  Earth's modern climate is characterized by wet, rainy deep tropics, however
paleoclimate and planetary science have revealed a wide range of hydrological
cycle regimes connected to different external parameters. Here we investigate
how surface wetness affects the tropical hydrological cycle. When surface
wetness is decreased in an Earth-like general circulation model, the tropics
remain wet but transition from a rainy to rain-free regime. The rain-free
regime occurs when surface precipitation is suppressed as negative evaporation
(surface condensation) balances moisture flux convergence. The regime
transition is dominated by near-surface relative humidity changes in contrast
to the hypothesis that relative humidity changes are small. We show
near-surface relative humidity changes responsible for the regime transition
are controlled by re-evaporation of stratiform precipitation near the lifting
condensation level. Re-evaporation impacts the near-surface through vertical
mixing. Our results reveal a new rain-free tropical hydrological cycle regime
that goes beyond the wet/dry paradigm.
