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Wang, S., J.-C. Golaz and Q. Wang, 2008:
Effect of intense wind shear across the inversion on stratocumulus clouds
Geophys. Res. Lett.,
35, L15814, doi:10.1029/2008GL033865.
(pdf, 231K)
Abstract
A large-eddy simulation model is used to examine the impact of the intense cross-inversion
wind shear on the stratocumulus cloud structure. The wind shear enhanced entrainment mixing
effectively reduces the cloud water and thickens the inversion layer. It leads to a reduction
of the turbulence kinetic energy (TKE) production in the cloud layer due to the weakened
cloud-top radiative cooling and the formation of a turbulent and cloud free sublayer within
the inversion. The thickness of the sublayer increases with the enhanced wind shear intensity.
Under the condition of a weaker inversion, the enhanced shear mixing within the inversion
layer even lowers the cloud-top height and reduces the entrainment velocity. Finally,
increasing wind shear or reducing inversion strength tends to create an inversion layer with
a constant bulk Richardson number (~0.3), suggesting that an equilibrium value of the Richardson
number is reached.
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Teixeira J., B. Stevens, C. S. Bretherton, R. Cederwall, J. D. Doyle, J.-C. Golaz,
A. A. M. Holtslag, S. A. Klein, J. K. Lundquist, D. A. Randall,
A. P. Siebesma, and P. M. M. Soares, 2008:
Parameterization of the atmospheric boundary layer: A view from just above the inversion
Bull. Amer. Meteor. Soc.,
89, 453458.
(pdf, 655K)
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Golaz, J.-C, V. E. Larson, J. A. Hansen, D. P. Schanen, and B. M. Griffin, 2007:
Elucidating model inadequacies in a cloud parameterization by use of an ensemble-based
calibration framework Mon. Wea. Rev.,
135, 4077-4096.
(pdf, 2.1M)
Abstract
Every cloud parameterization contains structural model errors. The source of these errors is
difficult to pinpoint because cloud parameterizations contain nonlinearities and feedbacks.
To elucidate these model inadequacies, this paper uses a general-purpose ensemble parameter
estimation technique. In principle, the technique is applicable to any parameterization that
contains a number of adjustable coefficients. It optimizes or calibrates parameter values by
attempting to match predicted fields to reference datasets. Rather than striving to find the
single best set of parameter values, the output is instead an ensemble of parameter sets. This
ensemble provides a wealth of information. In particular, it can help uncover model deficiencies
and structural errors that might not otherwise be easily revealed. The calibration technique
is applied to an existing single-column model (SCM) that parameterizes boundary layer clouds.
The SCM is a higher-order turbulence closure model. It is closed using a multivariate probability
density function (PDF) that represents subgrid-scale variability. Reference datasets are
provided by large-eddy simulations (LES) of a variety of cloudy boundary layers. The calibration
technique locates some model errors in the SCM. As a result, empirical modifications are
suggested. These modifications are evaluated with independent datasets and found to lead to an
overall improvement in the SCM's performance.
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Larson, V. E., A. J. Smith, M. J. Falk, K. E. Kotenberg, and J.-C. Golaz, 2006:
What determines altocumulus dissipation time? J. Geophys. Res.,
111, D19207, doi:10.1029/2005JD007002.
(pdf, 726K)
Abstract
This paper asks what factors influence the dissipation time of altocumulus clouds. The
question is addressed using three-dimensional, large-eddy simulations of a thin, midlevel
cloud that was observed by aircraft. The cloud might be aptly described as "altostratocumulus"
because it was overcast and contained radiatively driven turbulence. The simulations are used to
construct a budget equation of cloud water. This equation allows one to directly compare the
four processes that diminish liquid: diffusional growth of ice crystals, large-scale subsidence,
radiative heating, and turbulent mixing of dry air into the cloud. Various sensitivity studies
are used to find the "equivalent sensitivity" of cloud decay time to changes in various
parameters. A change from no sunlight to direct overhead sunlight decreases the lifetime of
our simulated cloud as much as increasing subsidence by 1.2 cm s-1, increasing ice number
concentration by 780 m-3, or decreasing above-cloud total water mixing ratio by 0.60 g kg-1.
