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Users should inspect upper level wind and height increment charts to identify potential sources of significant changes in the downstream evolution (e.g. Fig4.2-2, Fig4.2-3, Fig4.2-6).

Fig4.2-1: To view Analysis Increments:

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Fig4.2-2:  Rapid growth of uncertainty (in the background forecasts of the Ensemble of Data Assimilations (EDA)) for PV on the surface where potential temperature=315K (shaded as scale).   Also shown are the CTRL forecast PV=2 on 315K (red contour) and 850hPa wind vectors, and ensemble mean precipitation (dots; size indicates rate).   Rapid growth of uncertainty can be associated with cyclogenesis and warm conveyor-belts.  Mesoscale convective systems (e.g. over USA) can also distort the upper flow significantly.  The ENS perturbations may not capture such rapid growth adequately and the upper flow may well become modified more than modelled.  This can cause significant downstream differences at a later time in consequence.  Energetic, fairly large convective systems or strong dynamic upslope motions in warm front conveyors can have an impact on IFS performance.

Both charts show large increments associated with a major convective system over the Southern United States.  These show where observations departed significantly from the background field and the IFS adjusted its analysis to a significant degree in response.

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Taken together, Fig4.2-3(left) and Fig4.2-3(right) show a pattern typical of spring and early summer over the USA, when MCS activity is significant.  Often the IFS model under-repesents represents the associated net upward mass flux (in convective updraughts).  This in turn shows itself as a lack of divergence at upper levels where the updraughts spread out.  The upper level increments then look divergent as a result.  At the same time the upper level height field may not be high enough (due to latent heat released in the updraughts) .  This is commonly indicated as positive (red) upper level height increments.

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