How do the small-scale details of the radiative transfer shape the development of stratocumulus?

Stratocumulus clouds are particularly common in the subtropics and cover one-fifth of the Earth’s surface [1]. They regulate the climate and create a cooling effect on the surface through reflecting the incoming solar radiation. Stratocumulus coverage will decrease with the current warming conditions, but how much remains unclear. The strong coupling between meter-scale processes in the cloud-top region, the free troposphere and the SST complicates the projections [1, 2, 3, 4]. Numerical models with insufficient resolution overestimate mixing and mask the sensitivity of stratocumulus to changes in the environmental conditions [6]. How reliable are then these models when used to study the role of stratocumulus in the climate system?

This research focuses on the stratocumulus cloud adjustment mechanisms associated with radiative transfer. We use direct numerical simulations with increased resolution to study the radiative transfer, cloud-top turbulence and entrainment. As resolution increases, representation of mixing is improved and radiation effects can be better assessed. DNS has already proved successful to disentangle the interactions between turbulence, radiative cooling, and sedimentation [5, 6].

A simplified radiative transfer scheme, which considers the contribution from liquid water to the longwave radiative flux [7, 8], has been considered in the previous work due to the computational expensiveness of high-resolution simulations. This scheme has been shown to simulate the radiative transfer of liquid clouds in LES well [8]. However, it has neglected the potential contribution of water vapour to the radiative flux and might be insufficient to represent important aspects of stratocumulus, e.g., the diurnal cycle, cloud holes, cloud break-up, and regime transitions, which eventually influence the climate sensitivity. Using a detailed line-by-line radiative transfer model ARTS [9], we show that previous simulations might have underestimated the cloud-top radiative cooling and the capping inversion. Preliminary results show that this leads to a more turbulent boundary layer and slower breakup of the stratocumulus. These results should provide insight on sensitivity studies related to radiative feedback mechanisms, such as the change of downwelling longwave radiation due to increasing greenhouse gas concentrations.

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