How Do Small-Scale Updrafts Such as KH Waves Affect the Seedability of Clouds near Complex Terrain?

Effective cloud seeding requires the presence of super-cooled liquid water (SLW) and the absence of natural ice production. When no natural ice production exists, updrafts increase the environmental supersaturation and the amount of SLW. The western slopes of the mountains in the western US typically experience upslope flow and a clean Pacific moisture supply in the cold season, favoring the development of layers of high SLW concentrations, suitable for glaciogenic cloud seeding. On the scale of individual terrain ridges, updrafts associated with vertically propagating waves may enhance the amount of SLW and improve a cloud’s seedability. On an even smaller scale, orographic clouds often encounter locally enhanced wind shear leading to Kelvin Helmholtz (KH) waves (Grasmick and Geerts 2019, under review). The repeated vertical velocity fluctuations and shear-induced turbulent mixing associated with KH waves may (1) locally enhance SLW amounts, (2) broaden droplet size distributions and accelerate drizzle development, and (3) naturally produce ice through ice initiation and/or ice multiplication processes. If active, such drizzle production and/or natural ice production may reduce the effectiveness of cloud seeding by removing super-cooled liquid from clouds.

KH waves are remarkably common in deep stratiform precipitation systems especially in the vicinity of complex terrain, as is evident from transects of vertical velocity and 2D circulation, obtained from an airborne profiling Doppler radar, the Wyoming Cloud Radar. The high resolution of this radar (~30 m) allows detection and depiction of KH waves in fine detail. In this study, airborne Doppler radar data are used to explore the kinematics of KH waves, including the response of thermodynamic and kinematic variables above and below the instability layer. These waves are observed in a variety of wavelengths, depths, amplitudes, and turbulence intensities. KH waves are frequently locked to the terrain, and occur at various heights, including within the free troposphere, at the boundary layer top, and close to the surface. They are observed not only upwind of terrain barriers, as has been documented before, but also in the wake of steep terrain, where the waves can be highly turbulent.

This presentation will first establish the kinematic and dynamic framework of KH waves in orographic clouds, and will then address the question whether the presence of such waves improves seedability, or renders artificial seeding redundant.