  Condensation in cumulus clouds plays a key role in structuring the mean,
non-precipitating trade-wind boundary layer. Here, we summarise how this role
also explains the spontaneous growth of mesoscale ($>O(10)$ km) fluctuations in
clouds and moisture around the mean state in a minimal-physics, large-eddy
simulation of the undisturbed period during BOMEX on a large ($O(100)$ km)
domain. Small, spatial anomalies in latent heating in cumulus clouds, which
form on top of small moisture fluctuations, give rise to circulations that
transport moisture, but not heat, from dry to moist regions, and thus reinforce
the latent heating anomaly. We frame this positive feedback as a linear
instability in mesoscale moisture fluctuations, whose time-scale depends only
on i) a vertical velocity scale and ii) the mean environment's vertical
structure. In our minimal-physics setting, we show both ingredients are
provided by the shallow cumulus convection itself: It is intrinsically unstable
to length scale growth. The upshot is that energy released by clouds at
kilometre scales may play a more profound and direct role in shaping the
mesoscale trade-wind environment than is generally appreciated, motivating
further research into the mechanism's relevance.
