Observations of Supercooled Liquid Water Layers at the top of Snow-Producing Nimbostratus Clouds During the IMPACTS Campaign

Supercooled liquid water (SLW) has been frequently observed during the NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign that took place in the winters of 2020, 2022, and 2023. The maintenance of SLW in the presences of otherwise fully glaciated clouds is not well understood. Past theoretical research has suggested that moderate magnitudes of updrafts associated with uniform ascent, waves, or turbulence may produce liquid saturation from an initially ice saturated cloud, generate an imbalance between the condensate supply and ice crystal growth, and result in SLW layers or mixed phase clouds (Rauber and Tokay, 1991; Korolev and Field, 2008).

In this study, we analyze airborne remote sensing and in-situ observations to understand the microphysical properties and dynamical processes that are involved in the production of a SLW layer on top of a snow-producing nimbostratus cloud. Three well-coordinated NASA ER-2 and P-3 flight legs sampled through the SLW layer multiple times within a 500-m deep cloud top for 1 hour. The NASA Cloud Physics Lidar (CPL) penetrated through an upper-level cirrus cloud and detected the SLW top of the lower nimbostratus. Wave activity within the nimbostratus was apparent in the W-band reflectivity and Doppler velocity fields. In-situ measurement from cloud probes and the Turbulent Air Motion Measurement System (TAMMS) observed large liquid water content (LWC) on the order of 0.5 g m^-3 and vertical air motions of 1-7 m s^-1 within the top 500 m of the cloud. In addition to the observations, high-resolution model analysis suggests that an optimum dynamical and thermodynamical environment may have contributed to wave activities and turbulence that provided sources of sufficient updrafts to generate the SLW layer.