A. Displacements

Features in simulated WV images are displaced relative to those in real WV images when both images show the same pattern but in different geographic positions. This might be an indication that the model physics have basically grasped the synoptic situation, but either not in the correct place or not at the correct time step.

Displacements for example may occur when the centre of an upper level low is located on different positions when comparing both data sources. For a rapidly propagating cold front on the other hand, a time shift may be the manifest reason for differences in the position of the frontal zone. In practical work, the comparison with a WV loop can be of help to discriminate between time shift and misallocation. When the position of the WV feature in simulated WV imagery lies in the path of propagation, the model lagging or being too fast is a more likely explanation for the discrepancies.

Figure 2 shows the typical case of the model lagging behind the real development for a warm front located over the Benelux and western part of Germany. Comparison between MSG WV channel and the corresponding simulated WV image from ECMWF shows the propagation of the warm front much further to the south-east than model data would suggest.

The same time shift is visible for the cold front's passage over southern Ireland. While in reality the cold front cloud band had passed the island by 06:00 UTC, model data still places it over southern Ireland.

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Figure 2a and 2b: Simulated (background) and real (overlay) WV 6.2 µm satellite image from 22 December 2012 at 06:00 UTC.

A comparison of real and simulated WV images clearly shows the time shift of the model data compared to the actual situation.

The second example (figure 3) shows a pronounced upper level low (ULL) over western France. The WV satellite image from 18:00 UTC on 11 February 2013 shows the dark centre of the ULL inland near Bordeaux, while the 500 hPa geopotential isolines show a pressure minimum off-shore and more to the north. When comparing the simulated WV image of the same time stamp with model 500 hPa height, a better match is visible.

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Figure 3a and 3b: Simulated (background) and real (overlay) WV 6.2 µm satellite image from 11 February 2013 at 18:00 UTC.