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 medium range 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 medium range forecasting systems; they have two applications:
Re-forecasts are made every four days during each month on 1/5/9/13/17/21/25/29 (excluding 29 February). These are the re-forecast dates for medium range.
The set of re-forecasts for the M-climate s 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 at forecast intervals of 6 hours for each forecast parameter, forecast lead-time and for each calendar start date and location. They are used to define the M-Climate.
Running on fixed days of the month allows direct comparison between re-forecasts produced at different resolutions and/or in different years.
The M-climate is used in association with the ensemble forecast:
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:
In Cy49 (introduced in autumn 2024) and later, re-forecasts are made every four days during each month on 1/5/9/13/17/21/25/29 (excluding 29 February). There remain problems using each newly updated M-climate, particularly in spring and autumn when mean temperatures change most rapidly day by day.
In Cy48 and earlier (prior to autumn 2024) the re-forecasts were run twice per week with M-climate updates on Mondays and Thursdays. There were problems using each newly updated M-climate, particularly in spring and autumn when mean temperatures change most rapidly day by day.
Cy41r1 (introduced in spring 2015), the M-climate was constructed from only 500 re-forecasts was more prone to sampling errors and as a result.
ECMWF uses different reference periods but essentially the same re-forecast runs to build the M-Climate and the SUBS-M-Climate. The key difference is that those runs are grouped and used in different ways:
(FUG Associated with Cy49r1)