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Large "increments" in geopotential, wind, temperature and humidity fields show where observations where observations have caused the caused the analysis to depart significantly from the background.  Often during manual analysis of model behaviour one will focus on upper level increments. Alteration  Alteration to the initial upper flow may well induce a corresponding modification in the evolution downstream as the forecast progresses. Particularly  Particularly large upper level increments, that which can occasionally be seen, are often associated with areas of substantial, organised convection.  Examples are:

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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.

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Fig4.2-3A(left):IFS background 1000hPa heights have been adjusted (purple) in response to observations. The arrows show the vector differences of the analysed 200hPa winds from the background 200hPa winds.  In this case a major distortion of the upper flow was not evident in the background field.

Fig4.2-3A(right): IFS background 200hPa heights have been raised (red) or lowered (blue) in response to observations.  The lines show anomalies of the analysed 200hPa height field from the background 200hPa height field.  In this case a low level background field has been adjusted into a less deep and more relaxed trough.

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The increments are defined as analysis minus background.  They show where observations have caused the analysis to depart significantly from the background.  The purple arrows are the vector differences between background and the analysis.  Both charts show large increments associated crements associated with a major convective system over the Southern southeastern United States.  These show where observations departed significantly from the background field and the IFS adjusted its analysis to a significant degree in response.

Fig4.2-33B(leftright):IFS background 1000hPa heights have 200hPa velocity has been adjusted (purple) in response to observations. The arrows show the vector differences of the analysed 200hPa winds from the background 200hPa winds.  In this case a major distortion of the upper flow was not evident in the background field.

Fig4.2-33B(rightleft): IFS background 200hPa heights have been raised (red) or lowered (blue) in response to observations.  The lines show anomalies of the analysed 200hPa height field from the background 200hPa height field.  In this case a low level background field has been adjusted into a less deep and more relaxed trough.

Taken together, Fig4Figs4.2-3 (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-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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