Finally, interactions among the terms in the cloud water budget are summarized in a "budget
term feedback matrix." It is able to diagnose, for instance, that in our particular simulations,
the diffusional growth of ice is a negative feedback.
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Beare, R. J., M. K. MacVean, A. A. M. Holtslag, J. Cuxart, I. Esau, J.-C.
Golaz, M. A. Jimenez, M. Khairoutdinov, B. Kosovic, D. Lewellen, T. S. Lund,
J. K. Lundquist, A. McCabe, A. F. Moene, Y. Noh, S. Raasch, and
P. Sullivan, 2006: An intercomparison of large-eddy simulations of the stable
boundary layer. Bound.-Layer Meteor., 118, 247-272.
(pdf, 657K)
Abstract
Results are presented from the first intercomparison of large-eddy simulation
(LES) models for the stable boundary layer (SBL), as part of the Global Energy
and Water Cycle Experiment Atmospheric Boundary Layer Study initiative. A moderately
stable case is used, based on Arctic observations. All models produce successful
simulations, in as much as they generate resolved turbulence and reflect many of
the results from local scaling theory and observations. Simulations performed at
1-m and 2-m resolution show only small changes in the mean profiles compared to
coarser resolutions. Also, sensitivity to subgrid models for individual models
highlights their importance in SBL simulation at moderate resolution (6.25 m).
Stability functions are derived from the LES using typical mixing lengths used in
numerical weather prediction (NWP) and climate models. The functions have smaller
values than those used in NWP. There is also support for the use of K-profile
similarity in parametrizations. Thus, the results provide improved understanding
and motivate future developments of the parametrization of the SBL.
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Golaz, J.-C., S. Wang, J. D. Doyle, and J. M. Schmidt, 2005: COAMPS®
-LES: Model evaluation and analysis of second and third
moment vertical velocity budgets. Bound.-Layer Meteor., 116,
487-517.
(pdf, 975K)
Abstract
The Naval Research Laboratory Coupled Ocean/Atmosphere Mesoscale Prediction
System (COAMPS®) has been extended to perform as a large-eddy simulation
(LES) model. It has been validated with a series of boundary-layer experiments
spanning a range of cloud nighttime, and includes a nighttime stratocumulus case,
a trade wind cumulus layer, shallow cumulus convection over land, and a mixed
regime consisting of cumulus clouds under broken stratocumulus. COAMPS-LES results
are in good agreement with other models for all the cases simulated. Exact numerical
budgets for the vertical velocity second (w'2) and third moment (w'3) have been
derived for the stratocumulus and trade wind cumulus cases. For the w'3 budget in
the stratocumulus, the buoyancy contribution from the updraughts and downdraughts
largely cancel each other due to their similar magnitudes but opposite signs.
In contrast, for the cumulus layer, the negative buoyancy contribution from the
environmental downdraughts is negligible and the positive contribution from the
updraughts completely dominates due to the conditional instability in the environment.
As a result, w'3 is significantly larger in the cumulus than in the stratocumulus layer.
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Larson, V. E., J.-C. Golaz, H. Jiang, and W. R. Cotton, 2005: Supplying local
microphysics parameterizations with information about subgrid variability:
Latin hypercube sampling. J. Atmos. Sci., 62, 4010-4026.
(pdf, 997K)
Abstract
One problem in computing cloud microphysical processes in coarse-resolution numerical
models is that many microphysical processes are nonlinear and small in scale.
