Joint Centre for Mesoscale Meteorology (JCMM)
Use of water vapour imagery and PV inversion to adjust initial conditions in NWP
by Sean J Swarbrick
It is considered generally desirable to make greater use of satellite imagery in the generation of NWP analyses. This project is exploring one possible way of doing this, using the Meteosat water vapour channel. The technique attempts to utilise the alleged relation between WV imagery and upper-tropospheric PV fields. This relation, if properly understood, should facilitate the detection of errors in the initial state of a forecast.
The PV field on an appropriate isentropic or isobaric surface, at the initial forecast time, is compared with the corresponding WV image in order to reveal errors in the PV field. The PV field is then adjusted accordingly, and PV inversion is used to derive wind and temperature adjustments from the PV adjustments. The PV adjustments may involve scaling and/or displacement of PV anomalies, on a range of pressure levels. The resulting wind and temperature adjustments are applied to the initial forecast fields, and the forecast is rerun from the adjusted initial state.
A number of case studies have been carried out, and some positive impacts have been achieved, although the results as a whole seem to suggest that upper-tropospheric PV adjustment by itself may not lead to a positive impact in general.
It has also become apparent that the relation between PV fields and water vapour images is not well understood, and that a correct understanding of this is essential to further progress. Some significant qualitative insights on this issue have emerged.