Creation of M-Climate

The M-Climate is derived from a set of medium range re-forecasts.  These are created using the same calendar start dates over several years for data times either side of the time of the ensemble run itself.  The re-forecast runs are at the same resolution as the medium range ensemble (currently 9km) and run over the 15-day medium range ensemble period.   

There is merit in examining the real-time performance of a forecasting system.   But the sample sizes created for one system are far too small to conclude anything about its true performance levels.  Re-forecasts are used to increase the available data to produce a model climate.   The results of forecast system may be compared with this model climate.  

Re-forecasts are a fundamental component of all seasonal forecasting system; they have two applications:

Selection of medium range re-forecasts

The set of re-forecasts is made up from:

In total, each set of re-forecasts consists of 20 years x 9 runs x 11 ensemble members = 1980 re-forecast values.  These are available for each forecast parameter, forecast lead-time, calendar start date, location, at forecast intervals of 6 hours. 

The M-climate is used in association with the ensemble forecast:

Values evaluated in M-climate

The same M-climate set is used for 00UTC and 12UTC ensemble runs.  This is to avoid inconsistencies between the validity period of the ensemble and M-climate.  So, for example:


Limitation of twice weekly updates to M-climate

M-climate is updated twice per week on Mondays and Thursdays.  So M-climate quantile plots for the same lead-time from two forecast runs on e.g. 00UTC Thursday and 00UTC Friday will be slightly different.  This limitation of twice weekly updates to the M-climate can be significant.  It can be particularly evident in spring and autumn when mean temperatures are changing most rapidly day by day.

Different reference periods for M-Climate and ER-M-Climate

ECMWF uses different reference periods but essentially the same re-forecast runs to build the M-climate and the ER-M-climate.   The key difference is that those runs are grouped and used in different ways:  


Note before Cy41r1 in spring 2015, the M-climate was constructed from only 500 re-forecasts was more prone to sampling errors and as a result.