What is RDC?
First uploaded on 2022/09/16
Last updated on 2023/02/13
Copyright(C)2022 jos <jos@kaleidoscheme.com> All rights reserved.
The Earth's troposphere maintains a radiative-convective equilibrium
temperature, which is neither isothermal nor radiative equilibrium.
Radiatively Driven Circulation (RDC) is the circulation that the
time-averaged component of the multi-dimensional mass flux around
the cumulus cloud must have in order for the tropospheric atmosphere to
maintain its current radiative-convective equilibrium state.
This RDC alone can explain the mass/heat/water vapor transport from
the inside to the outside of cumulus domain.
The name is derived from the calculation method for the RDC flow field.
The RDC is:
-
circulation in the equilibrium atmosphere
(not forced circulation due to instability)
-
circulation of time-averaged components
(not circulation of the disturbance component)
Because of these features,
the equation of motion and dynamical detrainment
in non-cumulus-resolving models cannot produce the same effect as RDC.
Although they may be able to maintain the thermal equilibrium,
they are unable to express the mass circulation that should accompany it,
so they vaguely regulate the equilibrium state.
In order to represent RDC,
the RDC process must be extracted explicitly and calculated
separately from the dynamical flow in the non-cumulus-resolving model.
The two conditions that the RDC must satisfy are:
-
local thermodynamical balance in the atmosphere outside the cumulus
-
local mass continuity in the atmosphere outside the cumulus
Based on these conditions, the RDC can be calculated as follows.
-
First, the local thermodynamical balance that
maintains the current equilibrium for a given radiative
cooling rate and a temperature lapse rate yield
the atmospheric subsidence velocity.
-
This subsidence velocity (obtained in 1.) multiplied by the atmospheric
density gives the vertical mass flux,
which generally has a mass divergence field.
This is because the atmospheric density has a large
exponential vertical gradient while the subsidence velocity
(in 1.) does not change rapidly in the vertical direction.
Such a vertical mass flux cannot be sustained on its own
because it creates a vacuum in the atmosphere.
-
To compensate for the divergent vertical mass flux (in 2.),
a convergent horizontal mass flux is induced from the
cumulus domain, which is the only source of mass in the atmosphere.
Field of such a horizontal mass flux can in principle be computed
as a boundary value problem on a horizontal plane at each altitude.
-
The vertical mass flux (in 2.) and the horizontal mass flux (in 3.) form
circulation around the cumulus cloud.
This is the RDC.
Due to the characteristics of the horizontal flow (in 3.),
the RDC inevitably involves mass/heat/water vapor transport
from inside the cumulus to the vast atmosphere outside the cumulus.
You will find detailed description (with figures and equations) in
A New Scheme of Cumulus Parameterization Based on RDC.
Contact Us
Exhibited on 2022/09/16
Last Updated on 2023/02/13
Copyright(C)2022 jos <jos@kaleidoscheme.com> All rights reserved.