**REPORT ON**

**First CLIVAR Global Synthesis and Observation Panel Workshop on Ocean
Velocity Measurements and their Application**

**(GVW-1)**

Martin Johnson House\
Scripps Institution\
5 -- 7 December 2007.

Rick Lumpkin, NOAA/AOML, Miami, USA

Rick.Lumpkin@noaa.gov

Brian King, NOC, Southampton, UK

b.king@noc.soton.ac.uk

[This draft is version 2]{.mark}

[11 Dec 2007]{.mark}

# Contents

Contents [2](#contents)

1\) Introduction, scope and expected outcomes of GVW-1
[3](#introduction-scope-and-expected-outcomes-of-gvw-1)

2\) Measurements [3](#measurements)

2.1) Surface drifters [3](#surface-drifters)

2.2) Floats [4](#floats)

2.3) Moored instruments [6](#moored-instruments)

Timeliness of data release [6](#timeliness-of-data-release)

2.4) Shipboard and Lowered ADCPs [7](#shipboard-and-lowered-adcps)

Assembly of data at the Hawaii DAC
[8](#assembly-of-data-at-the-hawaii-dac)

Exploitation of ADCP data [9](#exploitation-of-adcp-data)

Lowered ADCP [9](#lowered-adcp)

2.5) Remotely derived currents [10](#remotely-derived-currents)

2.6) Gliders [11](#gliders)

3\) Exploitation of velocity measurements
[12](#exploitation-of-velocity-measurements)

3.1) Velocity measurements in adjoint assimilation efforts
[12](#velocity-measurements-in-adjoint-assimilation-efforts)

3.2) Operational Forecasting
[13](#regional-models-and-operational-forecasting)

3.3) Linear inverse models [13](#linear-inverse-models)

4\) Conclusions and recommendations
[13](#conclusions-and-recommendations)

Annex -- 1 List of workshop participants
[14](#annex-1-list-of-workshop-participants)

Annex -- 2 Second Announcement and Agenda
[15](#annex-2-second-announcement-and-agenda)

# 1) Introduction, scope and expected outcomes of GVW-1

From the workshop announcement:

1.2 Scope of workshop:

To establish the requirements for ocean velocity measurements as part of
an observing system for climate. This is a smaller task than a review of
all possible future operational use of ocean velocities

1.3 Expected outcomes:

1\) review, document and characterize ocean velocity data presently
available or likely to become available in the near future;

2\) review current dissemination and storage of ocean velocity data
streams and identify enhancements to make their exploitation more
effective;

3\) identify critical ocean velocity data streams that may constitute a
global network;

4\) develop the scientific justification and requirements that define
the need for the inclusion of ocean velocity measurements in the suite
of ocean observations used in climate research.

# 2) Measurements

The varied uses of velocity measurements include: assimilating models
for state estimation and seasonal to long-term forecasts; operational
ocean models; linear inverse models; comparisons with historical
measurements. In all cases, users emphasized the essential need for the
measurements to be accompanied by error estimates, and in particular,
any potential sources of biases (e.g., wind slip, mooring motion
rectification, etc.) as well as random errors. These estimates must be
made available to the user community.

## 2.1) Surface drifters

The global drifter array is composed of \~1250 drifters, \~1050 funded
by NOAA's Office of Climate Observations, with a goal of five degree
resolution. The GOOS/GCOS (OceanObs99) requirement for current
measurements is one measurement per month, per 5 box, with an accuracy
of 2 cm/s. So long as their drogue remains attached, drifters measure
currents at 15m depth, with a downwind slip of 1.2 cm/s at 10 m/s wind.
Slip in larger wind and wave conditions has not been measured. Drogue
detection is done with a submergence sensor or tether strain. The former
can be difficult to interpret and cannot be done in near real time.
Drifters without drogues experience considerably more downwind slip,
which has been quantified in drogued/undrogued studies in moderate wind
conditions. Drifter positions are distributed in real time on the GTS,
and in delayed mode with quality control, interpolated to regular 6h
intervals, by the drifter Data Assembly Center (DAC). Delayed-mode data
have a 2 to 4 month lag. These data are also archived at NODC and MEDS.
Service Argos provides position fixes for each drifter at a nominal
temporal resolution of 1.2 hours globally. Drifters collect total
velocity measurements that are relatively accurate, but are spatially
inhomogeneous, and include the wind-driven Ekman component.

Drifter data can map mean ocean surface circulation at the decadal time
scale, and eddy kinetic energy at high resolution globally, for
comparison with e.g., altimetry estimates. Drifter currents can be used
to evaluate the regional accuracy of models and distinguish which models
perform most realistically. Drifter velocities resolve seasonal and
interannual differences in circulation patters, for example the NAO and
ENSO related patterns. Because drifters measure total currents,
including the Ekman component, their data can be used to test
parameterizations of wind-driven upper ocean mixing. Drifter data can be
synthesized with altimetry and satellite winds to provide global,
homogeneously-gridded current time series of spatially-varying accuracy
(e.g., see "remotely derived currents" below), and for estimating the
geoid.

**Recommendation**: Careful evaluation of drifters' water-following
characteristics in high wind and wave conditions is necessary. These
conditions are experienced routinely in high latitudes (e.g. the
Southern Ocean), and in hurricanes and typhoons, but current estimates
of "slip" (movement of drifter relative to water at drogue depth) are
based on extrapolation of measurements at lower wind/wave conditions.

**Recommendation** : Since only drifter measurements are used for
estimating currents only when they are drogued, drogue lifetimes should
be as long as feasible for the drifters. Statistics should be gathered
to identify any systematic difference sin performance between
manufacturers. Pressure on manufacturers should be maintained to insure
that "shortcuts" are not taken. Drogue presence detection is critical,
and should be as robust as possible. Manufacturer diversity is
encouraged (1) to ensure the Global Drifter Program is not unduly
exposed to risk of failure by one supplier (2) ensure incentive for
manufacturers to respond to GDP requirements and to maintain cost
effectiveness, performance and hardware quality control.

