Ice-nucleating particle versus ice crystal number concentrationin altocumulus and cirrus layers embedded in Saharan dust:a closure study

Abstract

Abstract. For the first time, a closure study of the relationship between the ice-nucleating particle concentration (INP; INPC) and ice crystal number concentration (ICNC) in altocumulus and cirrus layers, solely based on ground-based active remote sensing, is presented. Such aerosol–cloud closure experiments are required (a) to better understand aerosol–cloud interaction in the case of mixed-phase clouds, (b) to explore to what extent heterogeneous ice nucleation can contribute to cirrus formation, which is usually controlled by homogeneous freezing, and (c) to check the usefulness of available INPC parameterization schemes, applied to lidar profiles of aerosol optical and microphysical properties up to the tropopause level. The INPC–ICNC closure studies were conducted in Cyprus (Limassol and Nicosia) during a 6-week field campaign in March–April 2015 and during the 17-month CyCARE (Cyprus Clouds Aerosol and Rain Experiment) campaign. The focus was on altocumulus and cirrus layers which developed in pronounced Saharan dust layers at heights from 5 to 11 km. As a highlight, a long-lasting cirrus event was studied which was linked to the development of a very strong dust-infused baroclinic storm (DIBS) over Algeria. The DIBS was associated with strong convective cloud development and lifted large amounts of Saharan dust into the upper troposphere, where the dust influenced the evolution of an unusually large anvil cirrus shield and the subsequent transformation into an cirrus uncinus cloud system extending from the eastern Mediterranean to central Asia, and thus over more than 3500 km. Cloud top temperatures of the three discussed closure study cases ranged from −20 to −57 ∘C. The INPC was estimated from polarization/Raman lidar observations in combination with published INPC parameterization schemes, whereas the ICNC was retrieved from combined Doppler lidar, aerosol lidar, and cloud radar observations of the terminal velocity of falling ice crystals, radar reflectivity, and lidar backscatter in combination with the modeling of backscattering at the 532 and 8.5 mm wavelengths. A good-to-acceptable agreement between INPC (observed before and after the occurrence of the cloud layer under investigation) and ICNC values was found in the discussed three proof-of-concept closure experiments. In these case studies, INPC and ICNC values matched within an order of magnitude (i.e., within the uncertainty ranges of the INPC and ICNC estimates), and they ranged from 0.1 to 10 L−1 in the altocumulus layers and 1 to 50 L−1 in the cirrus layers observed between 8 and 11 km height. The successful closure experiments corroborate the important role of heterogeneous ice nucleation in atmospheric ice formation processes when mineral dust is present. The observed long-lasting cirrus event could be fully explained by the presence of dust, i.e., without the need for homogeneous ice nucleation processes.

Long-term profiling of aerosol light extinction, particle mass, cloud condensation nuclei, and ice-nucleating particle concentration over Dushanbe, Tajikistan, in Central Asia

“…The POLIPHON methodology could be applied to 328 nighttime observations. The technique was recently discussed with a focus on desert dust by Ansmann et al (2019b). A case study from Dushanbe was shown in that article to provide an overview of the potential of the POLIPHON method.…”

“…In this article, we mainly concentrate on the retrieval of height profiles of dust mass concentrations and cloudrelevant dust properties (CCN and INP concentration). The required dust conversion factors and parameters are derived from extended AErosol RObotic NETwork (AERONET) observations at Dushanbe (Ansmann et al, 2019b). For the nondust extinction-to-volume conversion, we used typical finemode conversion factors as presented by Mamouri and Ansmann (2017) for central Europe (Leipzig, Germany).…”

“…In the latter retrieval, the ice supersaturation level during the ice crystal nucleation process is required, and a typical value of 1.15 (115 % relative humidity over ice) is assumed. Dust CCN concentrations are estimated from profiles of number concentration of dust particles with a radii larger than 100 nm n 100,d (Ansmann et al, 2019b;Lv et al, 2018). In the estimation of the aerosol-pollution-related CCN concentration, the dry activation radius is assumed to be 50 nm.…”

“…Conversion to dust and non-dust particle mass, number, and surface area concentrations from dust and non-dust extinction coefficient 5 Estimation of CCN concentration from dust (r > 100 nm) and non-dust number concentration (r > 50 nm) 6 Estimation of INP concentration using dust number concentration (r > 250 nm) and temperature (immersion freezing) and using dust surface area concentration and temperature (deposition nucleation) Table 2. Applied values of the conversion parameters required in the POLIPHON retrieval Ansmann, 2016, 2017;Ansmann et al, 2019b). The conversion factors are explained in the text and are needed to convert particle extinction coefficients into particle mass concentrations and cloud-relevant parameters (CCN and INP concentrations).…”

“…2, we briefly provide technical details to the Polly lidar and the data analysis. The POLIPHON (Polarization Lidar Photometer Networking) data analysis scheme Ansmann, 2016, 2017;Ansmann et al, 2019b) was applied to de-rive aerosol-type-dependent particle optical properties, dust and non-dust (haze, smoke) mass concentration profiles, and cloud-process-relevant aerosol parameters such as cloud condensation nuclei (CCN) and ice-nucleating particle (INP) concentrations. In Sect.…”