Level 3 data (velocity point-measurements)
| Page status: Draft | |
|---|---|
| Author(s): | Cynthia |
| Reviewer(s): | |
| Comment: | |
The required dimensions and variables associated with the Level 3 spectra processing level (NetCDF group) 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).
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-centered |
| FREQ | frequency | FREQ | Frequency in Hz for the spectra |
| 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_SPEC | power_spectrum_density_of_velocity_
in_the_analysis_frame_or_reference |
TIME, FREQ, N_VEL_COMPONENT, N_VEL_INSTRUMENT | Integrating these spectra across all frequencies should yield the variance of the timeseries used in computing the spectra. Please specify the analysis frame of reference in the variable attribute and/or group's attribute. |
| PSPD_REL | platform_speed_wrt_sea_water | TIME, N_VEL_INSTRUMENT | Used for converting from frequency to wavenumber domain. Typically the mean speed measured by the velocity-meter i.e., velocity past the sensor. |
| DOF | degrees_of_freedom | Constant [or TIME, N_VEL_INSTRUMENT] | This quantity should remain constant in time unless different spectral techniques are being used (unevenly least-square FFT vs standard FFTs) |
| PERGD | percentage_of_samples_good_in_segment | TIME, N_VEL_INSTRUMENT | Useful to identify is the spectra were computed largely from replaced bad samples. |
| UVW_VEL_SPEC_FLAGS | status_flag | TIME, N_VEL_COMPONENT, N_VEL_INSTRUMENT | Boolean flags about non-stationary tests criteria, noise tests, percent good threshold etc. |
| UVW_VEL_STD | standard_deviation_of_water_velocity_
in_the_analysis_frame_of_reference |
TIME, N_VEL_COMPONENT, N_VEL_INSTRUMENT | Calculated over each segment. This is exactly the same as RMS of the detrended signal. Used for surface waves analysis and for estimating ratio of whether Taylor's hypothesis is valid. |
| HEIGHT or DEPTH[math]\displaystyle{ \dagger }[/math] | See Level 1 table | TIME, N_VEL_INSTRUMENT | |
| Optional surface wave variables | |||
| Enter surface wave stats required/useful for analysing turbulence | TIME, N_VEL_COMPONENT, N_VEL_INSTRUMENT | ||
| UVW_VEL | water_velocity_
in_the_analysis_frame_of_reference |
TIME, N_VEL_COMPONENT, N_VEL_INSTRUMENT | Time-averaged over each segment. Needed for determining the inertial subrange model for spectral fitting. |
| Optional variables [math]\displaystyle{ \ddagger }[/math] | |||
| 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\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] 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{ \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\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. |
| |
| spectral_calculation_type | |||
| spectral_windowing | |||
| spectral_fft_length | |||
| Optional group attributes [math]\displaystyle{ \ddagger }[/math] | |||
| stationarity_testing | |||
| noise_testing | |||
| comment (optional) | Any additional information pertinent to other users who test their algorithms against the file. | ||
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