New look at humidity trends

Past studies have revealed a discrepancy between observed and simulated humidity trends in the satellite era. Especially over arid and semi-arid land regions, no trend in specific humidity is discernible in observations, whereas both uncoupled and coupled climate model simulations from the last CMIP exercise show a moistening trend. We revisit trends in specific humidity using global simulations that were conducted at a grid spacing of 10 km over multi decades. We consider two different models (IFS and ICON) as well as coupled and AMIP-type simulations. The coupled historical IFS simulation shows a moistening trend over the semi-arid and arid regions, similar to the result of the past coarse-resolution climate models. In contrast, the AMIP ICON simulation shows no discernable trend, in agreement with observations. One key difference between the two models is that IFS still uses parameterizations for shallow and partly deep convection, whereas ICON does not. Using the output of the two models, we further explore reasons for this distinct trend behavior between the two models.