Chapter 8: Measuring sealevel

Sea Level and PressurePressure and sea level measurements are of special interest in geophysical studies, and few other records are “as continuous or span such long timescales”

But relation between pressure and sealevel is not trivial. Really we want

Thus the quantity of dynamical interest is pressure on a geoidal surface just beneath the sea surface. This is called SYNTHETIC SUBSURFACE PRESSURE (SSP)

The height equivalent of SSP is called

ADJUSTED SEA LEVEL (ASL)

Sea Level and Bottom Pressure

• Bottom pressure observations have been made with increasing frequency over the past decades availability of stable pressure sensors

• They measure absolute pressure directly, but

- including atmospheric component (different from sealevel measurement)

- Including pressure changes due to density fluctuations of the water column above the sensor (important if DEEP sensor, negligible if shallow)

So:

To compare BP measurements to SSP measured from coastal level stations, need to convert BP also to SSP.

Sea Level and Bottom Pressure

)()(

zgz

zP

Hoatmbot dzgHgPP ')(

0

'H

dzApproximated here as since

0'dz is small

Density is a function of Temperature:

oT 14103 Cx owith

n

nnobotBP hTgPSSP )(

Assume hydrostatic pressure distribution in the vertical

integrating over the water column

SSP Baroclinic Pressure

So SSP determined from bottom pressure is

( is sea surface height)

Inverse barometer effect

SSP from sea level and from bottom pressure:

n

nnobotBP hTgPSSP )(

gPSSP oatmSL

Tide gauge

Device to measure sea level

Mean Sea Level (‘still water level’)

>> Measured wrt land so changes can result

- from a real change in sea level

- from a change in height of the land on which the

tide gauge operates

Level of the sea with motions such as wind waves and tides, averaged out.

Tide Gauges

(1) Stilling well and float

(2) Pressure systems

(3) Acoustic systems

(4) Radar systems

Tide gauge

Kelvin type tide gauge:

-Used pencils and springs

-Had to wind springs daily

-Paper set vertically

(prone to damping and strecthing as it had no cover)

Tide gauge

Tide gaugeSome factors that complicate determining the sea level change from tide gauge data:

• Settling of the site

• Vertical motions of the region due to post-glacial rebound, tectonic uplift or crustal subsidence

In long term assessments of sea level change, variables such as barometric pressure and windspeed have to be monitored so they can be eliminated.

Stilling Well

National Tidal and Sea Level FacilityUK Tide Gauge Network (Ilfracombe)

Pressure Systems

• Need knowledge of seawater properties

• Two main types:– Pneumatic bubbler gauges– Pressure sensor gauges

Pneumatic Bubbler Gauges

Pneumatic Bubbler Gauges

Pressure Sensor Gauges

• Installed directly in the sea to measure sub-surface pressure– Absolute pressure (sea level and

atmosphere), which requires a separate barometer synchronized with sensor to correct for atmosphere

– Differential pressure transducers, which have a vented cable that is exposed to the atmosphere (but they are known to have blockage issues)

Acoustic Tide Gauges

Radar Gauges

Installation considerations

• Withstanding ice, winter storms• Stable ground (no erosion, land subsidence,

etc.)• Areas of representative tides (not headlands)• Not over sandbars/long sloping beaches with

unusual wave patterns• Not near breakwaters, construction, shipping

traffic, outfalls, estuaries• Need nearby benchmarks for leveling

Comparison(1) Acoustic sensors

- In sounding well - $8-12,000, <1 cm accuracy- Open air - $5-8,000, >1 cm accuracy

(2) Pressure- Single transducer - $5-12,000, ~mm precision - Multiple transducers - $12-20,000, ~mm precision & accuracy - Bubble gauge - $8-12,000, ~1 cm accuracy

(3) Stilling Well - Float gauges - $5-12,000, ~1 cm accuracy

(4) Radar Systems - radar gauge - $12-20,000, expected ~1 cm accuracy

IOC Recommendations

• Mid/low latitude– Acoustic gauge with sounding tube– Radar tide gauge – Multiple transducer pressure system

• High latitude– Absolute pressure sensor & barometer– Bubbler pressure gauge

GLOSS Establishing high quality global and regional sea

level networks for application to climate, oceanographic and coastal sea level research.

Global core network:

290 sea level stations around the world

http://www.pol.ac.uk/psmsl/programmes/gloss.info.html

1 – ‘Operational’ (latest data 2005 and later)

2 – ‘Probably Op.’ (within 1995-2004)

3 – ‘Historical’ (earlier than 1995)

4 – No data www.gloss-sealevel.org/station_handbook/

University of Hawaii Sea Level Center

http://uhslc.soest.hawaii.edu/

To collect, process, distribute, and analyze in-situ tide gauge data from around the world in support of climate research.

