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Robert Tardif
(NCAR/RAP)
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Peter Zwack
(UQÀM)
1. Introduction
2. The COBEL 1D model
3. Model studies and
realizations
4. Related publications
1. Introduction
Improved understanding of complex
atmospheric boundary layer interactions are obtained through careful analysis of
measurements gathered during detailed field experiments and through
modeling studies. With the ever increasing computer power, sophisticated 3D
models, such as Large Eddy Simulation (LES) and Direct
Numerical Simulation (DNS), are now used to gain understanding into
boundary layer processes in a research environment. But their use as forecast tools is beyond what is
currently possible.
Gains in forecast accuracy of
boundary layer parameters can
be obtained by employing 1D high-resolution models. When used in
a consistent manner, these models can be useful in a number of
ways. When coupled to 3D mesoscale
models or analyses, high-resolution column models can provide
detailed and accurate simulations and forecasts of the boundary
layer structure (Musson-Genon, 1989, Guédalia and Bergot, 1994). Also,
sensitivity experiments and case studies using data from
field experiments can be performed to acquire a better
understanding of the importance of physical processes and their
interactions in improving forecast accuracy. Also,
column models, being simple to use yet comprehensive enough to
represent most atmospheric boundary layer processes, can easily
by used as teaching tools in classrooms to illustrate basic
concepts on boundary layer meteorology at the graduate or even
undergraduate levels.
2. The
COBEL 1D model
2.1 General considerations
COBEL is an acronym that stands for "COuche
Brouillard Eau Liquide". It is a high-resolution 1D (column)
model originally designed to simulate the evolution of the very
stable atmospheric boundary layer vertical structure at the local
scale. It can be coupled to a state-of-the-art 3D mesoscale model
or analyses to take into account the effects of possible horizontal
inhomogeneities in the mean state of the atmosphere. The effect of these
inhomogeneities are viewed as external forcings by the column model. The
external influences taken into account are the horizontal pressure force
(geostrophic wind), advection of temperature and humidity,
vertical motion and local pressure tendencies, which can all act
to modify the dynamic and thermodynamic structure of the boundary
layer (Figure 1). Using these mesoscale forcings, COBEL, with its adapted parameterization of radiative transfer and
turbulent mixing, among others, can be used as a local
dynamical adaptation engine for the boundary layer. High-resolution
representations of boundary layer vertical structure can be
obtained using this 3D-1D coupling methodology. The study
of Sharan and Gopalakrishnan (1997) has shown that the use of a the formulation of eddy exchange coefficients developed for, and
implemented in, COBEL can result in very good simulations of the
mean structure of the nocturnal boundary layer under a variety of
wind conditions. These results confirm those obtained by Estournel and
Guédalia (1985, 1987)
Figure 1. Coupling
strategy between a 3D mesoscale model and 1D high-resolution
boundary layer model.
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2.2 Summary of model characteristics
COBEL was originally designed at the Laboratoire
d'Aérologie, Université Paul Sabatier, Toulouse, France. Since
the initial motivation for its development was the study of
physical processes within the very stable nocturnal boundary
layer, COBEL incorporates standard as well as special (for very
stable boundary layers) turbulent mixing parameterizations based
on Turbulence Kinetic Energy (TKE) and mixing length (1.5 order
closure), as well as a sophisticated high spectral resolution
(232 channels) infrared radiation scheme (Vehil et al.,
1989) for accurate computations of radiative flux divergence in
the surface layer. The turbulent mixing parameterization is
especially adapted for the very stable stratification through the
use of a mixing length formulation for large Richardson numbers (Estournel and
Guédalia, 1987). Other formulations adapted to
the weakly stable, neutral and unstable boundary layer are also
part of the COBEL model. Other model features include an "all or nothing"
condensation scheme, the parameterization of the sedimentation of
fog droplets, as well as the Kessler (1969) parameterization of
precipitation physics for the drizzling process. The soil model
is based on the diffusion equation for soil temperature and
incorporates the Oregon
State University soil model of Mahrt and
Pan (1984) for soil moisture. Solar radiation is computed using
the Fouquart and Bonnel (1980) scheme developed at the Laboratoire d'Optique Atmosphérique,
Université des Sciences et Technologies de Lille, France. The complete set of physical
processes simulated by COBEL is shown schematically in Figure 2
and listed in Table I.
