Ensemble

a collection of NWP runs (typically in excess of 15) from the same start time (t=0) and using the same model physics, but each run (or 'member') having a slightly perturbed (altered) set of initial conditions. The alterations are constrained within limits which are calculated in various ways - one example being that of performing a separate short-range model run and identifying the errors that would grow most over a 48 hr period. These errors are then applied in varying amounts to the initial conditions before performing the operational ensemble run. Another technique is to use (known) errors from a previous run and applying these in small amounts to the initial conditions of the new run. Ensemble mean: an average of the ensemble output from a particular computer run - this is usually more accurate than just following one of the individual forecasts that make up the average. Further, by comparing the individual members 'spread' about the mean, some estimate can be made of the reliability of the forecast: if there is strong agreement and therefore small divergence from the mean solution, then high confidence can be assigned to the average solution. Wide divergence, or clustering of groups of individual members well away from the mean will lead to considerable caution regarding using the output too slavishly and lower confidence in issued forecasts.