Geoscience Reference
In-Depth Information
Fig. 7. Reflectivity measurement from the weather radar near Dodge City, Kansas, at 2358
GMT, February 12, 2007. Blockage caused by a wind farm to the southwest results in a
reduction of precipitation echoes downrange from the wind farm (most clearly visible at a
distance of 70-100 km from the radar). Range rings in black are at 10-km spacing. Adapted
from Burgess et al. (2008). This image was obtained from NOAA/National Climatic Data
Center.
thus wider than the width of the tower (approximately 8.5 m). For the lowest elevation angle
of the radar the tower fills the entire beam height.
The average amount of precipitation per hour, for the period 1 November 2007 to 31 March
2011, is shown in Fig. 8. In this figure it is seen that some azimuth gates have considerably less
measured precipitation compared to their neighbours. These gates coincide with the location
of the tower.
One way to obtain a quantitative estimate of the reduction in expected precipitation due
to blockage is to assume that the precipitation can be considered constant over some
neighbouring azimuth gates. To validate this assumption a correlation analysis was
performed. The analysis revealed that the correlation between precipitation measurements
from neighbouring azimuthal radar cells depends on cross-range distance and accumulation
period. The correlation decreases as the cross-range distance increases and for the same
cross-range distance, shorter accumulation periods results in lower correlation. Applying
the correlation analysis to the precipitation measured by the Arlanda weather radar showed
that, for example, precipitation from radar cells with cross-range distances up to 5 km and an
accumulation period of 24 h had a correlation over 0.9.
To obtain a measure of how much precipitation varies locally the coefficient of variation of
accumulated precipitation from neighbouring azimuthal radar cells was calculated. In the
analysis for the Arlanda weather radar it was shown that the coefficient of variation increased
with increasing number of neighbouring azimuthal gates (i.e. window size) and decreased
with accumulation period.
