Challenges for high-resolution simulations of atmospheric flow over complex terrain

Abstract:

A number of challenges arise as numerical simulations of the atmospheric boundary layer flow move to higher and higher resolution. One is in the representation of the topography: at higher resolutions, more terrain details can be represented, and therefore the maximum resolved slope of the terrain increases to the point where it cannot be accommodated by traditional terrain-following coordinates. An immersed boundary method has been implemented in a mesoscale model for that purpose, so that terrain slopes of any magnitude can be included, such as urban geometries or steep mountains. Another challenge as models move to higher resolution is in the choice of the turbulence closure model. At coarse, mesoscale resolutions (~ 10 km horizontal spacing), a Reynolds-averaged approach is used. At fine resolutions (less than about 100 m), large-eddy simulation closures can be used. The intermediate scales are called the gray zone, and significant problems occur when either LES or RANS closures are used. These challenges are evaluated in the context of multi-scale simulations using grid nesting for atmospheric boundary layer flow studies.