Camille Risi

and 3 more

In observations and cloud-resolving model (CRM) simulations, large-scale domains where convection is more aggregated (clustered into a smaller number of clouds) are associated with a drier troposphere. What mechanisms explain this drying? Hypotheses involve dynamical and microphysical processes. The goal of this study is to quantify the relative importance of these processes. We use a series of CRM simulations with different dynamical regimes and different kinds of convective organization forced by external forcings (isolated cumulonimbi, tropical cyclones, squall lines). We interpret the simulation results in the light of a hierarchy of simpler models (last-saturation model, analytical model). In CRM simulations, the troposphere is drier in the environment of more aggregated convection (tropical cyclones and squall lines). A last-saturation model is able to reproduce the drier troposphere even in absence of any microphysical processes or horizontal motions. Cloud intermittence is the key factor explaining this drying: when clouds are more intermittent, subsiding air parcels are more likely to encounter a cloud. An analytical model highlights the key role of the duration of convective systems. Remoistening by microphysical processes contributes to the moister troposphere when convection is less aggregated, though its importance is secondary smaller than that of intermittence. We suggest that the observed anti-correlation between convective aggregation and relative humidity may, at least partially, be mediated by the duration of convective systems.