A comparison of methods to estimate vertical land motion trends from GNSS and altimetry at tide gauge stations
Tide gauge (TG) records are affected by vertical land motion (VLM), causing them to observe relative instead of geocentric sea level. VLM can be estimated from global navigation satellite system (GNSS) time series, but only a few TGs are equipped with a GNSS receiver. Hence, (multiple) neighbori...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-03-01
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| Series: | Ocean Science |
| Online Access: | https://www.ocean-sci.net/14/187/2018/os-14-187-2018.pdf |
| Summary: | Tide gauge (TG) records are affected by vertical land motion
(VLM), causing them to observe relative instead of geocentric sea level. VLM
can be estimated from global navigation satellite system (GNSS) time series,
but only a few TGs are equipped with a GNSS receiver. Hence, (multiple)
neighboring GNSS stations can be used to estimate VLM at the TG. This study
compares eight approaches to estimate VLM trends at 570 TG stations using
GNSS by taking into account all GNSS trends with an uncertainty smaller than
1 mm yr<sup>−1</sup> within 50 km. The range between the methods is
comparable with the formal uncertainties of the GNSS trends. Taking the
median of the surrounding GNSS trends shows the best agreement with
differenced altimetry–tide gauge (ALT–TG) trends. An attempt is also made
to improve VLM trends from ALT–TG time series. Only using highly correlated
along-track altimetry and TG time series reduces the SD of ALT–TG time
series by up to 10 %. As a result, there are spatially coherent changes
in the trends, but the reduction in the root mean square (RMS) of differences between ALT–TG and
GNSS trends is insignificant. However, setting correlation thresholds also
acts like a filter to remove problematic TG time series. This results in sets
of ALT–TG VLM trends at 344–663 TG locations, depending on the correlation
threshold. Compared to other studies, we decrease the RMS of differences
between GNSS and ALT–TG trends (from 1.47 to 1.22 mm yr<sup>−1</sup>), while
we increase the number of locations (from 109 to 155), Depending on the
methods the mean of differences between ALT–TG and GNSS trends vary between
0.1 and 0.2 mm yr<sup>−1</sup>. We reduce the mean of the differences by taking
into account the effect of elastic deformation due to present-day mass
redistribution. At varying ALT–TG correlation thresholds, we provide new sets
of trends for 759 to 939 different TG stations. If both GNSS and ALT–TG trend
estimates are available, we recommend using the GNSS trend estimates
because residual ocean signals might correlate over long distances. However,
if large discrepancies ( > 3 mm yr<sup>−1</sup>) between the two methods are
present, local VLM differences between the TG and the GNSS station are likely
the culprit and therefore it is better to take the ALT–TG trend estimate.
GNSS estimates for which only a single GNSS station and no ALT–TG
estimate are available might still require some inspection before they are
used in sea level studies. |
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| ISSN: | 1812-0784 1812-0792 |