Geoscience Reference
In-Depth Information
Average rain rate [mm/h]
0.08
220
0.07
200
180
0.06
160
0.05
140
120
0.04
100
0.03
80
60
0.02
40
0.01
20
0
50
100
150
200
250
300
350
400
Azimuth gate
Fig. 8. Average amount of precipitation per hour from the lowest elevation angle (0.5
◦
)for
the weather radar at Arlanda airport, Sweden. Blockage caused by a nearby air traffic control
tower can be seen near azimuth gate 271.
For example, for an accumulation period of 1 hour and a window size of 13 gates
(corresponding to 11.7
◦
in azimuthal angle) the coefficient of variation did not decrease lower
than approximately 0.25 at any distance from the radar whereas for an accumulation period
of 1 month the coefficient of variation for the same number of gates was lower than 0.05 at
10 km from the radar. The coefficient of variation can be compared with the blockage caused
by an obstacle.
To find a quantitative estimate of the reduction in expected precipitation echoes caused by
the Arlanda tower a 13-gate wide window was applied to the data, accumulated over the
entire three-year-period. The measurements in the window were normalized over azimuth
and range to the average value of unaffected gates. In Fig. 9 it is seen that the measured
precipitation is reduced by close to 30% in the most severely affected gate.
Comparing the blockage of the Arlanda tower with the coefficient of variation for various
accumulation periods it was found that on average between 24 hours and 1 week was needed
for the coefficient of variation of the local precipitation to be lower than 30%. On individual
radar images it may therefore be difficult to see the effects of the Arlanda tower blockage.
3.2.2 Models
Modelling of electromagnetic shadow effects can be done with varying accuracy and
complexity. Methods and results of modelling blockage and shadow effects downrange
of wind turbines can be found in, e.g., Belmonte & Fabregas (2010); Greving & Malkomes
