A detailed look at the cumulus-valve mechanism and its potential implications for cloud-base cloudiness

Most uncertainty in the warming response of trade-wind cumuli in climate models occurs near cloud base and is associated with model diversity in the strength of shallow convective mixing. In contrast to climate models, cloud-base cloudiness in large-eddy simulations (LES) and in observations is relatively insensitive to changes in the environment. The cumulus-valve mechanism provides a conceptual framework for understanding changes in cloud-base cloudiness in response to changes in the shallow-convective mass flux (M)—an important measure for convective mixing. The mechanism assumes that M keeps the mixed-layer top close to the lifting condensation level, which could explain a larger cloud-base cloudiness with larger M if the increase in M was mostly due to an increasing area fraction of cumuli. Here we use real-case LES over the tropical Atlantic to understand if cloud-base cloudiness increases with increasing M.

We find that M explains a lot of the variations in cloud-base cloudiness (correlation coefficient R=0.86), but the maximum relative humidity at the mixed-layer top (RH_max) needs to be considered additionally to explain the nighttime behavior of cloud-base cloudiness (R=0.95). The coupling of M and RH_max through adjustments in the sub-cloud layer depth is crucial for regulating cloud-base cloudiness. Inability of GCMs to adjust the sub-cloud layer depth in response to a change in M may likely contribute to their overestimated trade-cumulus cloud feedback. The simulated relationships will be compared to measurements from the EUREC⁴A field campaign.