Idealized model simulations suggest that between one quarter and one third of the total clear-sky long-wave cooling of the Earth to space occurs within the Far InfraRed (FIR) spectral region. An even larger fraction of this cooling is predicted to occur in the FIR under all-sky conditions, because the presence of clouds causes lower emitting temperatures, and hence a shift to longer wavelengths of the peak of the black-body emission. For this reason, a detailed study of the Earth’s radiation budget requires accurate, frequent, and global-coverage measurements of the total upwelling radiance.

In November 2017 the European Space Agency (ESA) selected FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) as one of the two instrument concepts to be developed further and to compete to be the ninth Earth Explorer mission. FORUM will be a Far-Infrared Spectrometer measuring the upwelling spectral radiance emitted by the Earth across the most relevant infrared part of the electromagnetic spectrum from polar satellite. In particular, the instrument will cover the spectral range from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), covering the FIR, including the 100 – 667 cm−1 band, which has never been spectrally resolved from space before.

The main scientific objectives of FORUM include:
• Disentanglement and characterization of several climate forcing variables that contribute to the Outgoing Longwave Radiation (OLR) flux in the FIR spectral region, with a particular focus on water content in the Upper-Troposphere Lower-Stratosphere (UTLS) range, and cirrus clouds.
• Improvements in cloud models: need to achieve consistency of models in the Far- and Mid- InfraRed parts of the spectrum, retrieval of cloud micro-physical properties.
• Improvement of spectroscopy, with a particular focus on: achievement of inter-consistency between spectral band intensities in the Middle InfraRed (MIR) and FIR regions. Improvement of atmospheric continuum models in the FIR.
• Spectrally-resolved measurement of the emissivity in the FIR, of ice/snow in Polar regions.

As part of ESA Phase A activities, several studies are being carried-out to consolidate the FORUM requirements needed to meet the above scientific objectives. In this work we show how FORUM measurements, thanks to their high radiometric accuracy, will permit to obtain an accurate estimate of the upwelling FIR integrated radiance. In addition, the spectral resolution of the measurements will enable to study the individual contributions driving the outgoing long-wave flux: the atmospheric composition versus height, clouds and surface type and temperature.

Furthermore, FORUM mission will fly in tandem with the MetOp satellite, hosting the Infrared Atmospheric Sounding Interferometer – New Generation (IASI-NG). Since IASI-NG will measure only the MIR part of the upwelling atmospheric spectrum (from 645 to 2760 cm-1), the matching FORUM FIR spectra can be effectively used to synergistically complement the IASI-NG measurements which are less sensitive to the water vapor content in the UTLS, an important driver of the OLR flux. In this work results of (synthetic) synergistic FORUM and IASI-NG retrievals will be presented, along with an assessment of the information gained when FORUM measurements are available in addition to those of IASI-NG.

Area C - Board 298

The FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) mission concept has been selected as one of the two candidates missions for the ESA’s Earth Explorer 9 mission. FORUM will fly on a polar orbit with a ground footprint of 15 km diameter. The payload of FORUM is a Fourier Transform Interferometer (FTI) and an imager to be used to identify uniform scenes. The primary goal of the FORUM mission is the characterization of the Outgoing Longwave Radiation in the far-infrared and in the middle infrared regions, from 100 to 1600 cm-1. However, the measurement of the spectrally resolved emission of the Earth in that spectral region may be exploited to measure ice clouds, the vertical distribution of several trace gases (i.e. water, ozone) and the surface properties, especially over the polar regions where the relatively dry conditions enable to directly observe the Earth surface in the far-infrared. Moreover, the relatively high spectral resolution of FORUM will help in improving both the spectroscopy and the continuum knowledge in this so far underexploited spectral region.

FORUM is currently undergoing phase A and several studies are ongoing to address the capability of the proposed instrument to provide information on important climate variables like surface temperature and emissivity, vertical distributions of humidity, temperature, trace gases, and ice clouds. Among them the FORUM-Req project is devoted to the consolidation of the instrumental requirements for the FTI and the imager in order to ensure that the science goals of FORUM will be met by the developed payload. In this paper we summarize the results obtained in the frame of the FORUM-Req project with the simulations of FORUM measurements in clear sky conditions. A set of representative atmospheric and surface conditions have been selected and used to simulate the radiation that FORUM will measure in its lifetime. These simulations have been used to study the accuracy with which FORUM will measure the UTLS distribution of trace gases and the polar surface properties and the sensitivity of the obtained results to systematic biases.