Consequently, there are inaccuracies if microphysics parameterizations are forced
with grid box averages of model fields, such as liquid water content. Rather,
the model needs to determine information about subgrid variability and input it
into the microphysics parameterization. One possible solution is to assume the shape
of the family of probability density functions (PDFs) associated with a grid box and
sample it using the Monte Carlo method. In this method, the microphysics subroutine
is called repeatedly, once with each sample point. In this way, the Monte Carlo method
acts as an interface between the host model's dynamics and the microphysical
parameterization. This avoids the need to rewrite the microphysics subroutines.
A difficulty with the Monte Carlo method is that it introduces into the simulation
statistical noise or variance, associated with the finite sample size. If the family
of PDFs is tractable, one can sample solely from cloud, thereby improving estimates
of in-cloud processes. If one wishes to mitigate the noise further, one needs a
method for reduction of variance. One such method is Latin hypercube sampling, which
reduces noise by spreading out the sample points in a quasi-random fashion. This
paper formulates a sampling interface based on the Latin hypercube method. The
associated family of PDFs is assumed to be a joint normal/lognormal
(i.e., Gaussian/lognormal) mixture. This method of variance reduction has a
couple of advantages. First, the method is general: the same interface can be
used with a wide variety of microphysical parameterizations for various processes.
Second, the method is flexible: one can arbitrarily specify the number of hydrometeor
categories and the number of calls to the microphysics parameterization per grid box
per time step. This paper performs a preliminary test of Latin hypercube sampling.
As a prototypical microphysical formula, this paper uses the Kessler autoconversion
formula. The PDFs that are sampled are extracted diagnostically from large-eddy
simulations (LES). Both stratocumulus and cumulus boundary layer cases are tested.
In this diagnostic test, the Latin hypercube can produce somewhat less noisy
time-averaged estimates of Kessler autoconversion than a traditional Monte Carlo
estimate, with no additional calls to the microphysics parameterization. However,
the instantaneous estimates are no less noisy. This paper leaves unanswered the
question of whether the Latin hypercube method will work well in a prognostic,
interactive cloud model, but this question will be addressed in a future manuscript.
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Larson, V. E. and J.-C. Golaz, 2005: Using probability density functions to
derive consistent closure relationships among higher-order moments. Mon.
Wea. Rev., 133, 1023-1042.
(pdf, 621K)
Abstract
Parameterizations of turbulence often predict several lower-order moments
and make closure assumptions for higher-order moments. In principle, the low-
and high-order moments share the same probability density function (PDF). One
closure assumption, then, is the shape of this family of PDFs. When the higher-order
moments involve both velocity and thermodynamic scalars, often the PDF shape has
been assumed to be a double or triple delta function. This is equivalent to
assuming a mass-flux model with no subplume variability. However, PDF families
other than delta functions can be assumed. This is because the assumed PDF
methodology is fairly general. This paper proposes closures for several third-
and fourth-order moments. To derive the closures, the moments are assumed to
be consistent with a particular PDF family, namely, a mixture of two trivariate
Gaussians. (This PDF is also called a double Gaussian or binormal PDF by some
authors.) Separately from the PDF assumption, the paper also proposes a simplified
relationship between scalar and velocity skewnesses. This PDF family and skewness
relationship are simple enough to yield simple, analytic closure formulas relating
the moments. If certain conditions hold, this set of moments is specifically realizable.
By this it is meant that the set of moments corresponds to a real Gaussian-mixture
PDF, one that is normalized and nonnegative everywhere. This paper compares the
new closure formulas with both large eddy simulations (LESs) and closures based
on double and triple delta PDFs. This paper does not implement the closures in
a single-column model and test them interactively. Rather, the comparisons are
diagnostic; that is, low-order moments are extracted from the LES and treated
as givens that are input into the closures. This isolates errors in the closures
from errors in a single-column model. The test cases are three atmospheric boundary
layers: a trade wind cumulus layer, a stratocumulus layer, and a clear convective
case. The new closures have shortcomings, but nevertheless are superior to the double
or triple delta closures in most of the cases tested.
