This document provides an overview and validation of Re.SunTM, a software tool for assessing solar resources worldwide using mesoscale weather modeling and coupling techniques. Re.SunTM runs the WRF mesoscale numerical weather model to generate meteorological data, then applies clear sky models incorporating aerosol and gas data to estimate solar radiation indices. It was validated against measurements from 9 complex sites, showing a mean bias of 0.64% and root mean square error of 4%. Statistics demonstrate Re.SunTM can estimate average annual global solar radiation to within 4.14% of measured for 80% of cases. The document concludes Re.SunTM is useful for solar applications requiring site-specific resource assessment.

1. Re.Sun™ -Validation White Paper, March 2013
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Introduction
Understanding the solar resources in areas where local measurements are scarce or unavailable
is nowadays possible with state-of-the-art mesoscale numerical weather models and public high
quality global weather databases.
Since 2009, MEGAJOULE INOVAÇÃO has
developed and tested an internal methodology
using mesoscale modelling to feed a process
of coupling using broadband Clear Sky models
(ESRA, Ineichen, REST2) and solar radiation
decomposition techniques allowing the
assessment of Solar resources worldwide with
more detail than usual approaches: Re.Sun™.
The Re.Sun™ approach was developed internally and counts with the cooperation of the Physics
research department of Aveiro University for validation and optimization of the mesoscale
parameterisations.
Re.Sun™ - Approach
Re.SunTM
is a code based on coupling techniques developed to ingest results from WRF–Weather
Research and Forecasting mesoscale numerical model. The WRF model is presently used by
numerous institutions across the world, and its versatility permits its use in a variety of
applications. It consists of a series of modules, which allow the simulation of atmospheric
conditions at a large (meso) scale (>1km), as well as assimilation of observed data, through the
solution of several equations that describe the dynamics of the physical phenomena. WRF was
developed by the U.S. National Center for Atmospheric Research (NCAR) as an evolution of the
original MM5. It incorporates the latest scientific developments from the leading research
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2. Re.Sun™ -Validation White Paper, March 2013
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:: Re.Sun.cfg ::
> n WRF mesh points to incluide (points to interp)
> Transport Radiation model(Global or Indivudual
components of Radiation
> WRF Mesh aod/TL2 Interpolation
> Clear Sky model (ESRA; Ineichen; REST2)
> Decomposition model (20 in total ::DIRINT, BRL,
Skartveit & Olseth, Maxwell, Reindl and others)
:: WRF runs to Re.Sun
=>data inputs :: FNL ; CLC Land Use and SRTM for
topography)
=>set up highlights
(2/3 levels nesting ; 2-ways feedback; same radt for all
nests; RRTM to LW& Dudhia to SW model)
=>output modified (wrfout * tuned, only variables to
use in coupling process; 10 min or others time steps)
WRF runs wr fout_ReSun_d0X_YY-MM-DD_00_00_00.nc
> Tur bidity, Aer osols, Ozone and Nitr ogen
Diox ide Sour ces
:: SODA (Solar Radiation Data)
:: Aer onet (Aer osol Robotic Networ k)
:: MACC Re-analysis (Monitor ing atmospher ic
composition & climate, ECMWF)
:: OMI V2.1 (Nitr ogen Diox ide, NASA/Goddar d
Space Flight Center )
Horizon code
>Topogr aphy gr id
> number of sector s
Hor izon data (*netcdf or
*.hdf5)
centers, and is the result of decades of research. Over the last years, MJ INOVAÇÃO has “tuned”
the WRF configuration for best results in solar assessment.
Precursor mesoscale simulations are performed using WRF to solve all meteorological
interactions, rendering available fields like Short Wave Radiation, Temperature, Vapour Mixe
Ratio and Surface Pressure for posterior use by Re.SunTM
.
Re.SunTM
applies Clear Sky simulations which include current information about aerosols and
atmospheric gases (Ozone and Nitrogen Dioxide, in case of the REST2 model), pressure
correction and orographic horizon effects. From these runs correction factors are estimated,
which are interpolated to the interest points to correcting the local Clear Sky simulations. The
correction factors can be estimated in terms of Solar Global index (Global RadiationWRF / Global
RadiationClear Sky) or Solar Direct and Diffuse indexes, making use, in this case, of a decomposition
model.
A flowchart of the process is presented in the figure below.
The global weather databases for WRF simulations currently available at MJ INOVAÇÃO are: FNL
(Final) Operational Global Analysis from NCEP, ERA-Interim from ECMWF or Reanalysis I and II
from NCEP/NCAR. The domain configuration and model parameterizations are studied for a set
of cases but they can change according to site features.
All WRF simulations make use of SRTM (Shuttle Radar Topography Mission) topography
information. Simulations in the European context use Terrain Cover data from the Corine Land
Cover 2006 programme, outside Europe, terrain cover data from the Global Land Cover
Characteristics Database (USGS, National Center for Earth Resources Observation and Science –
EROS, Joint Research Centre of the European Commission) is used.