Area C - Board 299

It has been acknowledged for more than three decades that cirrus clouds play an important role in the radiative balance of the Earth’s atmosphere, yet their radiative forcing on a global scale remains highly uncertain owing to the complexity of their ice microphysical properties. Harries et al. (2008) estimated that approximately 35% of the energy emitted to space occurs at far-infrared wavelengths (here defined as wavelengths between 16 and 100 microns), which increases to greater than 50% in the presence of cirrus clouds. In order to better understand their impact on the Earth’s energy balance, it is important to know their radiative properties across the visible and infrared spectral regions. The ability to simulate the mid- and far-infrared radiation spectrum emitted by cirrus clouds relies on the capability of optical models to represent the scattering and absorption properties of their complex ice crystals. There have been several recent advancements in cirrus cloud optical models by Baum et al. (2014) and Baran et al. (2014) that attempt to capture the bulk microphysical properties of these clouds spanning the entire visible and infrared wavelengths. Whilst there are many satellite observations of the reflected visible and emitted near- and mid-infrared radiation in the presence of cirrus clouds, there have been no such observations that span the far-infrared. This has severely restricted the ability to test the abilities of these models to represent the radiative signatures of cirrus in the far-infrared.

Here we exploit aircraft based measurements of nadir radiances above cirrus clouds, spanning the near- to far-infrared, obtained during the Cirrus Coupled Cloud-Radiation Experiment (CIRCCREX) to test the ability of the ice cloud bulk scattering models to represent the radiative signatures of cirrus clouds. We focus on observations obtained during the CIRCCREX campaign on the 13th March 2015 over a decaying band of cirrus of the north coast of Scotland, UK. Radiance spectra above the cirrus clouds are simulated for a variety of cases using a combination of in-situ and model data to constrain the cloud and atmospheric properties. Sensitivity of the simulated radiance spectra in the mid- and far-infrared to ice crystal size and habit are investigated, and comparisons are then made between the simulated radiance spectra and those observed by a near- and mid-infrared spectrometer (Airborne Research Interferometer Evaluation System, ARIES) and a far-infrared spectrometer (Tropospheric Airborne Fourier Transform Spectrometer, TAFTS). For the cases considered, results indicate that whilst it is possible to simulate the radiance spectra above cirrus clouds in the mid-infrared to within instrument uncertainties, such simulations were then unable to agree with measurements in the far-infrared to within the estimated uncertainties. Furthermore, it is found that multiple solutions for the cloud optical properties could satisfy agreement to within measurement uncertainties in the mid-infrared, whilst significant sensitivity in the far-infrared could be used to inform on the most consistent solution across the mid- and far-infrared.

These results demonstrate the need for more observations of the far-infrared spectrum to enable the further development and validation of cirrus cloud bulk optical models, which in turn will lead to an improved understanding of the impact of cirrus clouds on the Earth’s radiation balance.

References
Baran A. J., P. Hill, K. Furtado, P. Field and J. Manners. A Coupled Cloud Physics–Radiation Parameterization of the Bulk Optical Properties of Cirrus and Its Impact on the Met Office Unified Model Global Atmosphere 5.0 Configuration. J. Climate, 27, 7725-7752, 2014.
Baum, B.A, P. Yang, A.J. Heymsfield, A. Bansemer, B.H. Cole, A. Merrelli, C. Schmitt and C. Wang. Ice cloud single-scattering property models with the full phase matrix at wavelengths from 0.2 to 100µm. J. Quant. Spec. Rad. Trans., 146, 123-139, 2014.
Harries, J.E., B. Carli, R. Rizzi, C. Serio, M. Mlynczak, L. Palchetti, T. Maestri, H. Brindley and G. Masiello. The far-infrared Earth. Rev. Geophys. 46, RG4004, 2008.