Figure 2. Graphical
depiction of physical processes included in the COBEL model
parameterizations.
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Table I. List of
physical processes represented into the COBEL column model.
External forcings are indicated by *
Physical
processes
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Shortwave radiative transfer
in clear and cloudy atmospheres (scattering,
transmission, reflection, absorption)
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Horizontal advection of
temperature, humidity and momentum*
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Vertical advection by
mesoscale vertical motion*
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Pressure tendency*
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3. Model
studies and realizations
A brief historical look at the COBEL model
development achievements is presented in this section. As
indicated above, COBEL was originally designed to study the
physical processes within the very stable nocturnal boundary
layer (BL). Once suitable parameterizations were developed for
such very stable boundary layers (Estournel and Guédalia, 1987),
the column model was used to perform a number of sensitivity
studies, comparisons with other models and simulations of real
cases from various field experiments (Estournel, 1988). The focus
was on the evening transition from neutral to very stable
stability regimes, over relatively flat terrain. The simulations
allowed a quantitative assessment of the relative contributions
of radiative flux divergence and turbulent flux divergence to
vertical distribution of the cooling throughout the boundary
layer.
After the initial development and
validation of the COBEL model prototype for the study of the dry
nocturnal BL, efforts were undertaken to adapt the model to
perform short-term forecasts of radiation fog events in the Nord-Pas de Calais region in Northern France
(Bergot and Guédalia, 1994). The goal was to develop a numerical tool able
to provide guidance to help the forecaster determine if fog will
form during the upcoming night. Efforts were focused on the
development of the model's capabilities to predict the onset and
development of a fog layer 6 to 12 hours in advance.
Using data from special field experiments
to provide initial conditions, and data from a mesonet to
determine the regional temperature and humidity advections,
several well-documented fog and near fog (100% humidity but no
fog) events were successfully simulated with COBEL (Guédalia and Bergot, 1994. Tests performed with COBEL indicate that the most
crucial process to take into account is the rate at which dew is
deposited at the surface. Even if relative humidity is near 100%
at sunset, if the rate at which the dew formation process
depletes the lower atmosphere of its humidity is greater than the
rate of increase of sursaturation produced by surface radiative
cooling, then no fog will form.
COBEL was also tested in an operational
mode (operational 3D mesoscale model providing initial
conditions, and forecast cloud cover and advections) over three
winters worth of data. Results showed that short-term fog
forecasts were greatly improved over forecasts issued by Météo
France's regional weather center (see
Table II below).
Table II. Statistical
verification of fog forecasts. 229 days, 54 observed fog events
(from Bergot and Guédalia, 1996)
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Regional weather center
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COBEL
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Probability of Detection
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67%
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85%
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False Alarm Ratio
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57%
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25%
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In addition, an error analysis indicated
that most of the prediction errors were related to erroneous
cloud cover forecasts from the 3D mesoscale model.
Bechtold et al., 1996 showed that
results from suitable high-resolution 1D ensemble-average models
similar to COBEL compare favorably well with Large Eddy
Simulation data. Indeed, similar temperature, humidity and cloud
water profiles were obtained with 1D and LES models for an
idealized cloud-topped boundary layer. Furthermore, a 1D model
very similar to COBEL reproduced very well the turbulence kinetic
energy computed with LES data.
Recognizing the fact that the COBEL column
model is able to provide accurate predictions of the evolution of
the very stable boundary layer structure during evening
transitions, work toward the adaptation of the model for the
simulation of the morning BL transition were performed at
Université du Québec à Montréal, in collaboration with the Massachusetts
Institute of Technology Lincoln Laboratory. This work has been funded by the Federal Aviation
Administration (FAA) as part of efforts toward the improvement of
ceiling & visibility forecasts; and by the National
Aeronautics and Space Administration (NASA) as part of
development efforts of the Aircraft Vortex Spacing System. Simple data
assimilation schemes were also implemented in the model. The new capabilities of the COBEL model
were validated using data from NASA funded Wake Vortex field
measurement programs at the Memphis and Dallas-Fort Worth
airports (Tardif and Zwack, 1997; 1998). Data from
operational 3D mesoscale models were used to drive the column model for these
validation experiments.
