engleski

Sensitivity of the ECMWF model northern winter climate to model formulation

The impact of various model formulations on model climate during boreal winter is studied using ensembles of seasonal integrations for four different versions of the ECMWF NWP model. The model versions, cycles 36, 46, 48 and 12r1, differ primarily in the representation of physical parametrization. In addition, higher vertical resolution was used for the 12r1 integrations. In the more recent cycles (48 and 12r1) a strong systematic overestimation of zonal flow over the northeastern Pacific has been dramatically reduced, contributing to a more realistic representation of the Pacific block. This improved representation of blocking, particularly in cycle 12r1, is linked to a more efficient diabatic response of the model to the warm SSTs in the western tropical Pacific. In contrast, over the Atlantic/European region a slight deterioration of blocking frequency in cycle 12r1 is associated with the strengthening of the Atlantic jet. The improvement in the Southern Hemisphere circulation, already evident in cycle 46, is not seen in the Northern Hemisphere, so it is argued that the impact of radiation changes introduced between cycle 36 and cycle 46 (inclusive) is influenced by seasonal cycle. A strong cooling of the southern (summer) polar stratosphere has been steadily reduced and in cycle 12r1 is about half of that seen in cycle 36. A reduction of errors in zonally averaged zonal wind and eddy kinetic energy is also clearly seen. In the tropics, the Hadley circulation has become more intense with the later cycles. This is associated with an intensification of convective rainfall within relatively narrow tropical convergence zones. Finally, it was found that the representation of interannual variations between strong positive and negative ENSO-index winters was most successful in cycle 12r1.

model formulation; model climate; seasonal ensembles; ECMWF

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