Three existing classi®cation schemes of daily circulation patterns are considered: (i) the subjective Grosswetterlagen; (ii) an objective scheme, the Jenkinson classi®cation, which produces weather types similar to the subjective Lamb classi®cation for the British Isles; and (iii) the objective P27 classi®cation scheme developed at the Royal Netherlands Meteorological Institute. The comparison between these schemes is based on the mean-squared-error skill score (percentage of explained variance) and the correlation coef®cient between the observed and predicted values. The relationship between these performance measures is examined.
For the Grosswetterlagen and the P27 classi®cation the class averages of the daily temperatures at De Bilt explain about 40 per cent of the variance of the daily temperatures at that location in the period 1949±1993. The corresponding ®gure for the Jenkinson classi®cation is 29 per cent. The Grosswetterlagen perform less well for the daily temperatures prior to 1949, when upper-air data were not available to de®ne the daily weather types.
For daily precipitation characteristics the three classi®cation schemes perform almost equally well. The class averages of the daily amounts explain 13 (summer season) to 20 per cent (winter season) of the variance of the local precipitation at De Bilt and about 25 per cent of the variance of the area-average precipitation over The Netherlands. A somewhat higher skill is achieved for precipitation occurrence. There is some evidence of systematic changes in the relationships between rainfall characteristics and weather types.
The class averages of the daily values have been used to estimate monthly values. Especially for the precipitation characteristics the estimates of the monthly values have higher skill than those of the daily values. Nearly 60 per cent of the variance of the monthly area-average rainfall over The Netherlands is explained. Almost the same skill is achieved with a regression on the monthly number of cyclonic and anticyclonic days or a regression on the monthly mean vorticity index in the Jenkinson classi®cation.