Variations in parametric sensitivity for wind ramp events in the columbia river basin

This work investigates the sensitivity of wind speed forecasts during wind ramp events to parameters within a numerical weather prediction model boundary layer physics scheme. In a novel way, it explores how these sensitivities vary across 1) ensemble members with different initial conditions, 2) different times during the events, 3) different types of ramp-causing events, and 4) different horizontal grid spacing. Previous research finds that a small number of parameters in the surface layer and boundary layer schemes are responsible for the majority of the forecast uncertainty. In this study, the values of parameters within the Mellor-Yamada-Nakahishi-Niino (MYNN) boundary layer scheme and the MM5 surface layer scheme of the Weather Research and Forecasting (WRF) Model are perturbed in a systematic way to evaluate parametric sensitivity for two types of specific ramp-causing phenomena: marine pushes and stable mix-out events. This work is part of the Department of Energy's Second Wind Forecast Improvement Project (WFIP2). A major finding of this study is that there are large differences in parametric sensitivity between members of the same initial condition ensemble for all cases. These variations in sensitivity are the result of differences in the atmospheric state within the initial condition ensemble, and the parametric sensitivity changes over the course of each forecast. Finally, parametric sensitivity changes between event type and with model resolution. These conclusions are particularly relevant for future sensitivity studies and efforts at model tuning.