Ensemble averaging provides more than simple spatial averaging. It is generally true that larger scale features are more predictable than smaller scale features. However, there are lots of variations on this theme. Barring model errors for a moment, predictability of a particular feature depends on two factors: (1) initial value related unceratinty in the upstream area that affects the forecast feature at verification time; and (2) the instabilitiy of the flow that connects the upstream area and verifying feature - i. e., all the dynamics connecting the initial and final time features. (Note that targeted observations, where data are collected in areas that influence particular downstream features, is a discipline devoted to studying these temporal/spatial connections.)
The propagation of forecast uncertainty in time and space is a very complex problem that leads from time to time to highly predictable smaller scale features and poorly predictable larger scale features. The ensemble, through its perturbed forecasts, can capture these variations and its mean will selectively filter out those unpredictable features that vary from member to member while retain those predictable features that are lined up in most member forecasts. Indiscriminate spatial filtering will remove some predictable smaller scale features while retaining some unpredictable larger scale features.
An example of a highly predictable smaller scale low pressure wave, verifying at 01101700 near the Great Lakes can be seen on a 168 hour lead time ensemble mean and Relative Measure of Predictability (RMOP) chart. The smaller scale wave south of the Great Lakes is well predicted at shorter lead times as well (144 and 120 hours). However, the considerably larger scale waves over the western half of the US and the eastern Pacific were well predicted only at 96 hours and shorter lead time. The red area associated with the short scale wave near the Great Lakes at all lead times in the above figures suggests that most individual ensemble members forecast this feature. This is confirmed by, for example, the 144 hour lead time 500 hPa 5520 m height spaghatti chart. Note on the lower right panel that all NCEP ensemble members gave a shorter scale low pressure wave around the Great Lakes whereas over the Canadian Rockies and the Gulf of Alaska the individual ensemble members show larger scale waves 180 degrees out of phase (troughs and ridges in the same area). The large uncertainty in these areas correspond with generally bluish colors and low probability values on the RMOP charts.
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