ESTIMATES OF SPATIAL DEGREES OF FREEDOM

The spatial degrees of freedom (dof) of atmospheric flows are estimate d by comparing the variance of the theoretical standardized chi-square d distribution with the sum of the squared eigenvalues of a spatial co rrelation matrix, dof = N-2/Sigma(i=1)(N) lambda(i)(2). The dof statis tics are applied to monthly anomalies of ten-year datasets using daily 1000-mb heights of the T-21 representation of observations (ECMWF ana lyses 1980-89) and model simulations (ECHAM2) for the Northern Hemisph ere mid- and higher latitudes (NH) and the eastern North Atlantic/Euro pean sector. Scales are distinguished by using unfiltered, low-, and b andpass filtered datasets. The following results are of interest: (i) The dofs of the observations are in qualitative agreement with the num ber of distinct weather types defined by phenomenological studies of t he synoptic climatology on the hemispheric and regional scale. (ii) Th e larger number of dofs in summer (than in winter) can be associated w ith the reduced forecast performance of NWP models in the anomaly corr elation sense. (iii) The larger number of ECHAM2 dofs for bandpass fil tered anomalies (compared with observations) reveals the model's inabi lity to activate as few modes as the atmosphere. (iv) Interannual vari ability is characterized by dof differences between daily anomalies ta ken from individual monthly averages and from the climate mean.