## 2.2) Floats

The Argo float array reached the nominal target of 3000 active floats in
November 2007, providing nominal spatial resolution of 3 degrees and
generating approximately 100,000 displacement vectors per year. The most
common mission profile has a float drifting at 1000 decibars, with a
10-day cycle consisting of approximately 9 days at the drift depth, 6 to
8 hours for each of ascent and descent, and 6 to 24 hours at the
surface. Details of these timings vary between float hardware types and
depend on the mission parameters selected by the float operator.

Floats transmit data while at the surface. In 2007, the majority of
active floats use the Argos satellites for both position and data
telemetry. A few experimental floats use Iridium for data telemetry, and
GPS for position. These floats require minutes rather than hours at the
surface. In 2007, the majority of new float deployments are still Argos,
so Argos will remain the dominant technology of the active fleet for
many years into the future. Any possible transition from Argos to
Iridium will depend on the rate at which float operators choose to
incorporate Iridium in the float hardware. Most floats use the Argos
multisatellite service, to provide the greatest possible number of data
telemetry opportunities and position fixes. Nevertheless, there can be
periods of up to a few hours between the actual surface
arrival(departure) and the first(last) reported Argos surface position.

Floats provide the potential to infer velocity at the drift depth and at
the ocean surface. The errors and unknowns contributing to the drift
depth velocity are: Argos position error; unmeasured displacement during
the parts of surface cycle not bounded by the Argos fixes; unknown shear
(∂u/∂z, ∂v/∂z) during the ascent and descent time. The single inferred
drift vector is an integral of velocity over the drift period. Error in
the surface velocity estimate arises from Argos position error, and the
floats not following the water ("slip"). The latter can be caused by
wind and wave action. Floats could thus be regarded as undrogued
drifters in a qualitative sense, although the exact slip is not yet
known.

Each of the errors described in the previous paragraph can be estimated
or corrected to some extent, using knowledge of float behaviour. The
techniques for these corrections require knowledge of all the Argos
position reports for a float surface cycle. At present, the only float
data transmitted on the GTS are the 'profile' data. The profile messages
include T and S profiles and a single position used to label the
profile. Therefore if differences in the GTS position for successive
profiles were to be used to infer drift velocity, this could only be
done in the crudest manner. Superior estimates can be made by using the
full set of Argos positions, which are available in real time at the two
Argo GDACs, and by the application of algorithms that are under
development.

Ocean velocities are thus regarded as a 'product' in the Argo program.

At present, the most widely circulated velocity product from Argo floats
is YoMaHa (<http://apdrc.soest.hawaii.edu/projects/yomaha/index.html>).
YoMaHa includes subsurface and surface velocity estimates. The product
is generated from traj.nc files at the Argo GDACs and uses the full set
of Argos position fixes. It does not allow for surface extrapolation
outside the first and last fix, or for the shear during descent/ascent.
Upper bounds on these errors have been estimated. Surface ocean velocity
error due to wind and wave slip have not been estimated. The ocean
surface velocity is a single linear vector fit to the reported
positions, so it may be contaminated by non-straight-line transients
including inertial, tidal and other motion.

Algorithms have been published that allow for the required surface
extrapolation to ascent_end and descent_start time, and to separate the
surface motion into a linear velocity and a circular inertial motion.
Implementation of these algorithms is under development. Methods for
handling the baroclinic shear have been discussed, but there are no
agreed algorithms. If there was an explicit requirement, the surface
fitting algorithms could be run automatically, to generate a real-time
velocity product available at the Argo GDACs.

The Argo subsurface velocities have potential value for state
estimation, assimilation and inverse model use. Assimilation models are
very sensitive to bias errors: systematic error translates into large
transport error. Some assimilation methods can make correct use of
Lagrangian data, i.e. allow for the fact that the submerged displacement
is an integral over 10 days. The technology exists for ocean basins to
be ensonified with a modest number of subsurface sound sources. Floats
could then listen and calculate their full subsurface trajectory with
daily or even hourly resolution. There are added costs for maintaining
sound sources, float hardware, and float energy budget leading to
reduced float lifetime. Funding proposals could be developed if the user
requirement is demonstrated.

**Summary:** Data reduction efforts to provide Argo-derived subsurface
velocities are of high value for a range of users. Surface velocity
products are of potential use in supplementing drifting buoy
observations, provided the error due to slippage was quantified.

**Recommendation**: Resources should continue to support the evolution
of Argo float velocity products, to incorporate improved error estimates
and where possible correction for the effects of surface advection,
baroclinic shear during ascent/descent.

**Recommendation**: More rapid turnaround, towards the limit of
near-real-time distribution, would allow these velocity fields to gain
value in state estimation and ocean forecasting efforts.

**Recommendation**: The necessary investigation should be undertaken to
improve value and confidence in the interpretation of Argo surface
displacement vectors as ocean velocity, paying particular attention to
float slip. Careful comparisons of floats, undrogued drifter and drogued
drifter motion, along with wind and possibly wave products, should be
conducted. This may enable Argo surface data to supplement drifters in
an Integrated Ocean Observing System.

**Recommendation**: User requirements for products, such as gridded
fields with error analysis reflecting spatio-temporal variations, should
be monitored. Where GSOP becomes aware of such potential requirements,
they should be communicated to the Argo Data Management Team, with a
request that ADMT facilitates the generation of such products.

## 2.3) Moored instruments

Moored current measurements are made with single point current meters
(CMs) or acoustic Doppler current profilers (ADCP) that provide a
profile over a vertical range that can be as small as 5 m or as large as
a few hundred metres. Measurement error is a combination of instrument
error, typically 1 - 2 cm/s, and sometimes unknown mooring motion; thus,
field accuracy is mooring dependent. Acoustic side-lobe interference
means that ADCP instruments cannot measure near the boundary, whether
the boundary is the sea surface or the sea bed. Moorings at a typical
depth of 300m miss the top 30m of the water column. In several
ATLAS-type moorings, a point acoustic CM is included at 10m depth. This
is different from the drifter-standard drogue depth of15m.

