In this study we analyze the surface wind variability over selected areas of the Greek territory by comparing a 3-Km spatial resolution simulation performed with the Weather Research and Forecasting (WRF) model for the summer months of 2013 with actual surface measurements. Daily 36hrs runs at 12 UTC were driven by FLN (1 deg x 1 deg) data for the period of 11 July 2013 to 17 July 2013. Various verification statistics such as BIAS,  RMSE and DACC for wind speed and direction were used to gauge the mesoscale model performance

Christakis, Nikolaos

Katsaounis, Theodoros

Kossioris, Georgios

Plexousakis, Michael

ACMAC

ACMAC’s PrePrint Repository
Evaluation of WRF performance for the analysis of surface wind speeds
over various Greek regions
Nikolaos Christakis and Theodoros Katsaounis and Georgios Kossioris and
Michael Plexousakis
Original Citation:
Christakis, Nikolaos and Katsaounis, Theodoros and Kossioris, Georgios and Plexousakis, Michael
(2014)
Evaluation of WRF performance for the analysis of surface wind speeds over various Greek regions.
(Submitted)
This version is available at: http://preprints.acmac.uoc.gr/314/
Available in ACMAC’s PrePrint Repository: May 2014
ACMAC’s PrePrint Repository aim is to enable open access to the scholarly output of ACMAC.
http://preprints.acmac.uoc.gr/
Evaluation of WRF performance for the 
analysis of surface wind speeds over various 
Greek regions!
Christakis N.1, Katsaounis Th.2, Kossioris G.2, Plexousakis, M.2*!
1 Department of Physics, University of Crete, and Institute of Applied and Computational 
Mathematics, FO.R.T.H., Heraklion!
2 Department of Mathematics and Applied Mathematics, University of Crete, and Institute of 
Applied and Computational Mathematics, FO.R.T.H., Heraklion!!
*corresponding author e-mail: plex@tem.uoc.gr!
Abstract In this study we analyze the surface wind variability over selected areas 
of the Greek territory by comparing a 3-km-spatial resolution simulation 
performed with the Weather Research and Forecasting (WRF) model for the 
summer months of 2013 with actual surface measurements. Daily 36hrs runs at 12 
UTC were driven by FNL (1º" 1º)  data for the period of 11 July 2013 to 17 July 
2013.  Various verification statistics, such as bias, RMSE and DACC for wind 
speed and direction were used to gauge the mesoscale model performance.!
1 Introduction!
In this paper, we analyze the performance of the WRF model in estimating the 
near-surface wind variability over selected areas of Greece. Such an analysis is 
mostly missing for Greece, while being of utmost importance for the growing 
wind power industry and civil protection agencies, due the increased occurrence 
of extreme weather events.  The sensitivity of the WRF model for several 
parametrizations is measured against ten wind-measuring stations consisting of 30 
m masts equipped with vane anemometers.  We chose to simulate a typical 
summer period, 11-17 July 2013, characterized by Etesian winds of moderate to 
strong north or north-easterly winds.!!
The 11-17 July 2013 event, was typically characterized by Etesian winds. They 
are northern sector winds blowing over the Aegean Sea during summer and early 
autumn. They are mainly north-easterly in the northern Aegean, northerly in the 
central and southern Aegean, and  tend to become north-westerly near the south-
western Turkish coasts, see e.g. (Kotroni et al., 2001). The Etesians are the 
consequence of a high-pressure center in the central and south-east Europe and a 
low-pressure system that extends from Turkey to northwest India, see e.g. (Reiter 
et al., 1980), (Metaxas and Bartzokas, 1994). According to the simulations 
(surface plots  not shown here), during the event,  a high pressure system of 
1015-1020 hPa was prevailing in central and eastern Europe. A low-pressure 
center of 990-995 hPa was extended south-east of Turkey reaching Persian Gulf. !!
The remaining of the paper is organized as follows: Section 2 describes the 
acquisition of observational data used in the analysis and the set up of the 
simulations with the WRF mesoscale model. Section 3 contains an analysis of the 
mesoscale model performance using various verification statistics. Our findings 
are summarized in Section 4. !
2 Data and Methodology!
A total of 10 observational sites was used in this investigation, with wind speed 
and direction measurements recorded at 20m, 28m and 30m above ground level. 
All simulations reported here in were performed with WRF version 3.4 
(Skamarock et al., 2005).  The locations of the observation (surface) stations and 
their codes are shown in Fig. 1. This observational network provides good spatial 
coverage, sampling many important locations that are representative of the 
topographic characteristics of Greece.!!
!
Fig.1  Locations of the observational sites together with their codes.!
2.1 Data!
Wind speed and wind direction were recorded by vane anemometers, placed on 30 