Area C - Board 300

Water vapour is the most important greenhouse gas in the Earth’s atmosphere. An increase of 1K in the globally averaged surface temperature leads to an approx. 7 % increase in the averaged water vapour concentration in near-surface atmospheric layers. This figure approximately doubles the radiative effects of anthropogenic greenhouse gases such as CO2 and CH4 (water vapour feedback). While climate models can reproduce water vapour distributions observed in layers near the surface with good accuracy, there are still considerable uncertainties in the distributions in the upper troposphere and lower stratosphere and the underlying atmospheric processes. Due to the high sensitivity of atmospheric radiative forcing to changes in greenhouse gases in the cold tropopause region, even slight, not well understood, variations in water vapour in the lower stratosphere are an important source of the decadal variability of the surface temperature (e.g. Solomon et al., 2010). The talk gives an overview of our knowledge on the influence of dynamical processes on water vapour variability in the UTLS and possible contributions of FORUM.

Area C - Board 301

Retrieval of cirrus cloud properties from space will be possible for the first time using the far infrared if FORUM is selected for launch as an ESA explorer mission. Within this study, one of the current best cloud retrieval models that normally operates in the visible/mid-IR has been extended into the far infrared in order to investigate what advantages this spectra region will bring to our knowledge of cirrus. ORAC (Oxford-RAL Retrieval of Aerosol and Cloud) is a state-of-the-art optimal estimation based retrieval model and forms part of the Community Cloud Code for Climate (CC4CL). It is routinely used to retrieve cloud properties (cloud optical depth, particle size, cloud height) from a number of satellite instruments and is part of ESA’s Climate Change Initiative on clouds. ORAC uses the best available knowledge of the state of the atmosphere and surface and then combines this with a forward model that looks to simulate passive airborne or satellite measurements, enabling physical and spectral consistency. It also includes consideration of the uncertainties associated with the measurements and allows diagnostic information of the retrieved results.
Retrievals from ORAC are performed using aircraft measurements of upwelling far and mid infrared spectra over cirrus. The in-situ atmospheric measurements from drop-sondes and on board pressure, temperature and humidity instrumentation are also incorporated to attain the best knowledge of the atmosphere. This study focuses on a flight over the UK that was part of the Cirrus Coupled Cloud-Radiation Experiment (CIRCREX) campaign, flight B895 on 13th March 2015. High resolution far and mid infrared spectra were measured by the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS, 80-600cm-1 at 0.12cm-1 resolution) and Airborne Research Interferometer Evaluation System (ARIES, 550-3000cm-1 at 1cm-1 resolution) respectively. Narrow wavelength bands are selected from these spectra and used within ORAC to retrieve the cirrus cloud properties of optical thickness, effective radius and cloud height. The in-situ measurements of the cloud particles from aircraft instrumentation and Lidar are used to validate the retrievals. Sensitivity of the retrieval to ice habit is investigated, as well sensitivities to instrument noise, uncertainties and selection of channels.

Area C - Board 302

The far infrared (Far-IR) defined here as wavelengths > 15 μm, or equivalently wavenumbers < 667 cm-1 contributes approximately 50 % of the total outgoing longwave radiation to space in the global mean. Typically, strong water vapour absorption means that, even in clear-sky conditions, the surface is obscured when viewed from any significant altitude. However, the colder, drier conditions often encountered at high latitudes simultaneously shift the peak of the Planck function to longer wavelengths and mean that partially transparent micro-windows in the Far-IR open up, allowing surface emission to propagate further through the atmosphere. In theory this implies that Far-IR surface emission at these latitudes may be sensed from space, and hence that the polar energy budget at the top of the atmosphere is sensitive to Far-IR surface emissivity.

A series of papers using the Community Earth System Model (CESM) have illustrated the potential importance of incorporating realistic values of Far-IR emissivity. Feldman et al. (2014) introduces the intriguing possibility of an ‘ice-emissivity’ feedback, whereby the emissivity changes induced by melting sea-ice may accelerate subsequent surface warming by exposing the less emissive ocean below the ice. Kuo et al. (2018) shows how the magnitude of CESM’s cold wintertime bias in Arctic surface temperature, a common feature in climate models through the IPCC's AR3 and AR5, is reduced with the adoption of spectrally varying surface emissivities. Similarly, Huang et al. (2018) reports significant changes in sea-ice fraction due to the inclusion of spectrally varying infrared surface emissivity, and note that the sign of the sea ice-emissivity feedback, while small, depends critically on assumptions made concerning the surface geometry and overlying snow properties.

