The Prevalence of Precipitation from Polar Supercooled Clouds in Observations and Models

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 from 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. More than 85% (75%) of sampled supercooled layers are found to be precipitating over the Arctic (Antarctic) site, and more than 75% (50%) precipitating continuously to the surface. Such high frequencies can be quantitatively reconciled with previous lower estimates by considering differences in radar hydrometeor detection sensitivity. Moreover, we find evidence that our estimates should be considered a lower limit. The prevalence of weak precipitation is consistent with supercooled cloud longevity, and with well-observed and widely simulated case studies. We find that apparently non-precipitating cases are most common where supercooled layers are thinnest and warmest, and that precipitation occurrence exceeds 80% over a wide range of cloud top temperatures. Occurrence frequencies are only slightly reduced when cases with overlying ice (seeder-feeder) are omitted, indicating that supercooled layers are a sustained source of ice precipitation, consistent with persistent ice nucleating particle activation. We posit that these ground-based statistics offer valuable guidance for large-scale models. Finally, we compare the observational statistics with GISS ModelE3 climate model simulations using a state-of-the-art ground-based forward simulator.