m masts located at the observation stations mentioned above, for the week 
between 11 and 17 July 2013. The masts were deployed as part of the research 
project AKAIPRO, see (AKAIPRO, 2011).  The 10 min average wind speed and 
wind direction at 28 m above ground level were used for comparison with the 
WRF simulations. In turn, the WRF-simulated wind speed and wind direction 
were estimated at 28m by interpolation, specifically by using NCL's 
wrf_interp_3d function.!
2.2 Methodology!
Numerical experiments were conducted to simulate the aforementioned event 
using WRF with one-way nested domains. The initial and boundary conditions are 
derived from the National Centers for Environmental Prediction (NCEP) model 
analysis by WRF preprocessing. An improved representation of topography with 
60 m " 60 m  spatial  resolution  (Abrams et al., 2010, Toutin, 2008) and an 
updated land-use dataset with 24 land-use categories from the U.S. Geological 
Survey (USGS) were used in order to ensure more accurate surface conditions. 
The Noah land surface model was used.!!
Three one-way nested domains with horizontal grid resolutions of 27 km, 9 km 
and 3km, respectively were specified. The innermost domain covers the entire 
Greek territory while the outermost domain contains most of Europe. All 
modeling domains had 50 layers in the vertical dimension, with the model top 
being set at 50 hPa. The Kain-Fritsch scheme (Kain, 2004)  was used for 
parametrizing cumulus convection. Shortwave radiation processes were handled 
using a cloud radiation scheme  (Dudhia, 1989) and the Rapid Radiative Transfer 
Model (RRTM) scheme (Mlawer E.J. et al., 1997) was applied for longwave 
radiation processes.  A total of nine different parametrizations, referred to in the 
sequel as P1 to P9, using three different microphysics options and three different 
planetary boundary layer schemes were used in simulating the weather event in 
11-17 July 2013. Parametrizations P1 to P3 used the simple WRF single-moment 
six-class scheme for micro-physics (Dudhia et al., 2008; Hong et al., 2004; Lin et 
al., 1983). Parametrizations P4 to P6 used the Ferrier micro-physics option 
(Ferrier et al., 2002) and P7 to P9 used the New scheme (Thompson et al.,  2004). 
The PBL scheme used in parametrizations P1, P4 and P7 was the Yonsei 
University scheme (Hong et al., 2006). Parametrizations P2, P5 and P8 used the 
Mellor-Yamada-Janjic scheme (Janjic,  1994), while P3, P6 and P9 used the 
MYNN3 Nakanishi and Niino scheme (Nakanishi and Niino, 2006). !
The validation of the WRF model physics parametrizations is done by computing 
various statistics for both the wind speed and direction. Namely, we use four 
different metrics. The wind speeds are validated using the standard Bias and 
RMSE measures and  their relative counterparts, while for the wind direction we 
use the relative Bias and  the DACC  statistics, see (Santos-Alamillos et al., 2013). 
Let mi  and oi denote the modeled and observed values of wind speed, respectively. 
The first metric we use is the Mean Error or Bias which measures the overall 
overestimation or underestimation of modeled wind speed values and it is  defined 
by !!!
#    and   # !!
where # with N the number of records. The second metric we use 
is the Root Mean Square Error (RMSE) between the modeled and observed values 
and measures the amount of scatter of the wind speed errors:!!
#     and    # !!
To measure the directional accuracy of the wind, besides the relative BIAS metic, 
we also adopted the DACC  metric. For any two angles ! and  " the circular 
distance is defined as  $%(!, ")=min|!-", 360&-(!-")|.!
Then the DACC metric measures the percentage of times in which the circular 
distance between modeled and observed wind directions is lower than a threshold, 
chosen as 30&:!
# !!
3 Results!
We divided the ten observation sites in three groups, each group representing a 
geographical region of Greece and with the sited in each group being of related 
meteorological characteristics for the event under investigation, see Fig. 1. The 
group labeled Northern Greece, consisted of the stations Evzonoi (eyz), Kavala 
(kav), Loutro (ltr) and Ptolemaida (ptl). The group Western Greece consisted of 
the sites Delvinaki (dlv), Patra (ptr) and  Pylos (pyl).  Finally, the group labeled 
South-Central Greece consisted of the sites Kissamos (kis), Toplou (tpl) and Volos 
(vol). Figure 2 shows the relative BIAS and the relative RMSE of the wind speed 
measured at 28 m above ground level, as well as, the relative BIAS and the DACC 
for the wind direction. The relative wind speed bias is quite low, under 5%, for all 
sites in this group with the exception of the station of Pylos (pyl) where it is under 
( )! "
N
=i
ii omN
=BIAS
1
1 100  !
avgO
BIAS=BIASRelative
Oavg=
1
N ∑i= 1
N
oi
( )! "
N
=i
ii
2 om
N
=RMSE
1
21 100  !
avgO
RMSE=RMSERelative
100
otherwise   ,0
300 if    ,1
1
!"
#
$ %&%
