Geophysical and Astrophysical Fluid Dynamics

Abstract:

Geochemical and astronomical evidence demonstrates that planet formation occurred in two spatially and temporally separated reservoirs. The origin of this dichotomy is unknown. We use numerical models to investigate how the evolution of the solar protoplanetary disk influenced the timing of protoplanet formation and their internal evolution. Migration of the water snow line can generate two distinct bursts of planetesimal formation that sample different source regions. These reservoirs evolve in divergent geophysical modes and develop distinct volatile contents, consistent with constraints from accretion chronology, thermochemistry, and the mass divergence of inner and outer Solar System. Our simulations suggest that the compositional fractionation and isotopic dichotomy of the Solar System was initiated by the interplay between disk dynamics, heterogeneous accretion, and internal evolution of forming protoplanets.

Abstract:

The data comprise two 2-dimensional gridded maps of horizontal wind measurements covering the north and south polar regions of Saturn, as previously published by Antuñano et al. (2015). As fully described in that paper, these measurements were derived from sets of Cassini Orbiter Imaging Sub-System (ISS) Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) images using the continuum band CB2 and CB3 filters, acquired for the northern hemisphere in June 2013 and for the southern hemisphere using WAC CB2 and CB3 images taken in October 2006 and December 2008. Additional NAC images using the CB2 and red filters taken in July 2008 were also used to analyse the southern polar vortex. The WAC images covered a region extending from a planetocentric latitude of around 60-65 degrees to each pole (apart from a segment in longitude between around 35 - 110 degrees W) with a horizontal resolution equivalent to around 0.05 degrees latitude (around 50km) per pixel, while NAC images were mostly used for the polar vortices, with a resolution equivalent to around 0.01 degrees latitude (around 10 km) per pixel. Horizontal velocities were obtained using semi-automated image correlation methods between pairs of images separated in time by intervals of approximately 1-10 hours. The correlation algorithm used pixel box sizes of 23 x 23 (in the north) or 25 x 25 (in the south), leading to a spatial resolution of the velocity vectors equivalent to around 1 degree latitude or 1000 km outside the polar vortices, reducing to around 0.2 degrees or 200 km within the polar vortices themselves. The automatically generated velocity vectors were supplemented by a small number (around 1% of the total) of vectors obtained manually from the motion of visually identified cloud tracers. The estimated measurement uncertainty on each vector was around 5-10 m/s. The original velocity vectors from Antuñano et al. (2015) were interpolated onto a regular latitude-longitude grid using convex hulls and Delauney triangulation via the QHULL routine of the Interactive Data Language (IDL). The final datasets are held on a regular grid separated by 3-4 degrees in longitude and 0.23 degrees in latitude. Data are stored as two text files, tabulating the latitude and (west) longitude of each point and the eastward and northward velocity components respectively in units of m/s. Reference: Antuñano,A., del Río-Gaztelurrutia,T., Sánchez-Lavega,A., & Hueso, R. (2015). Dynamics of Saturn’s polar regions. J. Geophys. Res.: Planets, 120, 155–176. doi: 10.1002/2014JE004709

Abstract:

This archive contains the Python code used to analyse and plot the data in Hammond & Lewis 2021, "The rotational and divergent components of atmospheric circulation on tidally locked planets", as well as the data from the "terrestrial" simulation of the atmosphere of a rocky planet using the general circulation model ExoFMS. It contains three files: 1) HL21_plotter.ipynb This is a Jupyter notebook containing Python code. It reads the data from the ExoFMS simulation and finds its rotational and divergent parts. It then plots the figures used in Hammond & Lewis 2021. 2) data/rotdiv-terr-control-1000-2000_atmos_average_interp.nc The "terrestrial" simulation output, interpolated to uniform pressure levels. This is used to plot quantities such as velocity at a constant pressure. 3) data/rotdiv-terr-control-1000-2000_atmos_average.nc The "terrestrial" simulation output, on the raw model sigma-pressure levels. This is used to calculate the dry static energy budget. The paper also uses a "Hot Jupiter" simulation from the THOR GCM. This is from "THOR 2.0: Major Improvements to the Open-Source General Circulation Model" (Deitrick et al. 2020). The data is available on request to Russell Deitrick (russell.deitrick@csh.unibe.ch). The same analysis can be made using HL21_plotter.ipynb, with small modifications due to the different grid in THOR.

Abstract:

Images of the giant planets Jupiter and Saturn show highly turbulent storms and swirling clouds that reflect the intensity of turbulence in their atmospheres. Quantifying planetary turbulence is inaccessible to conventional tools, however, since they require large quantities of spatially and temporally resolved data. Here we show, using experiments, observations, and simulations, that potential vorticity (PV) is a straightforward and universal diagnostic that can be used to estimate turbulent energy transfer in a stably stratified atmosphere. We use the conservation of PV to define a length scale, LM, representing a typical distance over which PV is mixed by planetary turbulence. LM increases as the turbulent intensity increases and can be estimated from any latitudinal PV profile. Using this principle, we estimate LM within Jupiter's and Saturn's tropospheres, showing for the first time that turbulent energy transfer in Saturn's atmosphere is four times less intense than Jupiter's.

Abstract:

The wintertime Southern Hemisphere extratropical circulation exhibits considerable zonal asymmetries. We investigate the roles of various surface boundary conditions in shaping the mean state using a semi-realistic, atmosphere-only climate model. We find, in agreement with previous literature, that tropical sea surface temperature (SST) patterns are an important contributor to the mean state, while midlatitude SSTs and sea ice extent play a smaller role. Our main finding is that Antarctic orography has a first-order effect on the structure of the midlatitude circulation. In the absence of Antarctic orography, equatorward eddy momentum fluxes associated with the orography are removed and hence convergence of eddy momentum in midlatitudes is reduced. This weakens the Indian Ocean jet, making Rossby wave propagation downstream to the South Pacific less favorable. Consequently, the flow stagnates over the mid- to high-latitude South Pacific and the characteristic split jet pattern is destroyed. Removing Antarctic orography also results in a substantial warming over East Antarctica partly because transient eddies are able to penetrate farther poleward, enhancing poleward heat transport. However, experiments in which a high-latitude cooling is applied indicate that these temperature changes are not the primary driver of circulation changes in the midlatitudes. Instead, we invoke a simple barotropic mechanism in which the orographic slope creates an effective potential vorticity gradient that alters the eddy momentum flux.