PHIPS Single-Particle and Bulk Optical Properties of Cirrus Ice Crystals from the CIRRUS-HL Campaign

The dataset used in this study was collected during the Cirrus in High Latitudes (CIRRUS-HL) campaign, conducted using the High Altitude and Long Range (HALO) research aircraft from June 6 to July 28, 2021. The campaign aimed to investigate the microphysical and optical properties of cirrus clouds across both mid-latitude and Arctic regions. Measurements include ice crystal microphysical data and angular light-scattering data obtained with the Particle Habit Imaging and Polar Scattering (PHIPS) probe. The data are organized into two datasets. The single-particle (PbP) dataset provides particle- resolved information on crystal size, habit, aspect ratio, projected area, and the associated angular light-scattering function of cirrus ice crystals. A temperature threshold of –38°C was applied to identify cirrus cloud regimes. The second dataset contains asymmetry parameters (g), retrieved for cloud volumes or for particle size and habit classes. These datasets support the findings presented in Järvinen & Schnaiter, ACP, 2026. Link to the mission home page https://halo-db.pa.op.dlr.de/mission/125 2.0 Instrument Description PHIPS is a single particle cloud probe developed under the lead of Martin Schnaiter (schnaiter@uni- wuppertal.de) and commercially available through the research company schnaiTEC GmbH. The instrument is installed underwing in a standard PMS canister. The instrument is a combination of a stereo-microscopic imaging unit and a polar nephelometer [1, 2]. The imaging part consists of two camera assemblies (hereafter denoted as C1 and C2), that simultaneously capture micro- scopic, brightfield images of ice crystals under an angular viewing distance of 120°. Incoherent illumination, provided by a pulsed 643 nm diode laser, is used to avoid diffraction aberrations in the microscopic images. The size of each image is 1360 x 1024 pixel. In CIRRUS- HL a 6-times magnification (zoom setting 3) was used, which corresponds to a frame size of 1.47 x 1.10 mm, corresponding to an optical resolution of 6.26 μm. The sensitive area of the cloud probe is 0.2 mm2 and the used image maximum acquisition rate was 10 Hz. 3.0 Data Processing: Image Post-processing The image post-processing procedure consists of the following steps, as discussed in Schön et al. [4]: Sharpen the image • smooth the original image with a Gaussian filter• subtract the smoothed image from the original image• add the result of the previous step multiplied by a factor to the original image Rescale the grayscale pixel values, to optimize the contrast of the image cutoff the grayscale values, so that they are in the range [min_values, max_value] scales the gray scale values in to the range of 0 (black) to 255 (white), with a power-law Noise reduction • replaces each pixel with the median of the neighborhood pixel values • reduces Gaussian noise Horizontal flip 3.1 Data Analysis: Microphysical parameters Based on an automated edge detection, developed by Roland Schön [3], several microphysical parameters of the ice crystals are calculated from the stereo-microscopic image pairs. For each imaging view one set of microphysical parameters are calculated reflecting the geometry of the particle in the given orientation. Only particles, that are fully captured within the image frame can be considered, other particles get NaNs. 4.0 Data Format 4.1 Particle-by-particle microphysical and light scattering data The PHIPS_CIRRUSHL_PbP_cirrus.nc file contains particle-resolved microphysical and optical measurements of cirrus cirrus ice crystals. The dataset includes 53,269 individual ice particle records, each characterized by the following variables: Angular light scattering: ASF(particle, angle) measured by the nephelometer, with 20 scattering angles in degrees (angle variable). Particle habit and