Geoscience Reference
In-Depth Information
It may be noted that nocturnal
Ns
values were lower than noon values (about 5 N units in
the monthly means) and also the existence of a marked seasonal pattern with a peak in
August and a minimum in December. This yearly cycle may be explained by examining the
behaviour of the magnitudes considered in the computation of refractivity and also by
considering separately the dry and wet terms (Fig. 3).
Fig. 3. Evolution of surface refractivity (N
s
) and the wet (N
w
) and dry terms (N
d
) over
Barcelona (Bech, 2003).
Monthly variations of these magnitudes show different behaviours. While the temperature
follows a very clear seasonal pattern (highs in summer and lows in winter, as expected), in
the case of the pressure it is much weaker (approximately winter maxima and summer
minima). The humidity, changing constantly throughout the year, exhibits no apparent
pattern. These behaviours are reflected in the evolution of
N
d
and
N
w
. The first one,
proportional to
pT
-1
, is nearly constant with maxima in summer and minima in winter; the
second, proportional to
eT
-2
, is much more variable (because of
e
) but maxima and minima
are swapped with respect to
N
d
(because of
T
-2
). Therefore,
N
w
, which represents about 30%
of
N
, contributes mostly to its variation: at short scale, it adds variability and also, at
monthly scale, modulates the summer maximum and winter minimum cycle which is
slightly compensated by the opposite cycle shown by
N
d
.
Surface refractivity distributions in Barcelona are shown in Fig. 4, exhibiting larger
variations at 12 UTC (aprox. 265 - 385) than at 00 UTC.
Fig. 4. Surface refractivity distributions at 00 and 12 UTC in Barcelona.
