We focus on a case from the Olympic Mountains Experiment (OLYMPEX) that occurred during 12–13 November 2015, with observed precipitation amounts in excess of 370 mm on the western slopes of the Olympics. This case was associated with an atmospheric river-type storm and had several distinct precipitation periods (pre-frontal, warm sector, and post-frontal), useful for studying MP deficiencies across a range of thermodynamic environments. We run nested simulations of the event at convection-permitting horizontal grid spacing using the Weather Research and Forecasting (WRF) model configured similarly to the operational High-Resolution Rapid Refresh model. All simulations use Thompson-Eidhammer aerosol-aware MP. Sensitivity experiments are conducted by varying fixed parameters within MP that affect processes both above and below the melting level, such as snow fallspeed equation coefficients, snow capacitance, cloud water and snow particle size distribution shape parameters, and collection efficiencies of cloud water by snow and rain.
Fixed-value experiments are performed to determine appropriate perturbation magnitudes for individual parameters, then SPP ensemble simulations are run varying a set of these parameters. Results from the WRF simulations are compared to precipitation observations from OLYMPEX, including rain gauges and disdrometers. WRF forecasts are also evaluated against in situ and remote sensing OLYMPEX observations, such as the University of North Dakota Citation aircraft cloud probes and an array of ground-based scanning and profiling radars. In this manner, the amplitude of SPP performed on various MP parameters can be physically constrained by observations.