Recent efforts include the adaptation of
COBEL to forecast
marine stratus burn-off at the San Francisco airport. As part of
this work, techniques were developed to retrieve horizontal temperature and
moisture advections from
additional special airport observations combined with model
simulations. Realistic values of temperature and moisture advections have been obtained with
the current version of
the technique, but further validation is required.
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Related
publications
Model development and nocturnal boundary
layer studies:
Guédalia, D., C. Estournel and R. Vehil, 1984: "Effects of sahel
dust layers upon nocturnal cooling of the atmosphere (ECLATS
experiment)". J. Clim.
Appl. Meteor., 23, 644-650.
Estournel, C., 1988: Etude de la
phase nocturne de la couche limite atmosphérique. Ph. D.
thesis, no. 1361, Paul Sabatier University, Toulouse, France.
(In French).
Estournel, C. and D. Guédalia, 1985: "Influence of geostrophic wind on atmospheric
nocturnal cooling". J.
Atmos. Sci., 42, 2695-2698.
Estournel, C, R. Vehil and D. Guédalia, 1986: "An
observational study of radiative and turbulent cooling in the
nocturnal boundary layer (ECLATS experiment)". Bound. Layer. Meteor., 34,
55-62.
Estournel, C. and D. Guédalia, 1987: "A new parameterization of eddy diffusivities for
nocturnal boundary layer modeling". Bound. Layer. Meteor., 39,
191-203.
Vehil, R., J. Monneris, D. Guédalia
and P. Sarthou, 1989: "Study
of the radiative effects (long-wave and short-wave) within a
fog layer". Atmos. Res.,
23, 179-194.
Fog forecasting:
Bergot, T., 1993: Modélisation du
Brouillard à l'Aide
d'un Modèle 1D forçé par des Champs
Mésoéchelle: Application à la Prévision. Ph. D. thesis, no. 1546, Paul Sabatier
University, Toulouse, France. (In French).
Bergot, T. and D. Guédalia, 1994: "Numerical forecasting of radiation fog. Part I:
Numerical model and sensitivity tests". Mon. Wea. Rev., 122, 1218-1230.
Guédalia D. and T. Bergot, 1994: "Numerical forecasting of radiation fog. Part II:
A comparison of model simulations and several observed fog
events. Mon. Wea. Rev., 122,
1231-1246.
Guédalia D. and T. Bergot, 1996: "Evaluation de la qualité de la prévision du
brouillard par un modèle numérique". La Météorologie, 14, 27-35. (In French)
Other references:
Bechtold, P., S. K. Krueger, W. S.
Lewellen, E. van Meijgaard, C.-H. Moeng, D. A. Randall, A.
van Ulden and S. Wang, 1996: "Modeling
a stratocumulus-topped PBL: Intercomparison among different
one-dimensional codes and with Large Eddy Simulation". Bull. Amer. Meteor. Soc., 77 no. 9,
2033-2042.
Fouquart, Y. and B. Bonnel, 1980: "Computations of solar heating of the Earth's atmosphere: a new parameterization". Beitr. zur Phys. der Atmos., 53,
35-62.
Kessler, E., 1969: "On the distribution and continuity of water
substance in atmospheric circulations". Met. Monograph, Vol. 10, No. 32,
American Meteorological Society, Boston, 84 pp.
Mahrt, L and H.-L. Pan, 1984: "A two-layer model of soil hydrology". Bound. Lay. Meteor., 29, 1-20.
Musson-Genon, L., 1989: "Forecasting in the vertical using a local
dynamical interpretation method". Mon. Wea. Rev., 117, 29-39.
Sharan, M. and S. G. Gopalakrishnan,
1997: "Comparative evaluation
of eddy exchange coefficients for strong and weak wind stable
boundary layer modeling". J.
Appl. Meteor., 36, 545-569.
Tardif, R. and P. Zwack, 1996: Providing
Meteorological support to the Aircraft Vortex Spacing System
Using the COBEL Column Model. Internal Report, presented
to the Massachusetts Institute of Technology Lincoln
Laboratory. 31p.
Tardif, R. and P. Zwack, 1998: Providing
Meteorological support to the Aicraft Vortex Spacing System
Using the COBEL One-Dimensional Numerical Model. Report on
Development Activities for FY 1997. Internal Report,
presented to the Massachusetts Institute of Technology
Lincoln Laboratory. 53p.
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