Vertical Resolution Refinement in an Aqua‐Planet and Its Effect on the ITCZ

The General Circulation Model ECHAM is used to study the effects of three refined vertical resolutions on convection in the tropics and on the structure of the intertropical convergence zone (ITCZ). Additional vertical resolutions have 76, 134, or 192 levels, which is over four times the default resolution of 47 levels. New levels are placed in the troposphere only. The simulations are conducted on an aqua‐planet with equator symmetrical, time and zonal independent sea surface temperature, and without a yearly solar cycle. Whereas the default vertical resolution shows a double ITCZ, refining the vertical resolution yields an equatorward shift of the ITCZ. The ITCZ converges to its equatorial position with 134 levels. The sensitivity of the ITCZ to the vertical resolution is traced back first and foremost to the mixing formulation in the convection scheme. Here a higher number of vertical levels leads to a stronger mixing between the updraft and its environment by design, which favors an equatorward position of the ITCZ. Differences in the relative humidity profiles explain the remaining differences in the ITCZ location. These differences can mostly be eliminated by making clouds transparent. Key Points Refining the vertical resolution in a General Circulation Model leads to a converging single ITCZ With a mean tropospheric grid spacing of 11.4 hPa a single ITCZ emerges Vertical discretization errors in the cumulus parameterizations lead to weak mixing and a double ITCZ for a coarse grid