An Evaluation of Simulated Microphysics over Terrain during the OLYMPEX Field Campaign

Abstract

The OLYMPEX field campaign of winter 2015-2016 offers the opportunity to assess the performance of model physics over a coastal mountain range. This thesis explores precipitation and microphysics simulations, and investigates the ability of full-physics simulations from the Weather Research and Forecasting Model (WRF) to simulate Kelvin-Helmholtz waves. First, moist physics is assessed from the perspective of precipitation for a variety of microphysics and planetary boundary parameterizations. Over the period from November 2015 to February 2016, WRF and the 13-km NOAA/NWS GFS model precipitation accumulation was underpredicted over the windward (western) side and crests of the Olympic Mountains. Underprediction was greatest where observed precipitation was largest. Two major atmospheric river type events (November 12-15, November 16-19) were examined in detail using a variety of physics choices. Improved simulation skill was generally limited to increasing resolution from 36 to 12-km, with substantial variability among microphysics and PBL choices. For the two cases noted above, warm-period precipitation maxima occurred farther up the valley than observed, with a tendency for underprediction. Differences in simulated microphysics are also examined, with results indicating the tendency of WRF to produce rain particles that are larger and less numerous than observations. Next, two Kelvin-Helmholtz wave (KH; December 12th and December 17th) events were simulated to further evaluate model physics at short spatial and temporal scales. Waves were realistically simulated at 444-m grid spacing, including reproducing the location and structure of waves. Waves were shown to be resolution-dependent and only adequately represented at 444-m grid spacing due to their 3-5 km wavelengths. In both cases, waves developed as the result of an intense shear layer, caused by low-level easterly flow. The Olympic Mountains enhanced wave amplitudes, and removing the Olympic Mountains eliminated wave activity in the December 12th case. When waves were within the melting level (December 12th), simulated microphysical fields experienced considerable oscillatory behavior; when waves were below the melting level (December 17th), the microphysical response was attenuated. Turning off moist physics and latent heating resulted in weaker KH waves, while varying physics choices resulted in variability in the amount of hydrometeors produced and the strength of the waves’ vertical velocities.