Sub-gridscale effects in mesoscale deep convection: Initiation, organization and turbulence

Deep convection shows a tendency toward organized long-lived mesoscale cloud systems. These dynamically self-sustaining systems form an important element of atmospheric variability on the mesoscale and down to dissipative scales. As it is not possible to resolve all scales of motion involved in mesoscale cloud systems, the representation and modelling of organized mesoscale deep convection necessarily requires a rupture of the mesoscale spectrum and a separate representation of smaller scales. It is proposed here that a natural point for this rupture is the scale of initiation of deep convection, linked to the tropospheric scale height, O(10 km). Smaller scale motions that must be represented as sub-gridscale effects include the effects of turbulent motions; condensation and precipitation at sub-gridscales; the organization of the troposphere and planetary boundary layer on the convective scale. It will be shown that the initiation and organization of deep convection can be understood in a systematic manner by examining mechanisms by which convective-scale motions appear through transitions from forced motions at other scales.