations in F45OW ( approximately B) and F555W (approximately V) of the intermediate-age populous star clusters NGC 121, NGC 339, NGC 361, NGC 416, and Kron 3 in the Small Magellanic Cloud. We use published photometry of two other SMC populous star clusters, Lindsay 1 and Lindsay 113, to investigate the age sequence of these seven populous star clusters in order to improve our understanding of the formation chronology of the SMC. We analyzed the V vs B-V and M(sub V) vs (B-V)(sub 0) color-magnitude diagrams of these populous Small Magellanic Cloud star clusters using a variety of techniques and determined their ages, metallicities, and reddenings. These new data enable us to improve the age-metallicity relation of star clusters in the Small Magellanic Cloud. In particular, we find that a closed-box continuous star-formation model does not reproduce the age-metallicity relation adequately. However, a theoretical model punctuated by bursts of star formation is in better agreement with the observational data presented herein.

Mighell, Kenneth J.↗

Analysis of Aircraft, Radiosonde and Radar Observations in Cirrus Clouds Observed During FIRE II: The Interactions Between Environmental Structure, Turbulence and Cloud Microphysical Properties

Ways to determine the turbulence intensity and the horizontal variability in cirrus clouds have been investigated using FIRE-II aircraft, radiosonde and radar data. Higher turbulence intensities were found within some, but not all, of the neutrally stratified layers. It was also demonstrated that the stability of cirrus layers with high extinction values decrease in time, possibly as a result of radiative destabilization. However, these features could not be directly related to each other in any simple manner. A simple linear relationship was observed between the amount of horizontal variability in the ice water content and its average value. This was also true for the extinction and ice crystal number concentrations. A relationship was also suggested between the variability in cloud depth and the environmental stability across the depth of the cloud layer, which requires further investigation.

Smith, Samantha A.↗

GEWEX Cloud System Study (GCSS) Working Group on Cirrus Cloud Systems (WG2)

Status, progress and plans will be given for current GCSS (GEWEX Cloud System Study) WG2 (Working Group on Cirrus Cloud Systems) projects, including: (a) the Idealized Cirrus Model Comparison Project, (b) the Cirrus Parcel Model Comparison Project (Phase 2), and (c) the developing Hurricane Nora extended outflow model case study project. Past results will be summarized and plans for the upcoming year described. Issues and strategies will be discussed. Prospects for developing improved cloud parameterizations derived from results of GCSS WG2 projects will be assessed. Plans for NASA's CRYSTAL-FACE (Cirrus Regional Study of Tropical Anvils and Layers - Florida Area Cirrus Experiment) potential opportunities for use of those data for WG2 model simulations (future projects) will be briefly described.

Starr, David↗

Measuring the Internal Structure and Physical Conditions in Star and Planet Forming Clouds Cores: Towards a Quantitative Description of Cloud Evolution

This grant funds a research program to use infrared extinction measurements to probe the detailed structure of dark molecular cloud cores and investigate the physical conditions which give rise to star and planet formation. The goals of this program are to acquire, reduce and analyze deep infrared and molecular-line observations of a carefully selected sample of nearby dark clouds in order to determine the detailed initial conditions for star formation from quantitative measurements of the internal structure of starless cloud cores and to quantitatively investigate the evolution of such structure through the star and planet formation process.

Lada, Charles J.↗

Measuring the Internal Structure and Physical Conditions in Star and Planet Forming Clouds Core: Toward a Quantitative Description of Cloud Evolution

This grant funds a research program to use infrared extinction measurements to probe the detailed structure of dark molecular cloud cores and investigate the physical conditions which give rise to star and planet formation. The goals of this program are to acquire, reduce and analyze deep infrared and molecular-line observations of a carefully selected sample of nearby dark clouds in order to internal structure of starless cloud cores and to quantitatively investigate the evolution of such structure through the star and planet formation process. During the second year of this grant, progress toward these goals is discussed.

Lada, Charles J.↗

Microphysical Timescales in Clouds and their Application in Cloud-Resolving Modeling

Independent prognostic variables in cloud-resolving modeling are chosen on the basis of the analysis of microphysical timescales in clouds versus a time step for numerical integration. Two of them are the moist entropy and the total mixing ratio of airborne water with no contributions from precipitating particles. As a result, temperature can be diagnosed easily from those prognostic variables, and cloud microphysics be separated (or modularized) from moist thermodynamics. Numerical comparison experiments show that those prognostic variables can work well while a large time step (e.g., 10 s) is used for numerical integration.

Zeng, Xiping↗

Use of In Situ Cloud Condensation Nuclei, Extinction, and Aerosol Size Distribution Measurements to Test a Method for Retrieving Cloud Condensation Nuclei Profiles From Surface Measurements

If the aerosol composition and size distribution below cloud are uniform, the vertical profile of cloud condensation nuclei (CCN) concentration can be retrieved entirely from surface measurements of CCN concentration and particle humidification function and surface-based retrievals of relative humidity and aerosol extinction or backscatter. This provides the potential for long-term measurements of CCN concentrations near cloud base. We have used a combination of aircraft, surface in situ, and surface remote sensing measurements to test various aspects of the retrieval scheme. Our analysis leads us to the following conclusions. The retrieval works better for supersaturations of 0.1% than for 1% because CCN concentrations at 0.1% are controlled by the same particles that control extinction and backscatter. If in situ measurements of extinction are used, the retrieval explains a majority of the CCN variance at high supersaturation for at least two and perhaps five of the eight flights examined. The retrieval of the vertical profile of the humidification factor is not the major limitation of the CCN retrieval scheme. Vertical structure in the aerosol size distribution and composition is the dominant source of error in the CCN retrieval, but this vertical structure is difficult to measure from remote sensing at visible wavelengths.

Ghan, Stephen J.↗

A State-of-the-Art Experimental Laboratory for Cloud and Cloud-Aerosol Interaction Research

The state of the art for predicting climate changes due to increasing greenhouse gasses in the atmosphere with high accuracy is problematic. Confidence intervals on current long-term predictions (on the order of 100 years) are so large that the ability to make informed decisions with regard to optimum strategies for mitigating both the causes of climate change and its effects is in doubt. There is ample evidence in the literature that large sources of uncertainty in current climate models are various aerosol effects. One approach to furthering discovery as well as modeling, and verification and validation (V&V) for cloud-aerosol interactions is use of a large "cloud chamber" in a complimentary role to in-situ and remote sensing measurement approaches. Reproducing all of the complex interactions is not feasible, but it is suggested that the physics of certain key processes can be established in a laboratory setting so that relevant fluid-dynamic and cloud-aerosol phenomena can be experimentally simulated and studied in a controlled environment. This report presents a high-level argument for significantly improved laboratory capability, and is meant to serve as a starting point for stimulating discussion within the climate science and other interested communities.

Fremaux, Charles M.↗