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Stevens, B., C.-H. Moeng, A. S. Ackerman, C. S. Bretherton, A. Chlond,
S. de Roode, J. Edwards, J.-C. Golaz, H. Jiang, M. Khairoutdinov, M. P.
Kirkpatrick, D. C. Lewellen, A. Lock, F. Müller, D. E. Stevens,
E. Whelan, and P. Zhu, 2005: Evaluation of large-eddy simulations via
observations of nocturnal marine stratocumulus. Mon. Wea. Rev., 133, 1443-1462.
(pdf, 869K)
Abstract
Data from the first research flight (RF01) of the second Dynamics and Chemistry
of Marine Stratocumulus (DYCOMS-II) field study are used to evaluate the fidelity
with which large-eddy simulations (LESs) can represent the turbulent structure of
stratocumulus-topped boundary layers. The initial data and forcings for this case
placed it in an interesting part of parameter space, near the boundary where
cloud-top mixing is thought to render the cloud layer unstable on the one hand, or
tending toward a decoupled structure on the other hand. The basis of this evaluation
consists of sixteen 4-h simulations from 10 modeling centers over grids whose vertical
spacing was 5 m at the cloud-top interface and whose horizontal spacing was 35 m.
Extensive sensitivity studies of both the configuration of the case and the numerical
setup also enhanced the analysis. Overall it was found that (i) if efforts are made
to reduce spurious mixing at cloud top, either by refining the vertical grid or limiting
the effects of the subgrid model in this region, then the observed turbulent and
thermodynamic structure of the layer can be reproduced with some fidelity; (ii)
the base, or native configuration of most simulations greatly overestimated mixing
at cloud top, tending toward a decoupled layer in which cloud liquid water path and
turbulent intensities were grossly underestimated; (iii) the sensitivity of the
simulations to the representation of mixing at cloud top is, to a certain extent,
amplified by particulars of this case. Overall the results suggest that the use of
LESs to map out the behavior of the stratocumulus-topped boundary layer in this
interesting region of parameter space requires a more compelling representation
of processes at cloud top. In the absence of significant leaps in the understanding
of subgrid-scale (SGS) physics, such a representation can only be achieved by a
significant refinement in resolution--a refinement that, while conceivable given
existing resources, is probably still beyond the reach of most centers.
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Zhu, P., C. S. Bretherton, M. Köhler, A. Cheng, A. Chlond, Q.Geng,
P. Austin, J.-C. Golaz, G. Lenderink, A. Lock, and B. Stevens, 2005:
Intercomparison and interpretation of single-column model simulations of a
nocturnal stratocumulus-topped marine boundary layer. Mon. Wea. Rev.,
133, 2741-2758.
(pdf, 1.3M)
Abstract
Ten single-column models (SCMs) from eight groups are used to simulate a nocturnal
nonprecipitating marine stratocumulus-topped mixed layer as part of an intercomparison
organized by the Global Energy and Water Cycle Experiment Cloud System Study, Working
Group 1. The case is idealized from observations from the Dynamics and Chemistry of
Marine Stratocumulus II, Research Flight 1. SCM simulations with operational resolution
are supplemented by high-resolution simulations and compared with observations and
large-eddy simulations. All participating SCMs are able to maintain a sharp inversion
and a mixed cloud-topped layer, although the moisture profiles show a slight gradient
in the mixed layer and produce entrainment rates broadly consistent with observations,
but the liquid water paths vary by a factor of 10 after only 1 h of simulation at both
high and operational resolution. Sensitivity tests show insensitivity to activation of
precipitation and shallow convection schemes in most models, as one would observationally
expect for this case.
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Cheng, A., K.-M. Xu, and J.-C. Golaz, 2004: The liquid water oscillation in
modeling boundary layer cumuli with third-order turbulence closure models.
