A Rayleigh-Drag Representation of Moist Convection in a Jupiter GCM

One of the significant problems with General Circulation Models (GCMs) of giant planets is the uncertainty over the boundary condition at the model’s solid-lower boundary. In reality, giant planets such as Jupiter do not have such a boundary. Their vertical structure instead consists of a stably-stratified outer layer known as the ‘weather-layer’, which extends down to several bars in pressure, and a much deeper neutrally-stratified interior beneath. Solid-lower boundaries are, however, a practical necessity in current GCMs.

A common feature of weather-layer-only GCMs is to include a frictional force, in the form of a Rayleigh drag, at the lower boundary. The use of such a drag is sometimes motivated by the argument that Ohmic dissipation, found deep in Jupiter’s interior, could act frictionally on the weather layer via deep meridional circulations. However, uncertainty remains about the appropriateness, formulation and magnitude of such a drag, and whether it can indeed affect the weather layer. Various recent studies have therefore studied the impact of reducing the magnitude of the Rayleigh drag in Jupiter-like GCMs.

Here we consider the impact of a Rayleigh drag away from a Jupiter GCM’s lower-boundary, at levels within the weather-layer. Such a drag is motivated by the idea that Jovian moist convection could generate net anticyclonic vorticity in regions of cyclonic background flow, as suggested by the presence of lightning in Jupiter’s cyclonic belts.

Using a Jupiter GCM without an explicit representation of moisture, but with the mid-weather-layer drag representing its effects, we find that a mid-level drag of reasonable magnitude results in a mid-weather-layer meridional flow from cyclonic to anticyclonic regions. Such a flow has previously been suggested as an explanation of Jupiter’s moist convection distribution. We will therefore comment on the potential realism of our drag formulation, and discuss the effects of a more self-consistent and explicit representation of moist convection.