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• across North America, notably the centre of the USA (e.g. Fig4Fig42.2.3 C & Fig4Fig42.2.5E), indicating perhaps that the model has underestimated the jet stream level winds at short time ranges. This is often associated with large, deep and energetic convective outbreaks, the outflow from which distorts the upper flow. The subsequent forecast of upper flow can differ significantly from earlier forecast runs - e.g. a downstream upper ridge may amplify and/or a perturbation may propagate downstream through the jet. Convective outbreaks over the central United States have thus been known to be "responsible" for lower forecast skill across Europe.
• over western Africa, again related to an area of convection (e.g. Fig4Fig42.2.6G).  The westward propagation of this type of feature over the Atlantic can be an ingredient in tropical cyclone development (African Easterly Waves).
• over South America, notably Argentina, where extreme convection is quite commonplace during the summer half of the year

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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. Figs 4.2.5A and Fig42.A & Fig42.B).

Fig4Fig42.2.1A: To view Analysis Increments:

1. On Charts page, enter Analysis Increments.
2. Click on Analysis Increments diagram.. Display of product appears - increments (differences from the IFS background) for 1000hPa height. 
   
3. As desired select other base times or level -  increments (differences from the IFS background) for 200hPa winds. 

Fig4Fig42.2.2B:  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.  Meso-scale 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 has been modelled with significant downstream differences at a later time in consequence.  It is helpful to note the development of energetic and fairly large convective systems or strong dynamic upslope motions in warm front conveyors and assess the possible impact on IFS performance.

Fig4Fig42.2.3A C1 (left): Large increments associated with a major convective system over the Southern United States highlight where observations departed significantly from the IFS background, and where the IFS adjusted its analysis to a significant degree in response. The arrows are vector differences from the IFS background 200hPa winds showing a major distortion of the upper flow has occurred.

Fig4Fig42.2.3B C2 (right): The lines show anomalies of the anaysed 200hPa height field from the background 200hPa height field, (red means IFS background heights are too low and the analysed heights have been raised in response to observations; blue means IFS background height are too high and the analysed heights have been lowered in response to observations).  In this case the anomalies suggest a less deep and more relaxed trough.

Taken together, Fig 4.2.3A and Fig4.2.3B Fig42.C1 and Fig42.C2 show a pattern typical of spring and early summer over the USA, when MCS activity is significant.  Often the IFS model will under-repesent the associated net upward mass flux (in convective updraughts), which in turn manifests 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 released in the updraughts) and this is commonly reflected as positive (red) upper level height increments.

 Fig4Fig42.2.4D: 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.

Figs4Figs42.2A & Fig42.5A and B show an example where large increments over the mid-West of the USA have induced differences in the forecast upper flow over East Canada two days later and over Europe five days later.

Fig4Fig42.2.5AE: 200hPa wind increments at 00UTC 28 Aug 2019.  The large differences near 90W-95W indicate observations depart significantly from the IFS background based upon the previous forecast run 12hrs earlier.

Fig4Fig42.2.5BF: Forecast 500hPa heights based on 00UTC 27 Aug 2019 (red) and 00UTC 28 Aug 2019 (black).  These compare the evolution of 500hPa heights before and after incorporation of observations over the USA at 00UTC 28 Aug which departed significantly from the IFS background at that time.  The analysis at 00UTC 28 Aug has been adjusted significantly in order to better agree with the observations. The difference in 500hPa height between the analysis at T+0 and that from a previous run at T+24 (both verifying at 00UTC 28 Aug) is highlighted by the yellow/blue "dipole" over the eastern USA.  The subsequent evolution differs from the evolution of the earlier forecast run, first in the handling of the upper ridge over eastern Canada and then in the downstream trough moving over Europe at day5. This is an example in which differences moved and developed downstream, but did not grow substantially. Very occasionally, sequences of this type, following large increments, can show substantial non-linear downstream growth of the differences between the previous and current forecasts.

Fig4Fig42.2.6G: 200hPa wind increments at 00UTC 26 Aug 2019.  The large differences over West Africa indicate that observations depart significantly from the IFS background based upon the forecast run 12hrs earlier. 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, and can be caused by there being insufficient upward net mass flux in the convecting area, which 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 a problematic area for the IFS.

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