Geoscience Reference
In-Depth Information
elevation then
QI
PBB
is taken from the higher one multiplied by factor
b
set as e.g. 0.3
(Ośródka et al., 2012). An example of the algorithm running is presented in Fig. 2 for
Pastewnik radar which is located near mountains.
4.6 Attenuation in rain
Attenuation is defined as decrease in radar signal power after passing a meteorological
object, that results in underestimation of the measured rain:
Z
corr
(7)
A
10 log
10
Z
where
A
is the specific attenuation (dB km
-1
),
Z
corr
is the non-attenuated rain and
Z
is the
measured one (mm
6
m
-3
). Especially at C- and X-band wavelength the attenuation can
considerably degrade radar measurements. The aim of the algorithm is to calculate the non-
attenuated rain. Empirical formulae for determination of specific attenuation can be found
in literature. Using 5.7-cm radar wavelength (C-band radar) for rain rate the two-way
attenuation
A
in 18°C can be estimated from the formula (Battan, 1973):
1.17
A
0.0044
R
(8)
Reflectivity-based correction made iteratively (“gate by gate”) is a common technique of
correction for attenuation in rain (Friedrich et al., 2006; Ośródka et al., 2012). For a given
gate
i
the attenuation at distance between gate
i
-1 and gate
i
can be calculated taking into
account underestimations calculated for all gates along the beam from the radar site up to
the
i
-1 gate (based on Equation 8). Finally, corrected rain rate in the gate
i
is computed from
the attenuation and underestimations in all previous gates.
In case of dual-polarization radars specific attenuation for horizontal polarization
A
H
and
specific differential attenuation
A
DP
(in dB km
-1
) can be calculated using different methods.
For C-band radar typically specific differential phase
K
DP
is applied using a nearly linear
relation between the attenuation and
K
DP,
e.g. (Paulitsch et al., 2009):
0.99
1.23
A
0.073
K
A
0.013
K
,
(9)
H
DP
DP
DP
or a linear one.
The iterative approach can lead to unstable results because it is very sensitive to small errors
in both measurement and specific attenuation. Therefore, in order to avoid the instability in
the algorithm, certain threshold values must be set to limit the corrections. For dual-
polarization radar a ZPHI algorithm is recommended, in which specific attenuation is
stabilized by differential phase shift Ф
DP
(Testud et al., 2000; Gourley et al., 2007a).
Magnitude of the correction in precipitation rate can be considered as a measure of quality
due to radar beam attenuation (Ośródka et al., 2012).
4.7 Spatial variability of reflectivity field
Small-scale variability of precipitation field is directly connected with uncertainty because
heavy precipitation is more variable in space and time, as it can be especially observed in