The tropical ocean observing array includes TAO in the Pacific, PIRATA
in the Atlantic and the growing Indian Ocean array. ADCP data is
available only in delay mode, generally a few months after recovery.
Although the point current meter measurements are telemetered to shore
via Service Argos and made available in near-realtime through
<http://www.pmel.noaa.gov/tao/>, these data at present are not placed on
the GTS. Current meter measurements are made at a subset of the overall
array. In the Pacific, four equatorial sites have included current
measurements for the long term. Other current measurements are supported
through research grants and are not part of the sustained ocean
observing system. At present there are no current measurements on
off-equatorial TAO moorings.

Other mooring sites that have current measurements include the
OceanSITES network (<http://www.oceansites.org/>). These data have
uniform standards for data format, but in some cases, such as subsurface
current data from the Hawaiian Ocean Time Series, the data haven't yet
been released. In addition to being used in various different dynamical
and thermodynamical analyses, moored current measurements can be used as
a reference to evaluate models.The strengths of the moored current
measurements are that they are not aliased by diurnal jets and eddy
variability \-- high resolution time series at a fixed point resolve
motion associated with diurnal, intraseasonal, seasonal, and interannual
variability; and that the velocity data often have concurrent wind
stress, heat flux, stratification, and other measurements. Moorings with
ADCPs and/or sets of current meters can be used to determine the
vertical shear; mooring arrays can be used to map fields and estimate
horizontal gradients. A set of 5 current meters with temperature sensors
placed in the top 25 m of a mooring at 2N, 140W shows that the
near-surface shear is extremely sensitive to the near-surface
stratification. On average, during the afternoon weak stratification
averaging less than 0.2C between 1m and 25m was associated with a
diurnal jet in the direction of the wind with an average shear of more
than 10 cm/s between 5m and 25m, while at nighttime the water became
unstratified and the average shear was less than 4 cm/s. Surface current
measurements which do not resolve the diurnal cycle can have significant
errors due to aliasing.

In addition to the sustained observing system, many regional,
research-focused efforts, in the past or ongoing, some in critical
western boundary regions and ocean choke points, have included current
meter measurements. During WOCE, many of these data were assembled by
Oregon State University.

The British Oceanographic Data Centre (BODC) is identified as the CLIVAR
moored instrument DAC. However, since it doesn't assemble data from some
of the big mooring programs (e.g., the tropical mooring arrays), it does
not provide a one-stop-shop for users to access all moored data.
Activity at the BODC is resource-limited: 0.4 FTE confirmed until 2012.
They are thus limited to responsive, rather than proactive, acquisition
of data sets. The workshop noted that the WOCE OSU archive has not yet
been integrated with the post-WOCE CLIVAR archive assembled by BODC.
Access to the WOCE data is essentially through the structure of the WOCE
DVD.

### Timeliness of data release

There was extensive discussion of the issues surrounding the timely
release of moored datasets. PMEL efforts with the tropical ATLAS array
demonstrate the added value in releasing the data to a broad audience,
where they are used in climate research, state estimation, and
operational use such as numerical weather prediction. In contrast, data
withheld for PI-only analysis beyond the time when they have been
cleaned up become more limited in usefulness for the overall climate
research community. In particular, it was noted that adjoint model state
estimation projects are run at the limit of computational resources.
They keep reasonably 'up-to-date' with recent observations, and there is
a limit to how often they can be re-run multiple times as extra data
become available. Therefore data that are made available with many years
delay may be omitted from such efforts, and may not be included in runs
submitted to IPCC, for example. It was proposed that timeliness of data
availability must be integral to being considered part of the observing
system for climate.

**Recommendation:** A CLIVAR DAC for moored measurements needs the
funding and associated manpower to gather datasets and assemble them in
a uniform format with a uniform inventory. This effort should include
ICES, WOCE, pre-WOCE data, etc.). Recognising resource limitations, it
is necessary to prioritise the datasets that will be of greatest value
for climate science. These include key locations established in CLIVAR
plans, such as boundary currents and low latitudes, with special
emphasis on sites that will be maintained long term.

**Recommendation:** The ocean observing system in the Pacific should
include off-equatorial current measurements in the equatorial wave band,
to resolve the meridional structure of equatorial waves, tropical
instability waves, wind forced response in frontal regions, and more
generally the role of advection in the off-equatorial evolution of mixed
layer heat anomalies.

**Recommendation:** Noting that the standard drogue depth for the
drifter program is 15m, but that some major programs deploy moored
instruments at 10m, a better understanding is required of upper ocean
shear; consideration should be given to including point current meters
at 15m.

**Recommendation:** Funding agencies should be asked to apply and
maintain pressure on PIs, to ensure data are released in a timely
manner. Guidelines exist for this.

## 2.4) Shipboard and Lowered ADCPs

ADCPs use an active sound source in the range 38-300 kHz. Lowered ADCPs
are either 150 or 300 kHz. They work by acoustic scattering from
material in the water. The motion of water relative to the platform is
inferred from Doppler shift of return signal. Lower acoustic frequency
gives better range (less acoustic attenuation) but worse spatial
resolution. The measurement is remote from the instrument with a range
of 100-1200 metres, depending on acoustic frequency. Instantaneous
measurements are made and data logged, at intervals of a few seconds.
The instruments have multiple beams in four directions. The beams are
separated with a conical half-angle of 20 or 30 degrees which means that
the 'footprint' is 10s-100s of metres. The following table summarises
and compares aspects of shipboard and lowered ADCP operations.

