Harmony was one of the three missions that was selected as ESA Earth Explorer 10 mission candidates. After a down-selection process at the end of the Phase-0 studies, Harmony, proceeded as the only remaining candidate to a Phase-A, which will close in the summer 2022. This presentation gives an overview of the mission, reflecting its current status.

The Earth is a highly dynamic system where transport and exchanges of energy and matter are regulated by a multitude of processes. The non-linear nature of the governing physics results in couplings between processes happening at a wide range of spatial and temporal scales, with cascades of energy flowing from the larger to the smaller scales and vice-versa.

The Earth System cannot be understood or modeled without adequately accounting for small-scale processes. Indeed, the parameterisation of the unresolved, sub-grid physical processes in global or regional models remains one of the main sources of uncertainty in climate projections, in particular with respect to air-sea coupling, cryosphere and clouds

Harmony is dedicated to the observation and quantification of small-scale motion and deformation fields, primarily, at the air-sea interface (winds, waves, and surface currents), of the solid Earth (tectonic strain), and in the cryosphere (glacier flows and surface height changes).

The retrieval of kilometre and sub-kilometre scale motion vectors requires concurrent observations of its components, which will be achieved by flying two relatively light-weight satellites as companions to a Sentinel-1D, with a receive-only radar as main payload. The resulting line-of-sight diversity will be exploited in combination with repeat-pass SAR interferometry to estimate tiny deformation rates in the solid Earth, and for land ice processes. It will also be used in combination with Doppler estimation techniques for the retrieval of instantaneous ocean and sea ice surface velocities. Over oceans, geometry-diverse measurements of the radar backscatter will further allow the retrieval of surface (wind) stress and wave-spectra. The Harmony spacecraft will also carry a multi-beam thermal-infrared payload, which in the presence of clouds will allow the retrieval of height-resolved motion vectors. The combination of surface currents, surface wind-stress, along with TIR derived cloud-top height and cloud-top motion vectors will provide an unprecedented view of the MABL. In absence of clouds, the TIR payload will provide simultaneous observations of the sea surface thermal differences,
providing a unique window to look at upper-ocean processes and air-sea interactions on the small ocean scales.

The formation flying architecture of Harmony enables the unique capability to reconfigure its flight formation so that instead of being optimised for the measurement of motion vectors, it is optimised for the measurement of time-series of surface topography. This will, among other outcomes, result in a globally consistent and highly resolved view of multi-annual glacier volume changes between well defined epochs, needed to better quantify the climatic response of glaciers. At the same time, Harmony will allow studying the seasonal and sub-seasonal processes from space that play a role in such responses, for instance by measuring variations in lateral ice flow and associated elevation changes simultaneously over large areas for the first time.

In order to study upper ocean processes and air-sea interactions, Harmony will provide kilometer-scale surface roughness, root mean squared slopes, and surface kinematics, in different viewing perspectives, reflecting the imprint of Marine Atmospheric Boundary Layer (MABL) eddies on the ocean surface. This provides information about both the surface wind vector, as well as surface current vectors and swell, and, importantly, the thermal disequilibrium between air and ocean.

This will lead to a more precise understanding of small-scale (submesoscale) impacts on air–sea fluxes, especially CO$_2$ fluxes, momentum, ocean heat uptake and overall energy pathways, to reduce uncertainties for lateral dispersion of pollutants and tracers, vertical transport and nutrient pumping.

The scientific goal of Harmony for the cryosphere is to bridge existing observational gaps in order to improve our understanding of the physical processes causing the widespread shrinkage of the cryosphere. These conceptually new observations will push back the existing limits by refining the reconstructions of past and ongoing glacier changes, by improving the representation of the driving mechanisms in regional and global models of ice flow dynamics and mass balance, by describing the unresolved complex processes allowing calibration and validation of sea ice models with more realistic rheology or by improving our understanding of the permafrost dynamics.

Harmony aims at providing, for the first time, worldwide integrated measurements of elevation changes and ice flow on glaciers and ice-sheet coastal areas, as well as localised elevation measurements of icebergs and ice shelves.

A particular strength of the interferometric capabilities of Harmony that the mission is able to measure large topographic changes and lateral displacements (scale of metres and tens of metres) through repeat XTI-mode elevation models and SAR offset tracking, and, at the same time, small changes (cm-scale) thanks to the diversity of SAR lines-of-sight.

Harmony aims to provide an integrated view of the dynamic processes that shape the Earth’s surface. For the Solid Earth, the scientific goals are to improve our understanding of tectonic and magmatic processes by bridging existing observational gaps with regard to strain rates, which are currently hindered by the lack of sensitivity to the North-South deformation component, and with regard to rapid elevation changes.

