Understanding drivers of inter-model differences in tropical free-tropospheric humidity in global storm-resolving models

The dry subsidence regions of the tropics and subtropics play an important role in setting the Earth’s clear-sky climate sensitivity, as the clear-sky feedback in these regions is particularly sensitive to both the baseline relative humidity (RH) and small RH changes under warming. Therefore, it is crucial that climate models reliably simulate the RH and its response to warming in these regions. However, considerable inter-model differences in RH remain, also in global storm-resolving models, the newest generation of climate models with horizontal grid spacings sufficient to explicitly resolve deep convection. The goal of this study is to identify potential causes for these inter-model differences and understand the mechanisms behind it. For this we examine the effect of changes in different model parameters – including microphysical parameters and vertical grid spacing – on tropical free-tropospheric humidity in a global storm-resolving model, focusing on the dry subsidence regions. Back-trajectory calculations allow us to determine the characteristics of the last saturation points for dry tropical air masses as well as the magnitude of moisture sources and sinks during subsequent advection, and how both change in the sensitivity experiments. The trajectory analysis confirms that moisture gains and losses during advection play a secondary role in setting the RH distribution in tropical dry zones in the model, as suggested by earlier studies based on coarser models. This leaves changes in the points of last saturation, which are determined by the circulation and the temperature field, as the more likely driver of RH changes. Preliminary results from the sensitivity experiments indicate that particularly changes in the vertical grid spacing of the model can affect the RH in tropical subsidence regions. These RH changes are explained by changes in the temperature of the main outflow regions of deep convection in the upper troposphere, where most last saturation points are located. These results highlight the importance of circulation and temperature differences across global storm-resolving models in driving inter-model differences in RH.