Geoscience Reference
In-Depth Information
Dam and Herring
1994
; Petrov and Boy
2004
; Wijaya et al.
2013
in this topic), the
precise calculation of the Earth time-variable gravity field due to atmospheric mass
redistribution (Boy and Chao
2005
) and the de-aliasing of gravity mission satellite
data (Flechtner
2007
; Karbon et al.
2013
), the determination of the centre of mass of
the atmosphere with respect to the centre of mass of the total Earth (including oceans
and atmosphere) (Chen et al.
1999
), and the determination of the total atmospheric
mass (Trenberth and Smith
2005
) as well as its seasonal variations (Hoinka
1998
).
With respect to atmospheric loading corrections in particular, the following
requirements for the reference pressure can be identified:
•
The reference pressure should be unambiguously determinable now, and the same
pressure values should be obtained at any time in future.
•
Any method for a reference pressure determination should be straightforward but
accurate enough for present day requirements defined within the Global Geodetic
Observing System (GGOS) of the International Association of Geodesy (IAG),
i.e., 1mm position accuracy and 0.1mm/year velocity accuracy (Rothacher et al.
2009
).
•
The reference pressure should be accompanied by information about its corre-
sponding height.
•
The reference pressure should cause no (or minimal) biases compared to previous
results, i.e. to analyses which did not refer to a reference pressure. For instance,
if the reference pressure is, in an absolute sense, more accurate than 2hPa, the
resulting height bias would be less than 0.6mm, which is certainly acceptable in
all areas. The value of 0.6mm is derived by assuming an average value for the
regression coefficient of 0.3mm/hPa (Rabbel and Zschau
1985
).
2.7 Atmospheric Tides
Atmospheric tides are global-scale waves. They are excited by regular external influ-
ences, in particular by the differential heating of the Sun (the regular day/night
cycle in the insolation of the atmosphere) and—to a lesser extent—by the grav-
itational lunisolar tidal force. For detailed descriptions we refer to Chapman and
Lindzen (
1970
) and Volland (
1997
,
1988
). The dominant regular daily and seasonal
variations of atmospheric parameters like surface pressure, wind velocity or tem-
perature, are of thermal origin, depending on differential solar radiation due to the
Earth's rotation and the geometry of the Sun-Earth system. With the mean angular
velocity of Earth rotation
10
−
5
rad/sec, the basic frequencies are
Ω
=
7
.
292115
·
Ω
s
26 corresponding to the periods of
one solar day and one tropical year, respectively (Brzezinski et al.
2002
). But since
the heating does not follow a simple sinusoidal pattern (the diurnal cycle is close to
a slightly smoothed two-valued step function), and since there are orographic and
meteorological differences near the ground, harmonics are generated with frequen-
cies
k
=−
Ω (
1
−
1
/
366
.
26
)
and
Ω
a
=
Ω/
366
.
Ω
a
which are positive or negative multiples of the two fundamental
frequencies. These thermal waves are coherent with the gravitational tides, and thus
Ω
s
and
k
