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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.
Examples of large analysis increments
Large increments in geopotential, wind, temperature and humidity fields show where observations have caused the analysis to depart significantly from the background.
Such increments are frequently associated with deep and very active convection, often with meso convective systems. Often the IFS model struggles to forecast the intensity of organised convection and under-represents the associated net upward mass flux in the convective updraughts. This in turn shows itself as a lack of divergence at upper levels where the updraughts spread out. The analysis process tries to remedy this by incorporating information from observations by warming up the atmospheric column and increasing diffluent motions at the tropopause. The difference between the initial background fields and the final analysed fields are shown by the increments. These commonly are indicated as positive (red) upper level height increments and increasing diffluent motions at the tropopause. The upper level increments then look divergent as a result.
Deep active convection over southern and southeastern United States
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. deep and more relaxed trough.
Taken together, 3 and 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.
3
The diagrams, Fig4.2-3 & Fig4.2-4, 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.
Deep active convection over India
Widespread active convective activity over the Indian subcontinent can also be a cause of large increments. Increments can be at low levels as well as in the upper troposphere.
Fig4.2-5(left): 1000hPa analysis increments over India 12UTC 2 June 2024. Here surface heating and strong convection have induced lower 1000hPa heights (and surface pressure).
Fig4.2-5(right): Visible satellite imagery around midday UTC over India 2 June 2024.
Deep active convection over South America
Fig4.2-46: 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.
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.
Example of downstream effects of large analysis increments
Large increments Figs4.2-5 & Fig4.2-6 show an example where large increments over the mid-West of the USA . This has have induced differences in the forecast upper flow over East Canada two days later and over Europe five days later.
Fig4.2-58: 200hPa wind increments at 00UTC 28 Aug 2019. Large differences near 90W-95W indicate observations depart significantly from the IFS background.
Fig4.2-69: 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 that 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.
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.
Incorrect analysis increments
Fig4.2-810: Large 100hPa increments assigned to 12000hPa 1000hPa are incorrect and will not be used. The observation at 3200M above mean observations at these high altitudes above mean sea level is are near 700hPa but the terrain following height levels s of the model will suffer some modification.
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