Process-based microphysical characterization of a strong mid-latitude convective system using aircraft in situ cloud measurements

Abstract. Clouds in the mixed-phase temperature regime impose a large uncertainty onto climate prediction models, in part due to incomplete knowledge of the degree of glaciation affecting cloud radiative properties. To achieve a better representation of these clouds, it is crucial to improve the understanding of ice nucleation and growth as well as microphysical properties determining the cloud phase. In this case study, we provide a rare data set of aircraft in situ measurements in a strong mid-latitude convective system extending from the boundary layer to the tropopause and aim to extend the sparse database of such measurements. Data were obtained with the research aircraft HALO and cloud properties were probed with the Cloud and Aerosol Spectrometer (CAS-DPOL) and the Cloud Imaging Probe grayscale (CIPg) during the CIRRUS-HL mission above Southern Germany in July 2021. Microphysical properties of the convective cloud system were measured along a 58-minute stepwise descent between the ground weather stations of Hohenpeissenberg and Munich at temperatures of -35 °C, -23 °C, -13 °C, -7 °C, and -1 °C. A phase identification (liquid/ice) of particles with diameters > 50 μm was achieved using the particle images of the CIPg. Based on recent work, clouds were categorized into four groups with different microphysical properties: Mostly Liquid, Coexistence, Secondary Ice, and Large Ice. High concentrations of large ice crystals were observed in upper layers at temperatures between -35 °C and -13 °C, confirming the importance of the Wegener-Bergeron-Findeisen process for mid-latitude convection. Exceptionally high vertical motions for mid-latitudes of up to +/ 4 ms^-1 encountered in the convection promote various freezing and ice growth processes, which in this system led to high ice water contents of up to ~ 1.2 gm^-3 and to instrument icing. In contrast, low-level clouds near -1 °C encountered at lower vertical velocities were predominantly composed of liquid droplets and contained precipitated large ice in low concentrations. We find that mechanisms initiating ice nucleation and growth strongly depend on temperature, relative humidity, and vertical velocity and variate within the cloud system. Our measurements represent a unique in-flight data set on microphysical cloud properties of a strong midlatitude convective event and invite for detailed cloud model evaluations and radar intercomparisons with focus on the mixed-phase temperature regime.