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Thermal plumes are the energy containing eddy motions that carry heat andmomentum in a convective boundary layer. The detailed understanding of theirstructure is of fundamental interest for a range of applications, fromwall-bounded engineering flows to quantifying surface-atmosphere fluxexchanges. We address the aspect of Reynolds stress anisotropy associated withthe intermittent nature of heat transport in thermal plumes by performing aninvariant analysis of the Reynolds stress tensor in an unstable atmosphericsurface layer flow, using a field-experimental dataset. Given the intermittentand asymmetric nature of the turbulent heat flux, we formulate this problem inan event-based framework. In this approach, we provide structural descriptionsof warm-updraft and cold-downdraft events and investigate the degree ofisotropy of the Reynolds stress tensor within these events of different sizes.We discover that only a subset of these events are associated with the leastanisotropic turbulence in highly-convective conditions. Additionally,intermittent large heat flux events are found to contribute substantially toturbulence anisotropy under unstable stratification. Moreover, we find that thesizes related to the maximum value of the degree of isotropy do not correspondto the peak positions of the heat flux distributions. This is because, thevertical velocity fluctuations pertaining to the sizes associated with themaximum heat flux, transport significant amount of streamwise momentum. Apreliminary investigation shows that the sizes of the least anisotropic eventsprobably scale with a mixed-length scale ($z^{0.5}\lambda^{0.5}$, where $z$ isthe measurement height and $\lambda$ is the large-eddy length scale).

Revisiting the role of intermittent heat transport towards Reynolds stress anisotropy in convective turbulence