Dw. Waugh et al., PREDICTIVE SKILL OF AN NWP SYSTEM IN THE SOUTHERN LOWER STRATOSPHERE, Quarterly Journal of the Royal Meteorological Society, 124(551), 1998, pp. 2181-2200
The predictive skill of the Australian Bureau of Meteorology's Global
Assimilation and Prediction (GASP) system in the southern lower strato
sphere is examined using two different sets of diagnostics: (i) conven
tional verification statistics used in numerical weather-prediction st
udies (namely, root-mean-square (RMS) error, anomaly correlation, and
bias), and (ii) elliptical diagnostics of the polar vortex (defined us
ing potential vorticity on isentropic surfaces). Both sets of diagnost
ics indicate the same variation in predictive skill for forecasts duri
ng October 1994. The stratospheric forecasts are a large improvement o
ver persistence even at seven days, with the performance at seven days
being comparable to that in the troposphere of three-day forecasts. T
here is large daily variability in the forecast scores for seven-day f
orecasts, and the days with below-average scores occur when the flow (
vortex) is rapidly changing. Examination of the differences in the ell
iptical diagnostics show that the forecast vortex is weaker, less dist
urbed (i.e. closer to the pole and less elongated), and rotates faster
than the analysed vortex. Consistent with a weaker forecast vortex, t
he minimum polar temperature and maximum zonal wind are underpredicted
in the forecasts. The verification statistics in the stratosphere hav
e a large seasonal variation, although the variation is different for
different statistics. The GASP RMS errors are largest (smallest) in la
te-spring (summer) whereas both the ratio of GASP to persistence RMS e
rror and the anomaly correlation indicate that the performance relativ
e to persistence is best (worst) in late-spring (summer).