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Stochastic Forcing of Planetary Scale Flow

Joseph EggerMeteorologisches Institut der Universität München, Munich, FRG

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H-D. SchillingMeteorologisches Institut der Universität München, Munich, FRG

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Abstract

Using geopotential height observations we analyze the fluctuations of the barotropic vorticity transfer from synoptic scale flow (zonal wavenumber m > 5) to planetary scales (m ≤ 5). We hypothesize that this transfer can be seen as a stochastic forcing of the planetary flow modes which is essentially independent of the state at planetary scales. The covariance of the transfer and the planetary flow is presented for selected planetary modes. In parallel, the linear barotropic vorticity equation is solved for these modes where the transfer is prescribed as stochastic forcing with the statistical characteristics of the observed transfer. The covariance of forcing and response is derived and compared to the observed covariance. It turns out that there is good agreement for retrograde and quasi-stationary modes. The agreement is less satisfactory for the largest atmospheric mode and for prograde modes with relatively large wavenumber. 11 is shown that the transfer cannot be seen as a diffusive process. We conclude that the transfer can be seen as stochastic forcing at least for the quasi-stationary modes. This has implications for the modeling of the long-term variability of the atmosphere which is dominated by these modes.

Abstract

Using geopotential height observations we analyze the fluctuations of the barotropic vorticity transfer from synoptic scale flow (zonal wavenumber m > 5) to planetary scales (m ≤ 5). We hypothesize that this transfer can be seen as a stochastic forcing of the planetary flow modes which is essentially independent of the state at planetary scales. The covariance of the transfer and the planetary flow is presented for selected planetary modes. In parallel, the linear barotropic vorticity equation is solved for these modes where the transfer is prescribed as stochastic forcing with the statistical characteristics of the observed transfer. The covariance of forcing and response is derived and compared to the observed covariance. It turns out that there is good agreement for retrograde and quasi-stationary modes. The agreement is less satisfactory for the largest atmospheric mode and for prograde modes with relatively large wavenumber. 11 is shown that the transfer cannot be seen as a diffusive process. We conclude that the transfer can be seen as stochastic forcing at least for the quasi-stationary modes. This has implications for the modeling of the long-term variability of the atmosphere which is dominated by these modes.

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