Sea level changes around Thailand

Transcription

1 Final symposium 27 May Bangkok, Thailand Sea level changes around Thailand Marc Naeije

2 Contents GEO2TECDI- SONG: measuring sea level around Thailand Concept of satellite altimetry Concept of tide gauges Altimetry results around Thailand: periodic sea level and trends Sea level rise in the Gulf of Thailand Vertical land motions: altimetry minus tide gauges Conclusions Radar altimeter data system RADS Cryosat- 2 opportunities for the Gulf of Thailand 2

3 GEO2TECDI- SONG: measuring sea level around Thailand In the Thailand- EC GEO2TECDI- SONG projects we investigate the vertical land motion in Thailand and sea level change in the Gulf of Thailand and Andaman Sea. An important focus is on Bangkok which is situated in a river delta and average height is close to sea level is subsiding due to ground water extraction is experiencing post- seismic motion due to nearby mega- thrust Earthquakes suffers from rising sea levels due to global climate change Combined with excessive rainfall this poses a serious threat to Thai society and economy, and therefore we have to monitor the sea level around Thailand, for instance with satellite altimetry and tide gauges. 3

4 Altimetry: N + S = r H - (corrections) r = satellite height H = altimetric distance N = geoid S = dynamic topography r H S N Radar pulse reflecting at the sea surface. 4

5 Altimetry continued Averaging many pulses together reduces noise and results in typical waveform that can be analysed. The position of the rising edge centre corresponds to the (average) distance of the sea surface. The slope and the amplitude of the echo are related, respectively, to wave height and to wind speed. Approaching the coast the waveform is corrupted by land in the radar footprint (red curve). 5

6 Altimetry: what it measures 6

7 Tide gauges Measure sea level w.r.t. geodetic benchmark Most common is a float operated in a pipe or stilling well Move vertically with the land due to post- glacial rebound, tectonic uplift, and/or subsidence Often tied to GPS recei- ver for absolute datum No global/dense data; +/- 20cm sea level rise over last century CCAR, Univ. of Colorado Tide gauge station at Hua Hin 7

8 Processed region 8

9 Regional sea level dynamics A simultaneous fit of linear trend, annual, semi- annual cycle and a bias, subjected to IRLS has been applied to monthly- averaged altimeter meshes (animation shown). In a direct comparison with tide gauge and GPS data we have to take into account the discontinuity in land motion due to the 2004 Sumatra Earthquake. Though the sea level trend over the medium length period seems significantly affected by the lunar nodal tide effect (18.6 year cycle), the effect in tide gauge data is the same and cancels out when differenced. 9

10 Sea level annual cycle 10

11 Sea level semi- annual cycle 11

12 Sea level trend and error 12

13 Sea level rise in the Gulf of Thailand 13

14 Altimetry and tide gauges Direct comparison between tide gauge monthly- average time series and those from altimetry from (prior to 2004 Sumatra Earthquake). An arbitrary bias is removed. Solid circles have final weight < 0.5 and crosses get zero weight (outliers). The given numbers give distance to TOPEX- class track, RMSE, correlation, and percentage valid observations. 14

15 Vertical land motion: ALT- TG Vertical land motion from the difference between altimetry and tide gauge data: blue crosses the monthly averages and the red ones 12- month moving average filtered. A linear trend has been fitted. Ko Mattaphon seems to behave different from the other stations but is also furthest situated from a TOPEX- class track. 15

16 Vertical land motion: GPS Vertical land motions at CHON and BANH based on GPS campaign data. Heights have been corrected for seasonal variations estimated from nearby CGPS data. We see a combination of interseismicity and GIA uplift at both locations before 2005 and post- seismicity after the Sumatra Earthquake at the end of The difference in vertical motion regimes before and after the quake are striking and the post- seismicity clearly adds to the flooding threat. 16

17 Comparing altimeter & TG trends Tidal Station Altimimetry Altimetry Tide gauge VLM (alt- tg) Laem Singh 3.6 ± ± ± ± 1.6 N/A VLM (gps) ( ) Sattahip 4.8 ± ± ± ± ± 1.3 a Ko Sichang 5.8 ± ± ± ± ± 1.3 a Hua Hin 4.8 ± ± ± ± 2.5 N/A Ko Lak 3.6 ± ± ± ± 1.5 N/A Ko Mattaphon 3.2 ± ± ± ± ± 0.8 b Ko Prab 3.3 ± ± ± ± 2.1 N/A Songkhla 3.3 ± ± ± ± 2.6 N/A Geting 3.9 ± ± ± ± ± 1.0 Altimeter based sea level trends in the GoT at tide gauge stations and the tide gauge trends over the same period. In column 5 we subtract the tide solutions from the altimeter to derive the local vertical land motion (VLM), and we compare this with VLM based on GPS (rightmost column). 17

18 GEO2TECDI- SONG conclusions Before mitigation methods can be devised against the threat of relative sea level rise, all contributing effects have to be charted, separated, qualified and quantified by using GPS, InSAR and altimetry, and combining results with observations from tide gauges and leveling Adding GPS based vertical land motion to the tide gauge sea level registration reveals the absolute sea level change at a number of tide gauge stations around Thailand, which is confirmed by altimetry Combination of altimetry and GPS could solve problem of erroneous tide gauge readings and poor coverage In the Andaman Sea we find average absolute rise of about 3mm/yr ± 0.5 and in the GoT of ± 4.0 mm/yr ± 0.5, but near estuaries, like that of the Chao Praya River (Bangkok), and the Mekong delta (Ho Chi Min City), this mounts easily to 5 to 6 mm/yr, much faster than the global average. 18

19 RADS: Radar Altimeter Database System 19

20 RADS: 2 flavors RADS web portal: For anybody interested in sea level information from altimetry Local install of RADS: core user see manual at For anybody interested in research collaboration with TUDelft/DEOS (through MOUs, as project partners) being ESA PI or CoI and/or a CAT- 1 (ESA approved) proposal 20

21 Latest altimetry asset: CryoSat- 2 CryoSat-2 current geographical mode map with LRM (blue), SAR (green) and SARiN (purple) modes 21

22 dualxovers C2- J2: sea level anomaly 7.7cm 5.6cm 4.7cm 4.5cm RADS CryoSat-2 product nears Jason-1/2 quality and incorporates reduced SAR data (Gulf of Thailand!) 22