Use of potential vorticity fields, Meteosat water vapour imagery and pseudo water vapour images for evaluating numerical model behaviour

Abstract

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.