Geoscience Reference
In-Depth Information
2
y
a
2
2
2
ya
y
a
arcsin
a
2
PBB
.
(3)
2
a
Depending on the relative position of the beam height respect to topography,
y
may be
either positive or negative. According to these definitions, partial beam blockage occurs
when
-a < y < a
, total beam blockage means that
y ≥ a
, and finally,
y ≤ -a
implies there is no
blockage at all. Using the notation introduced above, it can be seen that integrating
dy'
partial beam blockage, PBB, may be written as an analytical expression (Bech et al. 2003):
On the other hand, the height of the centre of the radar beam,
h
, is given at a distance
r
by
the expression (see, for example, Doviak and Zrnic, 2006):
2
2
h
r
k R r k
2
R
sin
k RH
,
(4)
e
e
e
0
where
R
is the Earth's radius,
k
e
is the ratio between
R
and the equivalent Earth's radius,
θ
the antenna elevation angle and
H
0
the antenna height. Information about atmospheric
propagation conditions is contained in
k
e
, which may be written in terms of the refractivity
gradient as:
1
k
(5)
e
dN
1
R
dh
The usual value for
k
e
in the first kilometre of the troposphere, assuming the normal VRG
value of 40 km
-1
, is approximately 4/3. Substituting (5) and (4) in (3), an expression of the
beam blockage in terms of the propagation conditions is obtained (Bech et al. 2003).
Three clutter targets (MNT, LML and MNY), which presented partial beam blockage under
normal propagation conditions, were chosen to examine the effects of changing the VRG.
The Vallirana radar (41 22' 28'' N, 1 52' 52'' E) is a C band Doppler system with a 1.3 º beam
width antenna at 3 dB. The targets chosen are normally used to check the radar antenna
alignment on a routine basis and are located within the region of interest of radar QPE.
The targets were located at different ranges, had different heights and showed different
degrees of blockage, in order to be representative of the topography surrounding the radar.
They are located in the so called Pre-coastal Range sharing a similar propagation
environment and comparable to that obtained by the Barcelona radiosonde. For example the
area considered is usually influenced by a marked sea-breeze circulation pattern, just like
the city of Barcelona (Redaño et al., 1991).
4.3 Beam blockage correction
To evaluate the effects of anomalous propagation, the partial beam blocking correction
scheme used in the NEXRAD Precipitation Processing System has been considered. This
scheme (Fulton et. al, 1998) is applied to radar beams partially shielded. In particular, this
type of beam blockage correction is applied to radar pixels (or radar bins) whose shielding
ranges between 10% and 60% and it consists of modifying radar equivalent reflectivity
