Detailed dual-Doppler structure of Kelvin-Helmholtz waves from an airborne profiling radar over complex terrain. Part II: Evidence for precipitation enhancement from observations and modeling

• Published in: Journal of the atmospheric sciences 2021-11, Vol.78 (11), p.3455
• Main Authors: Grasmick, Coltin, Geerts, Bart, Chu, Xia, French, Jeffrey R., Rauber, Robert M.
• Format: Article
• Language: English
• Subjects: Airborne radar; Airborne remote sensing; Airborne sensing; Cloud microphysics; Clouds; Doppler sonar; Downdraft; Eddies; Evaporation; Fallout; Growth; Ice; Ice nucleation; Ice particles; Kelvin-Helmholtz waves; Laminar flow; Large eddy simulation; Large eddy simulations; Microphysics; Modelling; Moisture content; Nucleation; Oceanic eddies; Orographic precipitation; Precipitation; Precipitation systems; Radar; Reflectance; Simulation; Small-scale turbulence; Stratiform clouds; Sublimation; Supersaturation; Terrain; Turbulence; Turbulence measurement; Updraft; Vortices; Water; Water content; Wind measurement
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/JAS-D-20-0392.1

Kelvin-Helmholtz (KH) waves are a frequent source of turbulence in stratiform precipitation systems over mountainous terrain. KH waves introduce large eddies into otherwise laminar flow, with updrafts and downdrafts generating small-scale turbulence. When they occur in-cloud, such dynamics influence microphysical processes that impact precipitation growth and fallout. Part I of this paper used dual-Doppler, 2D wind and reflectivity measurements from an airborne cloud radar to demonstrate the occurrence of KH waves in stratiform orographic precipitation systems and identified four mechanisms for triggering KH waves. In Part II, we use similar observations to explore the effects of KH wave updrafts and turbulence on cloud microphysics. Measurements within KH wave updrafts reveal the production of liquid water in otherwise ice-dominated clouds, which can contribute to snow generation or enhancement via depositional and accretional growth. Fallstreaks beneath KH waves contain higher ice water content, composed of larger and more numerous ice particles, suggesting that KH waves and associated turbulence may also increase ice nucleation. A Large-Eddy Simulation (LES), designed to model the microphysical response to the KH wave eddies in mixed phase cloud, shows that depositional and accretional growth can be enhanced in KH waves, resulting in more precipitation when compared to a baseline simulation. While sublimation and evaporation occur in KH downdrafts, persistent supersaturation with respect to ice allows for net increase in ice mass. These modeling results and observations suggest that KH waves embedded in mixed-phase stratiform clouds may increase precipitation, although the quantitative impact remains uncertain.