Accuracy of tide gauge and bottom pressure measurements

Associated to a variety of errors

•Inaccuracies in the measurement

•Local peculiarities of the observational site

In the case of bottom pressure sensors: Possible vertical motions of the anchoring system: anchor drifts (real or by sinking).

-Harms and Winant (1994) Synthetic Subsurface Pressure Derived from Bottom Pressure and Tide Gauge Observations. Journal of Atmospheric and Oceanic Technology.

-Lentz (1993) The Accuracy of Tide-Gauge Measurements at Subtidal Frequencies. Journal of Atmospheric and Oceanic Technology.

-Wearn and Larson (1982) Measurements of the sensitivities and drift of Digiquartz pressure sensors. Deep-Sea Research.

Pressure Sensors• Strain gage – This is the simplest and cheapest instrument. Widely

used. Accuracy about ±1%

• Vibratron - Measures the natural frequency of a vibrating tungsten wire stretched in a magnetic field between diaphragms closing the ends of a cylinder. Accuracy is ±.1% or better when temperature controlled. Precision is 100-1000 times better than accuracy. The instrument is used to detect small changes in pressure at great depths.

• Quartz crystal – Measures the natural frequency of a quartz crystal cut for minimum temperature dependence. Accuracy is +/- .015% and precision is +/- .001%

• Quartz Bourdon Gage – has accuracy and stability comparable to quartz crystal. Used for long-term measurements of pressure in the deep sea.

SEABIRD (Washington)

SBE 26plus SEAGAUGE Wave and Tide Recorder ~ US$11080.-After 15% discount

http://www.seabird.com

STANDARD CONFIGURATION

-Plastic housing for depths up to 600 m.

-20 m (45 psia) Quartzonix temprature-compensated pressure sensor

-Accurate temperature sensor

-32 MB FLASH memory

-12 alkaline D-cell batteries. Battery compartment separated from electronics by moisture-proof seal

OPTIONS

-Digiquartz temperature-compensated pressure sensor ranges from 0.2 to 680 m ($900.-)

-Lowered price Druck temperature-compensated strain gauge pressure sensor ranges from 20 to 600 m (generally intended for wave sampling applications, won’t provide highest quality tide data)

-High accuracy temperature sensor

-Wet-pluggable connectors

-Mounting fixture

Range: 9 ranges from 0-0.2m (15 psia) to

0-680m (1000 psia)

Accuracy: 0.01% of full scale

Repeatability: 0.005% of full scale

SBE 53BPR Bottom pressure recorder ~ US$15363.75After discount

SEABIRD (Washington)

STANDARD CONFIGURATION

-Depths greater than 680 m. (up to 7000 m). Titanium housing

-2000 to 10000 psia Paroscientific Digiquartz temperature-compensated pressure sensor

-Accurate temperature sensor

-32 MB FLASH memory

-12 alkaline D-cell batteries. Battery compartment separated from electronics by moisture-proof seal

OPTIONS

-Conductivity sensor (SBE4M, cable, mount – add $4139.50)

-High accuracy temperature sensor

-Wet-pluggable connectors

Range: 0-1300, 2000, 4000, or 6800 m (2000,

3000, 6000 or 10000 psia)

Accuracy: 0.01% of full scale

Repeatability: 0.005% of full scale

http://www.seabird.com

~ US$3000.-

PAROSCIENTIFIC, INC. (Washington)

--Ranges up to 500 psia

-Choice of stainless steel or nylon buffer tubes

-Oil-filled vs. non-oil filled

http://www.paroscientific.com

FALMOUTH SCIENTIFIC, INC. (Massachussets)

-High accuracy druck pressure sensor

-Temperature-corrected calibration

-Housing for 500 dbar operating depth

-128 MB internal memory

-Programmable sampling capability

-20 meters of conductor cable

-2 meter test cable

With a surface mounted enclosed equipped with

-Gel-cell lead-acid rechargeable battery

-Battery charger

-Data download connector

-Connector from subsurface sensor

-Barometer

$3885.-

+ $2570.-

-50 psia pressure sensor

-25 m. operating depth

-0-600 cm/s range

-Selectable 1-5 Hz sampling

-Internal wave processing and compass

-X and Y tilt sensors

-Temperature sensor

-32 MB internal memory

-Windows-based interfase software

-1.5 ton mooring frame

-Alkaline battery

$17175.-

http://www.falmouth.com/products/index.htm

Inverted echosounder

η

0

-H

isopycnal

Travel time anomaly Δtt is

For Gulf Stream front, first term is about 1ms and second one 7ms, EACH WAY, so total signal approx. 16ms roundtrip

Inverted echosounder: empirical traveltime vsdynamic height relations

Altimetry

TOPEX/Poseidon & Jason satellites

-radar altimeter emitting 2 frequencies (range)

-microwave radiometer measuring at 3 frequencies for water vapor content

-DORIS, LRA, and GPS for precise orbital information