Semi-grey radiative modelling of a discrete cloud layer in the atmospheres of Jupiter and other giant planets

Abstract:

Cloud cover on the giant planets prevents remote sensing the deep meteorology below the tropopause. This is likely made up of intense jets, waves and vortices which appear to penetrate deep below the cloud layer. En route to to developing a 3D General Circulation Model (GCM) to answer these questions, we have developed a 1-D radiative-convective model suitable to giant planets to calculate rapidly the vertical temperature profile on a high-resolution grid - 66 vertical levels down to a pressure of 18 bar.

We investigate the radiative-transfer in Jupiter’s upper troposphere using a semi-grey radiative-convective scheme based on the previous model developed by Young et al. 2019. The model has two bands representing short-wave (SW) solar and long-wave (LW) planetary thermal radiation. We consider an atmospheric column with absorption of both LW and SW emission, including an internal heat source of 5.7 W/m2. A novel feature is the implementation of a cloud layer reproducing the Bond albedo. We focus on ammonia clouds as they are the highest on Jupiter and thus interact most strongly with the incoming solar radiation.

We show in Figure 1 the net fluxes and heating rates at the equator for a SW-reflective/LW-transparent cloud layer at different pressure levels, compared to the Galileo probe measurements (Sromovsky et al. 1998). In red are the LW and in blue the SW. Net fluxes increase towards low pressure, tending to the overall net output of Jupiter at around 13 W/m2 at TOA (above the top of these observations). The 0.28 bar-cloud reproduces the observed net SW-fluxes as the top cloud is more likely to interact with the incoming solar radiation while the 0.49 bar-cloud reproduces the net LW-fluxes as the LW emission occurs at the cloud base. The observed SW-heating rate exhibits a peak around 0.7 bar, not reproduced by our model as it doesn’t represents the other cloud inferred by Galileo at 1.3 bars, while the 0.7 bar-peak in LW cooling is well reproduced by our model and is consistent with the radiative-convective boundary at this pressure. Future work will be devoted to adding a latitudinally-varying internal heat source, distinct albedo for belts and zones, and implementing these radiative processes into our GCM.