Level 4 data (velocity point-measurements)

Page status: Draft
Author(s):	Cynthia
Reviewer(s):
Comment:

This will dictate the data required at the final processing level, where we store the estimated dissipation estimates [math]\displaystyle{ \varepsilon }[/math] along with quality metrics.

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).
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

Required variables

Short name	Standard name	Dimensions	Comment
EPSI	specific_turbulent_kinetic_energy_dissipation _in_water	TIME, N_VEL_COMPONENT, N_VEL_INSTRUMENT	Dissipation rate of turbulent kinetic energy per unit mass of water [W/kg] estimated from each velocity component.
EPSI_FINAL	specific_turbulent_kinetic_energy_dissipation _in_water	TIME, N_VEL_INSTRUMENT	Final dissipation rate of turbulent kinetic energy per unit mass of water. Typically the vertical component, unless anisotropy precludes from its use.
KVISC	kinematic_viscosity_of_water	[Scalar i.e., 1 value], or TIME, R_DIST	This variable is used for assessing length scales against Kolmogorov length scale.
HEIGHT or DEPTH[math]\displaystyle{ \dagger }[/math]	See Level 1 table	TIME, N_VEL_INSTRUMENT
Quality-control metrics
EPSI_FLAGS	specific_turbulent_kinetic_energy_dissipation _in_water_status_flag	TIME, N_VEL_COMPONENT, N_VEL_INSTRUMENT	Status boolean flags (0-255, 8-bit) that designates one of 8 reasons for rejecting an estimate.
MAD_SQRT_DOF	mean_absolute_deviation_multiplied_ sqrt_of_degrees_of_freedom	TIME, N_VEL_COMPONENT, N_VEL_INSTRUMENT	This variable is used for assessing the goodness of fit. The larger the value, the poorer the model fit. Create concept page for this...
Enter remaining entries once this bridge is crossed	K_SLOPE, TAYL, Kbounds [Kmin Kmax via extra dim N_Bounds], Conf intervals for epsi and K_slope.
Optional surface wave variables
Enter surface wave stats required/useful for analysing turbulence		TIME, N_VEL_COMPONENT, N_VEL_INSTRUMENT
[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.

Optional variables

One could re-write for convenience the optional variables in Level 1 after time-averaging. For instance, PRES (pressure) would be fairly common in surface wave studies.

Short name	Standard name	Dimensions	Comment
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 }[/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] 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 results associated with fitting the spectral observations with the inertial subrange model. The results for each velocity component, along with quality indicators and errors are provided. A final estimate for the turbulent kinetic energy dissipation is also provided.
inertial_subrange_model	Specify the spectral model used for fitting (e.g., surface waves with citation)	Enter example
spectral_fitting_method	Specify the statistical technique used for spectral fitting	Examples in include: Maximum likelihood estimator assuming chi-distributed spectra
kolmogorov_constant	Specify the kolmogorov constant used for spectral fitting	Examples in include: 0.5 as per Sreenivasan 1995
comment (optional)	Any additional information pertinent to other users who test their algorithms against the file.

Notes

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Return to: Level 3 spectra

Go to: NetCDF velocimeters format