Forecasting vortex filaments

Abstract

The accuracy of stratospheric forecasts from the United Kingdom Meteorological Office's (UKMO) assimilation system in the Southern Hemisphere (SH) are studied primarily for a period in October 1994 and also February 1995. Conventional root mean square error (RMSE) calculations for different regions show that stratospheric forecasts are a large improvement over persistence in October 1994 (SH winter) even at five days but not so during February 1995 (SH summer). Systematic errors in the temperature and zonal wind fields were found to occur in relation with the stratopause and polar jet respectively. Studies also show that in general the vortex minimum temperature is forecast too cool and the maximum wind in the polar jet is forecast too strong. An advection scheme on specialised parcel location fields is used to study the differences in the meridional component of the wind vector with results indicating the forecast winds are highly consistent with the analysed winds even after five days in most cases. A back-trajectory mapping technique is employed to generate high-resolution maps of isentropic potential vorticity to permit the study of small-scale structure. The overall structure in a total hemisphere field produced from forecast winds is very similar to that from analysed winds even for filamentary structure near the polar vortex. Qualitative comparisons of aircraft measured tracer structure during the Airborne Southern Hemisphere Ozone Experiment (ASHOE) 1994 with structure from the high-resolution potential vorticity maps shows that large-scale features are represented well by the back-trajectory mapping technique with possibly less success for small-scale structure.