View source for Level 3 data (velocity point-measurements)

{{ReviewStage
|toreview=Draft
|authors=Cynthia
|instrument_type=Velocity point-measurements
}}
The required dimensions and variables associated with the '''Level 3 spectra''' processing level (NetCDF group) for velocity measurements are described below. 

Only a few attributes for each variable are listed since the page's purpose is to describe the information layout within each NetCDF file. {{FontColor|bg=#fca1fd|text=Please refer to the [[NetCDF_parameter|complete list]] for the additional attributes}} related to each variable (e.g., units).

__TOC__

=Dimensions=
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<br>
The number of velocity instruments (N_VEL_INSTRUMENT) is here for completeness, but the benchmarks examples will have measurements only from one instrument stored. 

{| class="wikitable sortable"
|-
! Short name
! Standard name
! Dimensions
! Comments
|-
| TIME
| time
| TIME
| Units in Days since 1950-01-01T00:00:00Z, time-centred.  Provide bounds attribute to designate the variable containing the limits of each segment ([http://cfconventions.org/cf-conventions/v1.6.0/cf-conventions.html#methods-applied-to-a-timeseries-ex see CF-compliant example]).
|-
| FREQ
| frequency
| FREQ
| Frequency in Hz for the spectra
|-
| N_VEL_COMPONENT
| unique_identifier_for_each_velocity_component
| N_VEL_COMPONENT
| Maximum of 3 for u,v, w (east, north, up) velocities
|-
| colspan="4" style="text-align:center; font-weight:bold; background-color:#f9eddd"|  Optional variables <math>\ddagger</math>
|-
| N_VEL_INSTRUMENT
| unique_identifier_for_each_velocity_instrument
| N_VEL_INSTRUMENT
| Required only if storing multiple synchronized instruments in the same file 
|}
</div>

=Variables=
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<br>
{| class="wikitable" 
|-
! Short name
! Standard name
! Dimensions
! Comment
|-
| UVW_VEL_SPEC
| power_spectrum_density_of_velocity_
in_the_analysis_frame_of_reference
| TIME, FREQ, N_VEL_COMPONENT,  N_VEL_INSTRUMENT
| Integrating these spectra across all frequencies should yield the variance of the timeseries used in computing the spectra. {{FontColor|fg=red|text=Please specify the analysis frame of reference}} in the variable attribute and/or group's attribute.
|-
| PSPD_REL
| platform_speed_wrt_sea_water
| TIME,  N_VEL_INSTRUMENT
| Used for converting from frequency to wavenumber domain. Typically the mean speed measured by the velocity-meter i.e., velocity past the sensor.
|-
| DOF
| degrees_of_freedom
| Constant [or TIME,  N_VEL_INSTRUMENT]
| This quantity should remain constant in time unless different spectral techniques are being used (unevenly least-square FFT vs standard FFTs)
|-
| PERGD
| percentage_of_samples_good_in_segment
| TIME,  N_VEL_INSTRUMENT
| Useful to identify is the spectra were computed largely from replaced bad samples. 
|-
| UVW_VEL_SPEC_FLAGS
| status_flag
| TIME,  N_VEL_COMPONENT,  N_VEL_INSTRUMENT
| Boolean flags about non-stationary tests criteria, noise tests, percent good threshold etc. 
|- 
| UVW_VEL_STD
| standard_deviation_of_water_velocity_
in_the_analysis_frame_of_reference
| TIME,  N_VEL_COMPONENT,  N_VEL_INSTRUMENT
| Calculated over each segment. This is exactly the same as RMS of the detrended signal. Used for surface waves analysis and for estimating ratio of whether Taylor's hypothesis is valid.
|- 
| [[Level_1_data_(velocity_point-measurements)#Required_height_variable|HEIGHT or DEPTH]]<math>\dagger</math> 
| See [[Level_1_data_(velocity_point-measurements)#Required_height_variable|Level 1 table]]
| TIME, N_VEL_INSTRUMENT
| 
|-
| colspan="4" style="text-align:center; font-weight:bold; background-color:#f9eddd"|  Optional surface wave variables
|-
| Enter surface wave stats required/useful for analysing turbulence
| 
| TIME,  N_VEL_COMPONENT,  N_VEL_INSTRUMENT
| 
|- 
| UVW_VEL
| water_velocity_
in_the_analysis_frame_of_reference
| TIME,  N_VEL_COMPONENT,  N_VEL_INSTRUMENT
| Time-averaged over each segment. Needed for determining the [[Velocity inertial subrange|inertial subrange model]] for spectral fitting.
|- 
| colspan="4" style="text-align:center; font-weight:bold; background-color:#f9eddd"|  Optional variables <math>\ddagger</math>
|-
| BVFSQ 
| buoyancy_frequency
| TIME, N_VEL_INSTRUMENT
| Useful for assessing anisotropy <math>N=\sqrt{\frac{-g}{\rho}\frac{d\rho}{dz}}</math>, but requires ancillary sensors (e.g., thermistors at different heights or CTD profiling)
|-
| SH
| 
| TIME, N_VEL_INSTRUMENT
| Useful for assessing anisotropy {{FontColor|fg=white|bg=red|text=SH=enter equation}}, but requires ancillary sensors (e.g., ADCPs or many velocity-meters)
|-
| BURST_NUMBER <math>\dagger\dagger</math>
| unique_identifier_for_each_burst
| TIME, N_VEL_INSTRUMENT
| Only required when measuring in burst-mode. Integers of 1, 2, etc to designate which burst the velocities are associated with.
|-
| colspan="4" | 
<math>\dagger</math> A variable situating the instruments in the water column is required. This may be a nominal depth (or height) or a time-varying depth or height above the bottom.

<math>\ddagger</math> One could re-write for convenience the optional variables in [[Level_1_data_(velocity_point-measurements)#Optional_variables|Level 1]] after segmenting them into smaller chunks. For instance, PRES (pressure) would be fairly common in surface wave studies.

<math>\dagger\dagger</math> Variable required only when measuring in bursts e.g., 18 min of continuous measurements every hour.
|} 
</div>

=Group attributes (metadata)=
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This section describes attributes that may provide additional information about how the data was processed and manipulated at this stage.

{{FontColor|fg=white|bg=red|text=Work in progress. Refer to example Yaml file for now}} in the [https://bitbucket.org/efm_cb/netcdf/src/master/TestData/adv_atomix_metada.yml demo].

{| class="wikitable" 
|-
! Attribute name
! Purpose
! Suggested content
|-
| processing_level
| Boilerplate about the content of the NetCDF group.
| <blockquote>''This group includes the spectral observation and ancillary information required for estimatng the dissipation of turbulent kinetic energy by fitting the spectra's inertial subrange.''</blockquote>
|-
| spectral_calculation_type
| 
| 
|-
| spectral_windowing 
| 
| 
|-
| spectral_fft_length
| 
| 
|-
| colspan="4" style="text-align:center; font-weight:bold; background-color:#f9eddd"|  Optional group attributes <math>\ddagger</math>
|-
| stationarity_testing
|
|
|-
| noise_testing
|
|
|-
| comment (optional)
| Any additional information pertinent to other users who test their algorithms against the file.
| 
|}
</div>


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Go back to the previous level: [[Level 2 segmented (velocity point-measurements)| Level 2 segmented]]

Go to the next level: [[Level 4 data (velocity point-measurements)|Level 4 dissipation estimates]]

[[Category:Velocity point-measurements]]