Eddy-driven zonal jets and Rossby waves are common features of planetary atmospheres and oceans, organising the large-scale flow and influencing the dispersion and transport of material tracers and constituents. In the presence of relatively weak friction and forcing, zonal jets form a dominant component of the flow in a regime known as “zonostrophic”, characterized by strongly anisotropic energy spectra and the formation of slowly evolving systems of alternating zonal jets. This regime is characterized by two scales, Lβ ~ (Πε/β3)1/5 and LR ~ (Urms/β)1/2, where Πε is the transfer rate of the inverse energy cascade and β is the radial gradient of the Coriolis parameter. Their ratio is known as the zonostrophy index, Rβ = LR/Lβ. Zonal jets become discernible at Rβ ≥ 1.5 but are much stronger for Rβ > 2. Achieving such high values of Rβ in a laboratory is non-trivial, however. The atmospheres of gas giant planets are probably well within such a regime with Rβ ~ 5 [Galperin et al. Icarus 2014], though the Earth’s atmosphere and oceans are in a more friction-dominated state where Rβ ~ 1.5 – 1.8. In this study we have investigated the flow obtained in a rapidly rotating fluid on a topographic beta-plane in a cylindrical tank, subject to localised, periodic mechanical forcing along a radius. The experiments were carried out in the 5 m diameter rotating tank at the Turlab facility in Turin, Italy under the European High-Performance Infrastructures in Turbulence (EUHiT) programme. Velocity measurements were obtained using PIV in a horizontal plane a short distance below the free surface, while discrete particles floating on the surface were tracked to obtained their Lagrangian trajectories. The flow exhibited the spontaneous formation of persistent zonal jets, nonlinear topographic Rossby waves and intense vortical eddies (see image below). The large-scale flow was found to lie within the zonostrophic regime with Rβ ≥ 2.4. Diagnostics indicate the presence of an anisotropic dual (inverse/direct) KE cascade. The KE spectrum, however, seems unexpectedly consistent with recent f-plane turbulence models based on Quasi-Normal Scale Elimination [Galperin & Sukoriansky Phys. Rev. Fluids 2020], the implications of which will be discussed in the presentation.

A new dust data assimilation scheme has been developed for the UK version of the Laboratoire de Météorologie Dynamique (LMD) Martian General Circulation Model. The Analysis Correction scheme (adapted from the UK Met Office) is applied with active dust lifting and transport to analyze measurements of temperature, and both column-integrated dust optical depth (CIDO), τref, (rescaled to a reference level) and layer-integrated dust opacity (LIDO). The results are shown to converge to the assimilated observations, but assimilating either of the dust observation types separately does not produce the best analysis. The most effective dust assimilation is found to require both CIDO and LIDO observations, especially for Mars Climate Sounder (MCS) data that does not access levels close to the surface. The resulting full reanalysis improves the agreement with both in-sample assimilated CIDO and LIDO data and independent observations from outside the assimilated dataset. It is thus able to capture previously elusive details of the dust vertical distribution, including elevated detached dust layers that have not been captured in previous reanalyses. Verification of this reanalysis has been carried out under both clear and dusty atmospheric conditions during Mars Years 28 and 29, using both in-sample and out of sample observations from orbital remote sensing and contemporaneous surface measurements of dust opacity from the Spirit and Opportunity landers. The reanalysis was also compared with a recent version of the Mars Climate Database (MCD v5), demonstrating generally good agreement though with some systematic differences in both time mean fields and day-to-day variability.

We assimilate atmospheric temperature profiles and column dust optical depth observations from the ExoMars Trace Gas Orbiter Atmospheric Chemistry Suite thermal infrared channel (TIRVIM) into the LMD Mars Global Climate Model. The assimilation period is Mars Year 34 Ls = 182.3 - 211.4, covering the onset and peak of the 2018 global dust storm. We assimilated observations using the Local Ensemble Transform Kalman Filter with 36 ensemble members and adaptive inflation; our nominal configuration assimilated TIRVIM temperature profiles to update temperature and dust profiles, followed by dust column optical depths to update the total column dust abundance. The observation operator for temperature used the averaging kernels and prior profile from the TIRVIM retrievals. We verified our analyses against in-sample TIRVIM observations and independent Mars Climate Sounder (MCS) temperature and dust density-scaled opacity profiles. When dust observations were assimilated, the root-mean-square temperature error verified against MCS fell by 50% during the onset period of the storm, compared with assimilating temperature alone. At the peak of the storm the analysis reproduced the location and magnitude of the peak in the nighttime MCS dust distribution, along with the surface pressure diurnal cycle measured by Curiosity with a bias of less than 10 Pa. The analysis winds showed that, at the peak of the storm, the meridional circulation strengthened, a 125 m/s asymmetry developed in the midlatitude zonal jets, the diurnal tide weakened near the equator and strengthened to 10-15 K at midlatitudes, and the semi-diurnal tide strengthened almost everywhere, particularly in the equatorial lower atmosphere.