ノーマルモード関数展開を用いたECMWF, NMC, JMA全球解析データのエネルギー比較解析

Abstract

In this study, normal-mode energetics diagnosis was carried out for the 9-year period of ECMWF (European Center for Medium-Range Weather Forecasts) analysis during winters (DJF) of 1986 to 1994. The diagnosis was performed also for operational global analyses by JMA (Japan Meteorological Agency) and NMC (National Meteorological Center) during the winter of 1988/89 for comparison. Special attention is focussed on the total energy spectra for Rossby and gravity modes over the 3-D scale index. The result was compared with our former studies for other datasets, including original and re-analyses of the First GARP Global Experiment (FGGE). The result for the 9-year average of ECMWF shows that the slopes of the Rossby-mode energy spectra in the eigenfrequency domain are estimated as 2.2, 2.1, 3.5, 5.0, 5.5 for vertical modes m=0 to 4, respectively, whereas the corresponding slopes are -1.8, -1.7, -1.6, -1.3, -1.0, respectively, for the gravity mode. We confirm that the energy spectra of Rossby and gravity modes are connected with each other in the high-frequency range, at least for the vertical modes m=1 to 4. However, the mechanism to create the spectral peaks within the Rossby modes is not known. The intercomparison of the normal-mode energetics for JMA, NMC, and ECMWF shows that the Rossby-mode energy levels are virtually the same, having a minor discrepancy less than 8%. We find that the total energy level pertaining to the gravity modes is about 45 (1O3 Jm-2) and shows a minor discrepancy of only about 8% between the analyses. According to our former studies for the FGGE reanalyses, the corresponding energy levels for the gravity modes are 90 and 41 (103 Jm-2) for GFDL and ECMWF, respectively. Hence, we can conclude that the total energy levels of the gravity modes in the recent operational analyses are converging to the value of ECMWF FGGE data. Since the amount of the total energy of the atmosphere is about 6800 (103 Jm-2), the energy level of the gravity mode is found to be only 0.7% of the total energy.