Isentropic analysis of convective motions and their interactions with large-scale flow

We introduce here a new framework to analyze convective circulations in isentropic coordinates and apply this new approach to study three atmospheric flows: radiative-convective equilibrium, hurricanes and an idealized Walker circulation. In our approach, the upward mass transport by sorted air in terms of the equivalent potential temperature. By averaging the vertical mass flux at constant value of the equivalent potential temperature, one can compute an isentropic mass transport which filters out reversible oscillatory motions such as gravity waves. An isentropic streamfunction is then defined as the net vertical mass flux of all air parcels with an equivalent potential temperature.

The isentropic streamfunction is first studied in simulations of radiative convective equilibrium. In this case, convection appears as rising air at high equivalent potential temperature, and descending motions lower equivalent potential temperature. The approach can also be used to diagnosed the impacts on mixing and entrainment in the convective drafts and determine the diabatic heating tendencies. We then extent our approach to investigate the interactions and convection in two additional cases: a quasi steady hurricane and an idealized Walker circulations. For both, we identify the larger-scale flow by coarse graining, then compute the isentropic streamfucntion associated with the smaller scales. In the hurricane case, we found that a significant portion of the isentropic circulation is accounted for by the large-scale flow, albeit convection plays a significant role in the outer rainband between 100 and 200km away from the center of the storm. In the idealized Walker circulation, convection accounts for the bulb of the isentropic circulation occurs at the convective scale, with the planetary scale flow only accounting for a small portion of the total mass transport. Finally, we use a clustering technique to identify different convective regimes. We identify three dominate convective regimes, corresponding to shallow convection, congestus clouds and deep convection

We argue here that the proposed approach offers a novel way to analyze convective activity in high resolution simulation. It not only offers an objective way to determine the mass transport by convection but can also be used to study more detailed aspects of convection such as mixing, regime transition or the thermodynamic cycle in convection.