J. Atmos. Sci., 61, 1621-1629.
(pdf, 778K)
Abstract
A hierarchy of third-order turbulence closure models are used to simulate boundary
layer cumuli in this study. An unrealistically strong liquid water oscillation (LWO)
is found in the fully prognostic model, which predicts all third moments. The LWO
propagates from cloud base to cloud top with a speed of 1 m s-1. The period of the
oscillation is about 1000 s. Liquid water buoyancy (LWB) terms in the third-moment
equations contribute to the LWO. The LWO mainly affects the vertical profiles of cloud
fraction, mean liquid water mixing ratio, and the fluxes of liquid water potential
temperature and total water, but has less impact on the vertical profiles of other
second and third moments. In order to minimize the LWO, a moderately large diffusion
coefficient and a large turbulent dissipation at its originating level are needed.
However, this approach distorts the vertical distributions of cloud fraction and
liquid water mixing ratio. A better approach is to parameterize LWB more reasonably.
A minimally prognostic model, which diagnoses all third moments except for the vertical
velocity, is shown to produce better results, compared to a fully prognostic model.
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Larson, V. E., J.-C. Golaz, and W. R. Cotton, 2002: Small-scale and mesoscale
variability in cloudy boundary layers: Joint probability density functions.
J. Atmos. Sci., 59, 3519-3539.
(pdf, 574K)
Abstract
The joint probability density function (PDF) of vertical velocity and conserved
scalars is important for at least two reasons. First, the shape of the joint PDF
determines the buoyancy flux in partly cloudy layers. Second, the PDF provides a
wealth of information about subgrid variability and hence can serve as the foundation
of a boundary layer cloud and turbulence parameterization. This paper analyzes PDFs
of stratocumulus, cumulus, and clear boundar y layers obtained from both aircraft
observations and large eddy simulations. The data are used to fit five families of
PDFs: a double delta function, a single Gaussian, and three PDF families based on
the sum of two Gaussians. Overall, the double Gaussian, that is binormal, PDFs
perform better than the single Gaussian or double delta function PDFs. In cumulus
layers with low cloud fraction, the improvement occurs because typical PDFs are
highly skewed, and it is crucial to accurately represent the tail of the distribution,
which is where cloud occurs. Since the double delta function has been shown in
prior work to be the PDF underlying mass-flux schemes, the data analysis herein
hints that mass-flux simulations may be improved upon by using a parameterization
built upon a more realistic PDF.
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Golaz, J.-C., V. E. Larson, and W. R. Cotton, 2002a: A PDF based
model for boundary layer clouds. Part I: Method and model description.
J. Atmos. Sci., 59, 3540-3551.
(pdf, 195K)
Abstract
A new cloudy boundary layer single-column model is presented. It is designed
to be flexible enough to represent a variety of cloudiness regimes--such as cumulus,
stratocumulus, and clear regimes--without the need for case-specific adjustments.
The methodology behind the model is the so-called assumed probability density function
(PDF) method. The parameterization differs from higher-order closure or mass-flux schemes
in that it achieves closure by the use of a relatively sophisticated joint PDF of
vertical velocity, temperature, and moisture. A family of PDFs is chosen that is
flexible enough to represent various cloudiness regimes. A double Gaussian family
proposed by previous works is used. Predictive equations for grid box means and a number
of higherorder turbulent moments are advanced in time. These moments are in turn used to
select a particular member from the family of PDFs, for each time step and grid box. Once
a PDF member has been selected, the scheme integrates over the PDF to close higher-order
moments, buoyancy terms, and diagnose cloud fraction and liquid water. Since all the
diagnosed moments for a given grid box and time step are derived from the same unique joint
PDF, they are guaranteed to be consistent with one another. A companion paper presents
simulations produced by the single-column model.
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Golaz, J.-C., V. E. Larson, and W. R. Cotton, 2002b: A PDF based
model for boundary layer clouds. Part II: Model results. J. Atmos.
Sci., 59, 3552-3571.
