Turbulence properties inside and outside trade-wind cumulus clouds

Shallow trade-wind cumulus clouds originate from thermals which rise from the turbulent subcloud layer and penetrate high enough to reach their lifting condensation level. Those thermals transport heat and moisture into the cloud layer. Analogously, the subject of such a transport can be the small-scale turbulence.

Turbulence measurements near the cloud base and in the subcloud layer were performed in the course of the EUREC4A field campaign by the ATR research aircraft in a large number of repeatable flight segments (Brilouet et al., 2021). In this study, we exploit this extensive dataset to derive properties of turbulence corresponding to short 'local' domains, of the size of the order of 100 m, e.g. turbulence kinetic energy dissipation rate, anisotropy and inertial range scaling. Taking advantage of the substantial amount of data provided by the EUREC4A measurements, we compare the statistics of those parameters between the areas inside cumulus clouds, outside them at the same altitude and at three levels inside the subcloud layer.

Such a comparison indicates that the character of small-scale turbulence inside cumulus clouds can be considered comparable to the one observed in the subcloud layer but significantly differs from that observed at the same level outside the clouds. As the cloud fraction at cloud base is typically rather a small number (about 4% during EUREC4A), it is in consequence inherently difficult for large scale models to accurately parameterize the intensity of turbulence and mixing in the trade-wind regime.