Abstract.For the presentation and analysis of atmospheric boundary-layer (ABL) data, scales are used to non-dimensionalise the observed quantities and independent variables. Usually, the ABL height, surface sensible heat flux and surface scalar flux are used. This works well, so long as the absolute values of the entrainment ratio for both the scalar and temperature are similar. The entrainment ratio for temperature naturally ranges from −0.4 to −0.1. However, the entrainment ratio for passive scalars can vary widely in magnitude and sign. Then the entrainment flux becomes relevant as well. The only customary scalar scale that takes into account both the surface flux and the entrainment flux is the bulk scalar scale, but this scale is not well-behaved for large negative entrainment ratios and for an entrainment ratio equal to −1. We derive a new scalar scale, using previously published large-eddy simulation results for the convective ABL. The scale is derived under the constraint that scaled scalar variance profiles are similar at those heights where the variance producing mechanisms are identical (i.e., either near the entrainment layer or near the surface). The new scale takes into account that scalar variance in the ABL is not only related to the surface flux of that scalar, but to the scalar entrainment flux as well. Furthermore, it takes into account that the production of variance by the entrainment flux is an order of magnitude larger than the production of variance by the surface flux (per unit flux). Other desirable features of the new scale are that it is always positive (which is relevant when scaling standard deviations) and that the scaled variances are always of order 1-10.
For students, the transition between secondary school and higher education can be problematic. Their prior knowledge may be insufficient, they may lack the right attitude or not have enough skills for university. Especially gifted students often lack challenges to remain motivated. Moreover, it is not easy for secondary school students to get a good picture of a variety of further studies. For their teachers, it is difficult to keep students informed about actual research in science and technology, as they are not in touch with this research on a regular basis. In this paper, we report about experiences of more than ten years at Utrecht University, offering pre-university students opportunities to be involved in lectures, workshops, laboratories and research at the university. We report on students' motivation to register for this programme, and their experiences afterwards. Important categories of motivation and experience are: raising interest in science; the choice of, and preparation for, further studies; working with a group of similar, motivated students; the wish to be challenged; and the experience of doing research at the university. University lecturers involved in the programme like to work with these students, and are motivated to share their research experiences.
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