Summary of Vehicle Characteristics. Drifters- basics Refers to surface current followers Consists of...

Summary of Vehicle Characteristics

Drifters- basics

Refers to surface current followers Consists of a buoy and drogue with a line between the

two (may be a single integrated unit), usually drogued at depths <30m.

Challenger Expedition-1872 First Global GARP Experiment (FGCE)- 1978-79 World Climate Research Program (WCRP)- 1985 Coastal Ocean Dynamics Experiment- 1981-82 Price of drifter used in global array reduced by a factor

of 3 between 1993 and 2003 ($5475 to $1820) In FY 2004 the drifter array has increased to about 1000

elements (1031 elements on 8/16/04) and by June 2005 the desired 1250 element array will be in place.

Drifters- design goals

No slippage of water past the drifter No current rectification by the drifter itself Buoy should be a good surface follower

– To maintain good antenna placement– To avoid being submerged by waves

Should have good durability/duration Resistance to bio-fouling Ease of deployment for use with ships of opportunity Low cost (usually, but not always, expendable)

Drifters- buoys, tethers, drogues

Surface buoys- spars, boats, pill boxes, cylinders, garbage cans, inverted cones

Tethers- ropes, chains, wires electrical cables, Kevlar, Bungee cord

Drogues- parachutes, weather socks, window shades, butterflies, corner radar reflectors, holey socks, cruciforms

Drifters- tracking methods

Human recovery and reporting Visual (from land or ship) Radar ranging (from land or ship) Radio ranging and directing finding (RDF) ARGOS transmitter

– Via satellite– Via local user terminal (cheaper, but less accurate)

GPS or LORAN position transmitted via packet radio or via cell phone or Iridium satellite

SVP drifter- spherical surface buoy with holey sock drogue

CODE-type Drifter

Coastal Ocean Dynamics Experiement

Location is by GPS or ARGOS Two types- spherical float and a

radar corner reflector drogue or a cruciform drogue with four small floats

SST and SSS, current meters (ac) 2500 drifters of this type have been

deployed in the past 10 years

Modified WOCE Drifter

http://woce.nodc.noaa.gov/wdiu/diu_summaries/svp/index.htm

ADOS- Drifter

Autonomous Drifting Ocean Station Spherical surface float with three pods of

downward looking radiometers (light can be measured in 9 wavelength bands)

Thermistor/pressure chain to 120m Wind direction, wind speed, atmospheric

pressure and PAR Water-following capability is not known

Status of Global Drifter Array

Drifter Data


http://www.po.gso.uri.edu/rafos/general/history/index.html

Swallow Floats

Swallow- 1955 Aluminum pipe with a battery and

a timer circuit that would excite a magnetostrictive transducer, a ‘pinger’ hanging underneath

The high freq and lower power level of the acoustic pinger on Swallow’s float were such that it could only be tracked from a surface vessel nearby

SOFAR Floats

Stommel- 1955 First subsurface floats traceable

over a long range Emit a low freq note at user

selectable intervals requiring large amounts of power

Fixed subsurface listening stations range on the sound and triangulate to position the float

These autonomous listening station (ALS) must be recovered to get the data

These floats can only be used on or near the sound channel axis

An Autonomous listening station being deployed from RRS Discovery in the mid-1980’s

ALS consists of a 10m hydrophone unit (furthest from the ship) and recording unit (cylinder above the hydrophone)

The ALS was deployed at a depth of approximately 1000m on a mooring

SOFAR Floats- ALS

RAFOS Floats

SOFAR spelled backwards- Rossby et.al. 1986

Listen to sound sources on the bottom, of known position, then triangulate to position themselves.

Less power required and less expensive Easily deployed by anyone simply by throwing

over the side; do not need to be recovered Data recovered by satellite when the floats surface

at the end of their mission (months to years) Isopycnal float was developed- uses a spring-

loaded piston that adjusts the volume, allowing the density to stay constant so that the float stays on a given isopycnal surface

RAFOS Data

‘Meddies’ Warm salty

anticyclones centered at 1000m depth

Profiling Floats

Autonomous LAgrangian Current Explorer (ALACE) Profiling ALACE (PALACE) Salinity PALACE (S-PALACE) Autonomous Profiling Explorer (APEX)

Profiling ALACE Floats

ALACE concept aimed to supply a float that operated autonomously and could operate anywhere (not limited to areas of the ocean with acoustic sound sources like RAFOS floats).

Soon after ALACE floats were first launched, the idea naturally arose that the floats should be equipped with instrumentation to observe temperature on the way up > PALACE floats

S-PALACE Floats

S-PALACE- eventually salinity measurements were added The data is collected by the float and telemetered back to land via

satellite link Floats can last as long as 150 profiles or more, giving a 4-5 year

lifetime on a two-week cycle After deployment, float sinks to the depth for which is was ballasted

and drifts with the currents at that depth for a pre-programmed time. It then ascends to the surface measuring temperature and conductivity. Once at the surface, the float transmits the encoded profile data to passing satellites for a period of 24 hours before descending again.

APEX Floats

Powered by alkaline batteries and has power sufficient to execute about 200 profiles

Field proven and routinely deployed from C130 aircraft and merchant ships

Can be programmed to follow isobaric or isopycnal surfaces

‘Park and profile’ feature allows drift depth to be de-coupled from max profile depth (example: a float might be programmed to drift at 1000db, then descend to 2000db before profiling upward to the surface).

Project Argo

Argo Data- Float Profile

http://flux.ocean.washington.edu/argo/homographs/TP/TP.html

Argo Data- Float Trajectory

http://flux.ocean.washington.edu/argo/homographs/TP/TP.html

Gliders- there is no propeller!

