Anisotropic turbulence in marine cumulus clouds

Clouds are such manifestations of turbulent flow in the atmosphere, where different physical processes and effects such as radiation, entrainment, microphysical interactions, non-zero gradients of vertical profiles of temperature and wind speed influence the flow at different scales. When analysing turbulence characteristics in clouds, one often assumes homogeneity, isotropy, and equilibrium. For such complicated systems as clouds, in which many thermodynamical and microphysical processes take place, these assumptions might fall short. If the turbulence characteristics are calculated along short windows, which still capture bigger scales of the order of the size of the cloud, then deviations from isotropy are observed in cloudy regions of the flight leg.

In this work, anisotropy is studied using anisotropy invariant maps. An example of anisotropy invariant map is the Lumley's triangle, which is a type of plot where theoretically possible values of invariants of anisotropic stress tensor are contained in a triangle figure. By studying the values of the invariants, one may connect the results to ideal geometrical cases such as elongation or contraction along one of the dimensions. Studying the time dependence of the values of the invariants gives an insight into different regimes of anisotropy in clouds.

The data under consideration comes from the EUREC4A campaign, where Twin-Otter aircraft performed penetrations of marine cumulus clouds near Barbados in 2020. Conditional statistics are made using the RQA method to differentiate between the cloud and its dynamical surroundings. By employing that differentiation, the extent of the anisotropy is studied. The effect of the averaging window is also considered. The preliminary results suggest that the turbulence in clouds is anisotropic to a degree.