2007 ostst meeting - 1 - y. faugere (cls) j. dorandeu (cls) f. lefevre (cls) long period errors...
TRANSCRIPT
2007 OSTST meeting- 1 -
Y. Faugere (CLS)
J. Dorandeu (CLS)
F. Lefevre (CLS)
Long period errors observed at Envisat crossovers and possible impact of tides
2007 OSTST meeting- 2 -
Introduction
An annual signal is observed on the ascending/descending Sea Surface Height differences at crossovers on Envisat.
The purpose of this talk is to: • Describe the observed signal
• To show how a part of this signal is linked to oceanic tides
• Discuss this error with the tide community
• Participate to the discussion on the potential problems relative to sun-synchronous orbits
2007 OSTST meeting- 3 -
Ascending track crossing the equator at 10H00am local time
Descending track crossing the equator at 10H00pm local time
Description of the signal
• Envisat is a sun-synchronous satellite:– Inclination=98°– Repetitivity= 35 days– Altitude ~800km
2007 OSTST meeting- 4 -
•Orbit
•Diurnal atmospheric effects
•Ionospheric effects
•Tides
•Other ?...
annual signal of ~1cm of amplitude
A.Cos(2π/365.t+φ)
Possible sources of error:
Description of the signal• Cyclic mean of [ascending-descending] SSH differences at 10-day crossovers on Envisat
1 year1 year 1 year
2007 OSTST meeting- 5 -
Description of the signal
• The annual signal A.Cos(2π/365.t+φ) is estimated in 10°x10° boxes over the Envisat period
2007 OSTST meeting- 6 -
Amplitude A
Phase φ
Description of the signal
0 3cm
0 360deg
• The annual signal A.Cos(2π/365.t+φ) is estimated in 10°x10° boxes over the Envisat period
2007 OSTST meeting- 7 -
Wave name Aliasing ENVISAT (days)
Sa 365.259622
Ssa 182.621095
Mm 129.530031
Mf 79.9227517
Q1 132.806118
O1 75.0669737
P1 365.24219
K1 365.24219
N2 97.3929566
M2 94.4864493
S2 ∞
K2 182.621095
Impact of oceanic tides
K1, P1,Sa are aliased on Envisat at about1 year
An error on K1, P1 or Sa may have a signature at the annual period on the ascending/descending
SSH differences
• Reminder of the aliasing periods of ocean tidal waves on Envisat
2007 OSTST meeting- 8 -
Impact of oceanic tides
0 50cm
Amplitude A
From SSH without
ocean tide correction
2007 OSTST meeting- 9 -
Impact of oceanic tides
0 50cm
Amplitude A
From SSH without
ocean tide correction
Amplitude of K1
Oceanic tidal wave
• Without correcting the ocean tide, the map of A has the same patterns as the diurnal waves of oceanic tide
2007 OSTST meeting- 10 -
A (SSHGOT00.2)
Impact of oceanic tides
0 3 0 360
• Impact of the tide model used to correct the SSH
φ (SSHGOT00.2)
2007 OSTST meeting- 11 -
A (SSHGOT00.2)
Impact of oceanic tides
0 3cm
0 3cm
0 360deg
0 360deg
• Impact of the tide model used to correct the SSH
φ (SSHGOT00.2)
A (SSHFes04) φ (SSHFes04)
2007 OSTST meeting- 12 -
A (SSHGOT00.2) - A (SSHFes04) (cm)
Amplitude of K1 (SSHGOT00.2) – Amplitude of K1 (SSHFes04) amplitude (cm)
Impact of oceanic tides
-1.5 1.5cm
-1.5 1.5cm
• Some similarities are observed between the difference of amplitude of the annual signal and the difference of amplitude of K1 oceanic tidal wave.
2007 OSTST meeting- 13 -
• Results obtained with older tidal solution
Impact of oceanic tides
A (SSHGOT00.2)
A (SSHFES04)
•ERS-2 data have been assimilated in both FES02 and FES04 in coastal regions, but not in FES99 and GOT00 => is the differences of annual signal observed due to the use of ERS-2?
A (SSHFES99)
A (SSHFES02)
2007 OSTST meeting- 14 -
Conclusion
• A centimetric annual signal is visible on the cyclic mean ascending/descending SSH differences at crossovers on Envisat• The geographical distribution of this amplitude is not homogeneous (0-3cm)• An error on K1 or Sa may have a signature at the annual period on the ascending/descending SSH differences• Changing the oceanic tide model changes the map of amplitude
=> Question 1: is this signal a signature of an error on K1, P1 or Sa tidal wave?
=> Question 2: What is the impact of the use, and the method of assimilation of ERS-2 data in the tide model quality?
2007 OSTST meeting- 15 -
• There are also low frequency errors on both ascending and descending (thus not visible on ascending/descending SSH differences) => see poster CV7
2007 OSTST meeting- 16 -
• time varying ascending/descending SSH differences are also noticed on non sun-synchroneous satellite:
Cyclic mean of [ascending-descending] SSH differences at crossovers
• There are also annual signal on both ascending and descending (thus not visible on ascending/descending SSH differences) => see poster CV7