Xarray engine: 2D wave spectra dimensions

This notebook demonstrates how earthkit-data manages parameters related to 2D wave spectra.

import earthkit.data as ekd

We analyse a 2D wave spectra product at 2025-12-15 00 UTC and 03 UTC.

A specific feature of this dataset is that the fields are additionally indexed by

• parameter.wave_direction

• parameter.wave_frequency

fl = ekd.from_source("sample", "2d-wave-spectra_an.grib").to_fieldlist()

fl.ls(extra_keys=["parameter.wave_direction", "parameter.wave_frequency"]).iloc[[0, 1, -2, -1]]

	parameter.variable	time.valid_datetime	time.base_datetime	time.step	vertical.level	vertical.level_type	ensemble.member	geography.grid_type	parameter.wave_direction	parameter.wave_frequency
0	2dfd	2025-12-15 00:00:00	2025-12-15 00:00:00	0 days	0	mean_sea	0	regular_ll	5.0	0.034523
1	2dfd	2025-12-15 00:00:00	2025-12-15 00:00:00	0 days	0	mean_sea	0	regular_ll	15.0	0.034523
2086	2dfd	2025-12-15 03:00:00	2025-12-15 03:00:00	0 days	0	mean_sea	0	regular_ll	345.0	0.497852
2087	2dfd	2025-12-15 03:00:00	2025-12-15 03:00:00	0 days	0	mean_sea	0	regular_ll	355.0	0.497852

fl[0]

Field

data

number_of_values	84
array_type	ndarray
array_dtype	float64

parameter

variable	2dfd
standard_name	unknown
long_name	2D wave spectra (single)
units	meter ** 2 * second / radian
wave_direction	5.0
wave_direction_index	0
wave_direction_bounds	(0.0, 10.0)
wave_direction_units	degree
wave_frequency	0.034523
wave_frequency_index	0
wave_frequency_bounds	(0.032917, 0.036208)
wave_frequency_units	1 / second

time

valid_datetime	2025-12-15 00:00:00
base_datetime	2025-12-15 00:00:00
step	0:00:00

vertical

level	0
layer	None
level_type	mean_sea

ensemble

member

0

geography

grid_spec	None
grid_type	regular_ll
shape	(7, 12)
area	(90.0, 0.0, -90.0, 330.0)

Conversion to Xarray is straightforward:

ds = fl.to_xarray()
ds

<xarray.Dataset> Size: 1MB
Dimensions:                  (wave_direction: 36, wave_frequency: 29,
                              forecast_reference_time: 2, latitude: 7,
                              longitude: 12)
Coordinates:
  * wave_direction           (wave_direction) float64 288B 5.0 15.0 ... 355.0
  * wave_frequency           (wave_frequency) float64 232B 0.03452 ... 0.4979
  * forecast_reference_time  (forecast_reference_time) datetime64[ns] 16B 202...
  * latitude                 (latitude) float64 56B 90.0 60.0 ... -60.0 -90.0
  * longitude                (longitude) float64 96B 0.0 30.0 ... 300.0 330.0
Data variables:
    2dfd                     (wave_direction, wave_frequency, forecast_reference_time, latitude, longitude) float64 1MB ...
Attributes:
    Conventions:  CF-1.8
    institution:  ECMWF

xarray.Dataset

• Dimensions:
  • wave_direction: 36
  • wave_frequency: 29
  • forecast_reference_time: 2
  • latitude: 7
  • longitude: 12
• Coordinates: (5)
  • wave_direction
    (wave_direction)
    float64
    5.0 15.0 25.0 ... 335.0 345.0 355.0

    standard_name :

    sea_surface_wave_from_direction

    units :

    degrees

    long_name :

    wave direction

    array([  5.,  15.,  25.,  35.,  45.,  55.,  65.,  75.,  85.,  95., 105., 115.,
           125., 135., 145., 155., 165., 175., 185., 195., 205., 215., 225., 235.,
           245., 255., 265., 275., 285., 295., 305., 315., 325., 335., 345., 355.])

  • wave_frequency
    (wave_frequency)
    float64
    0.03452 0.03798 ... 0.4526 0.4979

    standard_name :

    sea_surface_wave_frequency

    units :

    s-1

    long_name :

    wave frequency

    array([0.034523, 0.037975, 0.041772, 0.04595 , 0.050545, 0.055599, 0.061159,
           0.067275, 0.074002, 0.081403, 0.089543, 0.098497, 0.108347, 0.119182,
           0.1311  , 0.14421 , 0.158631, 0.174494, 0.191943, 0.211138, 0.232252,
           0.255477, 0.281024, 0.309127, 0.340039, 0.374043, 0.411448, 0.452593,
           0.497852])

  • forecast_reference_time
    (forecast_reference_time)
    datetime64[ns]
    2025-12-15 2025-12-15T03:00:00

    standard_name :

    forecast_reference_time

    long_name :

    initial time of forecast

    array(['2025-12-15T00:00:00.000000000', '2025-12-15T03:00:00.000000000'],
          dtype='datetime64[ns]')

  • latitude
    (latitude)
    float64
    90.0 60.0 30.0 ... -60.0 -90.0

    units :

    degrees_north

    standard_name :

    latitude

    long_name :

    latitude

    array([ 90.,  60.,  30.,   0., -30., -60., -90.])

  • longitude
    (longitude)
    float64
    0.0 30.0 60.0 ... 270.0 300.0 330.0

    units :

    degrees_east

    standard_name :

    longitude

    long_name :

    longitude

    array([  0.,  30.,  60.,  90., 120., 150., 180., 210., 240., 270., 300., 330.])

• Data variables: (1)
  • 2dfd
    (wave_direction, wave_frequency, forecast_reference_time, latitude, longitude)
    float64
    ...

    standard_name :

    unknown

    long_name :

    2D wave spectra (single)

    units :

    meter ** 2 * second / radian

    level_type :

    mean_sea

    _earthkit :

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    [175392 values with dtype=float64]

• Attributes: (2)

  Conventions :

  CF-1.8

  institution :

  ECMWF