Abstract
Abstract. The diurnal evolution of a cloud free, marine boundary layer is studied by means of experimental measurements and numerical simulations. Experimental data belong to an investigation of the mixing height over inner Danish waters. The mixed-layer height measured over the sea is generally nearly constant, and does not exhibit the diurnal cycle characteristic of boundary layers over land. A case study, during summer, showing an anomalous development of the mixed layer under unstable and nearly neutral atmospheric conditions, is selected in the campaign. Subsidence is identified as the main physical mechanism causing the sudden decrease in the mixing layer height. This is quantified by comparing radiosounding profiles with data from numerical simulations of a mesoscale model, and a large-eddy simulation model. Subsidence not only affects the mixing layer height, but also the turbulent fluctuations within it. By analyzing wind and scalar spectra, the role of subsidence is further investigated and a more complete interpretation of the experimental results emerges.
Highlights
Measurements of large-scale divergence in the atmospheric boundary layer (ABL) are difficult and often contaminated by error (Lenschow et al, 2007)
As we have shown with the case study of a cloud-free marine boundary layer, subsidence can be responsible for shrinking the mixed layer depth by 600 m, from the height of about 1250 m recorded in the morning to that of about 500–600 m in the afternoon
We find that the (i) the slowly decreasing sensible heat flux at the sea surface can not be responsible for the boundary layer evolution during the case study, as shown by the control run; (ii) by means of a polynomial profile for the subsidence velocity, we are able to reproduce the mean field evolution of the scalars and the observed collapse of the boundary layer height, associated with global air warming; (iii) when looking at turbulent fluctuations, quantified in terms of the second-order moments, we find that subsidence modifies their amplitude and spatial organization
Summary
Measurements of large-scale divergence in the atmospheric boundary layer (ABL) are difficult and often contaminated by error (Lenschow et al , 2007). In some ABL studies, subsidence velocity is – for simplicity – neglected or considered to be negligible (Batcharova and Gryning, 1991; Margulis and Entekhabi, 2004); while in other studies it is explicitly considered (see, e.g., Batcharova and Gryning, 1994; Yi et al, 2001; Bellon and Stevens, 2012) When this is the case, a common parametrization is to assume horizontal divergence constant with height. Lidar measurements have revealed their potential to study boundary layer height variation and evolution (Eichinger et al, 2005; Di Liberto et al, 2012) These studies often rely on prognostic equations of the boundary layer height evolution such as the one derived in Batcharova and Gryning (1994), where large-scale subsidence velocity is needed as an input parameter.
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