The prevalence of precipitation from polar supercooled clouds

Israel Silber,Daniel A. Knopf,Andrew S. Ackerman,Grégory V. Cesana,Johannes Verlinde,Ann M. Fridlind

doi:10.5194/acp-21-3949-2021

Abstract

Abstract. Supercooled clouds substantially impact polar surface energy budgets, but large-scale models often underestimate their occurrence, which motivates accurately establishing metrics of basic processes. An analysis of long-term measurements at Utqiaġvik, Alaska, and McMurdo Station, Antarctica, combines lidar-validated use of soundings to identify supercooled cloud layers and colocated ground-based profiling radar measurements to quantify cloud base precipitation. We find that more than 85 % (75 %) of sampled supercooled layers are precipitating over the Arctic (Antarctic) site, with more than 75 % (50 %) precipitating continuously to the surface. Such high frequencies can be reconciled with substantially lesser spaceborne estimates by considering differences in radar hydrometeor detection sensitivity. While ice precipitation into supercooled clouds from aloft is common, we also find that the great majority of supercooled cloud layers without ice falling into them are themselves continuously generating precipitation. Such sustained primary ice formation is consistent with continuous activation of immersion-mode ice-nucleating particles (INPs), suggesting that supercooled cloud formation is a principal gateway to ice formation at temperatures greater than ∼-38 ∘C over polar regions. The prevalence of weak precipitation fluxes is also consistent with supercooled cloud longevity and with well-observed and widely simulated case studies. An analysis of colocated microwave radiometer retrievals suggests that weak precipitation fluxes can be nonetheless consequential to moisture budgets for supercooled clouds owing to small liquid water paths. The results here also demonstrate that the observed abundance of mixed-phase clouds can vary substantially with instrument sensitivity and methodology. Finally, we suggest that these ground-based precipitation rate statistics offer valuable guidance for improving the representation of polar cloud processes in large-scale models.

Highlights

Supercooled clouds exert substantial radiative impacts on the surface energy budget over polar regions (e.g., Dong et al, 2010; Miller et al, 2015; Shupe and Intrieri, 2004; Silber et al, 2019b), and play an important role in Arctic amplification and solar absorption over the Southern Ocean (e.g., Cronin and Tziperman, 2015; McCoy et al, 2014, 2015; Pithan et al, 2018; Tan and Storelvmo, 2019)
We use long-term sounding and ground-based radar measurements to characterize the properties of precipitation from supercooled clouds over North Slope of Alaska (NSA) and Antarctic (McMurdo) sites and examine the influence of radar sensitivity on apparent precipitation occurrence
– roughly half of the detected supercooled cloud layers are seeded by ice precipitation from aloft, precipitation occurrence is only roughly 10 % lower from unseeded relative to all detected supercooled layers, indicating that supercooled clouds are commonly a source of ice in polar regions

Summary

Introduction

Supercooled clouds exert substantial radiative impacts on the surface energy budget over polar regions (e.g., Dong et al, 2010; Miller et al, 2015; Shupe and Intrieri, 2004; Silber et al, 2019b), and play an important role in Arctic amplification and solar absorption over the Southern Ocean (e.g., Cronin and Tziperman, 2015; McCoy et al, 2014, 2015; Pithan et al, 2018; Tan and Storelvmo, 2019). Major uncertainties in their representation in climate models (e.g., Cesana et al, 2015; Tsushima et al, 2006) ensue from knowledge gaps concerning the active processes affecting cloud life cycles (e.g., Tan et al, 2016; Kay et al, 2018). Both ice and liquid precipitation can form in supercooled clouds.

Methods

Findings

Discussion

Conclusion