Effects of Varying Saturation Vapor Pressure on Climate, Clouds, and Convection

Abstract

Abstract We investigate how climate, clouds, and convection change as the amount of water vapor in the atmosphere is varied by altering the saturation vapor pressure (SVP) by a constant in a one-dimensional climate model. We identify four effects of altering SVP on clouds in an Earthlike climate with distinct layers of low and high clouds. First, the anvils of high clouds get higher as SVP is increased (and vice versa) because they are bound by radiative constraints to occur at a lower temperature. The vapor pressure path above the cold anvils does not change in Earthlike climates. Second, low clouds get lower as SVP increases (and vice versa) because they are coupled to a convective boundary layer (CBL) that shallows primarily from an increase in the tropospheric static stability. The third and fourth effects follow from the first two, namely, that single-layer cloud states exist both in vapor-poor states with a merged cloud deck and vapor-rich states with an elevated cloud deck. We identify two cloud instability parameters that determine the transitions between single- and double-layer cloud regimes. Qualitatively, sufficiently vapor-poor states have a deep, diffusive layer that overlaps with a weaker convective layer (topping out at the tropopause) that cannot maintain low relative humidity in the midtroposphere through the drying of descending air, thus causing the cloud layers to merge. Sufficiently vapor-rich states lose their low clouds as the shallowing CBL drops below the lifting condensation level.