267,423 research outputs found
Cloud boundary height measurements using lidar and radar
Using only lidar or radar an accurate cloud boundary height estimate is often
not possible. The combination of lidar and radar can give a reliable cloud
boundary estimate in a much broader range of cases. However, also this
combination with standard methods still can not measure the cloud boundaries in
all cases. This will be illustrated with data from the Clouds and Radiation
measurement campaigns, CLARA. Rain is a problem: the radar has problems to
measure the small cloud droplets in the presence of raindrops. Similarly, few
large particles below cloud base can obscure the cloud base in radar
measurements. And the radar reflectivity can be very low at the cloud base of
water clouds or in large regions of ice clouds, due to small particles.
Multiple cloud layers and clouds with specular reflections can pose problems
for lidar. More advanced measurement techniques are suggested to solve these
problems. An angle scanning lidar can, for example, detect specular
reflections, while using information from the radars Doppler velocity spectrum
may help to detect clouds during rain.Comment: Reviewed conference contributio
Formation of giant molecular clouds in global spiral structures: The role of orbital dynamics and cloud-cloud collisions
The different roles played by orbital dynamics and dissipative cloud-cloud collisions in the formation of giant molecular clouds (GMCs) in a global spiral structure are investigated. The interstellar medium (ISM) is simulated by a system of particles, representing clouds, which orbit in a spiral-perturbed, galactic gravitational field. The overall magnitude and width of the global cloud density distribution in spiral arms is very similar in the collisional and collisionless simulations. The results suggest that the assumed number density and size distribution of clouds and the details of individual cloud-cloud collisions have relatively little effect on these features. Dissipative cloud-cloud collisions play an important steadying role for the cloud system's global spiral structure. Dissipative cloud-cloud collisions also damp the relative velocity dispersion of clouds in massive associations and thereby aid in the effective assembling of GMC-like complexes
Do Giant Molecular Clouds Care About the Galactic Structure?
We investigate the impact of galactic environment on the properties of
simulated giant molecular clouds formed in a M83-type barred spiral galaxy. Our
simulation uses a rotating stellar potential to create the grand design
features and resolves down to 1.5 pc. From the comparison of clouds found in
the bar, spiral and disc regions, we find that the typical GMC is environment
independent, with a mass of 5e+5 Msun and radius 11 pc. However, the fraction
of clouds in the property distribution tails varies between regions, with
larger, more massive clouds with a higher velocity dispersion being found in
greatest proportions in the bar, spiral and then disc. The bar clouds also show
a bimodality that is not reflected in the spiral and disc clouds except in the
surface density, where all three regions show two distinct peaks. We identify
these features as being due to the relative proportion of three cloud types,
classified via the mass-radius scaling relation, which we label A, B and C.
Type A clouds have the typical values listed above and form the largest
fraction in each region. Type B clouds are massive giant molecular associations
while Type C clouds are unbound, transient clouds that form in dense filaments
and tidal tails. The fraction of each clouds type depends on the cloud-cloud
interactions, which cause mergers to build up the GMA Type Bs and tidal
features in which the Type C clouds are formed. The number of cloud
interactions is greatest in the bar, followed by the spiral, causing a higher
fraction of both cloud types compared to the disc. While the cloud types also
exist in lower resolution simulations, their identification becomes more
challenging as they are not well separated populations on the mass-radius
relation or distribution plots. Finally, we compare the results for three star
formation models to estimate the star formation rate and efficiency in each
region.Comment: 21 pages, 14 figures. Accepted for publication in MNRA
Overlooked examples of cloud self-organization at the mesoscale
Stratocumulus clouds are common in the tropical and subtropical marine boundary layer, and understanding these clouds is important due to their significant impact on the earth's radiation budget. Observations show that the marine boundary layer contains complex, but poorly understood processes, which, from time to time, result in the observable self-organization of cloud structures at scales ranging from a few to a few thousand kilometers. Such shallow convective cloud features, typically observed as hexagonal cells, are known generally as mesoscale cellular convection (MCC). Actinoform clouds are rarer, but visually more striking forms of MCC, which possess a radial structure.
Because mesoscale cloud features are typically too large to be observed from the ground, observations of hexagonal cells historically date only to the beginning of satellite meteorology. Examples of actinoform clouds were shown in the venerable “Picture of the Month” series in Monthly Weather Review in the early 1960s, but these clouds were generally forgotten as research focused on hexagonal cells.
Recent high-resolution satellite images have, in a sense, “rediscovered” actinoform clouds, and they appear to be much more prevalent than had been previously suspected. We show a number of examples of actinoform clouds from a variety of locations worldwide. In addition, we have conducted a detailed case study of an actinoform cloud system using data from the Multiangle Imaging SpectroRadiometer (MISR) and the Geostationary Operational Environmental Satellite (GOES), including analysis of cloud heights, radiative properties, and the time-evolution of the cloud system. We also examine earlier theories regarding actinoform clouds in light of the new satellite data
Binary Formation in Star-Forming Clouds with Various Metallicities
Cloud evolution for various metallicities is investigated by
three-dimensional nested grid simulations, in which the initial ratio of
rotational to gravitational energy of the host cloud \beta_0 (=10^-1 - 10^-6)
and cloud metallicity Z (=0 - Z_\odot) are parameters. Starting from a central
number density of n = 10^4 cm^-3, cloud evolution for 48 models is calculated
until the protostar is formed (n \simeq 10^23 cm^-3) or fragmentation occurs.
The fragmentation condition depends both on the initial rotational energy and
cloud metallicity. Cloud rotation promotes fragmentation, while fragmentation
tends to be suppressed in clouds with higher metallicity. Fragmentation occurs
when \beta_0 > 10^-3 in clouds with solar metallicity, while fragmentation
occurs when \beta_0 > 10^-5 in the primordial gas cloud. Clouds with lower
metallicity have larger probability of fragmentation, which indicates that the
binary frequency is a decreasing function of cloud metallicity. Thus, the
binary frequency at the early universe (or lower metallicity environment) is
higher than at present day (or higher metallicity environment). In addition,
binary stars born from low-metallicity clouds have shorter orbital periods than
those from high-metallicity clouds. These trends are explained in terms of the
thermal history of the collapsing cloud.Comment: 11 pages, 2 figures, Submitted to ApJL, For high resolution figures
see http://astro3.sci.hokudai.ac.jp/~machida/binary-metal.pd
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