Coupled atmosphere and ocean boundary layer dynamics represents a crucial component of the Earth system. Air-sea interactions span several different time scales, from shorter scale weather pattern evolution and extreme meteorological events, to inter-annual/decadal climate variability and global change. Air-sea fluxes depend on many correlated properties. For instance, the exchange of momentum is caused by the shear resulting from the atmospheric wind and many other physical quantities, e.g. the ocean velocity, the sea state and the density stratification in the lower atmosphere and in the upper ocean. Similarly, heat fluxes depend on surface stress, roughness, temperature, and vertical transport in the turbulent atmospheric and oceanic boundary layers. Those physical characteristics and processes are often correlated, through the action of complex interactions. An increased coupling between surface winds and the stronger winds aloft emerges when warm sea surface temperature destabilizes the air column, influencing entrainment at the top of the boundary layer, and surface ocean currents affect roughness as demonstrated by the covariance of mesoscale features and surface wave-energy. Furthermore, secondary flows in the ocean and atmosphere boundary layers impact vertical transport and mixing, even producing non-gradient fluxes.

Despite the importance of the small scale processes in setting the conditions at the interface between air and water, the limited capabilities of present satellite and in situ instruments do not allow to properly characterize them. Indeed, most of our understanding of the dynamics at these scales comes from high resolution numerical modeling and theoretical studies, and only sparse observational analyses have effectively been carried out. Ocean fronts and eddies identified with aerial guidance, further seeded with drifters, occasionally provided some reference ranges and target requirements for future scientific missions. Reported analyses indicate divergence and vorticity values in the upper ocean that can largely, at time and very locally, exceed 5 and even 30 times the Coriolis frequency, which represent very extreme departures from balanced dynamics, and suggest intense vertical motion induced by sub-mesoscale features. Strong sub-kilometer scale variations of winds, associated with dynamical features such as atmospheric boundary layer rolls and convective plumes, have been reported, indicating that transport in the MABL depends on processes that are not represented in climate models.

To properly characterize the ocean and atmosphere boundary layer dynamics, it is thus crucial to identify - through the imprint they leave - small scale processes at the air-sea interface. Harmony will allow the retrieval of relative surface velocities, winds, and waves at kilometer to sub-kilometer resolution in target areas of high dynamical interest, providing information on divergence and vorticity both in the ocean and in the atmosphere. Simultaneous measurements of sea surface temperature under clear sky conditions and of cloud top heights & motion otherwise will either allow to link dynamical patterns to thermodynamic features or to estimate the thickness of the atmospheric layer directly affected by air-sea processes.

The identification of the presence of large variations in winds at sub-kilometer scale during intense events, such as in tropical cyclones, will also allow to identify the presence of local anomalies in air-sea fluxes and to link them to convective updrafts through the effect they have on relative vorticity near the surface, providing crucial information for the prediction of rapid intensification events in tropical cyclone development.

Even if the relatively long revisit clearly precludes the possibility to follow individual feature evolution outside the high latitude bands, these composite data will still represent fundamental new information that could be used to validate existing and future high-resolution numerical models and also to eventually downscale the observations from lower resolution products as those obtainable through satellite altimetry and scatterometers. This can be considered a realistic target considering the growing efforts towards the application and development of artificial intelligence tools combining techniques originally thought for computer vision tasks as super-resolution.

Spatially-detailed maps of three-dimensional surface displacements and topography change, and their temporal evolution, are essential for understanding and modelling geophysical processes that trigger earthquakes, landslides and volcanic events, and for the assessment of hazards arising from these phenomena. Current SAR missions are sensitive to vertical and east-west motions, but are extremely limited in their sensitivity to north-south motion. In its bistatic configuration, the Harmony mission will deliver 3D vectors of surface motion by means of differential repeat-pass InSAR methods. In areas where displacement is predominantly north-south, the ability to systematically measure the third dimension of displacement will reveal motions that have been invisible up until now, and in other areas will enable the resolution of ambiguities in the underlying physical processes that lead to earthquakes, landslides and volcanism. Resolving strain rates in tectonic regions down to 10 nanostrain/year will be a key target for the mission, which will encompass the majority of continental regions that lead to deadly earthquakes. In its cross-track configuration, as well as still providing 3D deformation maps, the Harmony mission will deliver time series of topographic change, providing high-resolution views of the active processes that reshape the Earth's surface. A key focus will be constraining changes that occur during volcanic eruptions, such as lava flows, dome growth and pyroclastic density flow deposits.

The unique bistatic configuration of the Harmony mission will allow the retrieval of 3D deformation maps with unprecedented accuracy. Current performance and end-to-end simulation results indicate that a deformation measurement performance close to 1 mm/yr @100 km can be achieved for all three dimensions. Similarly, the single-pass digital elevation models acquired in the cross-track configuration will have a point-to-point vertical accuracy of about 1-2 m at a resolution of 30 m x 30 m with the interferometric baselines being currently considered.

Besides presenting the main goals and motivations of the Harmony mission for solid Earth, this contribution will also show dedicated simulation results obtained with the performance simulator currently being developed in the frame of the science studies of the Harmony mission.

