Ensemble mean and spread charts

Special composite maps have been created to facilitate comparisons between the EM and the higher resolution deterministic forecast. Such maps normally show great consistency from one forecast to the next and can help forecasters judge how far into the future the EM can carry informative value for large synoptic patterns. The spread refers to the uncertainty of the values of geopotential height or temperature, not necessarily to the flow patterns. The four parameters that can be selected for display are:

·         Pressure reduced to mean sea level (labelled in 5 hPa intervals)

·         Wind speed at 850 hPa isobaric surface

·         Temperature at the 850 hPa isobaric surface   

·         Height of the 500 hPa isobaric surface

The ensemble spread tends to show a strong geographical dependence. Generally, it takes low values at low latitudes, where the variation of the above parameters is low, and high values at mid-latitudes, where the variability is higher. Since this latitude dependence tends to obscure the particularities of the current situation, a normalised standard deviation (Nstd) has been defined as

Nstd = Std/Mstd

where Mstd (the mean standard deviation) is a pre-computed field that represents the mean of the standard deviations of the 30 most recent 00 UTC ECMWF ensemble forecasts. It is a function of lead time and geographical location. The Nstd aims to highlight geographical areas of unusually high or low variance, where the uncertainty is larger or smaller than over the last 30 days.

If the spread in a particular area, for example at day 5, seems to be large but has recently tended to be equally large in the same area, then the Nstd is ≈ 1. Conversely, if the spread exceeds the spread that has been seen recently, then the Nstd is > 1.