Geoscience Reference
In-Depth Information
4.1 Radar beam blockage
Weather radars installed in complex orographic areas may suffer from partial or total beam
blockage caused by surrounding mountains. This effect can restrict seriously the use of the
lowest antenna elevation angles which typically provide the most useful information for
precipitation estimation at ground level - see for example Joss and Waldvogel (1990),
Sauvageot (1994), Collier (1996), or Smith (1998) among others. Therefore, in hilly terrain,
beam blockage correction schemes are needed to minimize the effect of topography if
quantitative precipitation estimations (QPE) are required. Such corrections are usually
included in operational QPE procedures as can be seen in, for example, Crochet (2009),
Harrold et al. (1974), Kitchen et al. (1994), Joss and Lee (1995), or Fulton et al. (1998) and may
be combined with correction techniques based in the analysis of the 3-D echo structure
(Krajewski and Vignal, 2001; or Steiner and Smith, 2002).
The idea that assuming normal propagation conditions for radar observations may not always
be a good choice and the use of local climatological refractive data for a specific radar site was
already proposed, for example, in the COST 73 Project (Newsome, 1992) and, in a different
context, evaluated by Pittman (1999) to improve radar height measurements. In this section the
effect of changing the radar beam propagation conditions upon an ordinary single polarization
reflectivity blockage correction is described - note that polarimetric radars allow other type of
corrections (Giangrande and Ryzhkov 2005; Lang et al. 2009). A simplified interception
function is proposed to simulate beam blockage and particular results for the Vallirana
weather radar, located at 650 m above sea level near Barcelona (NE Spain) in a complex
orography zone are obtained considering real atmospheric propagation conditions.
4.2 Beam blockage simulation
To describe in full detail the interception of the energy transmitted by the radar with the
surrounding topography, a precise description of the antenna radiation pattern is required.
As this pattern is rather complex, it is common to assume the usual geometric-optics
approach and consider that the radar energy is concentrated in the main lobe of the radar
antenna pattern (Skolnik, 1980). Then, when a radar beam intercepts a mountain, two
situations are possible: 1) only part of the beam cross section illuminates the intercepted
topography (partial blockage) or 2) the radar beam is completely blocked (total blockage).
The percentage area of the radar beam cross section blocked by topography may be
expressed as a function of the radius of the beam cross section,
a
, and the difference of the
average height of the terrain and the centre of the radar beam, y (Fig. 10).
Fig. 10. Elements considered in the radar beam blockage function:
a
, radius of the radar
beam cross section,
y
, difference between the centre of the radar beam and the topography,
dy'
differential part of blocked beam section and
y'
the distance from the center to
dy'
.
