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. Various physical mechanisms, including stable layers from melting ice at the 0°C isotherm, dry-air entrainment, and coupled thermodynamic-radiative processes invoked by the first-mover hypothesis, have been proposed to explain the mid-tropospheric detrainment of congestus clouds. Yet, the relative importance of these mechanisms likely varies across space and time.
Despite this uncertainty, both observations and cloud-resolving model simulations consistently show that congestus clouds (1) terminate and detrain, on average, between 5-6 km and (2) are more prevalent when the mid-tropospheric relative humidity is low. A robust theory of congestus clouds must be able to explain these patterns. To address this longstanding issue, we develop a clear-sky radiative-convective framework grounded in the discovery of an important spectroscopic property of water vapor.
This mass- and energy-conserving framework predicts a preferred level of mid-tropospheric detrainment based on radiative cooling processes in clear-sky regions. Convective detrainment maximizes at 5–6 km due to a sharp decline in radiative cooling, driven by water vapor spectroscopy—specifically, a reduction in the number of strong absorption lines in the water vapor rotation band. Using a simple spectral model, we show that this spectroscopic property is linked to the shape of the rotation band, which can be represented as the product of a power law and sinusoidal term capturing deviations of the rotation band from statistical log-linearity. Furthermore, the characteristic “C”-shaped relative humidity profile in the tropics strengthens outflow in dry mid-level conditions by amplifying vertical gradients in the clear-sky cooling rate. Strong RH gradients, particularly under the driest conditions, play an essential role in this process.