Dependencies between ice number concentrations and ice water content in mixed-phase clouds during ACME-V

Description

Mixed-phase clouds have not yet revealed all their secrets. One intriguing problem is how mixed-phase clouds can remain in a steady state over longer periods of time. To answer this question, in-situ measurements are necessary to accurately determine fundamental cloud properties such as size distributions and phase discrimination. Here, data is analyzed from the ARM Airborne Carbon MEasurements (ARM-ACME V), for which the ARM Aerial Facility deployed the Gulfstream-159 (G-1) research aircraft from June to September 2015 over the North Slope of Alaska. During ACME-V, the two-dimensional stereo probe (2DS) measured hydrometeors with diameters above 20 µm and starting at diameters of about 50 µm it is possible to distinguish between liquid and ice phase, although for small sizes not entirely accurately. Additionally, the Fast Cloud Droplet Probe (FCDP) measured cloud droplets with diameters up to 50 µm. Water content was measured directly with the Water Content Monitor (WCM), which can differentiate between ice and liquid water content. These instruments, combined with the temperature and relative humidity measurements, give us information about the existence of mixed-phase clouds in the Arctic. Comparing two flight segments, both at a constant altitude and a continuous cloud deck sampled for more than 30 km (or 5 min flight time) and two temperatures, at ca. -10 °C (June 10^th) and at ca. 0 °C (August 8^th) shows two different results. The ice crystals mixed with cloud droplets are larger in the warmer case. Looking at the relationship between ice crystal number concentrations (n_i) and ice water content (IWC), August 8^th exhibits two distinct clusters at two slightly different temperatures, with higher IWC for periods above -3 °C. On June 10^th, the observed IWC and n_i are enveloped by two slopes of 1 and 2.5 in the log-log space, which agrees well with theoretical predictions from modeling studies, predicting these two slopes as the boundaries. This theory results from modelling steady state mixed-phase clouds that have repeated ice crystal formation while larger ice crystals fall out. With these measurements an estimate of ice nucleation rates based on theory is possible. However, while for some cases during ACME-V, e.g. August 8^th, the slopes converge towards low IWC and n_i, for other cases, e.g. June 10^th, the converging point is at higher values. This work tries to identify the reasons for the differences in a variety of cases and the contributing factors.