An intercomparison of T-REX mountain wave simulations

Numerical simulations of flow over steep terrain using 11 different nonhydrostatic numerical models are compared and analyzed. The numerical model suite includes: ARPS, ASAM, BLASIUS, Bryan model, COAMPS (two different versions executed by two different groups), Durran-Klemp model, EULAG, RAMS, Unified Model, and WRF-ARW. Five test cases are simulated in a two-dimensional framework using an identical initial state, which is based on conditions on 25 March 2006 during Intensive Observation Period (IOP) 6 of the Terrain-Induced Rotor Experiment (T-REX). All of the numerical models use an identical horizontal resolution of 1 km, and a vertical grid increment that is stretched from 50 m near the surface to a constant of 200 m above the lowest 800 m of the model. The five simulated test cases use various terrain heights: a 1000-m idealized ridge that is steeper on the lee slope, a 2500-m asymmetric ridge, and a cross Sierra terrain profile. The models are tested with both free slip and no slip lower boundary conditions.

The results indicate a surprisingly diverse spectrum of simulated mountain wave characteristics, hydraulic jumps, and gravity wave breaking among the 11 different models. A statistical analysis of the model results will be presented including an intercomparison of the model-derived spectra. Mean and variance fields of the state variables and derived quantities such as momentum and energy flux will be presented as well. Implications of the results for mountain wave and wave breaking predictability will be addressed. The diversity among the various models may have important implications for ensemble design on the mesoscale.