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Identifying Rossby wave packets using local finite amplitude wave activity

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Upper tropospheric Rossby wave packets (RWPs) are often associated with weather systems and can thus have a strong influence on surface weather. They sometimes act as precursors to blocking or intense extratropical cyclones and are, in this sense, connected with severe weather episodes. Therefore, understanding the dynamics of RWPs is of fundamental importance in the context of predictability. This contribution presents local finite amplitude wave activity (LWA) as a novel diagnostic for RWPs. LWA , which is an extension of the finite amplitude wave activity of Nakamura and Solomon, is proportional to the local meridional displacement of contours of potential vorticity (PV) from zonal symmetry. The advantage of using LWA consists in the fact that its formulation does not make any small amplitude assumption; it is able to faithfully identify nonlinear phenomena such as Rossby wave breaking, blocking, PV streamers, or cutoffs. Furthermore, LWA has an exact conservation relation which allows one to formulate a budget equation for its evolution and to quantify the impact of non-conservative processes as a residuum from the LWA budget. Following the main idea of upscale error propagation, this diagnostic is applied to data from the global forecast model ICON to quantify the error growth at the planetary scale. At the same time, the LWA budget equation is used to estimate the magnitude of the non conservative term. A challenge in this context is the cascade of enstrophy to smaller scales, which results in an unavoidable sink term in the LWA budget due to di usion of vorticity. Preliminary results show that non conservative processes play a non-negligible role in the propagation of RWPs, suggesting that their misrepresentation in the models can lead to poor forecasts.

This talk is part of the DAMTP Atmosphere-Ocean Dynamics series.

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