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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.
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.
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, 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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