However, while the mid infrared (Mid-IR: 15 > wavelengths > 4 μm) surface emissivities used in these studies are based on observations, the Far-IR values are based on theoretical predictions and have not been directly validated. This missing evaluation is due to a lack of suitable spectrally resolved observations across the Far-IR from which surface emissivity can be inferred. Following on from initial work by Bellisario et al (2017), which exploited near surface flights to derive the first ever aircraft based estimates of Far-IR surface emissivity, here we present retrievals of infrared spectral surface emissivity made from the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) and Airborne Research Interferometer Evaluation System (ARIES), flying at an altitude of 9.2 km above sea-level over Greenland during March 2015. We describe the flight campaign, the atmospheric conditions, the instrumental setup and our retrieval method, including the quality control performed on the observations.

We find that the principal barriers to reducing uncertainty in our emissivity retrievals are instrument noise and calibration uncertainty, which also impact our ability to retrieve the underlying surface temperature. Part of the calibration issue is due to the lack of spectral overlap between TAFTS and ARIES. However, our results indicate that with the instrumentation available to us it is possible to retrieve emissivities from altitude with an uncertainty of ~ 0.01 or better across much of the infrared, with the frozen surfaces overflown showing distinct spatial and spectral variability. They show excellent consistency with retrievals from the same flight made by Bellisario et al. (2017) giving confidence in the methodology and results obtained for this more challenging viewing configuration. They also provide good insight regarding the expected ability of the Far Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission to retrieve high latitude surface emissivity.

Area C - Board 303

The SKIM “Sea surface KInematics Multiscale monitoring” mission is one of the selected mission for ESA Earth Explorer 9 Fast-Track programme. Thales Alenia Space is the instrument provider for this mission having developed relevant technology heritage for the SWIM instrument on-board CFOSAT launched late october 2018 and for KaRIn RF unit for the SWOT mission to be launched in 2021.

The SKaR radar under study by Thales Alenia Space is designed to measure total ocean surface velocity vector (primary goal), using a Doppler technique (so-called Pulse Pair processing) that is more direct than altimetry for accessing surface currents. From the measurements, directional surface spectrum using surface slope (given be power modulation spectrum as for SWIM) and orbital velocities (given by Doppler modulation spectrum) will be also obtained.

Accurate sea surface monitoring requires a complete measurement of the sea sate at different scale (from shortest wave components) to separate wave-current interactions. Therefore, SKaR combines in a single Ka-band instrument an accurate state of the art Nadir looking altimeter (sea level, wave height, ice freeboard, …) and a novel ocean wave+current spectrometer using 8 beams, one pointing at Nadir, two at 6° and five at 12° of incidence. A dense sampling of the swath is obtained thanks to a rotating system of the antenna feeds around the Nadir direction. The complete azimuth spread is covered by the conical scan at 3 to 10 rpm speed (value still to be selected at the time of this abstract) and an equivalent 300 km swath (Metop-SG orbit) is obtained.
The maximum incidence angle is driven by the instrument antenna design and also by the near incidence theory for observing the directional wave spectrum from space.

At the time of the "Living Planet - 2019” symposium, the on-going Phase A study results will be available and the Preliminary Requirements Review held. One of the major innovation is to implement on-board advanced ocean echo processing (usually performed on-ground) on space qualified re-programmable FPGA. Instrument detailed design, parameters optimisation, performances, interfaces and budgets as well as instrument technology maturity will be presented.

