Introduction

In ERA5, geopotential (z) is provided at the surface, but not on individual model levels (ml). However, geopotential on model levels can be computed using the procedure described below.

Inputs:

Output: Geopotential for each level, in m2/s2. To get the height in metres, divide the geopotential by the gravity constant 9.80665 m/s2.

In the procedure below the output data is written in GRIB format.

Prerequisites

You will need:

Step 1: Download input data

First we retrieve the required ERA5 data. We need:

We use a Python script to download the MARS catalogue ERA5 data through CDS API. The procedure is:

  1. Copy the script below to a text editor
  2. Amend date, type, step, time, grid and area in the script to meet your requirements
  3. Save the script (for example with the filename as 'get_data_geopotential_on_ml.py')
  4. Run the script
#!/usr/bin/env python
import cdsapi
c = cdsapi.Client()

# data download specifications:
cls     = "ea"         # do not change
expver  = "1"          # do not change
levtype = "ml"         # do not change
stream  = "oper"       # do not change
date    = "2018-01-01" # date: Specify a single date as "2018-01-01" or a period as "2018-08-01/to/2018-01-31". For periods > 1 month see https://software.ecmwf.int/wiki/x/l7GqB
tp      = "an"         # type: Use "an" (analysis) unless you have a particular reason to use "fc" (forecast).
time    = "00:00:00"   # time: ERA5 data is hourly. Specify a single time as "00:00:00", or a range as "00:00:00/01:00:00/02:00:00" or "00:00:00/to/23:00:00/by/1".

c.retrieve('reanalysis-era5-complete', {
    'class'   : cls,
    'date'    : date,
    'expver'  : expver,
    'levelist': '1/2/3/4/5/6/7/8/9/10/11/12/13/14/15/16/17/18/19/20/21/22/23/24/25/26/27/28/29/30/31/32/33/34/35/36/37/38/39/40/41/42/43/44/45/46/47/48/49/50/51/52/53/54/55/56/57/58/59/60/61/62/63/64/65/66/67/68/69/70/71/72/73/74/75/76/77/78/79/80/81/82/83/84/85/86/87/88/89/90/91/92/93/94/95/96/97/98/99/100/101/102/103/104/105/106/107/108/109/110/111/112/113/114/115/116/117/118/119/120/121/122/123/124/125/126/127/128/129/130/131/132/133/134/135/136/137',         # For each of the 137 model levels
    'levtype' : 'ml',
    'param'   : '130/133', # Temperature (t) and specific humidity (q)
    'stream'  : stream,
    'time'    : time,
    'type'    : tp,
	'area'	  : area, #example: [60, -10, 50, 2], # North, West, South, East. Default: global
}, 'tq_ml.grib')


c.retrieve('reanalysis-era5-complete', {
    'class'   : cls,
    'date'    : date,
    'expver'  : expver,
    'levelist': '1',       # Geopotential (z) and Logarithm of surface pressure (lnsp) are 2D fields, archived as model level 1
    'levtype' : levtype,
    'param'   : '129/152', # Geopotential (z) and Logarithm of surface pressure (lnsp) 
    'stream'  : stream,
    'time'    : time,
    'type'    : tp,
	'area'	  : area, #example: [60, -10, 50, 2], # North, West, South, East. Default: global
}, 'zlnsp_ml.grib')

The script produces two files in the current working directory:

Step 2: Compute geopotential on model levels

We then use a Python script to compute geopotential (z) for all model levels:

  1. Copy the script below to a text editor
  2. Save the script as 'compute_geopotential_on_ml.py'
  3. Run the script 'compute_geopotential_on_ml.py' with the correct arguments, i.e. :  python compute_geopotential_on_ml.py tq_ml.grib zlnsp_ml.grib -o z_on_ml.grib

This script is from ECMWF's generic article Compute geopotential on model levels .

Alternatively, there is a customer-supplied script that computes geopotential on model levels from NetCDF files (i.e. works on Microsoft Windows), and for a specific location. This script was written for the ERA-Interim dataset, but can be adapted to ERA5. Please see the article ERA-Interim: compute geopotential on model levels for details.


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