as regions of flow with horizontal divergence (associated [with]{} upflows) surrounded by a network of convergent flows [associated with downflows, e.g., @Duvall2000].

The origin of supergranulation as a dominant scale of convection remains unclear [see @Rincon2018 for a review]. Supergranulation as a particular length-scale might emerge because of thermal instabilities in the ionization zones [@Simon1964], as the largest buoyantly driven scale of convection in the Sun [@Cossette2016], or the largest convective scale not significantly constrained by rotation [@Featherstone2016]. Also unexplained is the evolution of the supergranulation pattern, which displays a wave-like behavior [@Gizon2003; @Schou2003] – although this is not the topic of this paper.

Comparisons between theory and observations of [supergranular-scale]{} flow fields are [usually]{} based on the power spectrum of horizontal surface flows [[e.g., @Rincon2017]]{}. However, it is clear that the power spectrum does not contain much information about the spatial pattern of the flow field, apart from the scale-dependence of the amplitude. For example, the power spectrum does not contain any information about the skewness of the underlying field [[see e.g.,]{} Figure 5 in @Nordlund1997]. Divergence maps of supergranulation have a positive skewness [@Duvall2000] which is due to the asymmetric distribution of upflows and downflows.

Such an asymmetry 