Unveiling the hidden heating layer at the top of stratocumuli

High-resolution simulations such as direct numerical simulations (DNS) are imperative to resolve the metre-scale variability in the stratocumulus-topped boundary layer (STBL). Previous research has considered the liquid water path (LWP) as a proxy for the longwave radiative transfer, to simplify the expensive radiative transfer calculations. However, the contribution to the radiative fluxes from the absorptive gases is thereby neglected. Utilising the line-by-line radiative transfer model ARTS, we show that the cloud top radiative cooling is underestimated by 30% in earlier simulations that employed the LWP parametrisation. Moreover, we identify a layer warming at 2.5 K h^-1 with thickness of merely about 5 m directly above the cloud top, which we call the cloud top inversional heating layer (CTIHL), the warming in which is attributed to the clear sky radiative effect. About the cloud top inversion, the absorptive gases, predominantly water vapour and carbon dioxide, exchange heat locally due to the strong temperature gradient. The clear sky radiative warming is compensated in the cloud layer by the cloud top radiative cooling due to the liquid water, resulting in the minimal thickness of the CTIHL, which poses challenge for observations. We investigate the environmental factors which modulate the strength of the CTIHL. It is determined that the warming magnitude of the CTIHL is sensitive to the strength of the inversion and water vapour content, but less to the concentration of carbon dioxide.