Conference Agenda

In August 2018, ESA’s Earth Explorer mission Aeolus has been successfully launched to space. Since then Aeolus has been demonstrating its capability to accurately measure atmospheric wind profiles from the ground to the lower stratosphere on a global scale deploying the first ever satellite borne wind lidar system ALADIN (Atmospheric Laser Doppler Instrument).

In order to identify and correct the systematic error sources, guarantee and enhance the performance of ALADIN and the data quality of the wind products, several calibration and validation campaigns were implemented.

In the aspect of ALADIN calibration, the ALADIN laser frequency stability and its impact on wind measurement was assessed and the correction of wind bias for ALADIN using telescope temperatures was conducted. By monitoring the ALADIN laser frequency over more than 2 years in space, excellent frequency stability with pluse-to-pluse variations of about 10MHz (root mean square) is evident despite the permanent occurrence of short periods with significantly enhanced frequency noise (> 30 MHz). Another systematic error source is related to small fluctuations of the temperatures across the 1.5 m diameter primary mirror of the telescope which cause varying wind biases along the orbit of up to 8 m s−1. To correct for this effect ECMWF model-equivalent winds are used as a reference to describe the wind bias in a multiple linear regression model as a function of various temperature sensors located on the primary telescope mirror. In cases where the influence of the temperature variations is particularly strong it was shown that the bias correction can improve the orbital bias variation by up to 53 %.

Shortly after the launch of Aeolus, co-located airborne wind lidar observations, which employed a prototype of the satellite instrument – the ALADIN (Atmospheric LAser Doppler INstrument) Airborne Demonstrator (A2D), were performed in central Europe, meanwhile ground-based coherent Doppler wind lidars (CDLs) net was established over China, to verify the wind observations from Aeolus. In the first airborne validation campaign after the launch and still during the commissioning phase of the mission, four coordinated flights along the satellite swath were conducted in late autumn of 2018, yielding wind data in the troposphere with high coverage of the Rayleigh channel. The statistical comparison of the two instruments shows a positive bias (of 2.6 m s−1) of the Aeolus Rayleigh winds (measured along its LOS*) with respect to the A2D Rayleigh winds as well as a standard deviation of 3.6 m s−1. In the validation campaign over China, by the simultaneous wind measurements with CDLs at 17 stations, the Rayleigh-clear and Mie-cloudy horizontal-line-of-sight (HLOS) wind velocities from Aeolus in the atmospheric boundary layer and the lower troposphere are compared with those from CDLs. Overall, 52 simultaneous Mie-cloudy comparison pairs and 387 Rayleigh-clear comparison pairs from this campaign are acquired. It is found that the standard deviation, the scaled MAD and the bias on ascending tracks are lower than those on descending tracks. From the comparison results of respective Baselines, marked misfits between the wind data from Aeolus Baselines 07 and 08 and wind data from CDLs in the atmospheric boundary layer and the lower troposphere are found. With the continuous calibration and validation and product processor updates, the performances of Aeolus wind measurements under Baselines 09 and 10 and Baseline 11 are improved significantly. Considering the influence of turbulence and convection in the atmospheric boundary layers and the lower troposphere, higher values for the vertical velocity are common in this region. The vertical velocity could impact the HLOS wind velocity retrieval from Aeolus.

Aeolus has the capability to measure wind profiles and aerosol optical properties profiles synchronously, which provides the possibility for studying the wind-driven evolution of aerosol. Combining the measurements of ALADIN/Aeolus and the data from other spaceborne sensors, together with NWP models, wind-driven dust aerosol transport and marine aerosol production are discussed, respectively.

Based on the observation of ALADIN, combined with the data of CALIOP, AIRS, ECMWF and HYSPLIT, a long-term large-scale Saharan dust transport event which occurred between 14 and 27 June 2020 is tracked and the possibility of calculating the dust mass advection is explored. The dust event's emission phase, development phase, transport phase, descent phase and deposition phase on 15, 16, 19, 24 and 27 June are captured by the quasi-synchronization observations of ALADIN and CALIOP, which is verified with the AIRS Dust Score Index data and the HYSPLIT trajectories. The dust mass advection of each transport phase is calculated.

Based on the observation of ALADIN and CALIOP, combined with the data from ECMWF, three remote ocean areas are selected and the optical properties at 355 nm of pure marine aerosol are derived. Then the optical properties are analyzed and discussed combined with the wind speeds. Eventually, the relationships between the marine aerosol optical properties and the wind speeds are explored at two sperate vertical atmospheric layers (0-1 km and 1-2 km, correspond to the heights within and above marine atmospheric boundary layer), revealing the marine aerosol related atmospheric background states. The optical properties present increasing trends and fitted by power law function with wind speed in all cases, implying that the atmosphere of the two vertical layers will both receive the marine aerosol input produced and transported by the wind and the turbulence. As derived data, the averaged marine aerosol optical depth and the averaged lidar ratio are acquired and discussed along wind speed bins.