(pdf, 662K)
Abstract
A new single-column model for the cloudy boundary layer, described in a companion
paper, is tested for a variety of regimes. To represent the subgrid-scale variability,
the model uses a joint probability density function (PDF) of vertical velocity, temperature,
and moisture content. Results from four different cases are presented and contrasted with
large eddy simulations (LES). The cases include a clear convective layer based on the
Wangara experiment, a trade wind cumulus layer from the Barbados Oceanographic and
Meteorological Experiment (BOMEX), a case of cumulus clouds over land, and a nocturnal
marine stratocumulus boundary layer. Results from the Wangara experiment show that the
model is capable of realistically predicting the diurnal growth of a dry convective
layer. Compared to the LES, the layer produced is slightly less well mixed and
entrainment is somewhat slower. The cloud cover in the cloudy cases varied widely,
ranging from a few percent cloud cover to nearly overcast. In each of the cloudy cases,
the parameterization predicted cloud fractions that agree reasonably well with the LES.
Typically, cloud fraction values tended to be somewhat smaller in the parameterization,
and cloud bases and tops were slightly underestimated. Liquid water content was generally
within 40% of the LES-predicted values for a range of values spanning almost two orders
of magnitude. This was accomplished without the use of any case-specific adjustments.
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Brown, A. R., R. T. Cederwall, A. Chlond, P. G. Duynkerke, J.-C. Golaz,
M. Khairoutdinov, D. C. Lewellen, A. P. Lock, M. K. MacVean, C.-H. Moeng,
R. A. J. Neggers, A. P. Siebesma, and B. Stevens, 2002: Large-eddy simulation
of the diurnal cycle of shallow cumulus convection over land. Quart. J.
Roy. Meteor. Soc., 128, 1075-1093.
(pdf, 601K)
Abstract
Large-eddy simulations of the development of shallow cumulus convection over land
are presented. Many characteristics of the cumulus layer previously found in simulations
of quasi-steady convection over the sea are found to be reproduced in this more strongly
forced, unsteady case. Furthermore, the results are shown to be encouragingly robust,
with similar results obtained with eight independent models, and also across a range
of numerical resolutions. The datasets produced are already being used in the
development and evaluation of parametrizations used in numerical weather-prediction
and climate models .
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Golaz, J.-C., H. Jiang, and W. R. Cotton, 2001: A large-eddy simulation study
of cumulus clouds over land and sensitivity to soil moisture. Atmos.
Res., 59-60, 373-392.
(pdf, 275K)
Abstract
A series of large-eddy simulations (LES) of non-precipitating cumulus clouds
over land was performed. These simulations were idealized from observed conditions
at the Southern Great Plains ARM site on 21 June 1997 and were intended to investigate
the effect of initial soil moisture on the structure of the cloudy boundary layer. The
surface fluxes were either dominated by latent heat or sensible heat flux, with the
transition between one regime and the other occurring over a very narrow soil moisture
range. The effect on clouds was mixed. Cloud fraction was nearly identical throughout
all experiments. Simulations with dominant sensible heat fluxes led to more turbulent
boundary layers and higher cloud bases. Simulations dominated by latent heat flux tended
to have fewer but stronger updrafts in the cloud layer.
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Larson, V. E., R. Wood, P. R. Field, J.-C. Golaz, T. H. Vonder Haar, and
W. R. Cotton, 2001a: Systematic biases in the microphysics and thermodynamics
of numerical models that ignore subgrid-scale variability. J. Atmos.
Sci., 58, 1117-1128.
(pdf, 184K)
Abstract
A grid box in a numerical model that ignores subgrid variability has biases in certain
microphysical and thermodynamic quantities relative to the values that would be obtained
if subgrid-scale variability were taken into account. The biases are important because
they are systematic and hence have cumulative effects. Several types of biases are
discussed in this paper. Namely, numerical models that employ convex autoconversion
formulas underpredict (or, more precisely, never overpredict) autoconversion rates,
and numerical models that use convex functions to diagnose specific liquid water content
and temperature underpredict these latter quantities. One may call these biases the
"grid box average autoconversion bias," "grid box average liquid water content bias,
" and "grid box average temperature bias," respectively, because the biases arise when
grid box average values are substituted into formulas valid at a point, not over an extended
volume. The existence of these biases can be derived from Jensen's inequality. To assess
the magnitude of the biases, the authors analyze observations of boundary layer clouds.
