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The Web API provides a way to access ECMWF's data archives in batch from outside ECMWF (users working at ECMWF or through ecgate should use their normal local MARS access). There are two services at present:

• Access to the ECMWF Public Datasets requires self-registration and allows access to the datasets described here
• Access to the MARS archive is only available to registered users

See ECMWF Web API Home for more information, including how to set up the service by installing an ECMWF key. Metview versions from 4.4.3 onwards have support for this service (but version 4.4.7 is recommended). See the Releases page for download.

Setup

First, whichever service you want, ensure that you have access to this service it by following the steps in Access ECMWF Public Datasets. Ensure that you have set up your ~/.ecmwfapirc key and that you are registered for the datasets you wish to access. If you can retrieve some data using a sample script provided there, then you are 90% of the way to accessing the service from within Metview. It is not required to install the Python client libraries, but it can help with testing the service before trying it with Metview.

Second, ensure that your Metview installation was built with Mars Web Access enabled. When configuring with CMake (Metview version 4.5.x), Mars Web Access will be automatically enabled if the curl development library is found on the system. When configuring with autotools (Metview version 4.4.x), the option is --enable-mars-web-access and is enabled by default. The only reason for disabling this option is if your system does not have the curl development library (libcurl) installed.

Using

Data can be retrieved using the Mars Retrieval icon or its Macro language equivalent, the retrieve() function. Either start with a supplied example icon, or create a new instance, either from the icon drawers at the bottom of the Metview desktop, or from the New Icon context menu. Edit the icon by selecting Edit from its context menu (or double-click the icon).

The important parameters parameter to set in order to use the Web API are Database (should be set to 'Data Server' for the public datasets, or 'Web API' for the core MARS archive) and Datasetpublic datasets is the Dataset parameter. Otherwise, the retrieval should be as described in the Web API guide. It may be useful to create a Mars Retrieval icon with the Database parameter set, and put it into an icon drawer (at the bottom of the Metview desktop) or the Mars Retrieval icon template drawer for future re-use.

To perform the retrieval, use the context menu of the icon: Execute will retrieve and cache the data; Visualise, Examine and Save will do the same, but will then perform more actions on the data. Please see the introductory Metview Tutorials for more information on how to use these features of Metview.

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Users who have started Metview for the first time with version 4.4.x onwards should already have some example icons and a macro in their Getting Started folder. The icons are also available for separate download - untar these into a Metview folder to start exploring them.

Note that in Metview versions prior to 4.5.5, it was necessary to add an additional parameter to the request:

 DATABASE = Data Server

This is no longer necessary, as Metview will automatically use the Web API if no local MARS connection is available.

Example retrieval for TIGGE data (icon parameters)

The following is the text behind an example Mars Retrieval icon for requesting TIGGE data (in this case, surface pressure - param 134). Either copy the values shown into the appropriate places in the icon editor, or switch the icon editor to Text View mode and copy the request in its entirety.

RETRIEVE,
    DATASET    = tigge,
    CLASS      = ti,
    EXPVER     = PROD,
    TYPE       = PFCF,
    LEVTYPE    = SFC,
    PARAM      = TP134,
    DATE       = 2016-06-12,
    TIME       = 20071001'00:00:00',
    STEP       = 12/18,
    ORIGIN  NUMBER   = ALLrjtd,
    GRID       = 10.5/10.5

Example retrieval for core archive data (icon parameters)

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# Metview Macro

#  **************************** LICENSE START ***********************************
# 
#  Copyright 20142016 ECMWF. This software is distributed under the terms
#  of the Apache License version 2.0. In applying this license, ECMWF does not
#  waive the privileges and immunities granted to it by virtue of its status as
#  an Intergovernmental Organization or submit itself to any jurisdiction.
# 
#  ***************************** LICENSE END ************************************
# 

era_data = retrieve
(
    dataset  : "interim",
    class stream   : "eioper",
    type     : "fc",
    class    : "ei",
    levtype  : "sfc",
    param    : "235.128",
    date     : 201305012013-05-01,
    step     : 12,
    gridtime     : [1.5,1.5]12
)

plot(era_data)

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creating-mars-icon creating-mars-icon

Creating a Mars Retrieval icon

The following images are taken from the new Desktop user interface, but the procedure is the same with the classic user interface. Note that prior to version 4.4.7, the New Icon menu did not allow the creation of Mars Retrieval icons outside ECMWF; in this case, one can be found in the Data Access icon drawersteps show how to use Metview's user interface to create and edit a new Mars Retrieval icon.

Right-click / Create new Icon	2. Select Mars Retrieval
3. The new icon appears on the Metview desktop	4. Edit the icon to set the retrieval parameters
	Image RemovedImage Added