What mechanisms explain the tropospheric drying associated with
convective organization? Insights from cloud-resolving and
last-saturation models
Abstract
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.