morphology: habit_class (integer flag) and habit_class_name (string), as well as habit family habit_family and habit_family_name, derived from stereo-microscopic images. Particlesizeandshape(fromstereo-microscopicimages):projectedarea(proj_area_C1/C2), perimeter (perimeter_C1/C2), sphere-equivalent diameter (diameter_C1/C2), maximum dimension (dmax_C1/C2), maximum dimension Dw that is perpendicular with respect to Dmax dw_C1/C2), aspect ratio (aspect_ratio_C1/C2), roundness (roundness_C1/C2), and image coordinates (x_Position_C1/C2, y_position_C1/C2). Particlefeaturesandclassification:flagssuchasAggregate,ChainAgg,Rimed,Pristine, Sublimating, Air_Inclusion, Hollow, Atypical_Facet_Angle, Multiple, and Dendritic_Growth derived from images. Environmentalvariables:Temperature(°C),Height(ma.s.l.),lat/lon(degrees),S_ice (ice supersaturation), vert_vel (m s−1), and RH_liq (%) provided by the HALO BAHAMAS basic data sensor. Imageandtriggermetadata:TriggerIntensity,ParticleTimeOfFlight,ImageCamera1, ImageCamera2, and DropletFlag. 4.2 Data Format: PHIPS_CIRRUSHL_g_cirrus.nc The PHIPS_CIRRUSHL_g_cirrus.nc file contains cloud-volume-resolved microphysical and op- tical properties derived from the CIRRUS-HL campaign using the PHIPS probe aboard the HALO research aircraft. The dataset comprises 475 averaged cloud volume intervals, each representing a period during which 50 consecutive intact ice crystals were measured. Asymmetry parameters: g and g_err provide the asymmetry parameter and associated retrieval uncertainty for each averaging interval. Size- and habit-resolved asymmetry parameters are included (g_size_ml, g_size_arctic, g_aggregate, g_plate, g_column, g_frozen_droplet, g_side_plane, g_bullet_rosette) with corresponding uncertainties, derived from particle-resolved PHIPS measurements in cirrus (PHIPS_CIRRUSHL_PbP_cirrus.nc). Icemicrophysicalproperties:Icewatercontent(phips_IWC_mean),effectiveradius(phips_reff_mean), mass-equivalent radius (phips_rm_mean), number concentration (phips_N_mean), mean extinction coefficient (phips_b_ext_mean), aspect ratios (AR_mean), and mean area-equivalent diameter (diameter). Minimum and maximum values for the time period are included as well. Environmental variables (from HALO BAHAMAS basic data sensor): Latitude (lat), mean, minimum, and maximum in-flight temperature (T_mean/max/min), and mean, minimum, and maximum ice supersaturation (RHi_mean/max/min) during the averaging interval. Time metadata: Start and end of the averaging interval (time_start, time_end) in seconds since 1970-01-01 00:00:00 UTC. Size and habit bin definitions: size contains the particle diameter bin centers; habit and habit_name enumerate the seven most frequent ice crystal habit categories. 5.0 Data Remarks Data acquisition for RF12 ended before the conclusion of the measurement flight due to full memory storage. References [1] Ahmed Abdelmonem, Emma Järvinen, Denis Duft, Edwin Hirst, Steffen Vogt, Thomas Leis- ner, and Martin Schnaiter. Phips–halo: The airborne particle habit imaging and polar scatter- ing probe–part 1: Design and operation. Atmospheric Measurement Techniques, 9(7):3131– 3144, 2016. [2] Martin Schnaiter, Emma Järvinen, Ahmed Abdelmonem, and Thomas Leisner. Phips-halo: the airborne particle habit imaging and polar scattering probe–part 2: Characterization and first results. Atmospheric measurement techniques, 11(1):341–357, 2018. [3] Roland Schön. Entwicklung eines optischen Messgeräts zur Erfassung der Struktur von Eis- partikeln sowie der winkelaufgelösten Streuung im sichtbaren Spektralbereich. PhD thesis, 2006. [4] Roland Schön, Martin Schnaiter, Zbigniew Ulanowski, Carl Schmitt, Stefan Benz, Ottmar Möhler, Steffen Vogt, Robert Wagner, and Ulrich Schurath. Particle habit imaging using incoherent light: a first step toward a novel instrument for cloud microphysics. Journal of Atmospheric and Oceanic Technology, 28(4):493–512, 2011.