Slocum Glider

Forward propulsion is accomplished by varying the vehicle’s buoyancy, with control surfaces transferring the vertical motion into forward motion.

Operate in a vertical sawtooth trajectory Dead reckons to waypoints from a pre-programmed mission When it rises to the surface it fixes location by GPS and

communicates data via Iridium SATCOM or FreeWave radio Any dead reckoning discrepancies upon surfacing are due to

current and are factored into the next segment of the mission Derives estimates of vertically averaged current in addition to

T/S data from CTD

Slocum Glider

Weight – 52 kg Length – 1.5 m Endurance – 30 d Vertical Speed ~ 20 cm/s Top Forward Speed ~ 0.77 kts (40 cm/s) Operating Depth – 4-200 m Range – 1500 km Sensors – CTD Navigation – GPS, internal dead reckoning, altimeter Operating Frequencies- FreeWave – 900 MHz, Telesonar

modem transmission – 9-14 kHz, Iridium Satellite – 1.6 GHz

Spray Glider

Length- 2 meters, Wingspan- 1.2 meters No external moving parts or motors Moves on a pre-programmed course vertically and

horizontally in the water by pumping mineral oil between two bladders, one internal and the other external to the hull> changes the volume of the glider, making it denser or lighter than the surrounding water.

The glider steers to waypoints by controlling its buoyancy and orientation, using the lift from its wings to move horizontally.

Spray Glider

To descend, approximately one liter (four cups) of mineral oil is drained from the external, pancake-shaped rubber bladder through a short tube and into an internal bladder. The movement of the oil and the subsequent shrinking of the external bladder decreases the glider's volume. Like a balloon drained of air, the glider then sinks.

To ascend, a hydraulic pump moves the oil from the internal bladder back into the external bladder, increasing the glider's volume.

At the beginning and the end of each dive, the glider obtains and records its position by rolling on its side to expose a Global Positioning System (GPS) antenna embedded the right wingtip. Researchers obtain data from the glider and send new instructions to it using a satellite phone system and an antenna embedded in the left wingtip.

To steer the glider, information from the compass and altitude sensors is used to move two battery packs, one to control pitch (fore and aft angle) and the second to control roll (rotation around the axis of the glider. This changes the orientation of the wings, similar to the way a pilot guides and operates a hang glider.

Velocity from Spray Glider

Average velocity between 0 and 400m depth offshore

Each color represents a different glider

Orientation sensors allow DR’ing. The difference between the DR displacement and the true displacement give the ocean current

ADCP will give vertical profiles of velocity

Seaglider

Fly through the water with extremely modest energy requirements using changes in buoyancy for thrust coupled with a stable, low-drag, hydrodynamic shape. Designed to operate at depths up to 1000 meters, the hull compresses as it sinks, matching the compressibility of seawater.

Result of a collaborative effort between APL-UW and the UW School of Oceanography. These small, free-swimming vehicles can gather conductivity-temperature-depth (CTD) data from the ocean for months at a time and transmit it to shore in near-real time via satellite data telemetry.

Make oceanographic measurements traditionally collected by research vessels or moored instruments, but at a fraction of the cost. They can survey along a transect, profile at a fixed location, and can be commanded to alter their sampling strategies throughout a mission.

Seaglider

1.8 m long and weighs 52 kg—a size and weight that allow easy launching and recovery by two people from a small boat.

Cylindrical hull is a series of arched anodized aluminum panels separated by ring frames. The hull is surrounded by a fiberglass fairing to give it a low drag shape.

After each dive Seaglider dips its nose to raise its antenna out of the water. It determines its position via GPS, calls in via Iridium data telemetry satellite, uploads the oceanographic data it just collected, then downloads a file complete with any new instructions.

Can travel at slopes as gentle as 1:5 or as steep as 3:1. At gentle glide slopes the vehicle transits most efficiently, while steeper slopes are used to maintain position and act as a "virtual mooring."

Global Surface Circulation and its Kinetic Energy Distribution Derived from Drifting buoys

need to further divide the temporal mean into the average over one grid and the departure

1864 drifting buoys analyzed to obtain the global surface circulation and its kinetic energy distribution on a 2degx2deg grid system

Permits a finer scale map than obtained in previous studies

More realistic description of the global surface circulation than before

Ex: jet-like structures of the western Boundary and equatorial currents

MKE>1000cm^2/s^2 in WBCs & ECs and EKE~MKE suggesting the presence of strong interactions between the mean flow and eddies


Eddy field compared with that from altimeter data and models Altimeter- qualitatively good agreement except within the tropical

region where the error of the eddy field estimated from the altimeter data is amplified due to the breakdown of the geostrophic assumption. Altimeter data tends to be smaller because of the long sampling inerval of altimetric observations and the disregard of the ageostrophic velocity component

Models- synoptic views of the mean fields are in rather good agreement with drifting buoy data, while the energy level is somewhat low. In the eddy field, model results are very quiet, i.e., the energy level in the eddy field is much smaller (by a factor of ~5). The difference of the energy level is larger than that in pervious studies using altimeter data.

Surface circulation field derived from drifting buoys provides useful information which can be used to make models more realistic.

References:

Near Surface Drifters, prepared for the Autonomous and Lagrangian Platforms and Sensors Workshop by Peter Niiler, Scripps Institution of Oceanography

Ishikawa, Awaji & Akitomo. Global Surface Circulation and its Kinetic Energy Distribution Derived from Drifting buoys. Journal of Oceanography, Vol. 53, pp. 489-516, 1997

Professor Rosenfeld’s presentations for Operation Oceanography (Summer 2005)

Summary of Vehicle Characteristics. Drifters- basics Refers to surface current followers Consists of...

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