  In Earth's current climate, moist convective updraft speeds increase with
surface warming. This trend suggests that very vigorous convection might be the
norm in extremely hot and humid atmospheres, such as those undergoing a runaway
greenhouse transition. However, theoretical and numerical evidence suggests
that convection is actually gentle in water vapor-dominated atmospheres,
implying that convective vigor may peak at some intermediate humidity level.
Here, we perform small-domain convection-resolving simulations of an Earth-like
atmosphere over a wide range of surface temperatures and confirm that there is
indeed a peak in convective vigor, which we show occurs near T_s ~ 330 K. We
show that a similar peak in convective vigor exists when the relative abundance
of water vapor is changed by varying the amount of background (non-condensing)
gas at fixed T_s, which may have implications for Earth's climate and
atmospheric chemistry during the Hadean and Archean. We also show that
Titan-like thermodynamics (i.e., a thick nitrogen atmosphere with condensing
methane and low gravity) produce a peak in convective vigor at T_s ~ 95 K,
which is curiously close to the current surface temperature of Titan. Plotted
as functions of the saturation specific humidity at cloud base, metrics of
convective vigor from both Earth-like and Titan-like experiments all peak when
cloud-base air contains roughly 10% of the condensible gas by mass. Our results
point to a potentially common phenomenon in terrestrial atmospheres: that moist
convection is most vigorous when the condensible component is between dilute
and non-dilute abundance.