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The aerosols or the TL2 (Linke Turbidity for an air mass equal to 2) can be derived from the
SODA (Solar Radiation Data) programme or from MACC Re-analysis (Monitoring Atmospheric
Composition & Climate) project (ECMWF). MEGAJOULE INOVAÇÃO has processed the two
databases and has created its own database. The SODA project has monthly Worldwide Linke
Turbidity Information already available, which is a result of assembling estimates of TL gathering
different background layers, METEOTEST (Redmund et al. 2003). The MACC Re-analysis project
does not contain TL2 information, but this can be estimated using the aerosols information for
different wavelengths and water column data. To obtain TL2 from MACC information we have
used two formulations: Redmund et al. 2003 and a multivariate fit calculated from beam
irradiance with Solis model and the Kasten pyrheliometric formula (1996).
Depending on the Clear Sky model used, we have different information to characterize the
atmosphere purity. In case of the ESRA model the equations make use of TL2. Simplified version
of Solis model (Ineichen model), use as input the aerosol optical depth information for a
wavelength of 700nm (AOD700).
Independently of the source of the aerosols or TL2, the data are used with a daily frequency.
The monthly values of SODA are interpolated linearly from monthly to daily values and the MACC
average daily values come from the insolation period with three hours of sampling.
Regarding the atmospheric gases, ozone data is obtained from MACC and Nitrogen Dioxide values
from OMI V2.1 programme (NASA/Goddard Space Flight Center).
The REST2 (Reference Evaluation of Solar Transmittance, 2 bands) model is currently in
implementation and its performance will be tested and results published in further editions of
Re.SunTM
White Paper. In a few words, the REST2 is a high performance model to predict
cloudless-sky broadband irradiance, illuminance and photosynthetically active radiation (PAR)
from atmospheric data (Christian A. Gueymard, 2007). REST2 was developed to precisely
account the effect of aerosols, using for that two bands separated at 700nm where band 1
covers the UV and visible ranges (290nm to 700nm) and band 2 covers the near-infrared range
(700nm to 4000nm).
The Re.SunTM
software thus provides a package of data that includes Solar Radiation (Global,
Direct Diffuse on horizontal/tilted plane and Beam) and Meteorological (Temperature, Relative
Humidity at 2m and Wind Speed/Direction at 10m) variables at site/project scale.

4. Re.Sun™ -Validation White Paper, March 2013
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Diffuse / Global Radiation ratio, (kd),
Complex/Coastal site, Corvo, 06
October 2011, Re.SunTM
Why no Pre-Calculated Solar Data ?
Mesoscale weather numerical modelling is a
complex science and in constant development.
Calculation domain and several model
parameterizations depend on the effects of
prevailing climate and local topography features.
The WRF model is also in continuous development
and constantly tested.
By not making use of pre-calculated solar maps or
series MEGAJOULE INOVAÇÃO ensures that our
Clients get the best modelling options available
for each case and at any given time.
Typical application from Re.Sun™
Typically, Re.SunTM
is best suited for:
• Country, Regional or Urban prospection of Solar Resource;
• Complex terrains (high altitudes and varied soil occupations), sites where the
atmospheric conditions are changing constantly;
• Urban environments where the shadow effects of buildings, aerosols and micro-climates
are features which affect the Solar Resource.
The approach proposed by MEGAJOULE INOVAÇÃO was conceived to gather and model the main
drivers which affect the Solar Resource modelling, performing a more realistic assessment than
usual techniques based on interpolations.
Traditional products are:
• Solar Resource Mapping at Country, Regional or Urban scale (flexible output data – GIS,
Google Earth, etc…);
• Solar Data Series TMY (Typical Meteorological Year) for annual energy estimates;
• Long Term reference solar data (for Long Term extrapolation of local measurements).
The main outputs are:
Global1
, Direct1
, Diffuse (Horizontal1
), Beam2
Radiation, Temperature, Relative Humidity at 2m
and Wind3
speed/direction at 10 m.
1,2,3 - The estimates in tilted planes, Solar Beam Radiation and Wind Speed/Direction are provided at an additional cost.

