Martin Singh

Program in Atmospheres, Oceans and Climate, MIT

Radiative-convective equilibrium at a wide range of temperatures

Simulation of radiative-convective equilibrium.

A simple construct in which to study moist convection is that of radiative-convective equilibrium (RCE). RCE allows for convective clouds to be investigated without the complicating effects of large-scale flow. The disadvantage is that this idealized situation is never truly observed in the atmosphere, and to date, cannot be simulated in the laboratory. Nevertheless, a full understanding of most radiative-convective equilibrium is a pre-requisite for understanding the much more complex behaviour of convection in the real world.

To this end a number of simulations of RCE have been performed using a cloud-permitting model (CM1, maintained by George Bryan of UCAR) over a wide range of surface temperatures. These simulations will be used to understand the behaviour of convective updrafts and rainfall as the temperature is changed. Here I share some animations of these simulations to give a flavour of how cloud behaviour changes as the surface temperature changes.

Vertical shifts of the general circulation in response to climate change

Schematic of the upward-shift transformation.

It is well known that the general circulation of the atmosphere exhibits an upward shift as the surface is warmed in simulations of climate change. One way of understanding this shift is through a transformation of the governing equations for the atmosphere, which allows for an upward shift of the circulation in response to an upward shift in atmospheric radiative cooling. We have recently shown that such a shift can explain a considerable component of the circulation changes in the middle and upper troposphere associated with a warming climate in simple and more complex general circulation models (see here).

The static stability in moist atmospheres

In an incompressible fluid, its stability to vertical displacements can be easily determined from the vertical density gradient. If light fluid overlies dense fluid the situation is stable, while dense fluid above light fluid leads to overturning. Slightly modified arguments can be made to determine the stabiltiy of a compressible fluid. However, when moisture, and the associated irreversible precipitation processes are included, the static stability can depend on whether the air is saturated, and therefore condensing, or unsaturated. In this case, understanding the effect of the vertical structure of density in the atmosphere on the dynamics is non-trivial. Recent work by Paul O'Gorman has described a method to calculate an "effective" static stability experienced by eddies in a moist environment. The effective static stabilty depends on the asymmetry between upwelling and downwelling regions of the eddies themselves. Understanding what sets this asymmetry, and under what conditions the resultant effective static stabilty can be applied are open questions, curently being investigated.