Level 2 data (velocity profilers)

Velocities have now been segmented and detrended. Each segment is stored separately from each other, which allows segmenting data using overlapping windows i.e., some velocity samples can belong to more than one segment.

The dimensions and variables for this processing level 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

Short name	Standard name	Dimensions	Comments
N_SEGMENT	unique_identifier_for_each_segment_in_the_ entire_available_timeseries	N_SEGMENT	Array of 1 to number of segments into which the data has been separated (Note TIME variable)
Z_DIST	distance_from_sensor_along_vertical	Z_DIST	bin centre distance (in meters) from the transducer along the instrument's vertical axis
N_BEAM	unique_identifier_for_each_beam	N_BEAM	Array of 1 to number of beams (3 to 5 typically)
N_SAMPLE	unique_identifier_for_each_sample_within_ the_segment	N_SAMPLE	Array of 1 to number of samples in each segment (segment length in seconds multiplied by the sampling rate in Hz)

Variables

Short name	Standard name	Dimensions	Comment
R_VEL_DETRENDED	water_radial_velocity_of_scatterers_ towards_instrument_detrended	N_SEGMENT, Z_DIST, N_BEAM, N_SAMPLE	High-frequency content of the along-beam velocities [m/s], which may include surface wave, motion contamination, in addition to the turbulence signal. Calculated from R_VEL by removing the segment mean or trend. Detrending method should be specified by detrending_method in group attributes (see below). Link to own detrending page. Corresponds to [math]\displaystyle{ b^\prime }[/math] in Nomenclature
TIME	TIME	N_SEGMENT, N_SAMPLE	Units in Days since reference time specified in variable attribute
THETA	beam_angle_from_instrument_z_axis	N_BEAM	Units in degrees, positive (usually ~20-30[math]\displaystyle{ ^\circ }[/math]) except vertical-pointing beams (0[math]\displaystyle{ ^\circ }[/math])
BIN_SIZE	instrument_measurement_volume_bin_size	N_BEAM	Vertical size of the ADCP bins [m]. Usually the same for diverging beams.
Optional variables [math]\displaystyle{ \ddagger }[/math]
PROFILE_NUMBER	unique_identifier_for_each_profile	N_SEGMENT, N_SAMPLE	This variable identifies each record (velocity) profile and is a proxy for TIME
R_VEL_DETRENDED_FLAGS	water_radial_velocity_of_scatterers_towards_ instrument_status_flag	N_SEGMENT, Z_DIST, N_BEAM, N_SAMPLE	This variable provides flags for outliers in each segment [math]\displaystyle{ \geq 2 }[/math] standard deviations.
[math]\displaystyle{ \ddagger }[/math] One could also re-write for convenience the optional variables in Level 1 after restructuring into segments.

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 velocities have been split into smaller segments for processing. The timeseries are also detrended to recover the turbulence, and potentially surface wave and motion contamination.
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
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
Optional group attributes
comment (optional)	Any additional information pertinent to other users who test their algorithms against the file.

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Return to: Level 1 raw

Go to Level 3 structure function