Geoscience Reference
In-Depth Information
The yearly minima of VRG, below -80 km
-1
sometimes reaching -120 km
-1
(maximum
superrefraction), at the end of the warm season is also appreciated in the VRG time series
plot of Barcelona shown in Fig. 7.
Fig. 7. Time series plot of VRG (N units km
-1
) for the period 2000-2006 in Barcelona.
The seasonal pattern noted in Barcelona is already indicated in the VRG World Wide maps
prepared by the International Telecommunications Union (ITU, 2003). In particular, in
August, an area of maximum superrefraction affects the Western Mediterranean region,
comparable in intensity to the maximum above the SW Pacific coast of N. America, and
somewhat weaker than the Arabian Peninsula -where the world maximum is located for
that month-. Using the Historical Electromagnetic Propagation Condition Data Base from
the US Naval Systems Ocean Center (Patterson, 1987) a comparison with ten radiosonde
stations located in the area was performed. Median monthly values allowed to check similar
patterns both in
Ns
and VRG. A related study was carried out recently by Lopez (2009)
using global analysis data from the European Centre for Medium-range Weather Forecasts
(ECMWF) to assess the occurrence of superrefraction, or with a similar approach, but at a
local scale, by Mentes and Kaymaz (2007) in Turkey, or Mesnard and Sauvageot (2010) in
France.
The frequency and cumulative probability distributions for Barcelona VRG are shown in Fig
8. A similar unimodal left skewed pattern, with stepper slopes for higher VRG values
(tending to super refraction), is shown for both 00 and 12 Z data. However, modal values
are very near the nominal standard propagation value of -40 N units/km (-49 N units/km at
night and -42 N/km units at noon).
Fig. 8. Frequency and cumulative probability distributions for the Barcelona VRG.