Often the biases are small, but the observations demonstrate that the biases can be large
in important cases. In addition, the authors prove that the average liquid water content
and temperature of an isolated, partly cloudy, constant-pressure volume of air cannot increase,
even temporarily. The proof assumes that liquid water content can be written as a convex
function of conserved variables with equal diffusivities. The temperature decrease is due
to evaporative cooling as cloudy and clear air mix. More generally, the authors prove that
if an isolated volume of fluid contains conserved scalars with equal diffusivities, then
the average of any convex, twice-differentiable function of the conserved scalars cannot increase.
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Larson, V. E., R. Wood, P. R. Field, J.-C. Golaz, T. H. Vonder Haar, and
W. R. Cotton, 2001b: Small-scale and mesoscale variability of scalars in
cloudy boundary layers: One-dimensional probability density functions. J. Atmos. Sci., 58, 1978-1994.
(pdf, 279K)
Abstract
A key to parameterization of subgrid-scale processes is the probability density function
(PDF) of conserved scalars. If the appropriate PDF is known, then grid box average cloud
fraction, liquid water content, temperature, and autoconversion can be diagnosed. Despite
the fundamental role of PDFs in parameterization, there have been few observational studies
of conserved-scalar PDFs in clouds. The present work analyzes PDFs from boundary layers
containing stratocumulus, cumulus, and cumulus-rising-into-stratocumulus clouds. Using
observational aircraft data, the authors test eight different parameterizations of PDFs,
including double delta function, gamma function, Gaussian, and double Gaussian shapes.
The Gaussian parameterization, which depends on two parameters, fits most observed PDFs
well but fails for large-scale PDFs of cumulus legs. In contrast, three-parameter
parameterizations appear to be sufficiently general to model PDFs from a variety of
cloudy boundary layers. If a numerical model ignores subgrid variability, the model has
biases in diagnoses of grid box average liquid water content, temperature, and Kessler
autoconversion, relative to the values it would obtain if subgrid variability were taken
into account. The magnitude of such biases is assessed using obser vational data. The
biases can be largely eliminated by three-parameter PDF parameterizations. Prior authors
have suggested that boundary layer PDFs from short segments are approximately Gaussian.
The present authors find that the hypothesis that PDFs of total specific water content
are Gaussian can almost always be rejected for segments as small as 1 km.
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Galli, G., F. Gygi, and J.-C. Golaz, 1998: Vibrational and electronic
properties of neutral and negatively charged C20
clusters. Physical Review B, 57, 1860-1867.
(pdf, 161K)
Abstract
We computed vibrational and electronic properties of the cage, bowl, and ring isomers of
neutral and negatively charged C20, within density-functional theory, using
fully optimized local-density and gradient-corrected geometries. Vibrational and electronic
spectra exhibit distinctive features, which could be used to identify a given isomer and
its charge state in molecular beams or thin films. Notable changes are observed in both
the Raman and infrared spectra when going from the neutral to the charged isomers. We
also calculated vibrational entropies from harmonic frequencies. Our results indicate
that, above a critical temperature, the ring isomer is always stabilized by entropic
effects, irrespective of the theoretical model used to compute the internal energy. In
particular, gradient-corrected functionals predict both the neutral and charged ring to
be the most stable isomer at all temperatures. Molecular-dynamics simulations were performed
to study the geometry of the ring at high temperature. Furthermore, we rationalized photoelectron
spectra of C2n clusters, n=9-12, in terms of differences in the electronic
structure for even and odd n.
