In-cloud characteristics observed in US Northeast and Midwest non-orographic winter storms with implications for ice particle mass growth and residence time

Abstract. The spatial distribution of surface snowfall accumulation is dependent on the 3D trajectories of ice particles and their residence times through regions of ice mass increases and decreases. We analyze 42 non-orographic, non-lake effect winter storms in the Northeast and Midwest United States from the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) and Profiling of Winter Storms (PLOWS) field campaigns. In situ aircraft measurements (1 Hz, ∼100 m horizontal distance) yield key data on vertical air motions, RH_ice, and number concentration. When suitable airborne radar data are available, we sort the in situ measurements by distance from cloud radar echo top.

90 % of updrafts (vertical air motion ≥ 0.5 m s⁻¹) were ≤ 1.2 km across. Measurements obtained within 3 km of cloud echo top were twice as likely (14 % versus 7 %) to have vertical velocities capable of lofting precipitation-sized ice compared to points sampled at lower levels. Below the near cloud top generating cell layer, most of the storm volume has RH_ice ≤ 95 % consistent with sublimation.

Rather than precipitation-ice growth within broad areas of vertical air motions, observations indicate that ice growth in these storms primarily occurs episodically within layers of overturning cloud-top generating cells with scales ≤ a few km. Below the generating cell layer, conditions for ice growth are rarer, and the ice particles usually either persist or shrink during most of their descent. The observed distributions of ambient in-cloud conditions provide benchmarks for evaluations of winter storm model output.