LAUSR

Abstract. Formation of ice particles in clouds at the temperatures of −10 °C or warmer was documented by using ground-based remote sensing observations. At these temperatures, the number concentration of ice nucleating particles (INP) is not only expected to be small, but also this number is highly uncertain. In addition, there are a number of studies reporting that the observed number concentration of ice particles exceeds expected INP concentrations, indicating that other ice generation mechanisms, such as secondary ice production (SIP), may play an important role in such clouds. To identify the formation of ice crystals and report conditions in which they are generated, W-band cloud radar Doppler spectra observations collected at the Hyytiala station for more than two years were used. Given that at these temperatures ice crystals grow mainly as columns, which have distinct linear depolarization ratio (LDR) values, spectral LDR was utilized to identify newly formed ice particles. Our results indicate that that the columnar ice production took place in 5 to 13 % of clouds, where cloud top temperatures were −12 °C or higher. For colder clouds, this percentage can be as high as 33 %. 40 ~ 50 % of columnar-ice-producing events last less than 1 hour, while 5 ~ 15 % can persist for more than 6 hours. By comparing clouds where columnar crystals were produced with the ones where these crystals were absent, we found that the columnar-ice-producing clouds tend to have larger values of liquid water path and precipitation intensity. The columnar-ice-producing clouds were subdivided into subcategories, using the temperature difference, Δ T, between the altitudes where columns are first detected and the cloud top altitude. The cases where Δ T  is less than 2 °C are typically single-layer shallow clouds where needles are produced at the cloud top. In multilayered clouds, where Δ T > 2 °C, columns are produced in a layer that is seeded by ice particles falling from above. This classification allows to study potential impacts of various SIP mechanisms, such as Hallet-Mossop process or freezing breakup, on columnar ice production. To answer the question whether the observed ice particles are generated by SIP in the observed single-layer shallow clouds, ice particle number concentrations were retrieved and compared to several INP parameterizations. It was found that the ice number concentrations tend to be 1 ~ 3 orders of magnitude higher than the expected INP concentrations.