  Shallow trade cumuli over subtropical oceans are a persistent source of
uncertainty in climate projections. Mesoscale organization of trade cumulus
clouds has been shown to influence their cloud radiative effect (CRE) through
cloud cover. We investigate whether organization can explain CRE variability
independently of cloud cover variability. By analyzing satellite observations
and high-resolution simulations, we show that increased clustering leads to
geometrically thicker clouds with larger domain-averaged liquid water paths,
smaller cloud droplets, and consequently, larger cloud optical depths. The
relationships between these variables are shaped by the mixture of deep cloud
cores and shallower interstitial clouds or anvils that characterize cloud
organization. Eliminating cloud cover effects, more clustered clouds reflect up
to 20 W/m$^2$ more instantaneous shortwave radiation back to space.