=
' =
N
DACC
N
i
i(
10%. WRF underestimates the wind speed at the near-coastal stations of Patra and 
Pylos while it overestimates the wind speed at the mountainous station of 
Delvinaki (dlv).  All stations are characterized by a positive relative BIAS for the 
wind direction indicating a clock-wise deviation of the simulated wind. The wind 
direction DACC indices range between 40% and 60% with that of Pylos being 
above 50%. The relative RMSE scores for the wind speed are between 35% and 
60% with Patras and Pylos being almost below 40%, consistently for all nine 
parametrizations. The high relative RMSE score for the Delvinaki site may be 
attributed to the high elevation and terrain complexity.!!
Fig. 2. Relative BIAS and relative RMSE of the wind speed measured at 28 m above ground 
level (top panel), as well as, relative BIAS and DACC for the wind direction at 28 m above 
ground level (bottom panel) for the sites in the group Western Greece.!
Figure 3 shows the relative BIAS and the relative RMSE of the wind speed 
measured at 28 m above ground level, as well as, the relative BIAS and the DACC 
for the wind direction also at 28 m above ground level, for the sites in the group 
Northern Greece. The relative wind speed bias is quite low, under 5%, for almost 
all parametrizations and sites in this group. All sites are characterized by a 
negative relative BIAS for the wind direction, except the Kavala site. In addition, 
all sites except Loutro have a very small relative BIAS, less than 5% in absolute 
value. The relative BIAS for the Loutro site is approximately -20%, consistently 
across all parametrizations. The wind direction DACC index in quite good for the 
Evzonoi site, almost acceptable for Loutro and poor for the Kavala and 
Ptolemaida sites. The relative RMSE scores for the wind speed are between 30% 
and 65% with Loutro and Evzonoi being below 40%, consistently for all nine 
parametrizations.!!
Fig. 3. As in Fig 2. but for the sites of the group Northern Greece.!
Figure 4 shows the relative BIAS and the relative RMSE of the wind speed 
measured at 28 m above ground level, as well as, the relative BIAS and the DACC 
for the wind direction also at 28 m above ground level for the sites in the group 
South-Central Greece. The relative wind speed bias is low, under 5%, for all sites 
in this group with exception of the station Kissamos (kis) where it is under 15%. 
The relative BIAS for the wind direction for the sites of Toplou and Kissamos is 
quite low, around 5%, but the Volos site shows a score between -10% and -18%. 
The wind direction DACC index for the Toplou site is extremely good. Figure 5 
shows the energy wind rose for the Toplou site for the parametrization P9, where 
the excellent agreement of the sectorial-wise distribution is clearly evident. Figure 
6 shows the energy wind rose for the Kissamos site for the parametrization P9. 
Although WRF captures the tendency of the prevailing NE or ENE wind direction, 
it does not capture as accurately the sectorial-wise distribution and misses 
completely a small easterly sector, see Fig. 6. The relative RMSE scores for the 
wind speed in the Toplou station are around 10% for all parametrizations, while 
Volos and Kissamos show scores around 40% and 30%, respectively. !!
Fig. 4. As in Fig 2. but for the sites in the group South-Central Greece.!
!
!
Fig. 5. Energy wind roses for the Toplou site for the parametrization P9. WRF-simulated wind at 
left, observational data at right.!
!
Fig. 6. Energy wind rose for the Kissamos site for the parametrization P9. WRF-simulated wind 
at left, observational data at right.!
4 Conclusions!
Regarding wind speed relative bias, parametrizations P9 and P7 give the lowest 
values for the stations of the group Northern Greece. P9 and P8 give the lowest 
relative bias in stations in the group Western Greece with P9 performing quite 
well also in the South-Central Greece group (see Fig. 4). In all the above 
parametrizations the microphysics option is Thompson with PBL parametrization 
NYMM3 for P9 (see Fig. 2, 3, and 4). Regarding relative RMSE, no 
parametrization has a clear overall better performance. Regarding the performance 
of parametrizations for wind direction, P2 and P8 perform quite well in all 
geographic regions.!!
Acknowledgement. G. Kossioris, Th. Katsaounis and M. Plexousakis have been 
partially supported by the European Union Seventh Framework Programme (FP7-
REGPOT-2009-1) under the Grant Agreement no.  245749 (Archimedes Center 
for Modeling, Analysis and Computation, University of Crete, Greece).!
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Evaluation of WRF performance for the analysis of surface wind speeds over various Greek regions

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Evaluation of WRF performance for the analysis of surface wind speeds over various Greek regions

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