+-----------------+------------------------+--------------------------+
|                 | > Shipboard            | > Lowered                |
+-----------------+------------------------+--------------------------+
| > Acquisition   | > Ship underway        | > Ship stationary        |
+-----------------+------------------------+--------------------------+
| > Extra         | > Precise ship heading | > GPS ship navigation    |
| > measurements  | > (better than         |                          |
| > required to   | > standard             |                          |
| > provide       | > gyrocompass)         |                          |
| > accurate      | >                      |                          |
| > velocity      | > GPS ship navigation  |                          |
+-----------------+------------------------+--------------------------+
| > Data          | > 5 or 10-minute       | > All data from one      |
| > reduction     | > averages             | > station processed to   |
|                 |                        | > single profile         |
+-----------------+------------------------+--------------------------+
| > Horizontal    | > 1-10 km              | > Determined by station  |
| > resolution    |                        | > spacing; typically 50  |
|                 |                        | > km open ocean; can be  |
|                 |                        | > a few km in boundaries |
+-----------------+------------------------+--------------------------+
| > Vertical      | > 10-20 metres         | > 10-20 metres           |
| > resolution    | > resolution           | > resolution             |
| > and coverage  | >                      | >                        |
|                 | > Upper ocean          | > Full ocean depth       |
|                 | > 200-1200m depth      |                          |
+-----------------+------------------------+--------------------------+
| > Universally   | > No                   | > No                     |
| > adopted data  |                        |                          |
| > reduction and |                        |                          |
| > reporting ?   |                        |                          |
+-----------------+------------------------+--------------------------+
| > Reliable      | > Sometimes on         | > No; Instrumental       |
| > error         | > research ships with  | > random and bias errors |
| > estimates     | > expert               | > poorly understood.     |
|                 | > interpretation       |                          |
+-----------------+------------------------+--------------------------+
| > QC procedure  | > Not universal        | > No                     |
| > well defined  |                        |                          |
| > and           |                        |                          |
| > implemented ? |                        |                          |
+-----------------+------------------------+--------------------------+
| > Suitable      | > Research ship        | > Research Ship          |
| > platforms     | > (dedicated           | > (dedicated expedition) |
|                 | > expedition)          |                          |
|                 | >                      |                          |
|                 | > Research ship        |                          |
|                 | > (unattended)         |                          |
|                 | >                      |                          |
|                 | > VOS (limited         |                          |
|                 | > success)             |                          |
+-----------------+------------------------+--------------------------+
| > Re            | > Not routinely        | > Could be available in  |
| al-time/Delayed | > transmitted in real  | > near real-time (few    |
| > Mode ?        | > time, but this could | > hours lag)             |
|                 | > be achieved if it    |                          |
|                 | > was required         |                          |
+-----------------+------------------------+--------------------------+
| > Strengths     | > Low cost on research | > As for shipboard. Full |
|                 | > ships; (but          | > ocean depth. Only      |
|                 | > significant capital  | > technique that         |
|                 | > investment on VOS).  | > provides detailed      |
|                 | >                      | > spatial description of |
|                 | > Provides direct      | > eg DWBCs.              |
|                 | > measurements of      |                          |
|                 | > absolute velocity    |                          |
|                 | > with good spatial    |                          |
|                 | > resolution; critical |                          |
|                 | > in boundary regions  |                          |
|                 | > where other          |                          |
|                 | > hydrographic methods |                          |
|                 | > are limited.         |                          |
+-----------------+------------------------+--------------------------+
| > Weaknesses    | > Contamination with   | > Contamination with     |
|                 | > transients;          | > transients;            |
|                 | >                      | >                        |
|                 | > Uncertain errors;    | > Uncertain errors       |
|                 |                        | >                        |
|                 |                        | > Equipment              |
|                 |                        | > vulnerability; recent  |
|                 |                        | > failures attributed to |
|                 |                        | > changes in             |
|                 |                        | > manufacturing process. |
|                 |                        | > Expected to be         |
|                 |                        | > addressed in 2008.     |
+-----------------+------------------------+--------------------------+

### Assembly of data at the Hawaii DAC

The ADCP DAC at University of Hawaii is presently supported by 15% of
Pat Caldwell's time. This is sufficient to keep 'ticking over' on the
ingestion of data that arrive in a recognisable format from the PI, and
which require minimal QC. The assumption is that the PI has done the
cleaning up and calibration.

The system for identifying and assembling important SADCP data was
effective during WOCE. Since then, the ADCP DAC has continued to work
closely with the CLIVAR and Carbon Hydrographic Data Office (CCHDO). As
with CCHDO, success since WOCE has been chiefly with cruises in the US
CLIVAR program, and a small number of non-US partners. Detailed
statistics were not available, but it was reported that while Japan has
been effective in submitting data, European nations have generally not.
The DAC receives all the SADCP data acquired on cruises in the US CLIVAR
program.

A German initiative will mean that the SADCP will be operated
continuously on their research ships. Some other countries have similar
initiatives in place, or are considering them.

Data reduction: A legacy of a decision that was appropriate when taken
at the start of WOCE is that data at the Hawaii DAC are interpolated
onto a 10-metre vertical grid and reduced to 1-hour averages. These data
are made available in NetCDF format. In addition, the DAC holds
full-resolution data (whatever the PI supplied) for data acquired in the
University of Hawaii binary CODAS format. CODAS format data may this be
held for most US cruises, and a number of other national cruises that
have adopted CODAS for the ADCP data. CODAS is a binary format,
requiring software and knowledge to extract the data. Thus the
accessible netCDF data are a significant reduction. Present-day ship
navigation means that data at 5- or 10-minute resolution data are likely
to be of research quality. Longer period averaging is no longer needed
or appropriate.

### Exploitation of ADCP data

Exploitation is mainly through providing reference level velocity for
hydrographic sections, especially boundary currents; inverse
calculations.

SADCP provides 'spatially continuous' sampling to aid interpolation and
interpretation of 'discrete' hydrographic sections.

Variability can be obtained from repeat sections (especially
'chokepoints' & 'jets'). For example repeat sections can be combined
with upper ocean temperatures from XBT to provide seasonal variation of
Ekman heat transport.

