During November and December 1998, ten cyclonic disturbances over the North
Atlantic and Mediterranean area were studied. Features in potential
vorticity (PV) fields were tracked in the analyses and short-range forecasts
of the Spanish version of the HIRLAM model (HIRLAM-INM) and the ECMWF
model. They were superimposed on the corresponding Meteosat water vapour
(WV) images with the aim of ascertaining the qualitative relationship
between Meteosat WV imagery and PV fields that might be used to assess
NWP model behaviour. Two cases predicted incorrectly by HIRLAM-INM were
studied in detail: a rapidly deepening depression over the Atlantic
(26–27 November 1998) and a cut-off low process over the western
Mediterranean (3–4 December 1998). For evaluating HIRLAM-INM model
behaviour, absolute vorticity at 2 PVU surface, positive PV anomalies
at 500 hPa as well as pseudo WV images were compared with Meteosat WV
pictures. Being synthetic products of the numerical model, the pseudo
WV images were used to indicate the strength of the relationship between
the Meteosat WV imagery and the PV fields, and to indicate whether any
mismatches correspond to real NWP model errors. It is demonstrated that
the HIRLAM-INM phase error at 3–4 December 1998 was clearly seen in the
comparison between Meteosat and pseudo WV images.