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Development of GRASP radiative transfer code for the retrieval of aerosol microphysics vertical-profiles from space measurements and its impact in ACE mission

Periodic Reporting for period 1 - GRASP-ACE (Development of GRASP radiative transfer code for the retrieval of aerosol microphysics vertical-profiles from space measurements and its impact in ACE mission)

Reporting period: 2018-03-01 to 2020-02-29

The focus of this project is on advancing in methodologies for atmospheric aerosols characterization. Actually, the Executive Summary of the 2013 IPCC's states that reducing the uncertainty in direct aerosol radiative forcing is a necessary step in reconciling estimates of radiative forcing and the equilibrium climate sensitivity of the Earth so that future predictions of surface temperature associated with climate change can be made with confidence. To that end, improved aerosol knowledge about vertically resolved measurements is essentia and particularly aerosol absorption characterization. Previous space missions (e.g. e.g. Terra, Aqua, POLDER, CALIPSO or CloudSat platforms) were not able to provide layer-resolved aerosol absorption. The NASA Aerosol-Cloud-Ecosystems (ACE) mission is 2007 Decadal Survey that addresses this problem. ACE was replaced by the Aerosol-Cloud-Convection-Precipitation (ACCP) mission following 2017 Decadal Survey and is already on phase of construction. ACCP includes multiwavelength lidar and polarimeter flying together but still there is no appropriate software for characterizing aerosol microphysical and optical properties from the space.

The main objective of this proposal is the development and implementation of the joint inversion in GRASP for space polarimeter + lidar instruments to retrieve independent and accurate vertical profiles of aerosol microphysical properties. Such an approach is critical for future space missions such as ACE and ACCP, and will allow aerosol microphysics vertical-profiles, particularly for absorption properties. To fulfill this objective we divide the goals in:
1. Study of capabilities of the joint inversion through new mathematical developments and sensitivity test analyses.
2. Evaluation of the GRASP joint inversion from field campaign data and their advantages over the classical stand-alone 3β+2α lidar inversion.
3. Exploring merging data of different satellite missions for implementing GRASP joint inversion.
4. Defining synergies in ground-based networks for the evaluation of aerosol microphysics satellite products.

The consortium to be created in this proposal fits with the general scope of Marie Curie RISE call to promote international and inter-sectorial collaborations as we proposed staff exchange among the institutions to share knowledge and ideas from the last research developments in aerosol sciences. We propose a set of secondments, workshop and training activities that will also foster a shared culture of research and innovation to promote creativity and to help creative ideas into innovative research, product and services.
During this first two-years period we carried out in depth study of capabilities of the regularization techniques developed by for the retrieval of vertical-profiles of aerosol microphysical properties from multiwavelength lidar system. Such developments based on the regularization technique such retrievals can be initially obtained by the well-known lidar configuration of measurements of three backscattering (355, 532 and 1064 nm) and extinctions (355 and 532 nm), known as the 3β+2α. During the studies carried out we study the need of constraining the 3β+2α configuration particularly for the range or radiuses and refractive indexes. The focus was for spherical particles and we found out that case-dependent optimized-constraints are the most suitable for accurate retrievals of aerosol absorption properties. The use of spectrally dependence of extinction-to-backscatter lidar ratio (LR) Angstrom exponent of extinction permitted to determine case-dependent optimized-constraints borrowing information from the analyses of AERONET inversion database, which is a more reliable inversion due to the larger information content. All previous developments were evaluated using NASA HSRL-2 measurements during DISCOVER-AQ in California, Texas and Colorado. The agreement with in-situ airborne data was very good being differences in single scattering albedo within the uncertainties. Eventually, the 3β+2α inversion approach just discussed was implemented in GRASP.

We also carried out theoretical development of the solution of the ill-posed problem for the combination of multiangle and multiwavelength polarimeter measurements and multiwavelength lidar. For obtaining aerosol microphysical properties (retrieved) from aerosol optical (measured) properties we need to solve an ill-posed problem that requires analyses. Current GRASP version already incorporates a numerical module based on least squares analyses for polarimeter measurements. We evaluated the capabilities of the inversion module for space lidar and polarimetry signal and implement more equations to fulfill the inversion.
The retrievals of aerosol microphysics from the stand-alone 3β+2α lidar inversion using case-dependent optimized-constraints opened new opportunities for the characterization of aerosol absorption vertically resolved. The implementation of the new approach of the 3β+2α lidar inversion in GRASP has expand the possibilities of GRASP software for aerosol retrievals and is allowing GRASP to become an universal software for characterizing aerosol microphysical and optical properties due to its versatility and multiple applications.

The development of a joint inversion that allows the retrieval of aerosol microphysical and optical properties vertically resolved combining multi-angle and multi-wavelength polarimetry with backscattering lidar measurements open new possibilities not explored until now. Such configuration is critical for space systems because the stand-alone 3β+2α lidar inversion faces with the limitations of deploying cost-effective space lidar systems capable of obtaining accurate extinction measurements. The joint inversion has already been implemented in GRASP and during the next two years of project we will perform an evaluation stage using airborne measurements. The success of the joint inversion development will made GRASP suitable for obtaining aerosol microphysics and optical properties from the polarimeter and backscattering lidar planned in the upcoming Aerosol-Clouds-Convection-Precipitation (ACCP) NASA mission.

The GRASP software has been used within the H2020 Aerosols, Clouds and Trace gases Research Infraestructure (ACTRIS-2) through the combination of ground-based sun photometry and backscattering lidar measurements to obtain aerosol microphysics and optical properties vertically-resolved. In the GRASP-ACE project we have extended such applications, and particularly for nighttime when no sun-photometry measurements are available. The approaches proposed are based on combining backscattering lidar measurements with moon aureole and direct moon measurements with moon-photometers. Also, the continuity with closed daytime retrievals with sun-photometry was studied. The results were very promising in the day-to-night continuity studies. These results are being taken as reference for future evaluations of satellite missions such as ACCP using extensive lidar networks such as EARLINET-ACTRIS or MPLNET which operate many lidar instruments, most of them being only backscattering lidar. Other approaches will be studied during the next two years of the GRASP-ACE project such as those that use sun-photometry measurements with spectroradiometers or with polarization capabilities.