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• the background field was possibly incorrect and the new observations have been used to improve the analysis. Or
• the observations were possibly incorrect and may have incorrectly influenced the analysis.  ECMWF "blocklists" observations or observation types if this happens repeatedly.  Blocklisted observations are excluded from the analysis until its the quality improves.

In either case, the subsequent evolution should be carefully monitored, or even treated with suspicion, as the instability in the structure of the IFS atmosphere transfers downstream.  In some cases there can be jumpiness in the forecast conditions for several days later at locations well away from the initial differences.  For this reason it can help to inspect the analysis increment data before committing to a forecast.

Large increments in many variables are also sometimes seen where observations become available near a vigorous pressure system (e.g. dropsondes near a hurricane).  These indicate there were shortcomings in the background forecast of the feature that the analysis system is trying to reduce.

There are some model uncertainties in representing tropical convection - not deep/strong enough, with resulting weak outflow. 

Large increments are often associated with MCAs or areas of deep active convection and/or intense cyclogenesis.   These often occur over southern USA in association with:

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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.

 Fig4.2-4: Analysis increments show the 200hPa vector differences in (purple) and height (red) between the IFS analysis and the IFS background.  The red areas show where the IFS background height was too low compared with observations.  Consequently 200hPa heights (black lines) have been raised in the region and the trough near and just to the west of the mass of active convective cloud is sharpened.

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Fig4.2-7: 200hPa wind increments at 00UTC 26 Aug 2019.  The large differences over West Africa indicate that observations depart significantly from the IFS background. The structure of these increments implies that divergence is being "added" to the upper level flow. This is a relatively common occurrence in convective regions.  It can be caused by insufficient upward net mass flux in the convecting area.  This in turn may be because the model's convection is insufficiently vigorous and/or organised.  MCS development commonly relates to this and is known to be problematic for the IFS.

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Fig4.2-8: Large 100hPa increments assigned to 12000hPa are incorrect and will not be used.  The observation at 3200M above mean sea level is near 700hPa but the terrain following height levels s of the model will suffer some modification.