Vertical distribution of ice optical and microphysical properties in Arctic low-level mixed-phase clouds during ACLOUD

Abstract. Low-level (cloud tops below 2 km) mixed-phase clouds are important in amplifying warming in the Arctic region through positive feedback in cloud fraction, water content and phase. In order to understand the cloud feedbacks in the Arctic region, good knowledge of the vertical distribution of the cloud water content, particle size and phase is required. Here we present seven in-situ vertical profiles of cloud microphysical and optical properties measured in the European Arctic during the ACLOUD campaign. Late spring- and summer-time stratiform clouds were sampled over pack ice, marginal sea ice zone and open ocean surface with cloud top temperatures varying between −15 and −1.5 °C. The results show that, although liquid phase dominates the upper parts of the clouds, ice phase was frequently observed in the lower parts down to cloud top temperatures as warm as −3.8 °C. In the studied vertical cloud profiles the average liquid phase microphysical properties, droplet number concentration, effective radius and liquid water content, varied between 22 and 120 cm⁻³, 16 and 27 μm, 0.09 and 0.48 gm⁻³, respectively. The average ice phase microphysical properties, ice number concentration, effective radius and ice water content, varied between 0.01 and 35 L⁻¹, 24 and 75 μm, 0.003 and 0.08 gm⁻³, respectively. The elevated ice crystal number concentrations and ice water paths observed for clouds with cloud top temperatures between −3.8 and −8.7 °C can be likely attributed to secondary ice production through rime-splintering. Low asymmetry parameters between 0.69 and 0.76 were measured for the mixed-phase ice crystals with a mean asymmetry parameter of 0.72. The effect of the ice phase for radiative transfer was investigated for the four cloud cases potentially affected by secondary ice production. Generally the ice phase has only a minor effect to the cloud transmissivity and albedo, except in a case where ice phase dominated the upper cloud layer extinction. In this case cloud albedo was increased by 10 %. The presented results highlight the importance of accurate vertical information of cloud phase for radiative transfer and provide a suitable data set for testing microphysical parameterisations in models.