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