Inhaltszusammenfassung

This thesis provides a quantitative view on the relative importance of individual processes to the flow-dependent amplification of forecast errors and forecast uncertainty. The focus is on the near-tropopause region of the midlatitudes, which is of particular importance to the evolution of midlatitude weather systems and the evolution of forecast errors. A potential-vorticity (PV) framework is developed to partition the contributions from individual processes. This partitioning builds on the ...This thesis provides a quantitative view on the relative importance of individual processes to the flow-dependent amplification of forecast errors and forecast uncertainty. The focus is on the near-tropopause region of the midlatitudes, which is of particular importance to the evolution of midlatitude weather systems and the evolution of forecast errors. A potential-vorticity (PV) framework is developed to partition the contributions from individual processes. This partitioning builds on the PV perspective for midlatitude dynamics and includes the influence of near-tropopause dynamics (nonlinear Rossby-wave dynamics), tropospheric-deep interaction (baroclinic growth), upper-tropospheric divergence (often associated with latent heat release), and nonconservative processes (direct PV modification by diabatic heating and nonconservative momentum change). In the first part of this thesis, the error growth of a state-of-the-art deterministic forecast is analyzed. In this case, localized mesoscale error maxima form near the tropopause until two forecast days. In the following days, this error pattern changes into a wave-like pattern that maximizes along the dynamical tropopause and reaches the scale of individual Rossby-wave anomalies after about six forecast days. An analysis based on the PV framework reveals that the error growth in this case is dominated by differences in the nonlinear Rossby-wave dynamics near the tropopause. Upper-tropospheric divergence makes a large contribution to error growth in localized regions where a surface cyclone is misrepresented. The direct impact of tropospheric-deep interaction, and thus baroclinic instability, is much smaller than the impact of near-tropopause dynamics. The mesoscale errors generated near the tropopause thus do not primarily project on differences in the baroclinic growth as stated in previous studies. Instead, they directly project on the tropopause evolution and amplify because of differences in the nonlinear Rossby-wave dynamics. The second part of this thesis investigates upscale error growth from the grid scale up to the planetary scale using global simulations with a stochastic convection scheme. Using the PV framework, a distinct sequence of the processes governing upscale error growth can be identified: In the first twelve hours, latent-heating differences induced by the convection scheme dominate the PV-error growth near the tropopause. In the following 1.5 days, the dominant error-growth contribution is given by upper-tropospheric divergence, which provides an effective mechanism to project errors from moist processes into the tropopause region. After two days, differences in the nonlinear near-tropopause dynamics dominate the error growth. A fourth stage of the error growth exists after about 14.5 days when the error grows from the synoptic scale of individual Rossby-wave anomalies up to the planetary scale of the Rossby-wave envelope. Compared to previous studies, a novel interpretation of the processes governing upscale error growth is provided. In the last part of this thesis, the amplification of ensemble spread is investigated for a medium-range forecast with large forecast uncertainty. The focus is on two aspects of the ensemble behavior: i) the mean divergence of the ensemble members indicating the general amplification of forecast uncertainty and ii) the divergence of the best and worst member indicating extremes in possible error-growth scenarios. To quantify the amplification of forecast uncertainty, a tendency equation for the ensemble variance of PV is derived. Averaged over the midlatitudes of the Northern Hemisphere, the variance amplification of this case is dominated by near-tropopause dynamics. Locally, however, there can be large differences as in the region where tropical storm Karl interacts with the Rossby-wave pattern during extratropical transition. In this region, the variance amplification is mostly related to the moist baroclinic cyclone development. The differences between the error growth in the best and worst member can, to a large part, be traced back in time to differences in the representation of a cutoff evolution, which further amplify in the highly nonlinear region of a large-amplitude ridge. This thesis provides novel insight into the processes governing the flow-dependent amplification of forecast errors and forecast uncertainty. This insight can be used to improve the interpretation, and possibly also the design, of future forecast systems. The results from this thesis can also be used to identify regions that are (intrinsically) prone to a large amplification of forecast errors and forecast uncertainty. As the first two stages of upscale error growth are not dominating error-growth mechanisms in the operational forecasts, one can expect that initial-condition errors are still of larger importance to error growth in operational forecasts than errors from moist processes with low intrinsic predictability.» weiterlesen» einklappen

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DDC Sachgruppe:
Naturwissenschaften