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- across North America, notably the centre of the USA (e.g. Fig4.2-3 & Fig4.2-5),suggesting underestimation of the jet stream winds at short time ranges. This is often associated with Often the outflow from large, deep and , energetic convective outbreaks , the outflow from which distorts the upper flow. The The subsequent forecast of upper flow can differ significantly from earlier forecast runs - e. g. a 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. Fig4.2-7). 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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Large increments in many variables are also sometimes noted seen where observational data has observations become available in the vicinity of near a vigorous pressure system (e.g. dropsondes in the vicinity of near a hurricane). These indicate there were shortcomings in the background forecast of the feature that the analysis system is trying to reduce.
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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. It is helpful to note the development of energetic and Energetic, fairly large convective systems or strong dynamic upslope motions in warm front conveyors and assess the possible can have an impact on IFS performance.
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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 will under-repesent represents the associated net upward mass flux (in convective updraughts), which . This in turn manifests 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) and this . This is commonly reflected indicated as positive (red) upper level height increments.
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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.
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.
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