GNSS Atmospheric and Ionospheric Sounding – Methods and Results - Global Navigation Satellite Systems: Signal,Theory and Applications - page 366

Although the ZWD is small accounting for about 10 percent of the total zenith tropospheric
delay (ZTD), the seasonal cycle in ZTD is due primarily to the wet component (ZWD).
Furthermore, the seasonal variation phases of ZTD are almost consistent with surface
temperature variations with the correlation coefficient of about 0.8. This reflects that annual
and semiannual variations of ZTD are due mainly to the ZWD variations, about 80% in the
surface temperature and 20% mainly in water vapour variations.

360

300

240

180

120

60

0

-90 -60 -30 0 30 60 90

Latitude (deg)

Fig. 7. Distribution of annual variation phase with the latitude. The phases are counted as
clockwise from the north

2.6 Diurnal and semidiurnal ZTD cycles
The 4-7 years of GPS-derived ZTD time series has been used to analyze the diurnal and
semidiurnal ZTD cycles and their features. Fig. 8a shows a colour coded map of diurnal
ZTD amplitudes derived from the GPS data. The diurnal cycle (S1) has amplitudes between
0.2 and 10.9 mm with an uncertainty of about 0.5 mm. The diurnal ZTD amplitudes reduce
with increasing latitude with the largest amplitudes appearing in the low-latitude equatorial
areas, in particular in tropical Asia and the Gulf of Mexico. At these low latitudes,
amplitudes of up to 10.9 mm are observed, while the high latitude areas reveal generally
lower diurnal ZTD amplitudes. The peak values of the diurnal cycles occur spreading over
the whole day (Fig. 9a). For the European stations there appears to be a preference for the
second half of the day. At the semidiurnal cycle (S2), amplitudes between 0.1 and 4.3 mm
with an uncertainty of about 0.2 mm are observed. Similar to the diurnal results mentioned
above, the largest semidiurnal amplitudes are also found in low-latitude equatorial areas.
The first peak of the semidiurnal cycle occurs typically around local noon. These diurnal
and semidiurnal cycles of ZTD may be due to certain short time scale physical processes
such as diurnal convection, atmospheric tides, general circulation and the coupling between
the lower and the middle and upper atmosphere.

From Eq. (4), the zenith hydrostatic delay (ZHD) can be written as ZHD=2.28 p0. The scale
factor k varies less than 1% even under severe weather conditions. As the hydrostatic