5. Re.Sun™ -Validation White Paper, March 2013
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Validation of Simulated Solar Data Series
During 2012 MJ completed its first validation programme for Re.Sun™, the solar coupling code
with mesoscale numerical model WRF, using Global Solar data from 9 locations.
This validation was based in WRF meshes of 5x5 km and 1x1 km
resolution, depending on site complexity. The weather input used
was the NCEP FNL (Final) Operation Global Analysis data set.
The simulated data was compared against local measurements for
9 complex sites, due to their challenging orogrophic and/or
atmospheric conditions, located in Portugal. One in continental
Portugal, in urban context, one in the Azores archipelago in Corvo
Island under the influence of the well-known Azores anticyclone
weather pattern. The remaining sites, 7, are located in Madeira
Island.
The climate throughout the year in Madeira is also directly
influenced by the Azores anticyclone weather pattern. Due the
privileged geographical location and east to west orientation the
island can be split in two climatic regions, the south, warm
subtropical, and the north, humid temperate. The two climatic
regions combine to produce an exceptionally mild climate, with an
overall average temperature of 22°C in the summer and 16°C in
the winter, along with a moderate level of humidity. The island
has many micro climates due to its extreme complex orography.
The local measurements in Corvo and Sintra were
carried out by MEGAJOULE and all measurements
in Madeira were courtesy of LREC (Laboratório
Regional de Engenharia Civil).
The test sites vary from flat to very complex,
coastal (< 10 km to shoreline) to inland and also
urban to rural environment. This set of tests contains all the main drivers in the context of solar
modelling. In the urban environment aerosols play an important role on the attenuation of
radiation, and the shadowing effects from neighbour buildings are crucial. The orographic
complexity is also present which normally leads to added shadowing effects, always considered
in Re.SunTM
simulations, as well as contributions from fog and cloud formation due to increased
humidity on the slopes not exposed to direct radiation.
Sintra
Corvo

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Example of mean Daily values of Global
Radiation Complex/Coastal site, Corvo
October 2011, Re.SunTM
(Orange),
Measured (Green)
Example of 10 minutes correlation of
Global Radiation Complex/Coastal site,
Corvo October 2011, Re.SunTM
Example of Cumulative Density Function
(CDF) of Global Radiation, sampling 10
minutes, Complex/Coastal site, Corvo,
October 2011, Re.SunTM
(Orange),
Measured (Green)
Results showed (table 1) a relative RMS error (Root Mean Square) of 4.00 % and a relative MB
error (Mean Bias) of 0.64 %.
The cumulative distribution of results (table 2) showed that for 80 % of cases, average Annual
Global Solar Radiation (in horizontal plane) was below 4.14 % and below 2.77 % for 50 % of
cases.
5 10 15 20 25 30
0
50
100
150
200
250
Day
Irradiance[W/m
2
]
Modelled
Measured
0 100 200 300 400 500 600 700 800
0
100
200
300
400
500
600
700
800
Measured [W/m2
]
Modelled[W/m
2
]
0 100 200 300 400 500 600 700 800
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Irradiance [W/m2
]
CDF
Modelled
Measured
Day
06
Day
17

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(Mapping of Mean Daily Global Radiation
based on time step of 10 minutes, 06
October 2011, Re.SunTM
(Mapping of Mean Daily Global Radiation
based on time step of 10 minutes, 17
October 2011, Re.SunTM
Table 1
Global statistics for Annual Average Solar Global Radiation comparison
rMB (%) rRMS(%) rSTD(%)
0.64 4.00 4.19
Table 2
Maximum relative Mean Absolute BIAS (rMAB) for different frequency levels
Frequency [%] Maximum rMAB (%)
50% 2.77
80% 4.14
90% 5.94

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Conclusions
The statistics above show the usefulness of Re.Sun™ code for: solar applications in Thermal
and Photovoltaic, solar series for energy building simulations and analysis of solar potential
at urban scale.
An 80 % probability was found that real average Annual Global Solar Radiation (in horizontal
plane) will be within an interval +/- 4.14 % of the simulated value and within +/-2.77 % for a
probability of 50%.
The complexity of the sites involved in the validation process included the main drivers in the
solar modelling context, rendering the worldwide application of this methodology more reliable.
Further releases of this White Paper will integrate: i) a new validation set based on WRF weather
dataset input ERA-Interim; ii) validation results of one year of energy production simulated
against real energy production for a PV central of 1MWp; iii) validation of Global, Beam and
Diffuse Radiation using stations from the SURFRAD network.
Further Info.:
www.megajoule.pt
www.facebook.com/MEGAJOULE
www.youtube.com/MEGAJOULEConsultants
www.linkedin.com/MEGAJOULE
www.twitter.com/MEGAJOULE
megajoule.inov@megajoule.pt

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Annex - List of sample sites
ID Complexity
Elevation
[m]
Data
Period
Data Recovery Rate
[%]
Sampling rMB R
1 Complex / Coastal / Rural 390 Oc11 - Sp12 78% 10min 1.7% 0.94
2 Complex / Coastal /Rural 675 Ja11-Dc11 98% 1hour -0.1% 0.95
3 Complex / Inland / Rural 1622 Ja11-Dc11 100% 1hour -3.0% 0.99
4 Complex / Inland / Rural 1017 Ja11-Dc11 99% 1hour -2.3% 0.97
5 Complex / Coastal / Rural 1020 Ja11-Dc11 90% 1hour 9.0% 0.98
6 Complex / Inland / Urban 260 Ja11-Dc11 100% 1hour -5.2% 0.94
7 Complex / Inland / Rural 1215 Ja11-Dc11 94% 1hour 2.8% 0.98
8 Complex / Inland / Rural 874 Ja11-Dc11 92% 1hour -0.5% 0.95
9 Flat / Inland / Urban 188 Ja11-Dc11 100% 1hour 3.4% 0.99