Convergence of aqua-planet simulations with increasing resolution in the Community Atmospheric Model, Version 3

The convergence of simulations from the Community Atmosphere Model with increasing resolution is determined in an aqua-planet context. Convergence as a function of scale is considered. Horizontal resolution (T42–T340) and time step (40–5 min) are varied separately. The simulations are sensitive to both. Global averages do not necessarily converge with increasing resolution. The zonal average equatorial precipitation shows a strong sensitivity to time step. Parametrizations should be applied in a range of time steps where such sensitivity is not seen. The larger scales of the zonal average equatorial precipitation converge with increasing resolution. There is a mass shift from polar to equatorial regions with increasing resolution with no indication of convergence. The zonal average cloud fraction decreases with increasing resolution with no indication of convergence. Equatorial wave propagation characteristics converge with increasing resolution, however a relatively high truncation of T170 is required to capture wavenumbers less than 16. Extremes are studied in the form of the probability density functions of precipitation. The largest half of the scales of the model converge for resolutions above T85.