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A comparison among four different retrieval methods for ice-cloud properties using data from CloudSat, CALIPSO, and MODIS
The A-Train constellation of satellites provides a new capability to measure vertical cloud profiles that leads to more detailed information on ice-cloud microphysical properties than has been possible up to now. A variational radar–lidar ice-cloud retrieval algorithm (VarCloud) takes advantage of the complementary nature of the CloudSat radar and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar to provide a seamless retrieval of ice water content, effective radius, and extinction coefficient from the thinnest cirrus (seen only by the lidar) to the thickest ice cloud (penetrated only by the radar). In this paper, several versions of the VarCloud retrieval are compared with the CloudSat standard ice-only retrieval of ice water content, two empirical formulas that derive ice water content from radar reflectivity and temperature, and retrievals of vertically integrated properties from the Moderate Resolution Imaging Spectroradiometer (MODIS) radiometer. The retrieved variables typically agree to within a factor of 2, on average, and most of the differences can be explained by the different microphysical assumptions. For example, the ice water content comparison illustrates the sensitivity of the retrievals to assumed ice particle shape. If ice particles are modeled as oblate spheroids rather than spheres for radar scattering then the retrieved ice water content is reduced by on average 50% in clouds with a reflectivity factor larger than 0 dBZ. VarCloud retrieves optical depths that are on average a factor-of-2 lower than those from MODIS, which can be explained by the different assumptions on particle mass and area; if VarCloud mimics the MODIS assumptions then better agreement is found in effective radius and optical depth is overestimated. MODIS predicts the mean vertically integrated ice water content to be around a factor-of-3 lower than that from VarCloud for the same retrievals, however, because the MODIS algorithm assumes that its retrieved effective radius (which is mostly representative of cloud top) is constant throughout the depth of the cloud. These comparisons highlight the need to refine microphysical assumptions in all retrieval algorithms and also for future studies to compare not only the mean values but also the full probability density function
Distributional Equivalence and Structure Learning for Bow-free Acyclic Path Diagrams
We consider the problem of structure learning for bow-free acyclic path
diagrams (BAPs). BAPs can be viewed as a generalization of linear Gaussian DAG
models that allow for certain hidden variables. We present a first method for
this problem using a greedy score-based search algorithm. We also prove some
necessary and some sufficient conditions for distributional equivalence of BAPs
which are used in an algorithmic ap- proach to compute (nearly) equivalent
model structures. This allows us to infer lower bounds of causal effects. We
also present applications to real and simulated datasets using our publicly
available R-package
Projection operator approach to spin diffusion in the anisotropic Heisenberg chain at high temperatures
We investigate spin transport in the anisotropic Heisenberg chain in the
limit of high temperatures ({\beta} \to 0). We particularly focus on diffusion
and the quantitative evaluation of diffusion constants from current
autocorrelations as a function of the anisotropy parameter {\Delta} and the
spin quantum number s. Our approach is essentially based on an application of
the time-convolutionless (TCL) projection operator technique. Within this
perturbative approach the projection onto the current yields the decay of
autocorrelations to lowest order of {\Delta}. The resulting diffusion constants
scale as 1/{\Delta}^2 in the Markovian regime {\Delta}<<1 (s=1/2) and as
1/{\Delta} in the highly non-Markovian regime above {\Delta} \sim 1 (arbitrary
s). In the latter regime the dependence on s appears approximately as an
overall scaling factor \sqrt{s(s+1)} only. These results are in remarkably good
agreement with diffusion constants for {\Delta}>1 which are obtained directly
from the exact diagonalization of autocorrelations or have been obtained from
non-equilibrium bath scenarios.Comment: 4 pages, 3 figure
Quantum Darwinism in quantum Brownian motion: the vacuum as a witness
We study quantum Darwinism -- the redundant recording of information about a
decohering system by its environment -- in zero-temperature quantum Brownian
motion. An initially nonlocal quantum state leaves a record whose redundancy
increases rapidly with its spatial extent. Significant delocalization (e.g., a
Schroedinger's Cat state) causes high redundancy: many observers can measure
the system's position without perturbing it. This explains the objective (i.e.
classical) existence of einselected, decoherence-resistant pointer states of
macroscopic objects.Comment: 5 page
Biases in Expansion Distances of Novae Arising from the Prolate Geometry of Nova Shells
(abridged) Expansion distances (or expansion parallaxes) for classical novae
are based on comparing a measurement of the shell expansion velocity,
multiplied by the time since outburst, with some measure of the angular size of
the shell. We review and formalize this method in the case of prolate
spheroidal shells. We present expressions for the maximum line-of-sight
velocity from a complete, expanding shell and for its projected major and minor
axes, in terms of the intrinsic axis ratio and the inclination of the polar
axis to the line of sight. For six distinct definitions of ``angular size'', we
tabulate the error in distance that is introduced under the assumption of
spherical symmetry (i.e., without correcting for inclination and axis ratio).
The errors can be significant and systematic, affecting studies of novae
whether considered individually or statistically. Each of the six estimators
overpredicts the distance when the polar axis is close to the line of sight,
and most underpredict the distance when the polar axis is close to the plane of
the sky. The straight mean of the projected semimajor and semiminor axes gives
the least distance bias for an ensemble of randomly oriented prolate shells.
The best individual expansion distances, however, result from a full
spatio-kinematic modeling of the nova shell. We discuss several practical
complications that affect expansion distance measurements of real nova shells.
Nova shell expansion distances be based on velocity and angular size
measurements made contemporaneously if possible, and the same ions and
transitions should be used for the imaging and velocity measurements. We
emphasize the need for complete and explicit reporting of measurement
procedures and results, regardless of the specific method used.Comment: 21 pages, LaTeX, uses aasms4.sty, to be published in Publ. Astron.
Soc. of the Pacific, May 200
Infrared and Raman Spectroscopy of the Dirhodium Tetraacetate Complexes Rh2(O2CCH3)4, Rh2(18O2CCH3)4, Rh2(O2CCD3)4 and Rh2(O2CCH3)4(H2O)2
The infrared (3500-50 cm-I), and Raman (3550-30 cm-I)
spectra of the dirhodium tetraacetate species Rh2(02CCHs)4,
Rh2(IB02CCHs)4, Rh2(02CCDS)4, and Rh2(02CCHs)4(H20)2 have been recorded and the key bands assigned, The oxygen-18 and deuteration studies, in particular, assisted with ma king the important band assignments for the anhydrous complex, for which 11 (RhRh)
occurs at 355-351 cm-I, and 11 (RhO) at 389-319 cm-I (Raman) and
398-341 cm-I (infrared). The band attributed to 11 (RhRh) is typically
intense, sharp, and relatively insensitive to either IBO_ or CDs-substitution. Thus 11 (RhRh) shifts only 4 cm-Ion either IBO
or CDs substitution whereas 11 (RhO) shifts 4-6 cm-I in the Raman
and 3-7 cm-I in the infrared spectra on IBO substitution but 12
cm-I in the Raman and 11-18 cm-I in the infrared spectra on CDs
substitution. Some preliminary isotopic work for the complex
Rh2(02CCHS)4(H20l2 is also presented
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