Level 2 data (velocity point-measurements)

From Atomix
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Author(s): Cynthia
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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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