Abstract
Recent advances in computational and global modeling technology have provided the potential to improve weather predictions at extended-range scales. In earlier studies by the author and his coauthors, realistic 30-day simulations of multiple African easterly waves (AEWs) and an averaged African easterly jet (AEJ) were obtained. The formation of hurricane Helene (2006) was also realistically simulated from Day 22 to Day 30. In this study, such extended predictability was further analyzed based on recent understandings of chaos and instability within Lorenz models and the generalized Lorenz model. The analysis suggested that a statement of the theoretical predictability of two weeks is not universal. New insight into chaotic and non-chaotic processes revealed by the generalized Lorenz model (GLM) indicated the potential for extending prediction lead times. Two major features within the GLM included: (1) three types of attractors (that also appeared in the original Lorenz model) and (2) two kinds of attractor coexistence. The features suggest a refined view on the nature of weather, as follows: The entirety of weather is a superset that consists of chaotic and non-chaotic processes. Better predictability can be obtained for stable, steady-state solutions and nonlinear periodic solutions that occur at small and large Rayleigh parameters, respectively. By comparison, chaotic solutions appear only at moderate Rayleigh parameters. Errors associated with dissipative small-scale processes do not necessarily contaminate the simulations of large scale processes. Based on the nonlinear periodic solutions (also known as limit cycle solutions), here, we propose a hypothetical mechanism for the recurrence (or periodicity) of successive AEWs. The insensitivity of limit cycles to initial conditions implies that AEW simulations with strong heating and balanced nonlinearity could be more predictable. Based on the hypothetical mechanism, the possibility of extending prediction lead times at extended range scales is discussed. Future work will include refining the model to better examine the validity of the mechanism to explain the recurrence of multiple AEWs.
Highlights
Due to the pioneering studies of Lorenz [1,2,3], the finite predictability of weather is well accepted.Subsequent studies have focused on how to estimate the limit of predictability and reveal the fundamental mechanisms responsible for limited predictability
Our analysis indicated that a quasi-equilibrium assumption within cumulus parameterizations may limit the scale interaction between convection and large-scale flows and, cause uncertainties in simulations of African easterly waves (AEWs) as well as hurricanes
The predictability of contained one-way interaction over the ocean
Summary
Due to the pioneering studies of Lorenz [1,2,3], the finite predictability of weather is well accepted. Weather systems at various scales and, have determined that the practical predictability of all dynamic models cannot be longer than two weeks. One may wonder how such a surprising result could be obtained while result could be obtained while the inherent limits for long-range forecasting that developed within the inherent limits for long-range forecasting that developed within the scientific literature remain the scientific literature remain (e.g., [4,7]). To address this question, the predictability problem that (e.g., [4,7]). Studies [17,18,19,20,21,22,23]
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