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This paper describes the parallelization process of the code PROMES, which represents a re- gional atmospheric model developed by some of the authors. The parallel code, called PROMESPAR, has been carried out under a distributed platform (cluster of PCs) and using Message Passing Interface (MPI) communication subroutines.
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PROMESPAR: A Parallel Implementation Of
The Regional Atmospheric Model PROMES
Juan E. Garrido1, Enrique Arias1, Diego Cazorla1, Fernando Cuartero1,
Iv´an Fern´andez2, Clemente Gallardo2
Abstrac t— This paper describes the parallelization
process of the code PROMES, which represents a re-
gional atmospheric model developed by some of the
authors. The parallel code, called PROMESPAR, has
been carried out under a distributed platform (cluster
of PCs) and using Message Passing Interface (MPI)
communication subroutines.
Keywords: Regional atmospheric model, paralleliza-
tion, message passing interface
1 Introduction
Climate change induced by human activities is one of the
topics to which more attention is devoted to scientific
research today. This is due, not only by the great com-
plexity involved in the processes affecting the climate,
but also to the threat involved in the serious impact that
occurs on the economics and the environment in many
parts of the planet. Three or four decades ago, it was
believed that the oceans would be able to absorb the pol-
lutants emitted by human activities; but today, maritime
degradation is undeniable. Even more recently, the idea
that humanity could induce a change in climate was a
hypothesis that received little scientific support. How-
ever, there is now a broad consensus among scientists,
about the evidence of anthropogenic climate change and
the need for better knowledge about likely developments
in the following decades.
To simulate the climate, we use numerical models repro-
ducing the main processes occurring in the five compo-
nents of the climate system: Atmosphere, hydrosphere,
geosphere, and biosphere, and the exchange of mass and
energy between them. The results obtained by the models
are evaluated and compared with the observed features of
the climate in recent decades. Once it is found the quality
level of the climate model is correct, we apply it to simu-
late potential changes in the climate, considering various
scenarios of anthropogenic emissions of greenhouse gases
This work has been supported by National Project CGL2007-
66440-C04-03. Instituto de Investigaci´on en Inform´atica de
Albacete 1and Instituto de Ciencias Ambientales 2,Univer-
sity of Castilla-La Mancha, Avda. Espa˜na s/n,02071-Albacete,
Spain, email for correspondence:{Enrique.Arias}@uclm.es, Tele-
phone number: +34-967-599200 Ext: {2497}, Fax number: +34-
967-599224.
and aerosols. Since this information, we can deduce the
potential impact of climate change produced in such a
hypothesis.
The history of weather forecasting is intimately associ-
ated to development of high performance and parallel
computing [9].
Is in the early stage of 1922, when L. F. Richardson pro-
vides a vision of how to partition the large amount of
computation required in this task, by using thousands of
computers. [1].
However, is in later forties, when the first steps towards
the use of computers in weather forecasting were done.
This beginning was made by von Neumann, Charney and
his colleagues in the computer ENIAC and its successors.
The work done by these researchers was so important
that, thereafter, it was considered the numerical weather
prediction methods as a whole discipline, and that was
the origin of the establishment of national prediction cen-
tres. In fact, today the major supercomputing centres
tend to focus on such tasks.
While the first steps in the weather prediction were bear-
ing fruit, it was thought to apply the same methodol-
ogy in predicting the a longer term, not only predict-
ing changes in the atmosphere (weather) but also in the
global system time (climate change).
Since the forties, there was a dramatic improvement in
numerical methods, algorithms and computer technol-
ogy, as well as physical models and science related with
weather and climate.
In fact, scientists working with models of climate and
weather are the main users of parallel platforms. How-
ever, it is necessary not only to have a platform, but also
the parallel algorithms suited to these platforms to ex-
ploit the full potential of resources. Scientists in these
areas were the first to make effective use of machines
with segmented architecture, such as IBM 360/195, Cray
1, Cyber 205, Cray YMP and Cray 90.
Until nineties, it was not made a serious attempt to de-
velop an operational parallel model. Americans were the
first to combine the efforts of experts in meteorology with
Proceedings of the World Congress on Engineering 2009 Vol I
WCE 2009, July 1 - 3, 2009, London, U.K.
ISBN: 978-988-17012-5-1
WCE 2009
experts in high performance computing, included in the
program of High Performance Computing and Communi-
cations (HPCC), in a more ambitious program such as the
Computer Hardware, Advanced Mathematics and Model
Physics (CHAMMP), a program of the U.S. Department
of Energy. The result was the development of a set of
models to use scalable parallel computer systems.
Thanks to the parallelization of weather prediction mod-
els, it is provided to scientists the ability to deal with
longer simulations, to increase the spatial resolution, etc.
