Abstract
A new method is introduced to identify coherent structures in the convective boundary layer, based on optimizing the vertical scalar flux in a two-fluid representation of turbulent motions as simulated by a large-eddy simulation. The new approach partitions the joint frequency distribution (JFD) of the vertical velocity and a transported scalar into coherent structures (fluid 2) and their environment (fluid 1) by maximizing that part of the scalar flux resolved by the mean properties in fluid 2 and fluid 1. The proposed method does not rely on any a priori criteria for the partitioning of the flow nor any pre-assumptions about the shape of the JFD. Different flavours of the optimization approach are examined based on maximizing either the total (fluid 1 + fluid 2) or the fluid-2 resolved scalar flux, and on whether all possible partitions or only a subset are considered. These options can result in different derived area fractions for the coherent structures. The properties of coherent structures diagnosed by the optimization method are compared to the conditional sampling of a surface-emitted decaying tracer, in which coherent structures are defined as having tracer perturbation greater than some height-dependent threshold. Results show that the optimization method is able to smoothly define coherent thermal structures in both the horizontal and the vertical. Moreover, optimizing the turbulent transfer by the fluid-2 resolved flux produces very similar coherent structures to the tracer threshold method, especially in terms of their area fraction and updraft velocities. Nonetheless, further analysis of the partitioning of the JFD reveals that, even though the area fraction of coherent structures might be similar, their definition can occupy different quadrants of the JFD, implying the contribution of different physical mechanisms to the turbulent transfer in the boundary layer. Finally, the kinematic and thermodynamic characteristics of the coherent structures are examined based on their definition criteria.
Highlights
In the convective boundary layer (CBL), the predominant source of turbulence is buoyancy resulting from the transfer of heat from the surface to the adjacent fluid
A new method for identifying coherent structures in the CBL has been presented. It is based on labelling the fluid in each grid box as either fluid 2 or fluid 1 in such a way as to maximize the contribution to some scalar flux that is resolved by the mean properties of the two types of fluid
Even though the optimization method makes no explicit use of the concept of a coherent structure, it is found that fluid 2 picks out the thermals with length scales comparable to the boundary-layer depth that are responsible for the non-local vertical transport in the CBL
Summary
In the convective boundary layer (CBL), the predominant source of turbulence is buoyancy resulting from the transfer of heat from the surface to the adjacent fluid. Buoyant plumes rise from the surface layer to form large coherent structures (thermals) that are responsible for transferring heat to the whole depth of the CBL by cooling the surface layer and warming the mixed layer. The overshooting (negatively buoyant) boundary layer thermals and the entrainment of potentially warmer air from the stable layer contribute to a pronounced heat-flux minimum in the vertical sensible-heat-flux profile. Through mass conservation, these coherent structures force a broader, compensating descent in the CBL
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