Investigating the Causes and Impacts of Convective Aggregation in a High Resolution Atmospheric GCM
A high-resolution, atmospheric general circulation model (GCM) is used to investigate the impact of radiative interactions on spatial organization of convection, the model’s mean state, and extreme precipitation events in the presence of realistic boundary conditions. Mechanism-denial experiments are performed in which synoptic-scale feedbacks between radiation and synoptic-scale dynamics are suppressed by overwriting the model-generated atmospheric radiative cooling rates with its monthly-varying climatological values. This enables us to examine the behavior of two versions of a model with nearly identical large-scale circulations but with differing degrees of convective organization.
When synoptic-scale radiative interactions are disabled, the mean circulation and precipitation remain unchanged, however tropical convection becomes less aggregated, with an increase in cloud fraction and relative humidity in the free troposphere but a decrease in both variables in the boundary layer. Although the distributions of clouds and humidity can be modulated by the degree of aggregation, they exhibit large sensitivity to radiative interactions. The less aggregated state is also associated with a decrease in the frequency of extreme precipitation events. A physical scaling diagnostic indicates that the reduction in extreme precipitation is coincident with a decrease in the dynamical contribution. At regional scales, the spatial contrast in radiative cooling between dry and moist regions diminishes when radiative interactions are suppressed, reducing the upgradient transport of energy, degree of aggregation, and frequency of extreme precipitation events.
Our simulations highlight the role of synoptic-scale radiative coupling in enhancing convective aggregation and extreme precipitation by increasing the horizontal gradient of radiative cooling which provides an upgradient transport of energy from dry to moist regions.