Reflectivity-Inferred Microphysical Response to Turbulent Layers in Winter Mountain Storms

Abstract Shear and buoyancy gradients, often observed in midlatitude baroclinic and orographic winter storms, produce discrete layers of turbulence. These turbulent layers modify the distribution of supercooled liquid water (SLW), whose presence enables hydrometeors to grow to precipitation sizes faster than through vapor-ice deposition alone. Both the Wegener–Bergeron–Findeisen process and riming require SLW—heterogeneously distributed in response to the dynamic forcings at all superposed scales. The University of Wyoming W-band cloud radar Doppler spectrum width characterizes the air motion turbulence intensity in mixed-phase layer clouds after avoiding fall speed dominated regions through comparison to coarser radar turbulence metrics. Embedded layers of turbulent air motion are compared to quiescent cloud regions in either/both the upwind and downwind directions. Median radar reflectivity profiles characterize the vertical growth of hydrometeors in the vicinity of identified layers, and differences in these vertical reflectivity gradients comparing turbulent to nonturbulent regions quantify enhanced hydrometeor growth over the layer. Over the entirety of the Seeded and Natural Orographic Wintertime Clouds—the Idaho Experiment, this parameter demonstrates a statistically significant increase, −13.6 dB Z e km −1 (from −1.7 to −24.5, 95% computed confidence), in radar reflectivity echo power with distance downward for embedded turbulent layers compared to quiescent cloud nearby. The increased vertical particle growth rate for turbulent layers appears to result from spatially heterogeneous phase partitioning, increased SLW mass and extent, and enhanced collision/collection rates in these layers. These first two conditions are examined individually where turbulent layers or fall streaks are sampled in situ, while the latter agrees with modeling results but can only be inferred herein. Significance Statement Turbulent mixing in mixed-phase clouds is understood to enhance cloud hydrometeor growth. This study quantifies these effects on observed airborne W-band radar reflectivity over an entire field campaign targeting midlatitude winter storms and calls into question whether this linkage is diagnosed properly if at all in forecast and bulk microphysical models with coarse (greater than 500 m) grid spacing or vertical resolution.