Search for: All records

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 dBZ_ekm⁻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 StatementTurbulent 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. 

Abstract High-resolution airborne cloud Doppler radars such as the W-band Wyoming Cloud Radar (WCR) have, since the 1990s, investigated cloud microphysical, kinematic, and precipitation structures down to 30-m resolution. These measurements revolutionized our understanding of fine-scale cloud structure and the scales at which cloud processes occur. Airborne cloud Doppler radars may also resolve cloud turbulent eddy structure directly at 10-m scales. To date, cloud turbulence has been examined as variances and dissipation rates at coarser resolution than individual pulse volumes. The present work advances the potential of near-vertical pulse-pair Doppler spectrum width as a metric for turbulent air motion. Doppler spectrum width has long been used to investigate turbulent motions from ground-based remote sensors. However, complexities of airborne Doppler radar and spectral broadening resulting from platform and hydrometeor motions have limited airborne radar spectrum width measurements to qualitative interpretation only. Here we present the first quantitative validation of spectrum width from an airborne cloud radar. Echoes with signal-to-noise ratio greater than 10 dB yield spectrum width values that strongly correlate with retrieved mean Doppler variance for a range of nonconvective cloud conditions. Further, Doppler spectrum width within turbulent regions of cloud also shows good agreement with in situ eddy dissipation rate (EDR) and gust probe variance. However, the use of pulse-pair estimated spectrum width as a metric for turbulent air motion intensity is only suitable for turbulent air motions more energetic than the magnitude of spectral broadening, estimated to be <0.4 m s⁻¹for the WCR in these cases. Significance StatementDoppler spectrum width is a widely available airborne radar measurement previously considered too uncertain to attribute to atmospheric turbulence. We validate, for the first time, the response of spectrum width to turbulence at and away from research aircraft flight level and demonstrate that under certain conditions, spectrum width can be used to diagnose atmospheric turbulence down to scales of tens of meters. These high-resolution turbulent air motion intensity measurements may better connect to cloud hydrometeor process and growth response seen in coincident radar reflectivity structures proximate to turbulent eddies.