Clear-Sky Convergence, Water Vapor Spectroscopy, and the Origin of Tropical Congestus Clouds

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. However, fundamental questions regarding their formation and prevalence remain unanswered. Here, we endeavor to answer how congestus cloud tops form by detraining preferentially at altitudes between 5 and 6 km and why this detraining outflow is invigorated by drier mid-tropospheric conditions. We construct a clear-sky radiative-convective framework of congestus cloud-top formation that is grounded in the discovery of an important spectroscopic property of water vapor. In this mass- and energy-conserving framework, convective detrainment maximizes at a height of 5 and 6 km due to a swift decline in radiative cooling in clear-sky regions. This decline is, in turn, a consequence of water vapor spectroscopy: more specifically, a drop in the number of strong absorption lines in the water vapor rotation band. In a simple spectral model, we link this spectroscopic property to the shape of the rotation band, which can be approximated as the product of a power law and a sine wave representing the band's deviation from statistical log-linearity. The characteristic “C”-shaped relative humidity profile in the tropics further strengthens the outflow in drier mid-level conditions by amplifying vertical decreases in the clear-sky cooling rate. Essential to this process are strong RH gradients, which are most pronounced under the driest conditions and induce a vertical decrease in the optical depth lapse rate across the mid-troposphere.