Geoscience Reference
In-Depth Information
For weather radar applications, if the vertical refractivity gradient of the first kilometre
(VRG) of the atmosphere is around
-1/4r
(i.e. -39 N units km
-1
or 118 M units km
-1
, where
r
is the Earth's radius) then standard propagation will occur for any angle of incidence
(Doviak and Zrnic, 2006). An increase in VRG bends the radar beam more slowly than
normal (subrefraction) and reduces the microwave radar horizon. With regard to ground
clutter echoes, subrefraction implies a decrease in their frequency and intensity. On the
other hand, a decrease in VRG generates the opposite effect, bending the beam faster than
normal (super refraction) for the interval between (typically) -78.7 km
-1
and -157 km
-1
(the
threshold to distinguish between standard propagation and superrefraction varies in the
literature around 80 km
-1
). Trapping, or ducting, the most extreme case of anomalous
propagation, occurs for values lower than -157 km
-1
, and in this case the microwave energy
may travel for long distances before intercepting ground targets producing anomalous
propagation (i.e., anaprop or AP) echoes. In fact the exact threshold for ducting depends on
the precise local value of the Earth radius, which means that it is not a constant value (for
example varies with latitude) - see Table 1 for a summary of ranges of refractivity and
modified refractivity gradients for different propagation conditions. As a reference, the two
examples of radar images shown in Fig. 2 were recorded with VRGs of -43 and -112 km
-1
.
Characteristic dN/dZ (km
-1
) dM/dZ (km
-1
)
Subrefraction (0,+) [157, +)
Normal (-79,0] (157, 79)
Superrefraction [-79,-157) [79,0)
Ducting [-157, -) [0,-)
Table 1. Effects upon propagation under different ranges of dN/dZ and dM/dZ (adapted
from Bech et al. 2007a).
On the other hand, a careful analysis of the fluctuation of target reflectivity may be a way to
monitor variations in atmospheric conditions (changes in moisture content, etc.) as shown
by Fabry et al. (1997). Subsequent research from that work triggered new interest in the
analysis and characterization of refractivity profiles near ground level - see for example
Park & Fabry (2011).
Superrefraction and ducting in particular, is usually associated with temperature
inversions or sharp water vapour vertical gradients. During cloudless nights, radiation
cooling over land favours the formation of ducts which disappear as soon as the sun heats
the soil surface destroying the temperature inversion. This process may be sometimes
clearly observed in the daily evolution of clutter echoes, as reported by Moszkowicz et al.
(1994) and others.
3. Propapagation condition variability
As radiosoundings have been traditionally the only source of upper air information
available on a routine basis, they have been used for years to calculate long term averages of
propagation conditions -see, for example, Gossard (1977) or Low and Huddak (1997)-. Since
1997, radiosonde observations have been made in Barcelona to support the operations of the
regional government's Subdirectorate of Air Quality and Meteorology, which later became
the Meteorological Service of Catalonia.
