AceCAST Version 1.3 New Feature Highlight: Adaptive Time Stepping
By: Taylor Trask, Meteorology Solutions Architect, TempoQuest
TempoQuest’s latest version of AceCAST implements key capabilities for users to apply to their custom operational or research-based weather forecasts. One of these features is adaptive time stepping, a critical option that enables users to maintain numerical stability while maximizing the time duration between which all the variables of the atmosphere are calculated by the model (timestep) to improve model efficiency.
Numerical Weather Prediction (NWP) models, such as the Weather Research and Forecasting (WRF) Model, work by solving (integrating) the meteorological governing equations in incremental timesteps throughout a simulation. In most applications, the timestep is a carefully selected value defined by the user that remains constant during a simulation. Choosing an optimal timestep value is challenging because the model horizontal and vertical stability criterion are highly sensitive to this parameter. A timestep that is too large will cause numerical instability and model failure, and a timestep that is too small will significantly increase the computing power required, resulting in inefficiencies and delayed forecast output. Generally, the more complex the simulation is, the smaller the timestep required.
In WRF applications, the common “rule of thumb” is to set the timestep (measured in seconds) to 6*dx, where dx is the grid size in the x-direction in kilometers. However, this recommendation is to be utilized with caution, noting that it is only a starting point and may need to be adjusted. For example, a domain over complex terrain (ie., mountains) will require a smaller timestep to comply with the numerical stability criterion, Courant-Friedrichs-Lewy (CFL) conditions. Additionally, simulations that use complex microphysics parameterizations will also require smaller timesteps.
Rather than having a static timestep, adaptive time stepping removes the “guess-work” required when determining the optimal timestep value since the timestep will dynamically adjust as the simulation progresses based on predefined minimum and maximum thresholds. The timestep adjustments are determined through the horizontal and vertical stability criterion equations, and how they evolve with time in the model domain. These automatic timestep adjustments provide computational cost effectiveness and enhanced model efficiency while maintaining numerical stability.
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.
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.