Impact of cloud physics on the Greenland ice sheet near�surface climate: A study with the Community Atmosphere Model

The Greenland Ice Sheet (GrIS) is losing mass in an accelerated fashion, which is for similar to 60% dominated by an increase in surface melting. Clouds exhibit an important control on the GrIS surface energy balance and surface melt. Therefore, to better simulate present and future GrIS climate, it is essential to represent clouds correctly in climate models. Here we use ground (at Summit Station) and satellite remote sensing (from CloudSat-CALIPSO) observations to evaluate GrIS cloud characteristics in several versions of the Community Atmosphere Model (CAM) over the period 2007-2012. Cloud cover, phase, water path, and radiative effects over the GrIS vary widely across the atmosphere models. Thanks to the inclusion of new cloud physical parameterizations, that is, ice nucleation and prognostic precipitation, in the most recent CAM version (CAM6), this model, amongst the various CAM versions, is best able to represent GrIS clouds, summer temperatures, and surface melting. However, CAM6 shows excessive rainfall over the ice sheet, which is an important outstanding model bias. Our study demonstrates the importance of simulating realistic cloud properties to improve climate model simulations of GrIS climate and climate change.