Global observations of column carbon dioxide concentrations and aerosol optical properties profiles are important for climate study and environment monitoring which is why China decided to implement the lidar mission ACDL (Aerosol and Carbon dioxide Detection Lidar) to measure CO2 and aerosol from space – has been launched to space successfully on 16 April 2022. The commissioning phase of ACDL is scheduled to be 6 months, during which the calibration and validation campaigns are implemented and the retrieval algorithms of column carbon dioxide concentration and aerosol optical properties profiles are improved. It is expected that with the calibrations and validations of ACDL and the updates of retrieval algorithms, the products of ACDL will be accurate and robust for science applications.

Remote sensing of ocean color over coastal waters is challenging and these difficulties can be placed in 3 categories: i) adverse atmospheric conditions associated with the presence of thin clouds or thick aerosol plumes (sometimes biomass burning), ii) challenging environments found over or around the water target (boundary conditions); iii) extreme conditions associated with the water content in optically active constituents (high concentrations of sediments). Evaluation and improvements of the estimation of bio-optical and biogeochemical parameters is an indispensable task for accurately monitoring the dynamics and the quality of coastal waters through the use of ocean color remote sensing. Especially, with the improvement of sensor ability and the advent of novel retrieval algorithms/models, ocean color is playing a more and more important role in understanding the utilization, the protection and the management of coastal environments. Ocean color data can thus provide biogeochemical data with known uncertainty, which is of great importance for quantitatively characterizing variation of key elements in coastal ecosystem and is required for input in modelling. Sentinel 3A/3B is new generation ocean color missions in Copernicus program in Europe, while HY-1C/1D is the first operational ocean color satellites in China. Their optical sensors (OLCI for Sentinel 3 and COCTS for HY-1) provide invaluable knowledge of ocean ecosystems due to their large swath and frequent coverage.

This project aims at tackling those issues over European (mainly French) and Chinese coastal waters. The main scientific objectives concern the monitoring of the quality of the French and Chinese coastal waters using OLCI and COCTS/CZI space-borne sensors. The project is divided into different tasks: (1) Characterization of uncertainty of OLCI and COCTS/CZI ocean color products in coastal waters; (2) Development of novel regional EO datasets in coastal waters. The first task aims at evaluating the atmospheric correction and bio-optical algorithms of OLCI and COCTS/CZI in our two areas of interest using in-situ measurements collected by both teams and the second task aims at developing regional bio-optical algorithms for the Chinese/French coastal waters according to specific spectral configuration of COCTS and OLCI.

During the symposium, we will present the validation results of OLCI and COCTS L2 products over different coastal waters across Europe and China. In this report, reference data including both aerosol and sea-water reflectance are acquired by an automatic photometer (CE318-TV12-OC, also called SeaPRISM) manufactured by CIMEL corporation (France). It measures the sun, sky and sea surface periodically, from which aerosol optical thickness (AOT) and Remote-sensing reflectance (Rrs) can be derived. This instrument has already been deployed in AERONET-OC network. Four CE318 are selected across Europe and China, two in Europe and two in China. They are all deployed on offshore platforms where sea water demonstrates different optical signatures. With temporal coverage spanning between January 2020 and December 2022, validation results show that (1) OLCI can provide AOT generally in agreement with in-situ data but tends to over-estimate AOT in both European and Chinese waters. Such over-estimation is more notable in Europe. (2) Irrespective of water types, AOT from COCTS shows no obvious over-/under-estimation in general, but demonstrates significant uncertainty (i.e., big dispersion). (3) Rrs from OLCI agrees very well with in-situ measurements in most visible-infrared bands. (4) COCTS tends to under-estimate Rrs across various waters. Furthermore, validation results for NASA L2 ocean color products (e.g., MODIS/AQUA) with same CE318 dataset will also be presented for inter-comparison. Also, consistency will be checked among ocean color products.

Finally, spatial-temporal analysis of the variability of the concentration of chlorophyll-a concentration is analyzed for the OLCI sensor over European coastal waters. This analysis is part of the PhD thesis of a young scientist. The trend, seasonal and intra-seasonal patterns are analyzed between 2016 and 2023. Future work plan and young scientist training will also be presented.