Prof. Peter Read

Abstract:

While its primary objectives were to study the interior of Mars and its present day seismic activity, the InSight lander also carried several meteorological sensors (primarily needed to differentiate true seismic signals from those produced by wind or passing vortices, or as part of a heat flow experiment) as well as cameras which could be used to monitor atmospheric and surface changes [1-6]. Although power became increasingly limited due to dust build-up on the lander’s solar panels [7], InSight’s Pressure Sensor measured nearly continuously at up to 20Hz for ~1.25 Mars years, giving the highest frequency pressure dataset yet obtained on Mars [8,9]. The Temperature and Winds for InSight (TWINS) instrument consisted of two booms pointing in opposite directions (such that at least one sensor would measure winds from a given direction with minimal influence from lander hardware). Each boom measured air temperature and winds at 1Hz nearly continuously for over one Mars year [8,10]. The Heat Flow and Physical Properties Package (HP3) regularly measured the diurnal variation of surface temperature [11,12], while aeolian observations revealed that vortices rather than linear wind stress were associated with the majority of particle motion events [10,13]. We will provide an overview of InSight’s meteorological and aeolian datasets, and show how we are using them to validate the predictions of four global and four mesoscale atmospheric models of InSight’s landing site in Elysium Planitia. The models used include Aeolis Research’s multiscale MarsWRF model (run at global and mesoscales) [14,15], the Open University’s global Mars model (in the form of the OpenMars reanalysis dataset, produced via data assimilation) [16], the global Mars version of LMD’s Planetary Climate Model [17], LMD’s mesoscale Mars model [18], and the Belgian version of the MarsWRF global model [19]. This work goes beyond previous pre-landing multi-model intercomparison and prediction efforts [e.g., 14] by assessing the performance of models against data and attempting to understand the reasons for differences, with the dual goals of better understanding the causes of weather phenomena at InSight and of improving Mars atmospheric model predictions of the near-surface environment. This is vital not only for improving future landing site predictions (which are key to planning Entry-Descent-Landing and surface mission operations), including the expected dust clearing from solar panels [7,20], but also for Mars science in general, such as improving the prediction of near-surface wind and dust lifting globally in order to better simulate the martian dust cycle and dust storms [21]

Abstract:

The regions of Saturn’s cloud-covered atmosphere polewards of 60^o latitude are dominated in each hemisphere near the cloud tops by an intense, cyclonic polar vortex surrounded by a strong, high latitude eastward zonal jet. In the north, this high latitude jet takes the form of a remarkably regular zonal wavenumber m=6 hexagonal pattern that has been present at least since the Voyager spacecraft encounters with Saturn in 1980-81, and probably much longer. The origin of this feature, and the absence of a similar feature in the south, has remained poorly understood since its discovery. In this work, we present some new analyses of horizontal wind measurements at Saturn’s cloud tops polewards of 60 degrees in both the northern and southern hemispheres, previously published by Antuñano et al. (2015) using images from the Cassini mission, in which we compute kinetic energy spectra and the transfer rates of kinetic energy (KE) and enstrophy between different scales. 2D KE spectra are consistent with a zonostrophic regime, with a steep (~n^-5) spectrum for the mean zonal flow (n is the total wavenumber) and a shallower Kolmogorov-like KE spectrum (~n^-5/3) for the residual (eddy) flow, much as previously found for Jupiter’s atmosphere (Galperin et al. 2014; Young & Read 2017). Three different methods are used to compute the energy and enstrophy transfers, (a) as latitude-dependent zonal spectral fluxes, (b) as latitude-dependent structure functions and (c) as spatially filtered energy fluxes. The results of all three methods are largely in agreement in indicating a direct (forward) enstrophy cascade across most scales, averaged across the whole domain, an inverse kinetic energy cascade to large scales and a weak direct KE cascade at the smallest scales. The pattern of transfers has a more complex dependence on latitude, however. But it is clear that the m=6 North Polar Hexagon (NPH) wave was transferring KE into its zonal jet at 78^o N (planetographic) at a rate of ∏_E ≈ 1.8 x 10^-4 W kg^-1 at the time the Cassini images were acquired. This implies that the NPH was not maintained by a barotropic instability at this time, but may have been driven via a baroclinic instability or possibly from deep convection. Further implications of these results will be discussed.

 

References

Antuñano, A., T. del Río-Gaztelurrutia, A. Sánchez-Lavega, and R. Hueso (2015), Dynamics of Saturn’s polar regions, J. Geophys. Res. Planets, 120, 155–176, doi:10.1002/2014JE004709.

Galperin, B., R. M.B. Young, S. Sukoriansky, N. Dikovskaya, P. L. Read, A. J. Lancaster & D. Armstrong (2014) Cassini observations reveal a regime of zonostrophic macroturbulence on Jupiter, Icarus, 229, 295–320.doi: 10.1016/j.icarus.2013.08.030

Young, R. M. B. & Read, P. L. (2017) Forward and inverse kinetic energy cascades in Jupiter’s turbulent weather layer, Nature Phys., 13, 1135-1140. Doi:10.1038/NPHYS4227

Abstract:

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:

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.