Level 3 data (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. Please refer to the complete list for the additional attributes related to each variable (e.g., units, bounds, cell_methods).

Dimensions

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

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 (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
Optional variables [math]\displaystyle{ \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

Variables

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. 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)
HEIGHT or DEPTH[math]\displaystyle{ \dagger }[/math]	See Level 1 table	TIME, N_VEL_INSTRUMENT
Quality-control metrics
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	power_spectrum_density_of_velocity_in_the_ analysis_frame_of_reference_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	water_velocity_in_the_ analysis_frame_of_reference_standard_deviation	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 verifying Taylor's frozen hypothesis is valid. Specify cell_methods attribute to indicate the calculation method e.g., "cell_methods= TIME:standard_deviation".
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 inertial subrange model for spectral fitting. Specify cell_methods attribute to indicate the calculation method e.g., "cell_methods= TIME:mean".
Optional variables [math]\displaystyle{ \ddagger }[/math]
BVFSQ	buoyancy_frequency	TIME, N_VEL_INSTRUMENT	Useful for assessing anisotropy [math]\displaystyle{ 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 SH=enter equation, but requires ancillary sensors (e.g., ADCPs or many velocity-meters)
BURST_NUMBER [math]\displaystyle{ \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.
[math]\displaystyle{ \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]\displaystyle{ \ddagger }[/math] One could re-write for convenience the optional variables in Level 1 after segmenting them into smaller chunks. For instance, PRES (pressure) would be fairly common in surface wave studies. [math]\displaystyle{ \dagger\dagger }[/math] Variable required only when measuring in bursts e.g., 18 min of continuous measurements every hour.

Group attributes (metadata)

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

Work in progress. Refer to example Yaml file for now in the demo.

Attribute name	Purpose	Suggested content
processing_level	Boilerplate about the content of the NetCDF group.	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.
spectral_calculation_type
spectral_windowing
spectral_fft_length
Optional group attributes [math]\displaystyle{ \ddagger }[/math]
stationarity_testing
noise_testing
comment (optional)	Any additional information pertinent to other users who test their algorithms against the file.

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