An Immersed Boundary Method for the Weather Research and Forecasting Model

Abstract

This paper describes an immersed boundary method that facilitates the explicit resolution of complex terrain within the Weather Research and Forecasting (WRF) model. Mesoscale models, such as WRF, are increasingly used for high-resolution simulations, particularly in complex terrain, but errors associated with terrain-following coordinates degrade the accuracy of the solution. The use of an alternative-gridding technique, known as an immersed boundary method, alleviates coordinate transformation errors and eliminates restrictions on terrain slope that currently limit mesoscale models to slowly varying terrain. Simulations are presented for canonical cases with shallow terrain slopes, and comparisons between simulations with the native terrain-following coordinates and those using the immersed boundary method show excellent agreement. Validation cases demonstrate the ability of the immersed boundary method to handle both Dirichlet and Neumann boundary conditions. Additionally, realistic surface forcing can be provided at the immersed boundary by atmospheric physics parameterizations, which are modified to include the effects of the immersed terrain. Using the immersed boundary method, the WRF model is capable of simulating highly complex terrain, as demonstrated by a simulation of flow over an urban skyline.

The Effect of Mesh Resolution on Convective Boundary Layer Statistics and Structures Generated by Large-Eddy Simulation

“…Applications include, atmosphere-land interactions (Patton et al 2005), boundary layers with surface water wave effects (Sullivan and McWilliams 2010;Sullivan et al 2007Sullivan et al , 2008, weakly stable nocturnal flows (Beare et al 2006), flow in complex terrain (Lundquist et al 2010), stratocumulus clouds (Stevens et al 2005), tropical boundary layers beneath deep convection (Moeng et al 2009), and coupling with mesoscale weather events (Bryan et al 2003), to mention just a few.…”

Intercomparison of terrain-following coordinate transformation and immersed boundary methods in large-eddy simulation of wind fields over complex terrain

“…For slopes lower than 30 degrees those errors are negligible, but at higher slopes model errors become large and can cause stability problems. Recently, the immersed boundary method (IBM) has become popular in numerical simulation of atmospheric flows over complex terrain in urban and mountainous areas [8][9][10][11][12][13][14][15][16]. The advantage of IBM is that it offers a simple strategy to use a regular computational grid while solving flow problems with complex geometries.…”

Some Requirements for Simulating Wildland Fire Behavior Using Insight from Coupled Weather—Wildland Fire Models

“…Most modeling systems standardly employ numerical filters on terrain elevation. These filters primarily smooth mountain peaks and valley lows but have little effect on steep slopes, another primary source of numerical instability, for which methods are being developed in WRF such as the immersed boundary method [52]. The CAWFE model, built upon the Clark-Hall atmospheric model optimized for use at very high resolution (100 s of m) in steep mountainous terrain, allows the user to filter either or both terrain elevation and the terrain gradient.…”