ml_to_hl

ml_to_hl(fs, z, zs, h, ref_level, method[, fs_surf])

Interpolates fs on model levels (i.e. on hybrid or eta levels used by the IFS) onto height levels (in m) above sea or ground level.

Parameters

  • fs (Fieldset) – fieldset to be interpolated. There is no restriction on the order or range of model levels in fs.

  • z (Fieldset) – geopotential fieldset on model levels (it must contain the same levels as fs but their order can be different)

  • zs (Fieldset or None) – surface geopotential field (if ref_level is set to “sea” it should be set to None).

  • h (list or Fieldset) – list of target height levels in metres (they can came in any given order). Values must be non-negative.

  • ref_level (str) – specifies the reference level for the target heights. The possible values are “sea” and “ground”. If it is “ground” a valid zs must be provided.

  • method (str) – specifies the interpolation method. The possible values are “linear” and “log”. For target height levels very close to 0 always a “linear” interpolation is used.

  • fs_surf (number or Fieldset) – (optional) specifies the field values on the surface. With this it is possible to interpolate to target heights between the surface and the bottom-most model level. If fs_surf is a number it defines a constant Fieldset. Only available when ref_level is “ground”. New in Metview version 5.14.0.

Return type

Fieldset

The input data (fs) must contain one field per model level only. It means that e.g. data containing multiple timesteps cannot be used as an input.

At gridpoints where the interpolation is not possible a missing value is returned. It happens where the target height level is below the bottom-most model level in fs or the surface when fs_surf is used. It also happens where the target height is above the top-most model level in fs. Please note that the model levels we are dealing with in ml_to_hl() are “full-levels” and the lowest possible model level does match the surface but it is above it. If you need to interpolate to height levels close to the surface use fs_surf.

Note

The actual ECMWF model level definition is stored in the “pv” array in the GRIB message metadata. You can figure out the total number of model levels in the given vertical coordinate system by using the len(pv)/2-1 formula. The typical values are 137 and 91. This can be then used to look up details about actual the model level definitions (e.g. approximate pressure and height values) on this page.

Note

Geopotential is not archived operationally on model levels in MARS at ECMWF. To compute it use mvl_geopotential_on_ml().

Example

This code illustrates how to use ml_to_hl() together with mvl_geopotential_on_ml() with data retrieved from MARS:

import metview as mv

# retrieve the data on model levels - surface geopotential (zs)
# is taken from the analyis on level 1!
ret_core = {
   "levtype": "ml", "date": 20191023, "time": 12 "grid": [2,2]}

fs_ml = mv.retrieve(**ret_core,
         type="fc",
         levelist=[1,"TO",137],
         step=12,
         param=["t", "q", "lnsp"])

t = mv.read(data=fs_ml, param="t")
q = mv.read(data=fs_ml, param="q")
lnsp = mv.read(data=fs_ml, param="lnsp")

zs = mv.retrieve(**ret_core,
      type="an",
      levelist=1,
      param="z")

# compute geopotential on model levels
z = mv.mvl_geopotential_on_ml(t, q, lnsp, zs)

# interpolate the t field onto a list of height levels above sea level
hlevs = [1000, 2000, 3000, 4000, 5000]
th_sea = mv.ml_to_hl (t, z, None, hlevs, "sea", "linear")


# interpolate the t field onto another list of height levels above ground
hlevs = [100, 200, 300, 400, 500]
th_ground = mv.ml_to_hl (t, z, zs, hlevs, "ground", "linear")