Testing the clear-sky convergence mechanism for congestus cloud formation using ORCESTRA data

Elevated moist layers and mid-level clouds around the triple point temperature lead to enhanced radiative cooling at their upper boundaries. This cooling propagates to surrounding areas and creates stable layers that act as buoyancy barriers for neighboring convection and thereby promote the accumulation of water vapor and clouds. Different theories for congestus cloud formation imply different entry points into this feedback loop. 

Here, we test the clear-sky convergence (CSC) mechanism -- which predicts a natural maximum horizontal CSC near the triple point caused by the structure of radiative cooling that is determined by the optical properties of water vapor -- using dropsonde measurements from the ORCESTRA field campaign in the Tropical Atlantic. We find that the vertical profile of CSC aligns well with the height of mid-level clouds in the ORCESTRA-East domain, but not in the West. The good agreement in the East is mostly caused by contributions from the stability structure and not from gradients in the radiative cooling. 

Additionally, we show using idealized experiments that for a moist adiabatic temperature structure a W-shaped relative humidity is necessary to achieve a positive CSC above the freezing level. This effect is caused by longwave radiative cooling and partly counteracted by shortwave radiative heating. Overall, we conclude that the cooling rate contribution to the CSC mechanism is not enough to trigger a congestus circulation, but can contribute to its maintenance in clear-sky regions with elevated moist layers.