ADCP data seem to be regarded mainly regarded as a 'research' data type.
Generally, the data are used by the PIs who acquired the data, or their
collaborators. This ensures a good level of expertise in exploitation of
the data, which is especially important for LADCP data because there is
no systematic agreement on the appropriate error fields. However, the
limited exploitation of the data centre holdings (in ocean state
estimation for example) also leads to a perception of lower urgency to
submit data to the data centre. Compared with other data types, there is
less extraction of data from DACs so the data delivery system is not
'exercised', nor stimulated to make improvements.

### Lowered ADCP

LADCP data were an 'orphan' during WOCE and continue to be so, with no
uniform agreed procedure for QC, data reduction, metadata storage and
error estimates. The longer this situation continues, the harder it will
be to assemble the datasets, even though these datasets can provide
critical information in undersampled boundary regions. Although some
useful discussion took place late in WOCE about LADCP archiving, there
remain critical unresolved questions about what is needed.

The archive of LADCP data will not be secure until the expert PIs
responsible for acquiring the data reach agreement on what should be
archived. In addition to 'processed profiles', PIs should agree the
amount of raw data and metadata required to enable future reprocessing
as methodology improves. The goal of a velocity product complete with
agreed reliable error estimates will not be achieved without a specific
initiative involving a range of PIs.

**Recommendation**: The UH ADCP DAC should be asked to review the SADCP
data reduction, and give consideration to serving NetCDF data with
better time resolution and without interpolating to 10-metre grids.
Archiving and serving data on the observed vertical grid is particularly
important with the diverse range of acoustic frequencies. It is
suggested that the DAC seek advice from the UH ADCP experts and develop
a plan in collaboration with them. When a new NetCDF resolution is
agreed for new acquisitions, an assessment should be made of the effort
required to translate historical holdings (with better resolution
presently in CODAS) into NetCDF.

**Recommendation**: The DAC should consider what steps could be taken,
and what resources would be required, to pursue missing data more
proactively. If greater resources are required, a proposal should be
generated. The priority datasets are from regions with multiple repeats
and an expectation of long-term support. Also those required
interpreting the long sections in the repeat hydrography program.

**Recommendation**: The DAC should identify ship operators that keep the
SADCP 'always operating' with a view to acquiring the data.

**Recommendation**: An international effort is required to enable PIs to
reach agreement on the processing and archiving of LADCP data, and the
generation of agreed processing protocols, QC procedures and error
estimates.

## 2.5) Remotely derived currents

Gridded surface current products, either geostrophic, wind-driven, or
total currents, are being derived from a synthesis of drifters,
altimetry, winds, and SST. An example of this synthesis is the Ocean
Surface Current and Analysis in Real time (OSCAR,
[http://www.oscar.noaa.gov](http://www.oscar.noaa.gov/)), which uses
gridded AVISO altimetry, gridded COAPS scatterometer winds, and Reynolds
SST to generate gridded surface currents from 1999 to the present, at 5
day temporal resolution. This product is updated every \~2 months.
Another product being developed at CLS is SURCOUF, which also uses
AVISO, Scatterometer winds, and also directly adds drifter velocities
through multivariate objective analysis.

Coefficients in OSCAR are tuned using drifter velocities, and the
ongoing product is evaluated against drifting and moored buoy current
measurements. Near the equator, OSCAR has less skill in deriving
meridional currents; at high latitudes, OSCAR current magnitudes are
systematically different from drifter velocities, with drifter
velocities being greater.

Some of the data streams used by OSCAR are vulnerable, especially those
from satellite missions subject to uncertainty. Evaluation and
improvements require maintenance of the global drifter array and
tropical moored array.

OSCAR maps show the evolution of the 2006---2007 El Nino, history of the
last three El Nino events, current La Nina anomalies in the tropical
Pacific basin, etc. The Pacific Decadal Oscillation may have undergone a
phase shift in 1998---1999; OSCAR currents can be used to evaluate the
role of heat and salt advection in upper ocean budgets (compared to 1D
terms). Work in progress: operational processing system being delivered
to NOAA for NCEP Ocean Prediction Center operational uses. Ongoing
upgrades and research focus on improved wind-driven current model and
wind product, ongoing validation with in-situ buoy measurements,
incorporating de-biased merged drifter and satellite-derived fields,
pseudo-Lagrangian statistics, generating nowcast SSH fields (and thus
currents) based on AVISO and Navy products, improving spatial resolution
to 0.5 degree, and focusing on along-track altimeter analysis. OSCAR is
used for wind speed corrections, oceanographic process studies,
assimilation in models, animal migration and fishery studies, and
educational purposes (e.g., NASA's
[http://www.oceanmotion.org](http://www.oceanmotion.org/)).

***[Question to Fabrice: can you clarify whether and where OSCAR output
is archived or secured ? since the OSCAR product is not 'data' it may
not have the usual backups used by data centres, and presumably the
output doesn't go to NODC. So, for example, if your lab has a disaster
and the disks are lost, is the output retrievable or can it easily be
regenerated ?]{.mark}***

The OSCAR product is funded year-by-year by NOAA, pending funds
availability in Research to Operations earmarks and the Office of
Climate Observations. It also has funding through 2008 from NASA, via
the Ocean Surface Topography Science Team and the Ocean Vector Winds
Science Team, with an additional proposal to NASA seeking to secure
funding through 2012. OSCAR also benefits from collaborations with Gary
Mitchum (USF) and Mark Bourassa (FSU)

**Recommendation**: Mesoscale-resolving ocean surface circulation
products are valuable tools for a number of uses. In order to support
the continued ability to create and improve these products, a number of
ongoing efforts must be continued. Ocean surface topography and vector
wind satellite observing systems are essential to resolve mesoscale
motion and wind-driven transport. The surface drifter array must be
maintained, as it provides key synergy with satellite-derived currents,
essential data for improving satellite-based dynamical models,
opportunities for global validation and error analysis, robust
statistics (\>1000 drifters), and the data needed for de-biasing and
optimal interpolation. Moored current meter data are essential for
quantifying and improving the skill of satellite-based currents at fixed
locations, particularly near the equator.

