Goal of the project is to “predict of the collapse of turbulence in the evening boundary layer and to understand the nonlinear coupling between the atmosphere and its underlying surface.” The plan of the proposal was followed in phase I: a 10-year dataset from Cabauw was used to classify nocturnal boundary layer regimes (and their degree of turbulence) in terms of the external forcings (large-scale pressure gradient, degree of cloud coverage). The resulting composites, published in Van der Linden et al. (2017) serve as a benchmark for numerical modeling. In Baas et al. (2017) we assessed the state-of-the-art of practical modeling with respect to this classification, using RACMO (basically the IFS scheme of the European Centre for Medium-range Weather Forecast). In the next stage, the impact of regime transition on fog formation was assessed.
Also, in accordance with the plan, we were successful in extending the maximum sustainable heat flux theory with land-atmosphere interactions, by introducing a new conceptual model (Van de Wiel et al., 2017; Figures below). As to contrast results from The Netherlands, also data from Antarctica (i.e. with snow coverage instead of grass) where analyzed with respect to collapsing turbulence and regime transitions in (Vignon et. al. 2017), and a comparison with the conceptual model was given in Van de Wiel et al. (2017). In the theoretical part of the project, simplified cases without atmospheric-surface interactions where studies using turbulence resolving techniques such as Direct Numerical Simulation in order to understand and predict the collapse of turbulence in those configurations. Successful ‘early warnings of critical transitions’ where reported in Van Hooijdonk et al. (2016). As, in reality, the collapse of turbulence is influenced by the turbulent state of the atmosphere during the preceding day and in the evening, we also investigated the impact of the evening transition in Van Hooijdonk et al. (2017) and Van Hooft et al. (2019). As stated in the proposal special emphasis has been given to conceptualization of land-atmosphere interactions and a generalized description of the diurnal cycle of the dry atmospheric boundary layer was given in Van Hooft et al. (2019), where indeed it was shown that (nighttime) boundary layer response was very different for different forcings.
To enable turbulence resolving simulations of the full diurnal cycle, we also investigated the potential of adaptive-grids in LES/DNS (Van Hooft et al., 2017). This technique appears to be promising in highly dynamic problems that suffer from a large-scale separation, such as our problem at hand. According to the research plan model results have been benchmarked against results reported in the literature. In summary: the project was executed according to the plan and significant progress was obtained, which has been disseminated in the scientific literature.
