Persistent Supercooled Drizzle at Temperatures Below −25 °C Observed at McMurdo Station, Antarctica

Abstract

The rarity of reports in the literature of brief and spatially limited observations of drizzle at temperatures below −20 °C suggest that riming and other temperature‐dependent cloud microphysical processes such as heterogeneous ice nucleation and ice crystal depositional growth prevent drizzle persistence in cold environments. In this study, we report on a persistent drizzle event observed by ground‐based remote sensing measurements at McMurdo Station, Antarctica. The temperatures in the drizzle‐producing cloud were below −25 °C and the drizzle persisted for a period exceeding 7.5 hr. Using ground‐based, satellite, and reanalysis data, we conclude that drizzle was likely present in parts of a widespread cloud field, which stretched more than ~1,000 km along the Ross Ice Shelf coast. Parameter space sensitivity tests using two‐moment bulk microphysics in large eddy simulations constrained by the observations suggest that activated ice freezing nuclei and accumulation‐mode aerosol number concentrations aloft during this persistent drizzle period were likely on the order of 0.2 L−1 and 20 cm−3, respectively. In such constrained simulations, the drizzle moisture flux through cloud base exceeds that of ice. The simulations also indicate that drizzle can lead to the formation of multiple peaks in cloud water content profiles. This study suggests that persistent drizzle at these low temperatures may be common at the low aerosol concentrations typical of the Antarctic and Southern Ocean atmospheres.

Nonturbulent Liquid‐Bearing Polar Clouds: Observed Frequency of Occurrence and Simulated Sensitivity to Gravity Waves

“…Yet the radiative cooling (Figure 3b3) is too weak to drive sufficient instability to generate turbulence by the end of the simulation. These results are similar to the model sensitivity tests in Silber, Fridlind, et al (2019) where N c was 10 cm −3 or less (see their fig. S5).…”

“…To illustrate the impact of aerosol particle sizes on cloud lifecycle, we utilize the Distributed Hydrodynamic Aerosol and Radiative Modeling Application (DHARMA) large‐eddy simulation model (Stevens et al, 2002). We use a model configuration following Silber, Fridlind, et al (2019), which is to our knowledge the only reported observationally grounded modeling case study of supercooled LBC initiation and evolution in an elevated, stable layer, representing a plausible scenario over both poles. We use periodic lateral boundary conditions on a grid size of 1 km ( x ‐axis) × 1 km ( y ‐axis) × 10 km ( z ‐axis) with a 50‐m horizontal and 25‐m vertical mesh between 500 and 2,000 m, linearly stretched above and below.…”

“…4) domain‐wide large‐scale ascent in a Lagrangian reference frame, specified here as w LS = −t /3 + 2, where t is elapsed time in hours and w LS units are cm/s, omitted after diminishing to 0 cm/s at 6 hr. As in Silber, Fridlind, et al (2019), all simulations use Morrison et al (2009) two‐moment microphysics with a single ice class, aerosol particle consumption from droplet coalescence is neglected owing to negligible influence on results (not shown), and idealized immersion‐mode ice initiation is limited to 0.2 L −1 using the approach described by Ovchinnikov et al (2014). See Silber, Fridlind, et al (2019), their Appendix B) for additional details.…”

“…In the baseline simulation, we examine the sensitivity of cloud properties to the aerosol PSD. Whereas Silber, Fridlind, et al (2019) used an idealized marine aerosol log‐normal PSD with a geometric standard deviation of 2 and a mean radius of 0.1 μm, here we use a smaller geometric standard deviation of 1.5 and a smaller mean radius of 0.03 μm. The hygroscopicity parameter is set to 0.5, and total aerosol number concentration is set to 64 cm −3 , ~3 times higher than the baseline simulation of Silber, Fridlind, et al (2019).…”

“…Clouds that are optically thin could, therefore, result from a low number density of aerosol particles that are all large enough to activate or a high number density of small aerosol particles that are mostly too small to activate. Processes such as ice and drizzle formation may also modify cloud characteristics such as N c and optical thickness (e.g., Silber, Fridlind, et al, 2019), and consequently, the cloud radiative heating rate profile and turbulence formation time scale. At the same time, these processes also affect cloud glaciation and dissipation time scales.…”