Sensitivity of Equatorial Convection to Horizontal Resolution in Aquaplanet Simulations with a Variable-Resolution GCM

Abstract

Climate simulations using idealized zonally symmetric boundary conditions (an aquaplanet) are performed in order to highlight possible deviations from zonality introduced by the spatial variations in resolution caused by the variable resolution technique. Simulations at three different uniform resolutions with spectral truncations T21, T63, and T106 are used as control simulations, and analyzed in order to describe the zonal climate simulated by this version of the ARPEGE-Climat general circulation model and establish its sensitivity to varying spatial resolution. The model simulates a single equatorial precipitation maximum in which the activity of the convective cells is modulated by westward and eastward propagating equatorial waves, depending on the resolution. A simulation with the variable resolution technique using a truncation T63 and a stretching factor of 3 with the maximum resolution pole placed at the equator is then compared to the uniform resolution simulations. Deviations from the zonally symmetric solution are produced by the sensitivity of tropospheric dynamics and moisture to the local resolution similar to those found at uniform resolution. An unsatisfactory feature of this simulation is the slowing down or reversing of the propagating convective cells in the low-resolution regions, which leads to an intensification of convective precipitation in this region, with the formation of a nonzonal Walker-type circulation in the equatorial plane. Analysis of four other simulations with different truncations and stretching factors (T42s2, T63s3, T95s3, T79s2.5) shows that this deficiency is strongly reduced by increasing the minimal resolution of the stretched grid to be equivalent to T31, which allows a more homogeneous representation of convection over the entire range of resolutions used. The T79s2.5 resolution seems to provide a good choice for producing a reasonable approximation of the zonally symmetric aquaplanet climate.