Geoscience Reference
In-Depth Information
Improvement in precipitation rate
R
estimation is noticeable using dual-polarization
parameters, especially for heavy rainfall. In addition to the horizontal reflectivity
Z
H
available
for single polarization radar, the specific differential phase
K
DP
and the differential reflectivity
Z
DR
can be applied (Bringi & Chandrasekhar, 2001). Typical forms of relationships for
precipitation estimation are as follows:
Rf K
(
)
,
Rf ZZ
(
, )
H
,
Rf K Z
(
,
)
, and
DP
R
DP
DR
Rf ZK
. These approaches for precipitation rate estimation are potentially
unaffected by radar calibration errors and attenuation, unbiased by presence of hail, etc.
(
,
, )
H
P
R
5.2 Bright band phenomenon
Vertical profile of reflectivity (VPR) provides very useful information for radar data quality
control. An averaged VPR is suggested to be taken from radar pixels lying at distance
between about from 30 to 80 km from radar site to obtain the profile valid for the whole
range of heights (Franco et al., 2002; Germann & Joss, 2004; Einfalt & Michealides, 2008). The
bright band is a phenomenon connected with the presence of the melting layer. It is
assumed that the melting layer is placed in range from the 0°C isotherm down to 400 m
below (Friedrich et al., 2006). The melting of ice precipitation into water drops and related
overestimation of precipitation rate results in errors of ground precipitation estimation. The
phenomenon is clearly visible in vertically pointing radar observations. For dual-
polarization radar a vertical profile of correlation coefficient (
ρ
HV
) is investigated instead of
reflectivity profile analysis (Tabary et al., 2006).
It is proposed that the relevant quality index equals 0 inside the melting layer due to bright
band, and equals 0.5 for measurement gates above the layer (Friedrich et al., 2006). In the
case when the melting layer does not exist (in winter season or within convective
phenomena) the quality index equals 1.
5.3 Data extrapolation onto the Earth surface
Information available from VPR can be used for another quality correction algorithm, which
is extrapolation of precipitation data from the lowest beam to the Earth surface, especially at
longer distances over 80 km. The averaged VPR is estimated for distance to radar site in
range from 30 to 80 km and then employed to extrapolate radar data from the lowest beam
to the Earth surface (Šálek et al., 2004). A quality factor which describes the relevant quality
index is the height of the lowest radar calculated from radar scan strategy, digital terrain
map (DTM), and the radar coordinates. It strongly depends on terrain complexity and
related radar beam blocking and is defined as a minimum height for which radar
measurement over a given pixel is feasible.
5.4 Orographic enhancement (seeder-feeder effect)
Orographic enhancement is a result of so called seeder-feeder mechanism which is observed
when ascent of air is forced by hills or mountains. The low-level clouds formed in this way
(feeder clouds) provide a moisture source that is collected by drops falling from higher
clouds (seeder clouds). Radar is not able to capture the enhancement, which occurs close to
the ground, as the measurement is performed at certain height over the hill. This effect can
be estimated by 3-D physical model taking account of information from numerical weather