Area C - Board 304

The Sea surface KInematics Multiscale monitoring (SKIM) mission proposal, currently in competition for the ESA Earth Explorer 9 opportunity slot, is intended to observe sea currents from space by means of direct Doppler measurements of the sea surface velocity. Its current baseline design includes a rotating plate with a set of 8 beams with incidence angles of 0º, 6º and 12º, providing an effective satellite swath of around 270 km. One of its challenges is to perform measurements of surface radial velocity with resolution better than 7 cm/s in a 30 x 30 km sea grid applied along any direction within the swath, what is requested by a specific requirement in the System Requirements Document. Such a requirement has a key role in evaluating the performance and needs to be addressed carefully, since it relates the instrument azimuth resolution with the observed surface. In this contribution we evaluate the impact of different design and performance parameters to the SKIM final spatial resolution by applying a fit to an analytical approach based on the assumption of constant true velocity vector inside each cell of the grid. Among the parameters considered, there are the plate spin rate, the orbit height and the measurement repetition rate.

Area C - Board 305

1. INTRODUCTION
Near nadir Doppler Ka-band measurements over the ocean have shown sensibilities to sea surface kinematics [1]. The surfacic motions due to waves and current are the main geophysical contributors to the measured microwave Doppler shift. However, these measured velocities have complex relations to oceanic motion and theoretical as well as numerical inspections are necessary to fully understand the undelying physical interactions. The understanding of this relation is at the heart of the Sea surface KInematic Multiscale (SKIM) mission measurement concept whose main objective is to provide the best estimate of the underlying sea surface current. The purpose of this work is to show how the strongest contribution of the overall Doppler shift is due to waves motion (the so called wave bais) and how it can be estimated to finally extract the current contribution. A theroretical approach is presented and compared to experimental results obtained from airborne campain. Numerical simulations are also presented to confirm theoretical results by including additional phenomena that can be missed by the theoretical approach.

2. THEORETICAL APPROACH

Asymptotic analytical solutions provide the expected statistical properties of both the normalized radar cross section and its associated Doppler frequency. These developments are discussed for radar measurements highly resolved in the range direction. In such a real-aperture-radar imaging configuration, time-varying complex reflectivity modulations emerge depending on the range resolved ocean wave direction.

The spectrum of the modulated Doppler weighted cross section stays close to the surface wave vertical velocity spectrum but can be significantly modified by the spatial slopes who interact through the second order incidence derivative of the cross section.
This mean complex reflectivity measurements dominated by wind-dependent short waves gives a quantitative knowledge of the ambient sea state conditions, namely long wave directional spectral properties and kinematics. Combined range-resolved radar cross section and Doppler anomalies open new strategies to separate wave-induced motions and surface current.

3. NUMERICAL APPROACH

Theoretical approach have been compared to experimental study and have shown a particularly good agreement. However, airborne campains always fail to reproduce spatial configurations leading inevitably to some difficul extrapolation of the results. A numerical approach including the maximum of physical phenomenon and sensors signal processing is thus more than welcomed to mimic acquisitions in spatial configuration. At time, the new and original Doppler measurements that will be performed by Skar sensor onboard SKIM has no equivalent in space. It is thus of major interest to numerically simulate the back scattered signal in SKIM configuration and develop algorithm in accordance. Some of the objectives of this approach can be summerized as follow:
• Validate stochastic approach
• Simulate existing signal (SWIM/KuROS -like signals)
• Validate closed-form of Doppler wave bias
• Test SKIM configuration possibilities (optimization) Antenna rotation, altitude, number of beams, define and scale OBP,...
• Evaluate sensitivity to parameters:
– Geophysical : wind, waves, current, ...
– Instrumental : antenna pattern, noise, ...
– Configuration : altitude, PRF, macrocycle, rotation rate, ...
• Investigate non-linear instrument effects (range-bunching, ...)
• Investigate non-linear wave effects
• Develop algorithms
– Forward model, Sketch of reconstruction
– Processing (Wave spectrum restitution, Pulse-pair, ...)
• Validate the overall SKIM concept

In order to investigate these aspects, several simulation tools have been developed with complementary objectives:
• A numerical simulator based on the Remote Sensing Spatial Simulator library ; it is operated on the High Performance
Computer of Ifremer, France (datarmor).

• The SKIM End-to-End Performance Simulator (SEEPS) includes a high fidelity instrument transfer function allowing for
performance analysis over realistic scenarios of ocean surface currents. A focus on the range modulated cross-section and Doppler signals will be presented. Inversion algorithms in terms of wave spectrum and resilient current Doppler shift will also be presented with performance figures.

Area C - Board 306