Creation of M-Climate

The M-Climate is derived from a set of medium range re-forecasts created using the same calendar start dates over several years for data times either side of the time of the extended ensemble run itself.  The re-forecast runs are at the same resolution as the medium range run itself (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 in order to avoid inconsistencies between the validity period of the ensemble and M-climate (e.g. day1 M-climate and day2 M-climate distributions are used with respectively, the T+0 to 24h and T+24 to 48h forecasts from 00UTC runs, and the T+12 to 36h and T+36 to 60h forecasts from 12UTC runs).

If for the same lead-time, one compares, the M-climate quantile plots (e.g. for a Thursday 00UTC run), and a run 24hrs later, they 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.