## 2.6) Gliders

Several types of gliders are presently being operated. Gliders measure
vertically-averaged absolute velocity, analogous to the ship-drift
method. Glider CTD profiles are used to calculate geostrophic shear.
There is the technical capability for gliders to carry ADCP, but as with
all instruments extra payload impacts on cost and energy budget. Gliders
make closely spaced (\~5 km) dives to \~1km; their speed capability is
\~1/2 knot and mission duration is up to \~4---6 months. Interpretation
of data can be challenging: gliders can be swept over considerable
distances in, e.g., western boundary currents; temporal variability can
be superimposed on the 'mean flow'. SIO and UW gliders are operating in
many areas. At present, there is no coordinated plan for global,
sustained observations from gliders. Current operations are exploratory.
Some tracks are starting to be repeated routinely, for example in the
California Current area. At present, active intervention required
throughout a glider survey.

Gliders are an order of magnitude more technologically sophisticated
than floats, meaning that mounting a glider survey requires considerable
expertise and investment. It is very hard to imagine an
internationally-coordinated effort within the next few years, because
the techniques are still in research and development. There are efforts
by countries such as Australia to include a glider facility as part of a
regional observing network, but we know of no similar initiatives
elsewhere. It's a labour-intensive problem, and the community may not be
big enough at present to generate a coordinated sustained program.
NOAA's Office of Climate Observations is interested in supporting glider
initiatives, with a budget placeholder for 50 or 100 gliders.

An example was shown of vertically-averaged absolute velocity from
glider tracks between New Caledonia and the Solomon Islands, showing the
North Vanuatu and North Caledonia jets.

**Recommendation:** Glider observations are an important new development
in measuring velocity that should be encouraged by the various funding
agencies.

# 3) Exploitation of velocity measurements

# 3.1) Velocity measurements in adjoint assimilation efforts

Two adjoint assimilation efforts were represented at the meeting: ECCO
and GECCO. The computational cost in including extra data in such models
is negligible. Modellers want access to all possible data. Currently,
GECCO uses drifter mean velocities, hydrographic sections, TAO
temperature profiles, global XBT data set, P-ALACE and Argo T, S
profiles, and SSS observations; mean wind stress, and SLA from altimetry
with Grace geoid. GECCO could also use subsurface velocity, e.g., from
Argo floats. In ECCO, the only velocity measurements assimilated at
present are the TAO moored measurements.

Issues with using velocity observations in state estimation efforts:

-   Technically, velocity observations are no more difficult to fit than
    T or S observations.

-   Is the ocean's memory with respect to velocity significantly shorter
    ?

-   New velocity observations need to have an uncertainty specified,
    which must include both measurement error and unresolved processes.

Will additional velocity measurements be used in ECCO? Yes, but:

-   [They should be as instantaneous and raw as possible. ***{ I'm not
    sure what point is being made here. Is it saying for example that a
    direct measurement that includes, say, Ekman and inertial transients
    is good, because those processes are also present in the model.
    Therefore a 'corrected' velocity with such processes removed might
    be inferior to the 'raw' observations ? This point was emphasized at
    several points by Matt -- clarification is needed}***]{.mark}

-   A processed velocity product may be preferable to individual
    platform trajectories. [{opposite statement to previous point. We
    need to clarify when raw vs. reduced is most valuable.}]{.mark}

-   Accurate error estimates are needed (instrument, sampling,
    unresolved processes).

-   Great care must be taken to allow for systematic biases in
    observations (e.g., wind-driven slip).

Unresolved processes are a major limitation on the potential value of
observations that include tides, internal waves, etc., and mesoscale
dynamics in non-eddy-resolving models. Biases such as slip must be
addressed by observationalists. Error bars must include unresolved
processes, e.g., varying EKE field associated with unresolved mesoscale
field.

The assimilated time-mean SSH in GECCO was compared to Niiler &
Maximenko's absolute SSH. Differences between model-derived and GRACE
absolute sea level are focused in boundary regions, western boundaries
in particular. Are these errors in GRACE? Velocity fields can address
this discrepancy. Improvements in the time mean and varying dynamic
topography and transports need to be tested against in-situ
measurements.

**Impact of velocity data in adjoint assimilation efforts**: the biggest
impact may be for mean currents and the seasonal cycle, especially in
boundary currents. Velocities at 1000 or 2000m depth would primarily
impact time mean and maybe seasonal cycle. Velocity analyses may need
synthesis results for temporal de-aliasing, for example recent work by
Russ Davis in Argo velocities.

**Obstacles to using velocity data:** concerns of potential biases in
the data are limiting aggressive use of the data in ECCO. M. Mazloff
presented a comparison of ECCO surface currents (both in a 1/6 Southern
Ocean run and a 1 global run) with drifter-derived climatological
currents, the latter with slip removed. In much of the model domain,
particularly in the Southern Ocean, systematic differences (up to
\~9cm/s) were seen, with drifter speeds larger. For the global run, the
drifter (and also OSCAR mean) zonal currents were consistently offset
from the ECCO 15 year averaged results, with the difference following
the zonal wind direction. Mazloff expressed concern that the data may
contain additional, undiagnosed downwind slip, and that assimilation of
these data could introduce artefacts such as spurious magnification of
the Antarctic Circumpolar Current transport. Niiler and others noted
that slip has been carefully quantified in lower wind conditions, seen
in many of the tropical one-degree bins of the global run, and that a
number of indirect tests (e.g., global absolute sea level from drifters
and altimetry, estimates of Ekman plus geostrophic currents, compared to
independent data such as Levitus climatology, vorticity balances, etc.)
have supported the accuracy of the drifter data. Niiler pointed to
potential model errors in parameterizing upper ocean mixing and the
structure of velocity shear. The Southern Ocean results could be
recalculated for the subset of drifters that experienced wind speeds
\<10m/s, to eliminate the role of undiagnosed slip in high-wind
conditions, or poor simulation of upper ocean physics in intense wave
breaking, bubble layers, etc. It was concluded that drifter velocities
will improve the model results when either (a) model parameterization of
upper ocean mixing improves, and/or (b) downwind slip is fully removed
from the drifter data (including in high wind conditions). Regardless,
an important point is that the models and data do not agree at present
... if they agreed fully in the absence of assimilation, the data would
bring little added value to the model.

