Frequency-Domain Statistics

Spectral analysis operations that estimate statistics from time-series data using Welch's power spectral density method.

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hmo

Frequency-domain significant wave height (Hm0) computed by integrating the power spectral density over one or more frequency bands.

Method: Welch's method is used to estimate the PSD S(f), then Hs is derived per band as:

Hs = 4 × √( ∫ S(f) df )

Parameter	Type	Default	Description
segsec	float	256	Welch segment length in seconds. Longer segments give finer frequency resolution.
bands	dict	see below	Mapping of band label to [fmin, fmax] in Hz. null for a bound means use the data's min/max frequency.

The sampling frequency is inferred automatically from the time coordinate spacing.

Output variables are renamed to hs_{band_label} and the result has a band coordinate.

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Default frequency bands

Label	Range	Description
0_25	0 – 0.25 Hz	Swell + wind sea
8_25	0.08 – 0.25 Hz	Swell band
25_120	0.25 – 1.20 Hz	Wind sea
25_300	0.25 – 3.00 Hz	Wind sea (extended)
tot	full spectrum	Total Hs

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Examples

Compute Hs over the default bands:

- func: hmo
  dim: time
  data_vars: [eta]
  segsec: 256

Override with custom bands:

- func: hmo
  dim: time
  data_vars: [eta]
  segsec: 512
  bands:
    swell: [0.04, 0.10]
    wind_sea: [0.10, 0.50]
    total: [null, null]

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Note

hmo expects a regularly sampled time series of surface elevation (or equivalent). Gaps or irregular sampling will produce inaccurate PSD estimates.

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API reference

gridstats.ops.frequency_domain.hmo(data: xr.Dataset, *, dim: str = 'time', segsec: float = 256, bands: dict[str, list[float | None]] = BANDS, group: str | None = None, **kwargs) -> xr.Dataset

Compute frequency-domain significant wave height (Hmo) per frequency band.

Uses Welch's method to estimate the power spectral density, then integrates it over specified frequency bands: Hs = 4 * sqrt(integral(S(f) df)).

Parameters:

Name	Type	Description	Default
data	Dataset	Input dataset. Variables must be time-series of surface elevation or equivalent.	required
dim	str	Time dimension name.	'time'
segsec	float	Welch segment length in seconds.	256
bands	dict[str, list[float | None]]	Mapping of band label to [fmin, fmax] in Hz. None values are replaced by the data's min/max frequency.	BANDS
group	str | None	Time component to group by (not typically meaningful for Hmo).	None

Returns:

Type	Description
Dataset	Dataset with variables named 'hs_{band_label}', each with a 'band'
Dataset	coordinate.