Simplicity of RDC

First uploaded on 2022/08/21
Last updated on 2023/02/18
Copyright(C)2022 jos <jos@kaleidoscheme.com> All rights reserved.

The reason for our belief in Radiatively Driven Circulation (RDC) is its simplicity. Within the RDC mechanism, the flow in three-dimensional space is simply determined to satisfy two relationships: local thermodynamical balance and local continuity at an arbitrary point in the troposphere. In RDC parameterization, this process is treated as a boundary value problem / spatial integral.

It is such a simple mechanism, yet it is not understood by most researchers. There is a reason for this. In order to understand RDC, it is necessary to understand that cumulus development is just a mixing process, that the developed cumulus is a mass of small vortices, and that this mixing domain does not have any forcing power for outflows.

Cumulus clouds have a special place in meteorology, especially in meteorological dynamics. And not only in meteorology. Every human being has experienced the fear of severe precipitation, gusty winds, thunderstorms, and lightning associated with cumulus clouds. More generally, we have been witnessing these violent weather phenomena since the beginning of life on Earth. Fear of cumulus clouds is a memory imprinted in the DNA of life on Earth.

Humanity has developed research to understand these phenomena of cumulus clouds, which are directly related to disasters. Since predicting disasters was naturally the most important issue, phenomena that occur inside cumulus clouds, such as those mentioned above, have been the focus of attention. Meteorology, the result of such research, has achieved remarkable results. Forecasts of the weather for the next 10 days are now reasonably accurate, and in the mid-latitudes, it is now possible to predict intense precipitation such as linear rain belt.

So-called meteorological phenomena are confined within cumulus clouds. Treating cumulus cloud as if it were a floating "object" in the atmosphere is probably correct for the purpose of disaster prediction. It is natural for the climate research community to assume that the problem of mass/heat/water vapor transport out of a cumulus cloud can also be understood by such an extension of meteorology. Many people may have no choice but to expect the transport capability of a cumulus cloud, since it is the only visible "object" in the large atmosphere. However, is it really the right approach to focus on this small-scale phenomenon when considering the outflow from the cumulus domain to the vast clear sky region?

We would like you to take off your meteorological preconceptions and look at a cumulus cloud. Cumulus cloud is nothing more than a thermal plume phenomenon in which hot, humid air in the lower atmosphere penetrates by buoyancy into cold, dry air in the upper atmosphere. The only thing that occurs in this process is mixing. Mixing is a process in which two heterogeneous fluids transition to one homogeneous fluid. With mixing, the motion with large-scale flow breaks up into small vortex motions. When a cumulus cloud is fully developed, the inside of the cloud is filled with vortices of small sizes that appear as lumpy/uneven pattern on the cloud surface. Cumulus cloud is just a mass of small vortices, being visualized with condensed cloud water/ice, which is far from something like a single floating "object". It is strange from the viewpoint of the second law of thermodynamics to think that such a small-scale vortices can be integrated and transformed into a physically meaningful large-scale flow again. Meteorologists definitely overestimate the effect of cumulus dynamics on climate phenomena. Meteorology is physics for weather forecasts, not physics for climate prediction. It should be considered that the outflow from the cumulus domain is determined by the field of a larger physical scale (both in space and time).

For twenty years, we have proposed the existence of an RDC mechanism that induces outflow from cumulus clouds over a large subsidence area that extends outside the cumulus clouds. RDC is a mechanism of the time-mean component of the atmospheric field. RDC has much larger physical scale than dynamical detrainment. Therefore, it is usually considered to be more effective in transport from the standpoint of fluid dynamics and thermodynamics. The problem is that RDC cannot be seen directly with the eyes of flesh like cumulus clouds. RDC can only be seen with the logical eyes of imagination. When looking at something, all we need to do is to discard preconceptions and think logically. RDC should be easy to accept if we only imagine a little about how the atmosphere maintains thermal equilibrium and mass continuity over a vast, invisible, subsidence area. Meteorological preconceptions are the single greatest barrier to understanding RDC.