The Prevalence of Precipitation from Polar Supercooled Clouds

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. 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.

Evaluation of the CAM6 Climate Model Using Cloud Observations at McMurdo Station, Antarctica

“…In addition, the observed liquid phase layers are generally located on top of the clouds and above ice phase (precipitation), while the simulated liquid and mixed phase layers are located underneath the ice phase at the bottom of the cloud layer. This observed feature is consistent with a previous study, which showed that ice precipitation at cloud base occurs frequently (∼75%) over McMurdo (Silber et al., 2021). It is likely that the vertical resolution of the model may be too coarse to represent the gradients necessary to maintain various dynamical processes.…”

“…Previous studies of in situ observations showed that mixed phase occurrence frequency increases when averaging the observations onto coarser spatial scales (D’Alessandro et al., 2019; Yang et al., 2020). Instrument sensitivities can also significantly affect the derived mixed‐phase cloud occurrence frequency, as demonstrated by comparisons between spaceborne and ground‐based radar observations (e.g., Silber et al., 2021). Thus, one should caution the spatial dependency of cloud phase distribution when comparing observations and model simulations at different spatial resolutions.…”

Predicting Frigid Mixed‐Phase Clouds for Pristine Coastal Antarctica

“…(2018) found INP concentrations several orders of magnitude lower than earlier values from Bigg (1973). The new findings seem to correspond with observations to the south over Antarctica that suggested INP concentrations are often sparse (e.g., Grosvenor et al., 2012; Hogan, 1986; O’Shea et al., 2017; Silber et al., 2021). The smaller INP concentrations should slow heterogeneous nucleation of liquid particles and could allow liquid clouds to persist longer.…”

“…In summary, accurate representation of INP concentrations appears to be critical for simulation of Antarctic clouds (see also Silber et al., 2021). Models such as the Community Atmosphere Model version 5 (CAM5) are already implementing prognostic cloud‐aerosol physics (Xie et al., 2017).…”

Habit‐Dependent Vapor Growth Modulates Arctic Supercooled Water Occurrence

“…This 60 m criterion also mitigates the influence of secondary ice production (SIP) mechanisms associated with fast falling ice and/or large drops, the general contribution of which to the total number of SIP events is still under active debate (e.g., Field et al, 2017;Korolev & Leisner, 2020;Luke et al, 2021). A cloud is flagged as "seeded" if, in any range gate within these 60 m, Ka-band ARM zenith radar (KAZR; Widener et al, 2012) echoes exist in at least 50% of the 2 s resolution measurements within 15 min after the radiosonde release time (see Silber et al, 2018Silber et al, , 2021. KAZR data are interpolated to the same 15 m vertical grid spacing as sounding data before use.…”