Simulated Kelvin-Helmholtz Waves Over Terrain and Their Microphysical Implications

Two Kelvin-Helmholtz (KH) events from the OLYMPEX field campaign over western Washington State were simulated and evaluated using observations. The events, 12 Dec 2015 and 17 Dec 2015, were realistically simulated by the WRF-ARW model at 444-m grid spacing, including qualitatively reproducing the location and structure of KH waves. Due to their wavelengths, waves were apparent in the 444-m domain but not the 4-km domain. In both cases, waves developed as the result of developing instability and an intense shear layer, caused by low-level easterly flow. The 12 Dec case had considerable blocking by the Olympic Mountains, which was the origin of its easterly flow, while in the 17 Dec event it was the result of the synoptic environment. As a result, removing the Olympics eliminated wave activity only in the 12 Dec case. Simulated microphysics were also evaluated. When waves were within the melting level, simulated microphysical fields – including precipitation –experienced considerable oscillatory behavior; when waves below the melting level, the microphysical response was attenuated. Turning off the model’s microphysics scheme and latent heating resulted in a decreased KH wave signal. Finally, a variety of microphysics and PBL schemes were evaluated for both events. Results showed that waves within the melting level influenced hydrometeor mixing ratios, particularly graupel, more substantially than waves below the melting level.