Observations of KH Wave Fall-streaks over Complex Terrain by Airborne Radar and In-situ Probes

Kelvin-Helmholtz (KH) waves are remarkably common in deep stratiform precipitation systems especially in the vicinity of complex terrain, as is evident from transects of reflectivity and vertical velocity obtained from an airborne profiling Doppler radar, the Wyoming Cloud Radar (WCR). The high range resolution of this radar (<40 m) allows detection and depiction of KH waves in fine detail. Terrain acts to locally enhance wind shear, both when low-level flow is blocked and when flow moves freely over terrain so that many scenarios exist that promote KH wave development. These waves are observed in a variety of wavelengths, depths, amplitudes, and turbulence intensities. KH wave trains 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.

While sampling precipitation systems over the Park Range in Colorado and the Payette mountains in Idaho, the UW King Air frequently observed in-cloud KH waves, often with plumes of enhanced reflectivity descending from the region of turbulent overturning. These fall-streaks are evidence for precipitation enhancement within KH waves and suggest that the vertical velocity couplets and turbulent mixing have microphysical implications. KH wave fall-streaks were observed at a range of temperatures in both super-cooled liquid and mixed-phase clouds. Observations indicate that the greatest impact, i.e. the largest reflectivity enhancement, occurs in cold clouds when updrafts contain liquid water. Ice mass can increase rapidly during these conditions from large supersaturation (deposition), collision-coalescence, and multiplication processes. Fall-streaks are especially useful because they can be tracked to connect a microphysical response to its kinematic source. When the KH waves are below flight-level, dual-Doppler synthesis reveals the 2D wind field along the flight track. Due to the ubiquity of KH waves and this evidence of precipitation enhancement, it is suspected that KH waves play an important role in the precipitation efficiency in orographic systems affecting hydrometeor size distributions, phase, and fallout.