M-climate, the ENS Model Climate
The M-climate is used in association with ENS forecasts, primarily to deliver the Extreme Forecast Index (EFI) and Shift of Tails (SOT) products and the 15-day ENS meteograms with a model climate (M-climate), to highlight significant forecast departures of 2m temperature, wind speed, cloudiness and precipitation from the norm for a given location and time of year.
To construct the M-climate, 9 consecutive re-forecast sets (covering a 5-week period) are used, the middle one of which corresponds to the preceding Monday or Thursday that is closest to the actual ENS run date. These re-forecast sets are each created using the same calendar start date for each of the last 20 years. Each set consists of 9 runs of an 11-member ensemble (a control and 10 perturbed members) run over the 15-day ENS forecast period. Therefore altogether 20 years x 9 runs x 11 ENS members = 1980 re-forecast values are available to define the M-climate for a given location at forecast intervals of 6hrs. Use of such a large number of runs is justified because for EFI and SOT computations it is essential to minimize sampling noise in the M-climate tails.
M-climate is updated twice a week, every Monday and Thursday, and is based on 00UTC runs only (there are no 12UTC re-forecast sets). The new files start to be used from the 00UTC run the next day. So if one compares, for the same lead-time, the M-climate quantile plots (eg for a Thursday 00UTC run), and a run 24hrs later, they will be slightly different. These limitations of twice weekly updates to the M-climate can be significant. These can be particularly evident in spring and autumn when mean temperatures are changing most rapidly day by day. The same M-climate set is used for 00UTC and 12UTC ENS runs in order to avoid inconsistencies between the validity period of the ENS and M-climate. As an example, we use the day1 and day2 M-climate distributions, with, respectively, the T+0-24h and T+24-48h forecasts from 00UTC runs, and the T+12-36h and T+36-60h forecasts from 12UTC runs.