The ATMOSFER campaign: synergistic observations for the preparation of the ESA FORUM mission

Abstract. The balloon-borne ATMOSFER campaign, supported by CNES, was held in Kiruna in June 2024. This campaign involved lightweight hygrometers (Pico-Light H₂O, NOAA GML frost point hygrometer, EN-SCI Cryogenic frost point hygrometer), optical particle counters and ice crystal imager (POPS, LOAC v1.5, LPC and NIXE-B), ozone sondes and the airborne demonstrator of the future ESA FORUM mission developed by CNR-INO (FIRMOS-B instrument). The instruments have been flown aboard Zero Pressure Balloon (ZPB) and small weather balloons (RS) by CNES and the scientific teams. The synergistic observations aboard stratospheric balloons captured the signature of multiple influences which are useful for preparation activities of the ESA FORUM mission. The measurements revealed stratospheric intrusions of sub-tropical air, the presence of upper tropospheric ice cirrus clouds and aerosol layers of small particles potentially originating from volcanic eruption (between 15 and 17 km of altitude) and forest fires (near the polar cold point tropopause), which will influence the vertical profile retrievals from FIRMOS-B radiances and provide interesting features for radiative closure studies. This article aims to give an overview of the observation potential of the campaign for further studies in the frame of the ESA FORUM mission. The datasets are publicly available in the HEMERA datacenter (https://data.hemera-h2020.eu/atmospheric-balloon-experiments/#/).