Process-Based Evaluation of Stochastic Perturbed Parameterization Tendencies on Convective-Permitting Ensemble Forecasts of the 1–2 June 2017 Mei-Yu Rainfall Event over Northern Taiwan

Abstract:

Heavy rainfall and flooding represents a nearly worldwide weather hazard and can be associated with significant societal disruption; therefore, it is imperative to understand the dynamical processes and predictability associated with these events. Predictability can be examined in a modeling framework using ensemble prediction systems (EPSs), which should account for errors and uncertainty both in the initial atmospheric state and the model formulation. Stochastic perturbed parameterization tendencies (SPPT) and independent SPPT (iSPPT) are designed to represent the uncertainty associated with subgrid-scale parameterization schemes by multiplicatively perturbing the contributed tendencies of these schemes at each time model step using a spatially and temporally-correlated random-noise pattern. While this method has been applied extensively in operational global EPSs and has been tested in regional convection-permitting experiments, relatively less attention has been given to the physical processes by which SPPT and iSPPT influence rainfall forecast variability. As a consequence, the goal of this research is to understand how stochastic perturbations to the radiation, microphysics, and boundary layer schemes affect the physical processes that determine the predictability of heavy rainfall in the WRF model. Generally speaking, iSPPT perturbations to microphysics tendencies yield the greatest rainfall forecast variability, while perturbations to the radiation temperature tendency are associated with the smallest ensemble spread. In the case of the 1–2 June 2017 mei-yu front event, stochastic perturbations to microphysics tendencies generate precipitation variability by modulating the efficiency of warm-rain processes. By contrast, perturbations to the turbulent mixing tendencies associated with PBL parameterization tend to indirectly alter rainfall over northern Taiwan by disrupting the upstream flow and low-level moisture transport.