Ground GNSS Atmospheric Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 46

Fig. 3.10 Distribution of
ZTD secular variation
velocities (trend) with the
altitude of all IGS sites

4

2

0

-2

-4

-6

0 1000 2000 3000 4000

Altitude (m)

the future. These secular ZTD variation characteristics reflect the total variations
of surface atmospheric pressure, temperature and relative humidity, atmospheric
vertical motions, etc.

3.3.2.2 Seasonal ZTD Variations

Meanwhile, the seasonal components are also obtained using Eq. ( 3.9 ), which can
be used to study the seasonal cycle, including amplitude and phase shift. The fitted
phase shift is used to determine in which month the seasonal maximum takes place.
The annual variation of ZTD ranges from 25 to 75 mm depending on the site, and
the average amplitude is about 50 mm at most sites (Fig. 3.11 ). The annual variation
amplitudes of ZTD at the IGS sites near Oceanic coasts are generally larger than
in the continental inland. In addition, larger amplitudes of annual ZTD variation
are mostly found at middle-low latitudes (near 20S ı and 40N ı ), and the amplitudes
of annual ZTD variation are especially smaller at higher latitudes (e.g. Antarctic
and Arctic) and the equator areas (see Fig. 3.12 ). Sites on the eastern Atlantic
and northeast Pacific coasts have lower annual variations, probably because of the
moderating effect of the ocean on climate. Sites on the lee side of the Alps have
higher annual variation, possibly due to the combined effects of a rain shadow in
the winter and high moisture from the Mediterranean in the summer (Haase et al.
2003 ; Deblonde et al. 2005 ). Figure 3.13 shows the annual phase distribution with
the latitude, where phase values are counted as clockwise from the north. It can be
seen that the phase of annual ZTD variation is almost found at about 60 ı (about
February, summer) in the Southern Hemisphere and at about 240 ı (about August,
summer) in the Northern Hemisphere, which is just a half-year difference.
The mean amplitude of semiannual ZTD variations is much smaller than annual
variations with about 10 mm. The amplitudes of the semiannual oscillations are
much smaller on the Southern Hemisphere than on the Northern Hemisphere.