Working Group 2 (WG2) has spent its first year building the scientific and organisational foundations needed to pursue the full potential of lidar data, both on its own and in combination with other atmospheric sensors. Focusing on aerosol microphysical retrievals and cross-instrument synergies, WG2’s early activities reflect a clear ambition: to improve how lidar products are exploited, made representative, and harmonised across the network.

A Structure Built for Synergy:

We have organized the WG2 into three complementary subgroups, each addressing a distinct segment of the data treatment puzzle. 

WG2a: Focuses on microphysical retrievals and algorithms.

WG2b: Explores the synergy between lidar and Automatic Lidar and Ceilometer (ALC) networks, with an emphasis on 4D representativeness. 

WG2c: Investigates the synergy between lidar and radar, targeting aerosol–cloud interaction (ACI). 

Together, these subgroups form the backbone of WG2’s central objective: increasing the exploitation of lidar products in microphysical retrievals and multi-sensor synergies including sun photometers.

Research and Coordination Objectives:

On the microphysical retrievals side, WG2 is working to identify the current state-of-the-art algorithms and  the differences between them, while developing a common description of the underlying challenge of nonsphericity. The group aims in defining quality control parameters that will determine whether lidar data are suitable for standard microphysical retrievals within operational networks.

On multi-sensor synergies, WG2’s efforts are twofold. For lidar/ALC synergy, the group is currently, reviewing state-of-the-art methods for layer identification—including AI-based techniques—and investigating how 4D representativeness through co-located measurements can be achieved 

For lidar/cloud radar synergy, WG2 is identifying  cloud-relevant aerosol products (aerosol typing, microphysics, CCN, INP) near clouds, using collocated lidar (fluorescence, HSRL, Raman) and cloud radar measurements. This work will feed into an open-access dataset of collocated aerosol, cloud, and water-vapour observations, intended to deeper explore the potential of the synergistic retrievals but also  to support modelling studies under EU and ESA projects including CERTAINTY, CleanCloud, and AirSense.

Beyond the research itself, WG2 has set a clear capacity-building pursuit: fostering knowledge exchange and a joint research agenda with algorithm developers to support the integration of microphysical retrievals into operational networks, and bridging traditionally separate disciplines within atmospheric science to achieve genuine synergy breakthroughs.

First-Year Activities and Deliverable 2.1:

WG2 activities during Year 1 have centred on two parallel efforts by all three sub-WGs. The first is a survey of existing algorithms, the most useful synergies, and available datasets, which will feed directly into Deliverable 2.1: a report covering existing algorithms, the most valuable synergies, datasets from networks and field campaigns, and emerging synergy approaches. 

The second is the development of a database (as described above) of multisensor synergy campaigns, built around clear selection criteria including geographical coverage, campaign duration, and the availability of synergistic datasets.

Connecting with ACTRIS:

As with WG1, WG2 has also identified strong potential for collaboration with ACTRIS working groups closely aligned with EARLICOST’s goals. 

This mapping already shows promising connections. On the EARLICOST side, WG1 and WG2 subgroups are led by a range of specialists: HSRL (M. Wirth, D. Bortoli), Fluorescence (B. Gast), Profiling of water vapour and temperature (D. Suma), Microphysical retrievals (G. Ciocan, Z. Bona), Synergy Lidar/ALC (A. Geiss, A. del Águila), and Synergy Lidar/Radar (J.A. Bravo and M. Sicard). 

On the ACTRIS side, relevant expertise spans bioaerosols (Qiaoyun Hu and Xiaoxia Shang for ARS; Heike Wex, Julia Burkart and Fiona Tummon for in-situ), lidar developments including multi-field-of-view, fluorescence, and water vapour techniques (Cristofer Jimenez and Kalliopi Voudouri), and aerosol–cloud interaction (Patric Seifert, with further contributors to be confirmed). 

This shared expertise underpins WG2’s broader ambition of creating scientific and technical hubs where knowledge of innovative lidar-based science can be exchanged freely.

Looking Ahead for Year 2:

WG2’s next steps centre on deepening engagement with key infrastructure communities, including EARLINET, CloudNet, and E-PROFILE, and on preparing and and on developing and circulating targeted surveys to identified users and domain experts, covering existing algorithms, available datasets, and synergy approaches across the subgroups.With these foundations in place, the group is looking forward to productive discussions in the period ahead as it works toward its first substantive deliverables.

To support this progress, WG2 has access to a full suite of networking and mobility tools, including Inclusiveness Target Country conference grants (ITCG), Meetings (M), Short-Term Scientific Missions (STSM), Training Schools (TN), Virtual Mobility (VM), Virtual Networking Support (VNS), and Workshops (W) resources designed to ensure that WG2’s ambitions for data treatment and synergy can be realised by a genuinely connected, pan-European community.