Level 2 data (velocity point-measurements)
Page status: Ready for review | |
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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. |
| |
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
| |
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
| |
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,
| |
comment (optional) | Any additional information pertinent to other users who test their algorithms against the file. |
Return to: Level 1 raw
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