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THINICE is an international airborne field campaign that occurred in Svalbard in August 2022 co-organized by French, British and US colleagues. It was dedicated to studying the dynamics of Arctic cyclones (ACs), moist intrusions, tropopause polar vortices (TPVs), their interactions with cloud microphysics, sea surface and sea ice. Two instrumented research aircraft were deployed: the ATR42 aircraft operated by SAFIRE, the French facility for airborne research, and the Twin Otter aircraft operated by the British Antarctic Survey (BAS). The objectives of the ATR42 flights were to observe the structure and evolution of Arctic cyclones over the whole tropospheric column with a focus on mixed phase clouds within Arctic cyclones. The objectives of the Twin Otter flights were more on boundary-layer processes by measuring turbulent fluxes, surface heat fluxes and sea-ice surface elevation. After providing an overview of the field campaign, the presentation will focus on ATR42 flights that crossed warm fronts within Arctic cyclones and during which mixed-phase clouds have been intensively observed. We will show how the measurements made by in-situ cloud microphysical probes and the remote-sensing platform RALI composed of the Doppler radar RASTA and the lidar LNG on board the ATR42 aircraft have been combined to get a comprehensive picture of mixed-phase clouds properties., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The microphysical properties of low stratus and fog are analyzed here based on simultaneous measurement of an in situ sensor installed on board a tethered balloon and active remote-sensing instruments deployed at the Instrumented Site for Atmospheric Remote Sensing Research (SIRTA) observatory (south of Paris, France). The study focuses on the analysis of 3 case studies where the tethered balloon is deployed for several hours in order to derive the relationship between liquid water content (LWC), effective radius (Re) and cloud droplet number concentration (CDNC) measured by a light optical aerosol counter (LOAC) in situ granulometer and Bistatic Radar System for Atmospheric Studies (BASTA) cloud radar reflectivity. The well-known relationship Z = α × (LWC)β has been optimized with α ϵ [0.02, 0.097] and β ϵ [1.91, 2.51]. Similar analysis is done to optimize the relationship Re = f(Z) and CDNC = f(Z). Two methodologies have been applied to normalize the particle-size distribution measured by the LOAC granulometer with a visible extinction closure (R² ϵ [0.73, 0.93]) and to validate the LWC profile with a liquid water closure using the Humidity and Temperature Profiler (HATPRO) microwave radiometer (R² ϵ [0.83, 0.91]). In a second step, these relationships are used to derive spatial and temporal variability of the vertical profile of LWC, Re and CDNC starting from BASTA measurement. Finally, the synergistic remote sensing of clouds (SYRSOC) algorithm has been tested on three tethered balloon flights. Generally, SYRSOC CDNC and Re profiles agreed well with LOAC in situ and BASTA profiles for the studied fog layers. A systematic overestimation of LWC by SYRSOC in the top half of the fog layer was found due to fog processes that are not accounted for in the cloud algorithm SYRSOC.

The German research aircraft HALO (High Altitude Long range aircraft) can be equipped with a remote sensing payload to study cloud properties and water vapor profiles of the atmosphere. This package was first flown during the NARVAL (Next-generation Aircraft Remote sensing for VALidation studies) mission in December 2013 and January 2014. New missions are planned for 2016. The HALO microwave package (HAMP) consists of a cloud radar and microwave radiometers. The cloud radar is a nadir pointing Ka-band radar (35 GHz). The microwave radar radiometers are operating at 26 frequency channels in the K-band, V-band, W-band, F-band, and G-band. The active and passive microwave measurements are complemented by a water vapour and high spectral resolution lidar. During the NARVAL campaign a joint flight with the French research aircraft Falcon 20 carrying the RASTA radar was performed over France over a distance of about 500 km. The RASTA cloud radar is a multi-beam radar operating at W-band (95 GHz). While the Falcon was flying at cloud top at about 10 km MSL, HALO was flying at 12.5 km MSL. The complete joint flight was over clouds which were also precipitating for about half of the joint flight track. The differences in reflectivities observed at both wavelengths are negligible for the 3 - 4 km deep non-precipitating cloud. The difference is getting more significant (app. 15 dB) in an about 2 km deep layer above the melting layer. In rain the difference is up to 30 dB, due to differential attenuation (in rain and above the melting layer) and the fact that reflectivity at W-band of raindrops is limited due to Mie scatter when the size of the drops is in the same order like the wave-length. Additionally, the HALO Ka-band radar provides measurements of the linear depolarization ratio LDR. The combination of both radar observations at different wave length allows for the estimation of the properties of cloud and precipitation particles due to differential attenuation. This can provide more information than will be possible with a single radar system. Thus, the combination of both systems provides unique possibilities for the understanding of cloud physics and the development and validation of retrieval algorithms for current (e.g. Cloudsat and Calipso) as well as future (e.g. EarthCARE) satellite missions.