## 3.2) Regional models and Operational Forecasting

Coastal radar and ADCP-derived velocities are currently being used in
regional models such as ROMS. Tropical mooring velocities are also
compared to models and used to optimize them. The value of TAO for
Pacific regional models was highlighted, as these observations are in a
region of surface divergence (e.g., few drifter measurements) where
geostrophy breaks down.

The Navy ocean data assimilation model is currently set up for
observations, but is not yet assimilating velocity data. It uses a 3D
multivariate optimum interpolation scheme for assimilation. Observations
are passed through QC procedures (comparison to prediction fields or
climatology), with potential feedback to observations such as adaptive
sampling guidance. At this point, velocity data QC needs to be
integrated into the system for velocity data assimilation. At present
the system infers geopotential from temperature and salinity profiles,
thus implying geostrophic surface velocities. This effort will require
estimates of instrument/measurement errors to be provided by the
observationalists; representation errors (e.g., the presence of tides
and internal waves not resolved by the model) can be derived at the
modelling end.

Timeliness: With every time increment, the global HYCOM/NCODA calculate
a hindcast to --120h and forecast to +96h. Velocity data can be received
with up to 5 days delay for use in this scheme. The delay is driven
primarily by the availability of altimeter data.

US Navy systems will probably look to assimilate all sources of velocity
observations. In particular, the need for velocities at the reference
level for geopotential calculations will drive assimilation of Argo
trajectories. At present, a mooring in the Florida Straits is collecting
velocity data that will be assimilated in the 1/25 degree HYCOM
operational model. The relative value of various velocity measurements
can be quantified in data-withholding experiments, ongoing in the GODAE
context.

## 3.3) Linear inverse models

Linear inverse models use extremely simple physics, namely conservation
of mass, salt, heat and net silica, to derive a plausible set of values
for unknowns: reference velocities for hydrographic station pairs, and
diapycnal transfers of the properties from one density layer to another.
The latest generation of global models incorporate diapycnal
transformation from air-sea fluxes in outcropping layers, and have been
used to infer the basin-scale distribution of interior mixing. Because
these models seek solutions to an underdetermined system, solutions are
generally sensitive to the starting point; improved initial
(pre-inversion) conditions can greatly drive down error bars on the
solution and help resolve transport and mixing estimates. Such models
can serve as a tool to propagate information from a regional set of
velocity estimates in a globally consistent manner.

R Lumpkin noted that there are key regions where measurements of
velocity in density layers (transports) are most important, generally
regions where the barotropic component plays a crucial role. Specific
recommendations for measurements in these regions may rest with regional
panels. Regardless, some key sites can be noted in particular: choke
points such as the Bering Strait, Denmark Strait, Faroe Banks, Florida
Current, and the Drake Passage; western boundary currents (surface and
deep) such as the Gulf Stream, Kuroshio, Agulhas, Kermadec Ridge and the
Kerguelen Plateau; bottom water chokepoints and cul-de-sacs in various
ocean ridges and rises; and dense flow out of the Weddell and Ross seas
and past Cape Farewell, Greenland.

Regional inverse models have incorporated ADCP measurements for improved
resolution of current structures; error estimates for the lowered ADCP
are dominated by representation errors, in particular the presence of
high frequency and ageostrophic motion.

# 4) Conclusions and recommendations

There may be an opportunity to assemble and publish the capability and
requirement for a velocity observing system as part of the Oceanobs 09
conference, which will be late in 2009. The Oceanobs planning group next
meets in early 2008, after which there will be more information
available about the papers that will be solicited for the conference. If
a paper is solicited on velocity measurements in a climate observing
system, the process of preparing it will take over from this workshop
the task of defining the requirement.

In that case, it is expected that GSOP would identify a collection of
investigators with the necessary expertise to undertake the task. Thus
the need for a further meeting of the group attending this workshop, or
of an alternative group focussed around an Oceanobs paper, will be
decided by GSOP as Oceanobs planning develops.

This workshop was mainly successful in describing the capability of the
present observing systems. The interaction between investigators
involved in acquiring and distributing velocity data, and those involved
in global-scale synthesis is expected to stimulate a number of valuable
developments in data assembly and delivery. Priorities were defined or
restated. Some shortcomings were identified. Of these, some can be
addressed with existing resources; others will require successful
proposals to generate extra resources.

The workshop discussion and this report of it fall short of 'defining
and justifying the requirement for ocean velocity measurement for
climate research and associated activities over the next ten years'.
Such documents are not satisfactorily written by committee. An Oceanobs
09 paper would be a powerful and timely tool for preparing such a
document.

The workshop regularly returned to the theme of western boundary
currents and equatorial current systems as a gap in the velocity
observing system. It is unclear whether the oceanographic community
knows precisely what should be observed, or how those observations
should be made. Analyses from process studies in these regions (e.g.
KESS, CLIMODE, SYNOP), however, may provide some guidance.

The main conclusions from the discussion have been highlighted in the
body of the report as 'recommendations'. Since the workshop has no
self-standing authority, we consider these recommendations to be for the
attention of GSOP, to generate action as they see fit.

The workshop report will be circulated to attendees in draft form soon
after the meeting. The consultation interval will be as short as
reasonably possible, after which the final report will be submitted to
the GSOP cochairs for their consideration.

The workshop closed at 1200 on Friday 7 December 2007.

