Analysis increments charts indicate areas where the analysis scheme may not have completely captured the true state of the atmosphere. They show where observations have led to a modification to the field. Hopefully this modification results in an improvement. But the increments do not suggest that a perfect description of the field has been attained. In any case, new data anywhere, unless fully rejected, will always induce some, usually slight, changes to the structure of the IFS model atmosphere.
The influence of any analysis modification by latest data, good or bad, often travels faster downstream than the synoptic systems themselves. For example, for forecasts over Europe:
- a two-day forecast may be affected by initial conditions over most of the North Atlantic.
- a five-day forecast may be affected by the initial conditions over the North American continent and easternmost North Pacific.
There is also an ever-present influence from the subtropical and tropical latitudes. In particular where:
- subtropical depressions, tropical storms or hurricanes enter the mid-latitude westerlies (extra-tropical transitions) or
- large areas of severe convection modify the upper flow (e.g. as in MCS over the United States).
The energy of the distorted flow propagates downstream faster than the ambient winds. Typically, but very roughly, the energy progresses at about 30° longitude per day. However, case studies show that the value can be very different in individual cases .
It is important to assess the potential influence of any inadequately resolved features and the subsequent downstream evolution and consequences. A judgement of the medium-range performance should be based upon large upstream areas and also involve the upper-air flow. A good or poor short-range forecast in an area does not mean the medium-range forecast there is necessarily similar.
Fig4.3-1: Schematic illustration of the typical propagation of forecast errors over the northern hemisphere towards Europe in situations with generally zonal flow. The errors propagate mainly along the storm track, which during the warm season, on average, is displaced polewards. Note however that in individual cases the propagation speed can vary greatly from what is shown here.
Jumpiness or errors in the forecast at Day3 over Europe typically have their origin over the eastern or north-eastern parts of the North American continent. Jumpiness or errors at Day5 over Europe often have their origin over north-eastern North Pacific and adjacent coastal regions. In rare cases, forecast failures at Day7 have been traced back even further. The Madden-Julian Oscillation can also have a significant downstream effect. During all seasons, but particularly in summer and autumn, forecast errors associated with disturbances in the tropics or subtropics can move into the zonal westerlies (shown schematically by the green arrows). Extra-tropical transition of low-latitude weather systems into the mid-latitudes can often cause substantial differences in subsequent evolution and significant forecast variability in forecasts over European areas.
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