Aerosol particles and clouds significantly contribute to the uncertainties in climate predictions. To improve the representation of aerosol particles and clouds in models, their properties and global distribution as well as their interactions will be investigated within the ESA/JAXA satellite mission EarthCARE, which is expected to be launched in 2023. For the first time EarthCARE will combine a cloud radar, a high spectral resolution lidar, a multispectral imager, and a broadband radiometer on one single platform. To use these measurements to their full extent, they have to be well calibrated, carefully validated, and fully characterized (e.g. with respect to sensitivity, spatial resolution, and sampling). Airborne measurements provide a unique opportunity to perform underpasses beneath the satellite track and to sample the same air masses over an extended area. Similar instrumentation to those used on EarthCARE can be employed on HALO, but with higher resolution and sensitivity.

PERCUSION is planned as a nine-week EarthCARE centered campaign that will focus both on EarthCARE characterization and on studies of deep convection over the tropical Atlantic close to the inter-tropical convergence zone (ITCZ) in mid to late summer (between August to October 2024). The objectives of PERCUSION are:

1. PERCUSION will validate EarthCARE both in terms of observed quantities (e.g. reflectivities and radiances) and derived products (e.g. water contents and cloud structures) to guide the use of its data in synergy with a new generation of geostationary satellites for global observation of tropical cloud structures in service of the second objective (O2).

2. PERCUSION seeks to understand how air-mass properties and meso-scale dynamical processes, influence the structure and dynamics of the ITCZ, and to quantify how the organization of both shallow and deep convection influences local and remote cloud radiative properties. In this respect, processes of special interest are: (a) Convective selfaggregation, (b) interactions of aerosol, cloud, and precipitation, (c) ice formation, and (d)
boundary layer dynamics and thermodynamics.

PERCUSION will consist of three parts: (i) test flights and EarthCARE validation flights out of Oberpfaffenhofen, Germany, (ii) research flights out of Barbados, and (iii) out of Cap Verde. One research flight will be used as ferry between (ii) and (iii). Along with two ferry flights the total flight hours amount 250h. All parts will contribute to the overarching goal to validate the new EarthCARE satellite. Oberpfaffenhofen (Germany) as operational base allows for measurement flights to the North Atlantic, Central Europe as well as into the Mediterranean area assuring an extensive validation and characterization of the satellite data under different conditions.

Measurements with the WALES system are well suited to discriminate different aerosol contributions and will help to investigate the quality of EarthCARE algorithms for aerosol classification and separation. WALES measurements were further used to derive the radiative effect of different aerosol layers, which is an essential step on the aim to achieve radiative closure. The cloud radar has demonstrated its high sensitivity and accuracy during former measurements, it is an ideal tool for water, mixed-phase, and ice cloud validation studies. A synergistic retrieval of combined radar and lidar measurements was adapted and further developed to be applied for HALO measurements. Thus, an independent algorithm as well as the EarthCARE algorithm can be applied on the HALO measurements to validate higher level products (Level 2b). In combination with the radiation measurements on board, HALO offers the full bunch of measurements to perform radiative closure studies, so that an EarthCARE-like payload on HALO enables to validate and characterize the whole chain of EarthCARE products.

During the tropical southern part of PERCUSION HALO will first be stationed at Barbados as to link the measurements to the ground observations from the Barbados Cloud Observatory (BCO) and also to the series of NARVAL campaigns, and to EUREC4A. Scientifically this part is complemented by sampling the Eastern Tropical Atlantic with stronger aerosol impact out of Cap Verde. PERCUSION has a different and broader perspective as its predecessor campaigns as it focuses on deeper convection close to the ITCZ and comprises different aerosol regimes over the entire tropical Atlantic.

PERCUSION plans to fly large circular flight patterns, encircling convective regions, with additional bisectional flights to sample the convective environment and measure the microphysical structure of the deep convective regions. It is intended to use the maximum ceiling of HALO to stay above possible cirrus outflow – typically at 13–14 km altitude. EarthCARE underpasses will build the core of these measurement flights. The large scale patterns are designed to measure and close the atmospheric energy budget about the ITCZ; which is closely related to the overall aim of EarthCARE, and relate this to the specific state of condensate (also temperature and water vapor) within the atmosphere, as deduced from combined active and passive observations. The lack of measurements in these environments, linking circulation to convection on different space and time scales, leaves a great number of questions that haven’t been answered by past studies. Measurements during PERCUSION will support associated modeling activities using storm resolving models and large-eddy simulations. Synergies with the ground based stations (BCO and measurements at Mindelo by the Leipzig Institute for Tropospheric Research (TROPOS) as well as the envisioned ship-based campaign BOWTIE further enhance research opportunities and activities.