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Abstract The vertical profile of clear-sky radiative cooling places important constraints on the vertical structure of convection and associated clouds. Simple theory using the cooling-to-space approximation is presented to indicate that the cooling rate in the upper troposphere should increase with surface temperature. The theory predicts how the cooling rate depends on lapse rate in an atmosphere where relative humidity remains approximately a fixed function of temperature. Radiative cooling rate is insensitive to relative humidity because of cancellation between the emission and transmission of radiation by water vapor. This theory is tested with one-dimensional radiative transfer calculations and radiative-convective equilibrium simulations. For climate simulations that produce an approximately moist adiabatic lapse rate, the radiative cooling profile becomes increasingly top-heavy with increasing surface temperature. If the temperature profile warms more slowly than a moist adiabatic profile in mid-troposphere, then the cooling rate in the upper troposphere is reduced and that in the lower troposphere is increased. This has important implications for convection, clouds and associated deep and shallow circulations.
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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.
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- Award ID(s):
- 2310364
- PAR ID:
- 10573142
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- AGU Advances
- Volume:
- 6
- Issue:
- 1
- ISSN:
- 2576-604X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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