AceCAST Version 1.3 New Feature Highlight: Stochastic Perturbation Schemes

AceCAST Version 1.3 New Feature Highlight: Stochastic Perturbation Schemes

By: Taylor Trask, Meteorology Solutions Architect, TempoQuest

An image showing some of the stochastic perturbation settings in the WRF input file.

TempoQuest’s most recent release of their premiere weather software product, AceCAST, which provides accelerated operational and research-based high resolution weather forecasts, provides several new capabilities to offer to users. One of these capabilities is a suite of stochastic perturbation schemes.

Stochastic perturbation schemes, also known as stochastic parameterization schemes, enables users to easily identify areas of model uncertainty in ensemble (group) simulations by applying a small perturbation (change) at every incremental step in time for which the governing equations are being solved to each ensemble member. Having the ability to identify model uncertainty is critical to improving the model forecast.

Users have the option to apply one or more schemes provided within the stochastic suite:

  • Stochastically Perturbed Physics Tendencies (SPPT): Used to minimize uncertainty in the physics parameterizations. This scheme randomly perturbs the boundary layer and convection physics tendencies of potential temperature, wind, and humidity, but does not perturb physics tendencies (of these variables) from the radiation or microphysics schemes.
  • Stochastic Kinetic Energy Backscatter Scheme (SKEBS): Used to minimize uncertainty in turbulent eddies (microscale wind processes). This scheme randomly perturbs potential temperature and rotational wind component.
  • Stochastically Perturbed Parameter Scheme (SPPS): Used to minimize the uncertainty in selected physics. This scheme randomly perturbs parameters in three selected physics parameterizations: the Grell-Freitas (GF) Convection Scheme, the Mellor-Yamada Nakanishi and Niino (MYNN) Boundary Layer Scheme, and the Rapid Update Cycle (RUC) Land Surface Scheme. This perturbation suite can be applied to one or more physics schemes.

When a stochastic option is activated, a perturbation field is generated. This three-dimensional Gaussian field is characterized by the overall perturbation amplitude, and the spatio-temporal correlations that a user defines in the Stoch section within the WRF input file (ie., namelist.input). Additionally, a user can define:

  1. The perturbation correlation length scale
  2. The temporal decorrelation of random fields
  3. The standard deviation of the perturbation field at each grid point
  4. The cutoff thresholds of the perturbation pattern above the threshold deviation (from 3)
  5. The seed for the random number stream in an ensemble simulation

Most evident in ensemble prediction systems, the benefits of stochastic perturbation schemes allow users to identify uncertainty in different aspects of the model, allowing for precise areas of meteorological representation enhancement in the model forecasts. Generally, more than one stochastic perturbation scheme is used to improve the probabilistic skill of weather forecasts on a short-to-medium timescale.

About AceCAST

AceCAST is a powerful cutting-edge software powered by Graphic Processing Units (GPUs) that enables the acceleration of the National Center for Atmospheric Research (NCAR) Weather Research and Forecasting Model (WRF). AceCAST is the product of a half-a-decade of punctilious research and development that empowers WRF users to secure striking performance optimizations using the superior massive parallelism of GPU hardware versus traditional Central Processing Unit (CPU) computation. AceCAST encompasses an ample set of refactored common WRF physics and dynamics modules, and namelist options with NVIDIA CUDA or OpenACC CPU programming techniques, allowing a wide swath of users to adopt AceCAST painlessly as a drop-in replacement for existing WRF configurations.

About TempoQuest

TempoQuest is an independent weather software vendor that was incorporated to revolutionize mesoscale weather forecast modeling and to simplify the forecast tasks for meteorologists. TempoQuest offers two software products, AceCAST, or accelerated WRF software, and WSV3, a highly advanced weather visualization and storm tracking software. Additionally, TempoQuest offers WRF On-Demand, a cloud application that greatly simplifies and accelerates the running of CPU and GPU WRF simulations.