Background
Background
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The EUMETSAT ROM SAF data is downloaded from the ECMWF public datasets service using web-api, which is a "programmatic approach" for data download with python scripts . We will not reproduce the all existing documentation here, but we will note some key steps required to download the data and provide the main links. Firstly, you will need to download the appropriate python modules to run the python download scripts. For example, the python scripts will contain "from ecmwfapi import" commands, like in this simple example, retrieving a monthly mean zonal wind at 1000 hPa, in netcdf format (More complex retrieval scripts are provided here):
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has produced two atmospheric reanalyses as part of its CDOP-2 activities. These reanalyses were recommended by the IROWG climate subgroup (see recommendation 6 to CGMS, in section 4.2), where it was stated that:
"Encourage NWP centers to engage in reanalysis activities based only on data types that are not bias-corrected, especially RO. ECMWF, e.g., carries out such an activity as part of the ROM SAF CDOP-2 plans."
The two atmopsheric reanalyses cover the the period 20070101 to 20151231. They have been run using the ERA5 reanalysis system, but at low resolution (T159 ~ 125 km grid), and using a static B matrix rather than ERA5's flow-dependent version which uses the Ensemble of Data Assimilations approach. The first ROM SAF reanalysis assimilates just reprocessed GPS-RO data provided the ROM SAF and AMSUA channel 14 to constrain the upper stratosphere. The second reanalysis also includes conventional measurements (e.g., sondes, synop, ...) that are not bias corrected as part of the variational bias correction (VAR-BC) scheme. Notably, this excludes aircraft temperature measurements that are bias corrected in VAR-BC.
The two atmospheric reanalyses provide:
- gridded daily analysis fields at the hours (00,03,06,09,12,15,18,21) (class=ea, stream=oper, type=an)
- monthly mean fields from 20070201 to 20151201 (class=ea, stream=moda, type=an)
- the observation departure statistics for the data assimilated in the reanalyses (class=ea, stream=oper, type=mfb)
The reanalyses can be summarised as follow:
| Experiment id | Name of reanalysis dataset | Description | Class | Dates |
|---|---|---|---|---|
| 21 | rom_saf_21 | Assimilating reprocessed ROM SAF GPS-RO, AMSU-A channel 14 and conventional data not used in VAR-BC | ea | 200701-20151231 |
| 22 | rom_saf_22 | Assimilating reprocessed ROM SAF GPS-RO, AMSU-A channel 14 | ea | 200701-20151231 |
This atmospheric reanalysis data is available for the surface, on 37 fixed pressure levels and on 137 model levels.
A selection of science results presented at the Joint 6th ROM SAF User Workshop and 7th IROWG Workshop are given here.
This data has also been used to demonstrate how QBO zonal wind information can be retrieved from GNSS-RO measurements - click here.
Access to the ROM SAF data using Web API
The ROM SAF datasets can be retrieved using the ECMWF Web API service. The Web API is a "programmatic approach" for data download using python scripts.
Frequently asked web api questions (FAQs) can be found here: Web API FAQ
General information on downloading with web api is given here: Web API
General information related to downloading the data via web-api, is described here:
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Specifically, follow the steps here to install the latest code version:
https://pypi.org/project/ecmwf-api-client/
Briefly, install this with the pip command:
pip install ecmwf-api-client
You will now have the modules needed in the python scripts. (I actually needed this command for the ECMWF system pip install --user ecmwf-api-client because of local permissions).
Login and retrieve your "key":
Login at:
https://apps.ecmwf.int/auth/login/
Retrieve your key at:
Create and paste the "key" information in here:
$HOME/.ecmwfapirc
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Retrieval efficiency when using Web API
The importance retrieval efficiency when using Web API is discussed here. This page includes Web API retrieval examples for other reanalyses, such as ERA5.
Specific examples of efficient, working scripts for the ROM SAF reanalyses are given here.
These are based on examples used for other reanalyses and they can be adapted easily to the users needs.