ANALYSIS OF BLOCKING EVENTS FROM OBSERVATIONS AND ECHAM MODEL SIMULATIONS

A new objective routine for the identification of individual blocking events has been developed which is applicable to data sets of differen t horizontal resolution. It is based on the 500 hPa geopotential heigh t anomalies with respect to a mean seasonal cycle. Three features of b locking are checked with this method: the intensity of the anomaly exc eeding some threshold, a certain extension prescribed by a peripheral value of geopotential height, and a duration of at least 5 days. The r outine is applied to ECMWF analyses and to climate simulations of the Northern Hemisphere, which were performed with the atmosphere model EC HAM3. In winter, the blocking statistics derived from the observations are in accord with the results of other identification techniques. In other seasons the differences are larger due to a different definitio n of blocking. It is shown that the model ECHAM3 simulates the observe d two-dimensional frequency distribution of blocking events quite real istically.