Level 2 data (velocity point-measurements)

Page status: Ready for review
Author(s):	Cynthia
Reviewer(s):
Comment:

The required dimensions and variables for the Level 2 segmented processing level within NetCDF ATOMIX format 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. Provide bounds attribute to designate the variable containing the limits of each segment (see CF-compliant example).
N_SAMPLE	unique_identifier_for_each_ sample_within_the_segment	N_SAMPLE	Max value is the segment length (seconds) multiplied by the sampling rate
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	water_velocity_in_the_ analysis_frame_of_reference	TIME, N_SAMPLE, N_VEL_COMPONENT, N_VEL_INSTRUMENT	This may be rotated in the direction of the mean flow, or left in XYZ_VEL if relying only on the vertical component when estimating [math]\displaystyle{ \varepsilon }[/math]. Please specify the analysis frame of reference in the variable attribute and/or group's attribute.
UVW_VEL_FLAGS	water_velocity_in_the_ analysis_frame_of_reference_status_flag	TIME, N_SAMPLE, N_VEL_COMPONENT, N_VEL_INSTRUMENT	Enumerated flag that denotes if and how missing samples were replaced
UVW_VEL_DETRENDED	water_velocity_ in_the_analysis_frame_of_reference_detrended	TIME, N_SAMPLE, N_VEL_COMPONENT, N_VEL_INSTRUMENT	These velocities in the analysis frame of reference have been detrended. The signal may include surface wave, motion contamination, in addition to the turbulence signal. Please specify how.
HEIGHT or DEPTH[math]\displaystyle{ \dagger }[/math]	See Level 1 table	TIME, N_VEL_INSTRUMENT
Optional variables [math]\displaystyle{ \ddagger }[/math]
ROT_AXIS	angle_of_rotation_from_east_to_x	TIME, N_VEL_COMPONENT, N_VEL_INSTRUMENT	Rotation axis [i,j,k] in the geographical coordinates system used to rotate from the geographical to the analysis frame of reference.
ROT_ANGLE	axis_of_rotation_from_east_to_x	TIME, N_VEL_INSTRUMENT	Angle of rotation from geographical coordinates to the analysis frame of reference around the rotation axis.
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{ \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 }[/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{ \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.

Attribute name	Purpose	Suggested content
processing_level	Boilerplate about the content of the NetCDF group.	In this group, quality-controlled and despiked velocities have been split into smaller segments for processing, and are often rotated into flow's frame of reference. Detrending may occur at this step if this wasn't completed at the previous processing level.
segment_length	Provide length in seconds of each segment	Usually about 300 to 600 s dependent on time and length scales of turbulence
segment_overlap_proportion	Provide proportion overlap of each segment, i.e., window overlap	Often set to 0 or 0.5
Optional group attributes [math]\displaystyle{ \ddagger }[/math]
detrending_method	Specify which filter or technique was used to detrend velocities, which is required for spectral and turbulence analysis.	Some examples include High-pass YY order butterworth filter with XX seconds cutoff frequency on the entire burst/timeseries Linear detrending on each segment
analysis_frame_of_reference	Specify the coordinate systems (e.g., geographical, instrument, or flow frame reference) used for analysis since the inertial subrange constants depends on the longitudinal, transverse and vertical component.	Some possible options Geographical coordinates Instrument coordinates rotated in the direction of mean flow rotated along the bathymetry/principal component
rotation_method	The basis of rotation (if performed) for the turbulence analysis. Were the velocities left in the measurement frame of reference? Was a new frame of reference defined that is more representative of the site conditions (e.g., cross-shelf or along-shelf). Were the velocities rotated over each segment (horizontal plane only i.e., vertical mean velocity was near zero)?	State whether the mean vertical velocity was assumed to be zero (flat bottom), and if the rotation depended on the measured velocities or was prescribed. For example, Velocities were rotated into the segment's mean velocity direction (including the contributions from vertical component) Velocities were rotated in the horizontal plane according to the segment's mean horizontal velocity Velocities were not rotated. Analysis relied on the vertical component in the intrument's measurement system.
comment (optional)	Any additional information pertinent to other users who test their algorithms against the file.

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