Throughout the last decade, several parallel approaches
have been developed. Among them, we remark [3] the
based on vectorial multiprocessors such as CCM2 and its
scalable variants [2, 12, 20], massively parallel comput-
ers (adaptation from the spectral model of the National
Meteorology Centre) [19], distributed memory multipro-
cessors [18] (integrated prediction system) and passing
messages [15].
Since 1995 until now, it has been followed different ways
in the application of parallelism to the weather predic-
tion. These paths have led us to new versions of the above
mentioned models (i.e. the last version of the CAM model
called CCM [7]), to applications in our area of interest,
such as GRID technology (IrisGRID [4] or CrossGrid [5]),
the apparition of Climateprediction.net [6] program, to
adaptations of different codes to the most powerful ma-
chine of the moment [17, 13, 14] and implementations of
meteorological aspects such as weather data assimilation
or transposing multidimensional vectors [10].
Special mention deserves the MM5 fifth generation
mesoscale model [8].It is relevant because it is the model
used as reference by Promespar (its implementation is
carried out taking into account only part of the compre-
hensive scheme conforming the full model).
Designed to work with high resolution (higher than 5km),
the MM5 consists on a model with very sophisticated
physical parameterizations schemes, but needing a huge
computational power. It was developed by the Univer-
sity of Pennsylvania (PSU) and the National Center for
Atmospheric Research (NCAR) in the United States.
The MM5 model, running on a parallel distributed mem-
ory platform, with massively parallel processor (MPP),
networks of workstations, etc., is called MM90 [16]. This
code was implemented in Fortran 90, using a communica-
tion library developed at Argonne National Laboratory
called RSL, library that corresponded to the ones pro-
vided by the seller (NX for Intel Paragon, or MPL for
IBM SP2), or MPI for other platforms. MM90 is the
successor of MM5 implementation of massively parallel
machine called MPMM [11].
The paper is organized as follows. Section 2 introduces
the regional atmospheric model PROMES, and in Section
3 the parallelization of PROMES is presented. The ex-
perimental results are outlined in Section 4. Finally, the
conclusions and future work are commented in Section
5.
2 The regional atmospheric model
PROMES
PROMES is a regional atmospheric model developed by
some of the authors and presented in [1]. In particular,
PROMES is a mesoscale forecast model over which sev-
eral physical phenomena which act on the atmosphere are
parametrized modifying its conditions and behaviour. Ir
becomes evident that due to the fact that the model is
represented by a set of equations, as bigger the number of
physical parameters to parametrized as complex its res-
olution; and obviously its accuracy. The complexity on
the solution makes necessary the used of parallel plat-
forms to solver the problem in order to obtain the results
in a reasonable time.
Figure 1 shows the physical parameters that are modelled
on PROMES.
Figure 1: Physical parameters modelled at PROMES
In order to make the computations easier, the model di-
vides the zone to be studied on a set of vertical columns,
each one with the atmosphere behaviour in an instant of
time. This division is known as grid of calculus and it is
shown on Figure 2.
Finally, and overview of the structure of PROMES code
is shown in Figure 3.
3 PROMESPAR: a distributed memory
implementation of PROMES
As it was previously commented, in order to obtain a
very accuare solution in a reasonable time, it is nec-
essary the use of parallel platforms. In this paper, a
distributed memory implementation of PROMES code,
called PROMESPAR, is presented.
La parallelization of PROMES consists on dividing the
Proceedings of the World Congress on Engineering 2009 Vol I
WCE 2009, July 1 - 3, 2009, London, U.K.
ISBN: 978-988-17012-5-1
WCE 2009
Figure 2: Grid of calculus
Figure 3: General squeme of PROMES code
domain on a set of subdomains getting out the work to
carried out into the different processors (see Figure 4).
Once the domain has been divided the processors just
exchange the frontier information.
In order to obtain an equally load balancing, a constrain
is applied to the size of the subdomain and the number of
processor to be used. This constrain is given by equation
2
ProcXBlockSize=(
OrXmatSize
X sizeP roc )±XBorderSize (1)
ProcY BlockSize=(
OrY matSize
Y sizeP roc )±YBorderSize (2)
where ProcXBlockSize and ProcY BloclSize mean the
size of blocks for each processor at Xor ycoordinate,
Figure 4: Squeme of spliting the domain into subdomains
respectively, which is computed from the original dimen-
sion of the matrix (OrXmatSize and OrY matSize)and
the number of processors by each coordinate (Xsiz eP roc
and Y sizeP roc), and taking into account the boundary
conditions (XBorderSize and YBorderSize).
However, processor 0 has additional tasks due to the fact
that it acts as master reading initial conditions, boundary
values for the domain, etc from files
In any case, the good load balancing could be affected
mainly by two factors:
Static imbalance. Those processors whose sub-
domains contain maritima zones have less compu-
tational load. This circunstance is due to the fact
that the computations needed for solving the fore-
casting model are simplest in this kind of cells (some
physical phenomena as the effect of orography, heat
exchange with the masses of plants, etc are not taken
into account).
