Report 37:    Intercomparison of the Seasonal Cycle in 200 hPa Kinetic Energy in AMIP GCM Simulations

Boyle, James S.
October 1996, 34 pp. 

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The 200 hPa kinetic energy is represented by means of the spherical harmonic components for the AMIP simulations, the NCEP/NCAR reanalysis and the ECMWF reanalysis(ERA). The data used are the monthly mean wind fields for 1979 to 1988. The kinetic energy is decomposed into the divergent (DKE) and rotational (RKE) components and emphasis is placed on examining the former.

The two reanalysis data sets show reasonable agreement that is best for the rotational kinetic energy. The largest difference in the divergent kinetic energy occurs during the northern summer. As might be expected, the two analyses are closest in regions where there are sufficient observations such that the effects of the model used in the assimilation cycle are minimized.

The observed RKE shows only a slight seasonal cycle with a maximum occurring during the northern winter. The DKE, on the other hand, has a very pronounced seasonal cycle with maxima at the solsticial seasons and minima during the equinoctial seasons. The most energetic season is the northern summer (JJA), the least energetic the northern spring (MAM). SON is a transitional season with the three months having a large spread in values. The largest jump in DKE is seen from May to June, evidently in response to the onset of the Asian summer monsoon. The spectral decomposition of the DKE indicates that the NCEP analysis has more energy in the zonal component of the flow than the ERA. This is most pronounced during the northern summer.

The model results show a very large spread in the magnitudes of the RKE and DKE although the models all evince a seasonal variation in phase with that observed. The median value of the models systematically overestimates the observed RKE throughout the year and underestimates the DKE during the northern summer. These relations can be reversed for individual models. There is also a marked tendency for the models to have too much DKE in the zonal flow as compared to the reanalyses, especially at the higher energy levels. It does not appear possible to associate any specific model discrepancy with the particulars of the model formulation.

The median values of the seasonal cycle of RKE and DKE for the models are superior to those of any individual model. Results are also presented for simulations following the AMIP protocol but using updated versions of the original AMIP entries. In most cases these new integrations show better agreement with the observations. (pdf file)

UCRL-MI-123395

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