Global monsoon in ICON: the scale-dependent response of Northern Hemisphere monsoons

Abstract

The global monsoon system is a lifeline for two-thirds of the world's population, as it is essential for tropical water security, food, and agriculture. However, its complex multiscale interactions challenge weather and climate models. This study investigates how horizontal grid spacing (80, 40, and 10 km) in the ICOsahedral Non-hydrostatic (ICON) model affects both the mean and the variability of Northern Hemisphere monsoons across diurnal, intraseasonal, and interannual timescales. All simulations show substantial skill in capturing the global monsoon system domain and its mean annual range of precipitation with a pattern correlation of > 0.7 and RMSE < 3 mm d−1. For the key Northern Hemisphere regional monsoons, South Asia (SAsiaM), West Africa (WAfriM) and North America (NAmerM), ICON achieves an accuracy > 80 % in capturing the observed monsoon domain. Crucially, the impact of grid spacing is strongly region-dependent and non-systematic. The finer grid spacing induces higher mean precipitation biases over continental SAsiaM, and WAfriM. Some of these biases are related to the intensity and location of moist monsoonal low-level jets, as well as their sensitivity to grid spacing. Furthermore, the fine grid spacing overestimates monsoon precipitation variability at interannual and intraseasonal scales, including intense precipitation frequency (> 10 mm d−1). This amplification stems primarily from enhanced grid-scale precipitation resulting from efficient microphysical processes, while convective precipitation exhibits limited sensitivity to grid spacing. Over NAmerM, biases are smaller and show minimal sensitivity to model grid spacing. Increased intraseasonal variance (2–30 d band) in the 10 km simulation is linked to more intense low-pressure synoptic systems over SAsiaM and intense African easterly wave activity over WAfriM. All simulations agree on the diurnal precipitation peak timing, with the 10 km simulation marginally performing better over continents. Our results demonstrate that fine grid spacing alone does not uniformly improve monsoon simulations. Some features, such as the precipitation diurnal cycle, are improved while existing biases in mean precipitation and variability are enhanced. This underscores the role of region-dependent sensitivity of grid spacing governing monsoon dynamics.