Dynamic imbalance. This kind of imbalence is
devoted by the initial conditions. For instance, the
effect of solar radiation could vary if a cloudy day or
a sunny day is considered. These effects are unpre-
dictable. However, other effects as the solar radia-
tion during the night are predictable.
Figure 5 shows the different libraries considered in the
implementation of PROMESPAR, all used under FOR-
Proceedings of the World Congress on Engineering 2009 Vol I
WCE 2009, July 1 - 3, 2009, London, U.K.
ISBN: 978-988-17012-5-1
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TRAN programming language. In particular, the follow-
ing libraries have been considered:
MPI: Messing Passing Interface use for communica-
tions purpose. This library supports the comuni-
cation between the different processors of the dis-
tributed memory platform.
NETCD: NetCDF (network Common Data Form) is
a set of software libraries and machine-independent
data formats that support the creation, access, and
sharing of array-oriented scientific data.
IOPSL: Library for input/output operations with
meteorological data.
Other physical libreries: computation of solar radia-
tion, heat exchange ground-atmosphere, etc.
<<Library>>
libmpi.a
libnetcdf.a
libioipsl.a
physical operations
<<Executable>>
PROMESPAR
I
I
Figure 5: Components squeme of PROMESPAR
Figure 6 represents the workflow of the parallel imple-
mentation of PROMES, PROMESPAR.
The workflow in Figure 6 is followed by each processor,
and the barriers on Figure 6 mean communication or syn-
chronization taks amount the different processors.
4 Experimental results
The experimental results have been obtained taken into
account 24 hours of simulation. The distributed memory
implementation has been run into a cluster of PCs with
16 Intel processors at 1.8GHz, each one with 512 MB of
main memory and interconnected by a Myrinet Network
using NFS file system.
The performance obtained in the parallel implementa-
tions are evaluated in terms of:
Execution time: Time spent in order to solve the
problem.
Figure 6: Workflow of PROMESPAR
Speed-up: The ratio of the time taken to solve a
problem on a processor to the time required to solve
the same problem on a parallel computer with piden-
tical processors.
Efficiency: A measure of the fraction of time for
which a processor is usefully employed; it is defined
as the ratio of the speed-up to the number of proces-
sors.
Most time consuming has been spent at main loop where
are contained the most caomputational cost operations.
In particular, apart from send and receive operations
for communication purpose, physical operations are in-
voqued. These operations are shown at Figures 3 and
6.
The experimental results considered in this section take
into account a 24 hour simulation, which is equivalent to
carry out 2881 iterations of main loop.
Figures 7, 8 and 9 show the results of the previous exper-
iment (24 hour simulation) in terms of execution time,
Proceedings of the World Congress on Engineering 2009 Vol I
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ISBN: 978-988-17012-5-1
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speed-up and efficiency.
Figure 7: Execution time of PROMESPAR
Figure 8: Speed-up of PROMESPAR
Figure 9: Efficiency of PROMESPAR
From the experimental results, te main conclusion is that
the best results, in terms of execution time has been ob-
tained considering 8 processors. However, in terms of
speed-up and efficiency best results are obtained for 2
processors. This is a normal circunstance due to the in-
fluence of the communications. However, for this partic-
ular applications the main goal is to reduce the execution
time.
As it was previously commented, the most time consum-
ing of PROMESPAR code is spend on main loop. Figure
10 show a detailled study of the time spend on main loop.
It is possible to observe that fisicapal,Coriolis and Difu-
sion functions spent the most quantity of time, and ob-
viously the parallelization approach allows to reduce this
execution time, overall from one to two processors. Any-
way, the reduction of execution time results quite good.
Figure 10: Execution time of the main loop of PROMES-
PAR for an hour simulation
5 Conclusion
PROMES is a mesoscale regional atmospheric model de-
veloped by some of the authors of this paper. However,
due to the high time consuming by PROMES code and
the necessity of having more accurate results, both cir-
cunstances justify the used of parallelism. In this paper,
a distributed memory implementation of the regional at-
mospheric model PROMES has been carried out. This
parallel implementation is called PROMESPAR.
The experimental results show a dramatically execution
time reduction by means of the use of a parallel plat-
form considering the same configuration that the orig-
inal PROMES code. These results leads to think that
either longer or more accurate simulations could be car-
Proceedings of the World Congress on Engineering 2009 Vol I
WCE 2009, July 1 - 3, 2009, London, U.K.
ISBN: 978-988-17012-5-1
WCE 2009
ried out spending the same time, or more complex models
could be considered. In fact, the authors are extend-
ing PROMES code in order to be able of making cli-
mate change studies. Climate change studies consider
100 years simulations spending, obviously, lot of time and
then if the researchers want to provide conclusions from
these studies the use of parallelism becames essential.
Acknowledgments
The authors would like to thank to the Madrid supercom-
puter and visualization center (CESVIMA) to be able to
use the supercomputer known as MAGERIT.
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