Département Eaux de surface

Understanding lake dynamics is crucial to provide scientifically credible information for ecosystem management. In this context, three-dimensional hydrodynamic models are a key information source to assess critical but often subtle changes in lake dynamics occurring at all spatio-temporal scales. However, those models require time-consuming calibrations, often carried out by trial-and-error. Through a new coupling of open source software, we present here a flexible and computationally inexpensive automated calibration framework. The method, tailored to the calibration data available to the user, aims at (i) reducing the time spent on calibration, and (ii) making three-dimensional lake modelling accessible to a broader range of users. It is demonstrated for two different lakes (Lake Geneva and Greifensee) with an extensive multi-variable observational dataset. Models mean absolute errors are reduced by up to ~50% over the baseline. Guidelines on heat and momentum transfer parameters are given with their dependence on the observational setup.

Guided by the working hypothesis that the Schwabe cycle of solar activity is synchronized by the 11.07-year alignment cycle of the tidally dominant planets Venus, Earth, and Jupiter, we reconsider the phase diagrams of sediment accumulation rates in Lake Holzmaar and of methanesulfonate data in the Greenland ice core Greenland Ice Sheet Project 2 (GISP2), which are available for the period 10000-9000 cal. BP. As some half-cycle phase jumps appearing in the output signals are, very likely, artifacts of applying a biologically substantiated transfer function, the underlying solar input signal with a dominant 11.04-year periodicity can be considered to be mainly phase‐coherent over the 1,000-year period in the early Holocene. For more recent times, we show that the reintroduction of a hypothesized "lost cycle" at the beginning of the Dalton minimum would lead to a real phase jump. Similarly, by analyzing various series of ¹⁴C and ¹⁰Be data and comparing them with Schove's historical cycle maxima, we support the existence of another "lost cycle" around 1565, also connected with a real phase jump. Viewed synoptically, our results lend greater plausibility to the starting hypothesis of a tidally synchronized solar cycle, which at times can undergo phase jumps, although the competing explanation in terms of a nonlinear solar dynamo with increased coherence cannot be completely ruled out.