AceCAST Version 1.3 New Feature Highlight: Analysis Nudging

AceCAST Version 1.3 New Feature Highlight: Analysis Nudging

By: Taylor Trask, Meteorology Solutions Architect, TempoQuest

An image showing some of the Analysis Nudging (Red) and Surface Analysis Nudging (Green) settings in the WRF input file.

TempoQuest announced last week their newest distribution of AceCAST, version 1.3, showcasing several new features including Stochastic Perturbation Schemes, Adaptive Time Stepping, Fractional Sea Ice, and Analysis Nudging. Each of these features provide additional options for users to apply to their custom AceCAST operational or research-based weather forecasts.

Analysis Nudging, also known as Grid Nudging, enables users to apply a Four-Dimensional Data Assimilation (FDDA) technique, which takes into consideration how the meteorological fields change with respect to space and time. This technique only allows nudging to be applied to the horizontal winds (i.e., U and V components), temperature, and water vapor (ie., moisture), and nudges the model towards the analysis by adding a corrective term to the predictive, time-varying equations. This additional term is based on the difference between the model field and a reference field, which is computed at each model grid cell. To use Analysis Nudging in a WRF simulation, an additional section called FDDA, and its respective settings, are required to be added to the WRF input file (ie., namelist.input). Analysis Nudging does not require the use of any additional data assimilation systems, such as the WRF Data Assimilation System (WRFDAS).

When activating Analysis Nudging, users have the capability to control:

  1. Which meteorological fields to nudge: The meteorological fields, horizontal winds, temperature, and moisture can be nudged individually, or as a combination.
  2. How strongly to nudge each meteorological field: The strength of nudging applied to a meteorological field is determined by a nudging coefficient specific to that field. Nudging coefficients that are lower applies weaker nudging and nudging coefficients that are higher applies stronger nudging.
  3. To nudge a meteorological field in the Planetary Boundary Layer (PBL) or whether to truncate nudging below a certain model grid level: It is common practice to apply nudging above the PBL because nudging applies a fake term to the meteorological governing equations and adding this to a sensitive dynamic budget area will produce unwarranted results. Additionally, having the capability to truncate where nudging is applied is a critical benefit because it will lower computational cost since nudging will only be applied to a portion of the (vertical) atmosphere and not its complete depth.
  4. To apply nudging throughout the entire simulation or only during a portion of the simulation
  5. To turn nudging off abruptly or gradually over a specified period: The recommended setting is to gradually turn nudging off because if nudging is abruptly turned off, it will create too much noise in the forecast output.

Analysis Nudging contains a sub-feature called Surface Analysis Nudging, which applies the same concept of Analysis Nudging but at the surface. Utilizing the Surface Analysis Nudging technique also requires the FDDA section, and respective settings to be added to the WRF input file. Additionally, the use of a separate WRF utility, known as OBSGRID, is required. OBSGRID is WRF’s Objective Analysis Program that generates the necessary WRF input files for Surface Analysis Nudging, Observational Nudging, and Objective Analysis.

Like Analysis Nudging, users can control several nudging parameters such as:

  1. Which meteorological fields to nudge
  2. How strongly to nudge each meteorological field
  3. To apply nudging throughout the entire simulation or only during a portion of the simulation
  4. To turn nudging off abruptly or gradually over a specified period
  5. To set the Radius of Influence (ROI) used to determine the confidence (ie., weights) for the analysis: The ROI is based on the nearest observations between each grid point. If there are no nearby observations to a specific grid point, then the analysis is used at a reduced weight.

Analysis Nudging and Surface Analysis Nudging are most effective when the analyses are representative of the scales resolved by the model and by the domain. The two features are commonly used for research applications in historical simulations and air quality models.

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.