The Virtual Effect of Water Vapor Helps Shape Basic Features of Convective Self-Aggregation

We explore how the virtual effect of water vapor influences the steady-state features of convective self-aggregation. In Earth’s atmosphere, moist air is lighter than dry air at the same temperature, pressure, and volume because the molecular weight of water is lower than the dry air. This is referred to as the virtual effect of water vapor. We will present a set of 2D convective self-aggregation simulations, in which we systematically vary the strength of the virtual effect. Two principal effects are observed: (1) the horizontal scale of convective self-aggregation changes significantly with the strength of the virtual effect (Figs. 1a & b), consistent with the boundary layer theory of Yang (2017); and (2) the vertical distribution of cloud fraction varies with the strength of the virtual effect, which might be related to the weak buoyancy gradient constraint in the free troposphere (Fig. 1c). We will show mechanism-denial experiments to explain the model results.