Wind Analysis – Sample Report
2015
TechWorks Marine Limited
Pottery Enterprise Zone
Pottery Road
Co. Dublin
Ireland
Overview
This sample report presents the results of a satellite wind anlysis study carried out by Techworks
Marine ltd. The data used in this study consists of merged satellite scatterometery data from both the
QuikSCAT and ASCAT sensors over the period from July 1999 to June 2011.
Figure 1. Annual mean variation in wind speed across the site.
Annual Mean Wind Speeds
(1999 - 2011)
Min
(m/s)
Max
(m/s)
Range
(m/s)
Standard
Deviation
9.9
11.7
1.8
0.49
Table 1. Basic wind statistics for the 12 year sampling period.
Satellite Wind Measurements
QuikSCAT is a satellite-borne scatterometer, which uses radar backscatter to infer ocean wind speeds
and direction. Sea surface winds modulate the ocean roughness, scattering the incident radar signal,
allowing the retrieval of surface wind speed and direction from the returned signal. As it is a microwave
sensor, data can be gathered in clear and cloudy conditions. The QuikSCAT record spans from 19 July 1999
to 21 November 2009, with two daily measurement samples.
A second scatterometer, ASCAT, also provides wind speed and direction measurements over the
oceans and has a continuous archive from 2007 to the present day. For this case study, gridded data was
used for the analysis spanning the period June 2007 to June 2011, as these data have been reprocessed to
improve their quality.
The QuikSCAT/Ascat instruments are calibrated to provide information on the wind field at a height
of 10m above the sea surface. The instruments provide point data measurements over gridded areas of
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approximately 25 X 25 km. The Charnock sea roughness equation for open seas (Charnock, 1955), assuming
a logarithmic wind profile and using an iterative calculation was used to lift the 10m satellite values to a
hub height of 100 m.
Figure 2. Seasonal wind speed variation
Figure 3. Monthly wind speed averages.
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Figure 4. Monthly wind speed averages.
January February
Min
9.7
8.6
max
16.3
15.8
Range
6.6
7.2
StDev
2.15
1.93
Min
max
Range
StDev
July
6
9.1
3.1
0.82
August
6.4
10.2
3.8
1.17
March
10.1
14.1
4
1.27
April
8.2
12.7
4.5
1.35
May
6.4
13.2
6.8
1.83
June
5.9
9.5
3.6
1.12
September October November December
7.6
9.6
11.6
9.5
12.2
16
15.6
15.1
4.6
6.4
4
5.6
1.52
1.49
1.42
1.75
Table 2. Basic wind statistics for each month.
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Figure 5. Wind Rose showing the wind direction, wind speeds, and frequency at each direction for the
sample site. Data spans from July 1999 to June 2011.
Weibull Distribution and Probability Statistics
We generated Weibull statistics to assess satellite long-term estimates of the wind resource
at the sample site. The Weibull distribution indicates the wind variability at the given site using the k
(shape) and A (scale) parameters. The A parameter is proportional to the mean wind speed at a given
location. The k unit less shape factor for a given distribution reflects the width of the distribution with
lower k values representing higher wind variability.
The frequency distributions in Figure 6 illustrate the spread in wind speed values across twelve
years for the QuikSCAT/Ascat archive. Wind power density based on both modelled (Weibull) and
actual values are also included in the plot and give a good indication of the wind power resource
available in the area.
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120
0.09
Probability Density
0.08
Probability Density
100
Modelled Weibull
Distribution
Wind Power Density
(W/m^2)
Modelled Wind Power
Density
80
0.07
0.06
0.05
0.04
0.03
60
40
0.02
20
0.01
0
0
1
3
5
7
Wind power density (W/m^2)
Weibull Distribution
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51
Bin Number (windspeed m/s)
Figure 6. Probability density plot of the satellite readings raised to 100m and modelled Weibull probability
density. Wind power density (Green) and modelled wind power density (purple) are also included.
Merged Quikscat and
Ascat Data
Shape
Parameter (k)
Scale
Factor (A)
2.17
12.37
Mean Wind speed
(m/s)
No. of
Recordings
10.83
6972
Table 3. Weibull statistics and mean wind speeds for the satellite data set.
Potential sources of error and accuracy analysis.
Satellite winds were measured at a height of 10m and were lifted to 100m (hub height) using the
Charnock profile. This technique does not consider atmospheric stability and its impact on the wind profile
or diurnal variation in the winds. Higher wind speed retrievals from Quikscat/Ascat can be demanding due
to the sparcity of high wind speed data for validation of the satellite signal and contamination of the signal
due to rain. The QuikSCAT sensor is more sensitive to rain contamination than the ASCAT sensor, but
information on rain events is included in the data quality flags and data affected is removed from analysis.
A comparison of both satellite datasets with buoy wind data (Bentamy et al, 2008) found an rms difference
of ~1m/s for both ASCAT and QuikSCAT data.
Each satellite makes an observation twice a day, so this analysis is based on two daily measurements
or 4 during the period of overlap between the satellites. A comparison of the k and A values and mean
wind speeds for the Fino1 test platform (10 minute observations from a met mast) and the Ascat/QuikSCAT
timeseries showed that they agree within a few percentage points. A study by Berge et al (2009) reported
a K factor of 11.25 (A=2.25) for in situ data and 11.12 (A=2.17) for model data (2km resolution) in a
comparison of the Weather Research and Forecasting (WRF) model output with the FINO1 data over the
years 2004-2007. This shows that the accuracy of the result using the satellite time series compared to in
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situ data is similar to that of a numerical model, and that the differences between the satellite and in situ
are similar to those found when using different periods of in situ data.
Data source
K factor
A parameter
Mean wind speed m/s
FINO-1 2004-2011
11.1
2.3
9.82
QuikAscat merged 19992011
11.0
2.2
9.70
Numerical model (Berge et 11.12
al, 2009) 2004-2007
2.17
FINO-1 2004-2007
2.25
11.25
Table 1. Weibull statistics and mean wind speeds for the QuikSCAT /Ascat and FINO-1 data sets.
References
Bentamy, A., D. Croize-Fillon, and C. Perigaud (2008), Characterization of ASCAT measurements based on
buoy and QuikSCAT wind vector observations, Ocean Sci., 4, 265–274, doi:10.5194/os-4-265-2008
Berge, E.; Byrkjedal, O.; Ydersbond, Y.; Kindler, D. Modelling of offshore wind resources. Comparison of a
meso-scale model and measurements from FINO-1 and North Sea oil rigs. Scientific Proceedings
EWEC'09 Marseille, March 2009.
Charnock, H. (1955), Wind stress on a water surface. Q.J.R. Meteorol. Soc., 81: 639–640.
TechWorks Marine
Pottery Road
Dun Laoghaire
Co. Dublin
Tel. +353 (0)1 236 5990
Fax. +353 (0)1 236 5990
[email protected]
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