Sensitivity Analysis of Wind Ramp Forecasts in Complex Terrain to Planetary Boundary Layer Parameters

As part of the second Wind Forecast Improvement Project (WFIP2) this study investigates the sensitivity of wind ramp events in the Columbia River basin to variations in parameters within the model physics as well as perturbations to the initial conditions.  Previous studies on boundary layer wind forecasts indicate that a small number of parameters in surface layer and planetary boundary-layer (PBL) schemes are responsible for the majority of the forecast uncertainty.  These parameters include the Prandtl number, turbulent length scales, surface roughness, and those related to the dissipation of turbulent kinetic energy.  In this study the values of parameters within the Mellor-Yamada-Nakahishi-Niino (MYNN) PBL scheme and the MM5 surface layer scheme of the Weather Research and Forecast (WRF) model are perturbed in a systematic way to evaluate the parametric sensitivity during high impact, wind ramp events.  The forecast variance from the physics ensemble is compared to the spread from an initial condition ensemble that uses an ensemble Kalman filter (EnKF) data assimilation framework to evaluate the relative contributions from initial condition and parametric uncertainty.  Finally, a subset of physics ensembles is created using the initial conditions from individual members of the EnKF ensemble to investigate the flow dependence of the parametric sensitivity.