Abstract

Congestus clouds, characterized by their vertical extent into the middle troposphere, are widespread in tropical regions and play an important role in Earth's climate system by their contribution to cloud radiative forcing, atmospheric humidification, and surface rainfall. However, their spatial distribution-in particular the abundance of stratiform clouds sourced by the outflow from congestus cloud tops is inaccurately captured by state-of-the-art climate models, suggesting that fundamental questions regarding their formation, dynamics, and climate impact remain unanswered. Here, we demonstrate the existence of a clear-sky water vapor absorption feature that lends insight into how congestus cloud tops form by detraining preferentially at altitudes between 5-6 km and why they are more prevalent in dry mid-tropospheric conditions. Convective detrainment maximizes at a height of 5-6 km due to a swift decline in radiative cooling in clear-sky regions. This decline is, in turn, a consequence of the absorption feature: more specifically, a non-uniform density of strong absorption lines in the water vapor rotation band. The increased prevalence of congestus clouds in drier mid-tropospheric conditions may be due to stronger vertical gradients in the clear-sky cooling rate, which lead to stronger outflow at 5-6 km. We speculate that, in partnership with stability and entrainment, radiation could significantly and systematically influence mid-tropospheric buoyancy and therefore congestus cloud top formation.