# Annex -- 1 List of workshop participants

Fabrice Bonjean, bonjean@esr.org, Earth & Space Research, Seattle, USA\
Pat Caldwell, caldwell@hawaii.edu, NODC/SOEST, U. of Hawaii, Honolulu,
USA\
Bruce Cornuelle, bdc@ucsd.edu, SIO, La Jolla, USA\
Meghan Cronin, Meghan.F.Cronin@noaa.gov, NOAA PMEL, Seattle, USA\
Jim Cummings, james.cummings@nrlmry.navy.mil, Naval Research Laboratory,
Monterey, USA\
Peter Hacker, phacker@hawaii.edu,
[IPRC](http://iprc.soest.hawaii.edu/)/SOEST,  U. of Hawaii, Honolulu,
USA\
Brian King (Co-convener), b.king@noc.soton.ac.uk, NOC, Southampton, UK\
Rick Lumpkin (Co-convener), Rick.Lumpkin@noaa.gov, NOAA AOML, Miami,
USA\
Matt Mazloff, mmazloff@mit.edu, MIT, Cambridge, USA\
Mary Mowat, mmow@bodc.ac.uk, BODC, Liverpool, UK\
Peter Niiler, pniiler@ucsd.edu, SIO, La Jolla, USA\
Dean Roemmich, droemmich@ucsd.edu, SIO, La Jolla, USA\
Detlef Stammer, detlef.stammer@zmaw.de, Institut für Meereskunde,
Hamburg, Germany\
Hiroshi Yoshinari, hyy@affrc.go.jp, National Res. Inst. of Fisheries
Science, Yokohama, Japan

With assistance from

Nico Caltabiano, caetano@noc.soton.ac.uk, ICPO, NOC, Southampton, UK\
Tomomi Ushii, tomomi@ucsd.edu, SIO, USA

# Annex -- 2 Second Announcement and Agenda

Second Announcement

16 November 2007

First GSOP Workshop on Ocean Velocity Measurements and their Application
(GVW1)

Convenors: Rick Lumpkin (AOML, Miami) and Brian King (NOC, Southampton)

Martin Johnson House, Scripps Institution\
December 5^th^, 6^th^, & 7^th^ (until 1545 if needed), 2007.

This second announcement includes the provisional agenda.

**Agenda**

***Wed 5 December***

**1 Introduction (Lumpkin, King, Stammer) \[0845-0900\]**

1.1 Local arrangements

1.2 Scope of workshop:

To establish the requirements for ocean velocity measurements as part of
an observing system for climate. This is a smaller task than a review of
all possible future operational use of ocean velocities

1.3 Expected outcomes:

1)  review, document and characterize ocean velocity data presently
    available or likely to become available in the near future;

2)  review current dissemination and storage of ocean velocity data
    streams and identify enhancements to make their exploitation more
    effective;

3)  identify critical ocean velocity data streams that may constitute a
    global network;

4)  develop the scientific justification and requirements that define
    the need for the inclusion of ocean velocity measurements in the
    suite of ocean observations used in climate research.

**2 Measurements \[0900-1700\]**

For each measurement the discussion should cover:

> \(a\) what/how the instrument measures velocity.
>
> \(b\) accuracy, time & space resolution of measurements, etc.
> strengths/ weaknesses/ advantages/ disadvantages from a CLIVAR
> perspective
>
> \(c\) how the data are used.
>
> \(d\) data reduction issues: quality control, interpolation, assigning
> error bars.
>
> \(e\) data archiving, distribution, availability/timeliness.

After discussing each measurement type, the workshop should be able to
evaluate and agree on the 'adequacy' of the observing system for this
data stream. Since the adequacy depends in some sense on the desired use
of the data, this evaluation will depend on Section 3 of the Agenda. We
should also establish ideas about the future (funding) security of the
observing systems and data management systems. Is the measurement
undertaken by research PIs using research budgets or is it under
financial control of 'operational' agencies? Will the answer to this
question change over the next 5-10 years?

Presentations (approximately 1 hour each, with additional time for
discussions)

2.1 Surface drifters (Niiler \[a,b,c\], Lumpkin \[d,e\])

2.2 Profiling floats (Hacker/Yoshinari \[a-e\])

2.3 Moored current meters (Cronin \[a,b,c\], Mowat \[d,e\])

2.4 ADCPs Shipboard and Lowered (King \[a,b,c\], Caldwell \[d,e\],
Okamaki \[d,e\])

2.5 Remotely-derived currents (Bonjean/Lagerloef \[a-e\])

2.6 Others

If there are major data streams that are of importance to CLIVAR that
have not been discussed, the workshop needs to identify them and propose
a strategy for filling the gaps.

Areas not covered in 2.1 to 2.6 at present: gliders, acoustically
tracked subsurface floats, inverted echo sounders, coastal radar arrays.

***Thu 6 December***

**3 Exploitation \[0830-1400\]**

Types of exploitation

1.  Modelling and Assimilation (introduction: Stammer)

> Mazloff: use of velocity measurements in ECCO
>
> Cornuelle: velocity measurements in ROMS

2.  Forecasting (introduction: Cummings)

3.  Inverse modelling, budgets and transports (introduction: Lumpkin)

3.4 Climate monitoring: direct comparisons between new and old data
(introduction: Roemmich)

For each type of data use we should try to establish:

a\) Number of user groups involved

b\) Requirements for access

c\) Dependence on particular data streams

d\) Capacity of data exploitation to refine sampling strategy

**4 Recommendations (discussion led by Lumpkin/King) \[1400-1700\]**

We will need to produce a meeting report. This should, amongst other
things,

-   Define the requirement for ocean velocity measurement for climate
    research and associated activities over the next ten years.

-   Identify and document present capability to meet the requirement.

-   Identify inadequacies in present observing/data delivery system.

-   Identify gaps in the workshop discussion (were there things we
    couldn't resolve because we didn't have the right knowledge?)

-   Establish the possible requirement for a future workshop: When might
    one be required, and what expertise might be needed

***Fri 7 December (morning)* \[0830-1200\]**

**5 Preparation of draft meeting report**