Comparison between RDC and Dynamical Detrainment

First uploaded on 2022/07/30
Last updated on 2023/10/09
Copyright(C)2022 jos <jos@kaleidoscheme.com> All rights reserved.

• In the Radiatively Driven Circulation (RDC), outflow from the supplying source (cumulus) domain occurs over all altitudes in the troposphere. This allows water vapor to be efficiently transported out of the supplying source domain. It is because the air parcels flow out of the cumulus flank at lower altitudes, without the water vapor being removed as precipitation during ascent within the cumulus.


• Since the RDC is induced by the divergence of the vertical mass flux, which occurs everywhere in the subsiding atmosphere, the outflow of the horizontal mass flux to compensate for the divergence is efficient to the boundary far from the supplying source domain.


• This may be more important to consider in real atmosphere than in modelled atmosphere: the flow of the dynamical detrainment is due to blow-off from the cumulus cloud, but blow-off flows tend to be turbulent and inefficient for transport. The RDC flow is created by suction in the vast area outside the supplying source domain, and the suction flow remains laminar and therefore efficient for transport.
  An unfortunate example is the fact that accidents are rare near the water inlet of a swimming pool, whereas they are more likely to occur near the drain outlet, because the suction due to the laminar flow provides efficient transport and generates strong forces.


• The essential difference between dynamical detrainment and RDC is evident in the calculation method. The RDC parameterization is solved as a boundary value problem, so that the effects of the flow leaving the supplying source domain extend to the boundary of the calculation area. This corresponds to the fact that the outflow is not essentially a forced flow from the cumulus, but a compensating flow to fill the vacuum (mass divergence) created in the subsiding atmosphere at the thermal balance.