Simulating Nonhydrostatic Atmospheres on Planets (SNAP): Formulation, Validation, and Application to the Jovian Atmosphere

Abstract

A new non-hydrostatic and cloud-resolving atmospheric model is developed for studying moist convection and cloud formation in planetary atmospheres. It is built on top of the Athena++ framework, utilizing its static/adaptive mesh-refinement, parallelization, curvilinear geometry, and dynamic task scheduling. We extend the original hydrodynamic solver to vapors, clouds, and precipitation. Microphysics is formulated generically so that it can be applied to both Earth and Jovian planets. We implemented the Low Mach number Approximate Riemann Solver (LMARS) for simulating low speed atmospheric flows in addition to the usual Roe and HLLC Riemann solvers. Coupled with a fifthorder Weighted Essentially Nonoscillatory (WENO) subgrid-reconstruction method, the sharpness of critical fields such as clouds is well-preserved, and no extra hyperviscosity or spatial filter is needed to stabilize the model. Unlike many atmospheric models, total energy is used as the prognostic variable of the thermodynamic equation. One significant advantage of using total energy as a prognostic variable is that the entropy production due to irreversible mixing process can be properly captured. The model is designed to provide a unified framework for exploring planetary atmospheres across various conditions, both terrestrial and Jovian. First, a series of standard numerical tests for Earth's atmosphere is carried out to demonstrate the performance and robustness of the new model. Second, simulation of an idealized Jovian atmosphere in radiative-convective equilibrium shows that 1) the temperature gradient is superadiabatic near the water condensation level because of the changing of the mean molecular weight, and 2) the mean profile of ammonia gas shows a depletion in the subcloud layer down to nearly 10 bars. Relevance to the recent Juno observations is discussed.

How Well Do We Understand the Belt/Zone Circulation of Giant Planet Atmospheres?

“…They point out that ammonia-rich air rising, and ammonia-poor air falling, cannot be in steady state-some process needs to close this circulation as rain does for water uplift at Earth's low latitudes. Ammonia snow would fall through the NH 3 cloud base into the warmer air below, eventually sublimating somewhere around 4 bar, re-releasing ammonia into the gas phase (Li and Chen 2019). To get the NH 3 all the way back down to 40-60 bar would require "hidden and dense" ammonia vapour downdrafts within the equatorial zone .…”

“…So thick clouds need not imply local upwelling. However, 'secondary' circulations associated with these temperature contrasts may support and enhance the contrast between the dry, aerosol-depleted belts and the moist, aerosolcovered zones, and indeed some mechanism is required to re-supply condensed volatiles that are removed from the upper troposphere via precipitation (e.g., Li and Chen 2019). In the tropics, the correspondence between cool zones and cloudiness is clear.…”

Uranus and Neptune are key to understand planets with hydrogen atmospheres

“…Stoker, 1986). Instead, most of the storms that we see must be powered by water condensation (see Hueso and Sánchez-Lavega, 2001;Hueso, Sánchez-Lavega, and Guillot, 2002;Sugiyama et al, 2014;Li and Chen, 2019) , at levels of ∼ 6 bar in Jupiter and ∼ 12 bar in Saturn. Juno's MWR instrument was able to probe these regions and deeper in Jupiter but the measurements are mostly sensitive to ammonia's absorption, now believed to be a complex function of depth, latitude and possibly even longitude (Li et al, 2017).…”

“…Cloud or cloud-ensemble models (e.g. Hueso and Sánchez-Lavega, 2001;Sugiyama et al, 2014;Li and Chen, 2019) do not include meridional motions and/or global scale winds. They also simplify the microphysics.…”

Reappraising the appropriate calculation of a common meteorological quantity: Potential Temperature

“…The Weather Research and Forecasting model (WRF) was extended to "planetWRF" to simulate the weather in the atmosphere of other planets. Here, the potential temperature is included in the prognostic model equations (Richardson et al, 2007), while it was pointed out by Li and Chen (2019) that this approach could suffer from not accounting for the temperature dependence of the isobaric specific heat capacity c p of the respective atmosphere's gas composition. The atmosphere of Jupiter's moon Titan, the only known moon with a substantial atmosphere, was comprehensively studied with frequent application of the potential temperature based on profile measurement of temperature and pressure in Titan's atmosphere by the Huygens-probe (Müller-Wodarg et al, 2014).…”