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-forecasts have the same resolution (currently 9km) and period (15 days) as the medium range ensemble.
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:
- medium range forecast verification metrics are based on the re-forecasts
- re-forecasts allow computation of the M-climate.
Selection of medium range re-forecasts
The set of re-forecasts is made up from:
- a set of re-forecasts using the same calendar start dates for each of the last 20 years. Re-forecasts are made every four days during each month on 1/5/9/13/17/21/25/29 (excluding 29 February).
- nine consecutive re-forecasts (covering a 5-week period). The middle one corresponds to the preceding date that is closest to the actual ensemble run date.
- each re-forecast is from an 11-member ensemble (1 control and 10 perturbed members) run over the 15-day ensemble forecast period.
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:
- to present the 15-day ensemble meteograms with the medium range climate (M-climate)
- to deliver the extreme forecast index (EFI) and shift of tails (SOT) products
- to highlight significant forecast departures of variables from the norm for a given location and time of year.
Values evaluated in M-climate
- 2m temperature.
- soil temperature.
- sea-surface temperature.
- mean sea level pressure.
- precipitation.
- cloudiness.
The same M-climate set is used for 00UTC and 12UTC ensemble runs. This is to avoid inconsistency between the validity period of the ensemble and M-climate. So, for example:
- Day1 M-climate is used with T+12h to 36h forecasts from the 12UTC run and T+0h to 24h forecasts from the 00UTC run.
- Day2 M-climate is used with T+36h to 60h forecasts from the 12UTC run and T+24h to 48h forecasts from the 00UTC run.
Updates to M-climate
In Cy49 and later, re-forecasts are made every four days during each month on 1/5/9/13/17/21/25/29 (excluding 29 February). Using fixed days of the month allows direct comparison between re-forecasts produced with different resolutions and/or in different years.
Mean temperatures change most rapidly day by day during Spring and Autumn. This can cause problems using newly updated M-climate, particularly in these seasons.
In Cy48 and earlier re-forecasts were run twice per week with M-climate updates on Mondays and Thursdays. The interval between creation of re-forecasts remained similar to that in Cy49 bringing similar problems in Spring and Autumn.
Cy41r1 in spring 2015, the M-climate was constructed from only 500 re-forecasts was more prone to sampling errors and as a result.
Different reference periods for M-Climate and SUBS-M-Climate
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:
- For shorter ranges, the priority is the best possible capture of the climatological distribution of the tails (e.g. extreme forecast index (EFI) and shift of tails (SOT)).
- For longer ranges, the priority is the correct representation of seasonal cycles.
(FUG Associated with Cy49r1)
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