Geoscience Reference
In-Depth Information
Fig. 10
Hydrostatic mapping
functions VMF1 and GMF
at 5
◦
elevation at Fortaleza,
Brazil. Phenomena such as the
El Niño event in 2009 cannot
be captured with empirical
mapping functions like GMF
that contain only average
seasonal terms
10.104
10.102
10.1
10.098
10.096
VMF1 hyd
GMF hyd
10.094
10.092
2005
2006
2008
2009
2010
the special meteorological conditions described by the VMF1 during the El Niño
event in 2009/10.
Niell (
2006
) compared the mapping functions VMF1, IMF, GMF, and NMF with
mapping functions derived from ray-tracing of radiosonde data in 1992, which were
assumed to be the most accurate reference possible. The standard deviation was
converted to station height scatter with the rule of thumb by Niell et al. (
2001
)
(one third of the delay at 7
◦
elevation angle). The best agreement was found for
the VMF1, both for the hydrostatic and the wet mapping function. All hydrostatic
mapping functions show the lowest scatter at the equator, because there are only
small pressure and temperature variations. The scatter increases with station latitude,
in particular for the empirical GMF and NMF, which of course cannot account for
the variations at synoptic time scales (
10 days). The situation is different for the
wet mapping functions where the scatter is largest at the equator. This is due to the
fact that the zenith wet delays are largest over the equator (up to 40 cm). At the
poles, there is hardly any humidity; thus, errors in the wet mapping functions are
not critical for the estimation of station heights (and zenith wet delays). Again, the
performance of the wet VMF1 is best. However, it has to be mentioned here that
the radiosonde data might have been assimilated in the NWM which are used to
determine the VMF1 and IMF. Consequently the station height scatter for these two
mapping functions may be too optimistic.
There have been many investigations comparing the application of different map-
ping functions in VLBI or GPS analysis. For example, Böhm et al. (
2007
) and
Steigenberger et al. (
2009
) compared various mapping functions in GPS analysis,
and Tesmer et al. (
2007
) compared them in VLBI analysis. Moreover, Kouba (
2008
)
compared the gridded VMF1 against the VMF1 determined for specific sites, or
Böhm et al. (
2009b
) assessed the accuracy of forecast VMF1 for the application in
real time analysis.
∼