Land ice applications are one of the primary scientific goals of the ESA EarthExplorer 10 candidate mission Harmony. Specifically, the mission aims to
(i) provide a consistent, temporally and spatially highly resolved global glacier mass balance, filling major spatial gaps in the current observation of mountain glaciers and outlet glaciers of the ice sheets;
(ii) give new insights into the physical processes associated with the coupling between glacier mass change and ice dynamics and through that substantially improve understanding and prediction of rapid or even abrupt glacier changes, and the balance between vertical ice flow and mass accumulation/ablation;
(iii) provide crucial large-area information on the spatial distribution, extent and magnitude of subsidence and erosion in permafrost areas in order to estimate permafrost degradation and its local and global impact.
To fulfil these goals, the Harmony mission design consists of two passive (receive-only) SAR satellites that fly in constellation with Sentinel-1D. The two Harmony spacecraft will perform measurements of the signals transmitted by Sentinel-1D in either single pass cross-track interferometry (XTI) or stereo formation. From two time series (each one year long, data points every 12 days) of single pass XTI acquisitions, the first series at the beginning (year 1) and the second at the end of the mission (year 4 or 5), world-wide glacier elevation changes will be computed, responding to goal (i). The same approach will be used to detect major erosional processes in ice-rich low-land permafrost, such as thaw slumps, over a period of 4-5 years (goal iii). At the higher temporal resolution of 12-days for the individual series, the interferometric XTI digital elevation models (DEMs) will serve to extract seasonal and sub-seasonal glacier elevation changes. Remarkably, these fast repeat DEMs will be simultaneous with lateral ice displacements derived from repeat Sentinel-1D data using offset tracking, enabling a major step forward for linking variations in ice flow with vertical surface elevation changes (goal ii) and in addition improving the precision of the displacement measurements through the concurrent elevation maps and multi-view redundancy. When interferometric phase coherence persists across the 12-day repeat interval of subsequent acquisitions with XTI but in particular the Harmony stereo formation, this will lead to interferometric repeat pass measurement of three-dimensional Earth surface motion and deformation. Being able to separate out the vertical ice flow component on glaciers will allow comparison with accumulation or ablation to detect mass imbalance (goal ii). Over permafrost areas, Harmony’s interferometric 3D surface motion will improve measurements of seasonal and multi-year frost heave and thaw subsidence, and help detect additional lateral components of these two processes, which so far are assumed to act mostly vertically.

The upper ocean is of intense interest. Almost all human interactions with the ocean occur in its first hundred meters in depth. Most ocean life is concentrated in the upper ocean, and is currently largely altered by modified exchanges between the atmosphere and the ocean. Air-sea interaction processes in the upper ocean are also essential factors in determining future weather and climate.
This makes the upper ocean an important arena for science which transcends the boundaries of physics, chemistry, biology, meteorology and climatology. Digital twins of the Earth systems (DTEs) shall then be key tools to provide integrated capabilities to improve local and global predictions. DTEs will then use any sort of models, data-driven and/or model-driven ones, that provide sufficiently accurate representations that are being twinned/replicated.

Ideally, where accuracy of numerical simulations would be perfect, DTEs would use model-driven simulations derived directly from physics principles. The barrier of computational costs to reach this high accuracy shall certainly not preclude these physics-based approaches. But air-sea interactions are characterized by a too large number of scales interacting over a wide range of time and spatial scales. No computer can encompass the interacting dynamics of all hydrodynamic scales involved in ocean-atmosphere interactions – ranging from the scale of the Sun’s heating (∼10,000 km) down to the turbulence dissipation scale (∼1 mm). Computers can only simulate some of the scales. The others, at the unresolved scales of motion and exchanges, must be parameterized for each type of phenomenon (wave, eddy, current, rain/slicks, ...), in terms of its effects on the resolved scales. These neglected sub-grid processes must then be properly 'calibrated' and further taken into account in order to achieve accurate energy transfers at the ocean-atmosphere interface, as well as to ensue stable numerical simulations. Some very high resolution physics-based model can likely reach the necessary high accuracy, and will certainly be used to generate sets of reliable results to refine surrogate models or metamodels to enter DTEs. Yet, we must admit that computational simulations may also reach some limiting “irreducible imprecision” compared to measured quantities at the ocean-atmosphere interface for different turbulent regimes at the ocean-atmosphere interface, e.g. very extreme events. This limiting feature explains the observed irreproducibility among different model schemes which are supposed to be solving the same problem.

In that context, especially marine-atmosphere extreme events benefit from ultra-high resolution satellite observations and have long been known to be essential to determine whether the results seen in high-resolution ocean-atmosphere coupled models are realistic. In this endeavor, the Harmony mission seeks to provide more direct observations to help quantify and calibrate fine-scale air-sea interaction processes. The Harmony mission will thus pave this new regime of high resolution observations of upper ocean and lower atmosphere dynamics to inform and drive the developments of the next generation of simulation schemes to enter DTEs. In particular, Harmony directional and combined-observations down to very high spatial resolution will deliver evidences to improve the representions of the rapidly fluctuating ocean-atmosphere components. The Harmony mission will thus further produce a new systematic capability for dealing with the changing regimes of uncertainty at the ocean-atmosphere interface.