Stochastic Physics

Stochastic physics is a component of the UFS Weather Model (WM) and contains a variety of stochastic schemes. The schemes add valuable scientific and numerical impact by addressing dynamical effects of unresolved scales in the UFS WM forecast system. Stochastic Kinetic Energy Backscatter (SKEB; Berner et al., 2009), Stochastically Perturbed Physics Tendencies (SPPT; Palmer et al., 2009), and Specific Humidity perturbations (SHUM; inspired by Tompkins and Berner, 2008) are what is known as “ad-hoc” schemes; their perturbations are prescribed based on a white-noise perturbation field and added to parameter tendencies or state fields to achieve ensemble spread.

In addition, there are the capabilities to perturb certain land model/surface parameters (Gehne et al., 2019 and Draper 2021), and a cellular automata scheme (Bengtsson et al. 2013), which interacts directly with the convective parameterization. Stochastic Parameter Perturbation (SPP) represents a more targeted stochastics physics method, where parameters within physics schemes are directly perturbed based on a perturbation field that is coherent in time and space. SPP achieves adequate spread through the perturbation of many parameters across multiple physics schemes. The land model/surface parameter perturbations are essentially an implementation of SPP within the land-surface model physics parameterization. The cellular automata scheme applies a stochastically-perturbed organization of sub- and cross-grid convection within the cumulus scheme of a given model. More detailed information on these stochastic physics schemes can be found in the Stochastic Physics User’s Guide.