Ground GNSS Atmospheric Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 52

Notably, these results are adjusted by a scale factor 2.28 mm/hPa, which are
accounted for in the comparison. These pressure data origin from more than 8,000
land and ocean weather stations including the Global Telecommunication System
(GTS) and marine reports from the Comprehensive Ocean-Atmosphere Data Set
(COADS) (Dai and Wang 1999 ). The plots of diurnal ZTD cycles (Figs. 3.20 and
3.21 ) and semidiurnal ZTD cycles (Figs. 3.22 and 3.24 ) show general similarities,
indicating that the diurnal and semidiurnal atmospheric tides are probably the main
driver of the diurnal and semidiurnal ZTD variations derived from GPS. However,
there are also some discrepancies with the global surface pressure estimates,
particularly in the S 1 . The larger discrepancies occur in the Pacific Ocean area and
tropical regions, e.g. tropical south-east Asia, but also in North America and Western
Europe. These discrepancies probably can be related to observation errors, different
locations of the weather and GPS observation sites, the underlying geophysical
signals and other processes.
On the one hand, reasons for differences in S 1 and S 2 between the GPS-
derived results and those calculated from surface pressure data are suspected to
lie in the applied mapping functions and the loading of the earth's crust, as these
have unique diurnal and semidiurnal characteristics that are different from the S 1
and S 2 pressure tides. However, a recent simulation study by Humphreys et al.
( 2005 ) showed that the impact of the mapping functions of Niell ( 1996 )isless
than 11 % on the amplitude of the subdiurnal oscillations in ZTD, assuming an
average distribution of elevation angles and an elevation cutoff of 7 ı . The effect of
atmospheric pressure loading was found to be less than 11 % of the amplitude of
the subdiurnal oscillations in ZTD (Humphreys et al. 2005 ). Also the estimated
errors due to the solid Earth tide (Watson et al. 2006 ) and the ocean loadings
(Dach and Dietrich 2000 ) on the amplitude of the subdiurnal oscillations in ZTD
are found to be as high as 17 %. However, these effects are mitigated by applying
corrections based on the solid Earth tide model IERS03 and ocean tide model
FES2004, respectively. In addition, the water vapor diurnal variations will affect
surface and atmospheric longwave radiation and atmospheric absorption of solar
radiation (Dai et al. 2002 ; Pramualsakdikul et al. 2007 ) and possible atmospheric
tides. However, this is difficult to verify as IGS stations have few co-located
meteorological observations which would allow us to determine the amount of water
vapor at the IGS stations independently. Further work is needed to investigate the
importance of diurnal and semidiurnal variations in water vapor on ZTD. On the
other hand, the differences in subdiurnal variations (particularly S 1 ) may be due to
other processes, such as the diurnal cycle of convection and atmospheric large-scale
vertical motion. Actually the classic tidal theory predicts that the diurnal tide S 1
is very complex and irregularly distributed (Chapman and Lindzen 1970 ). It needs
to further investigate the complex subdiurnal variations and mechanism with more
data in the future.