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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 ensemble 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 

Fig4.2-3(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.

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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.

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. 

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Fig4.2-5(right): Visible satellite imagery around midday UTC over India 2 June 2024. 

Deep active convection over South America

 Fig4.2-6: 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.

Example of downstream effects of large analysis increments

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.

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Fig4.2-9: 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.

Example of differing downstream effects

Sometimes greater uncertainty in a given area or region can be identified by the noting the differences in the downstream evolution between forecasts.  The weather in a downstream area can then be difficult to predict.

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Fig4.2-10: The plot shows the difference in 500hPa ensemble mean height initialised 00UTC 26 October 00UTC and 00UTC 28 October 00UTC.  At 26 October, T+168h the forecast has a trough over Scandinavia while 28 October, T+120h the forecast has something like a ridge, shown by a large negative (purple) difference.  The error can be traced back to north-eastern U.S.A and eastern Canada at 26 October, T+48h where compared with 26 October, T+0).  In this case it is difficult to identify severe weather or any significant analysis increments over that part of U.S.A.

The jump between the forecasts has a major impact on the forecast surface weather in Scandinavia.  The forecast of 26 October develops an intense low over Finland on Friday bringing strong winds and cold air with a cold snap over southern Scandinavia.  The forecast of 28 October develops a broader low and more zonal winds over southern Scandinavia with more neutral conditions.

Fig4.2-11: Comparison between the 26 October forecast and the corresponding analyses.  The 26 October forecast failed to capture the weak ridge over Scandinavia shown by a large negative (purple) difference from the analysis.  The 26 October forecast  was effectively a bust.

Incorrect analysis increments

Fig4.2-12: Large 100hPa increments assigned to 1000hPa are incorrect and will not be used.  The observations at these high altitudes above mean sea level are near 700hPa but the terrain following height levels of the model will suffer some modification.

(FUG Associated with Cy49r1)