Category Archives: MARS

Getting ECMWF data

So, you’ve heard ‘ECWMF is far better than  NCEP’, and you want a piece of some ECMWF data action. Where do you get get it? How do you find it?.  Here’s a list to help you (me) out.

There are multiple routes to ECMWF data heaven. The most flexible and powerful, but most complicated is MARS. To use MARS fully you need the client, a piece of open-source software covered by a GPL licences, but costs £100. It is used to communicate with the data servers at ECMWF, and can subset data on the server side to reduce the volume of data needed to be transferred.  Quicker way to MARS is via  Web MARS.

Some links on MARS
www.ecmwf.int/services/computing/training/material/mars.pdf
www.ecmwf.int/publications/manuals/mars/practice/
www.ecmwf.int/publications/manuals/mars/practice/
www.ecmwf.int/services/archive/

The other route is the ECMWF data server. This gives you straight access, no software needed, no (not many) questions asked. It is actually just a linux server sitting outside the ECMWF firewall running MARS, but only allowing access to the disk archive, and not the tape archive.

To grab a dataset requires a request. Some fields used in the request are:

class ECMWF classification (od, rd, e4, …)
stream originating forecasting system(oper, wave, enfo, seas, …)
expver version of the experiment (01 operational, 11, aaaa)
domain area covered by the data (Global, Mediterranean, …)
system seasonal forecast operational system (1, 2, 3)
type type of field (an, fc, …)
levtype type of level (pl, ml, sfc, pt, pv)
levelist levels for the specified levtype (off if levtype=sfc)
param meteorological parameter (t, temperature, 130, 30.128)
number ensemble member (1, 2, …)
channel brightness temperature frequency band
diagnostic iteration sensitivity forecast products
frequency direction 2-d wave spectra products
product section,
latitude longitude
ocean products

An example of a MARS request is:


retrieve,
class = ei,
stream = oper,
expver = 1,
date = 20071201/to/20071231,
time = 00/06/12/18,
type = an,
levtype = sfc,
param = sd,
target = “era-int.200712.sd”

The request is used to build a MARS tree, which describes the data hierachy to work through.

Truncation is done before interpolation to save resources, and is done depending on the final grid specified. The mapping between the final grid and the resolution truncated to is:
Grid increment Truncation
2.5  ≤ Δ T63
1.5  ≤ Δ < 2.5 T106
0.6  ≤ Δ < 1.5 T213
0.4  ≤ Δ < 0.6 T319
0.3  ≤ Δ < 0.4 T511
0.15 ≤ Δ < 0.3 T799
0.09 ≤ Δ < 0.15 T1279
0.0   ≤ Δ < 0.09 T2047

To subset an area use the boundaries North/West/South/East.

Archived data is available to all registered users. Real-time data only to the big spenders!