The effect of arctic mosses on the simulation of permafrost by the JULES land surface model

Thawing permafrost is a potential source of large quantities of carbon, which could lead to significant climate feedbacks in the future, so it is important to include this effect in climate models. JULES (Joint UK Land Environment Simulator) is the land surface model used in the Hadley Centre climate model, as well as a land model in its own right, which will soon be used to quantify the magnitude of carbon release from permafrost thaw. It is therefore imperative that JULES simulates permafrost realistically. It has been shown that the model currently overestimates the permafrost active layer thickness, suggesting that its summer soil temperatures in permafrost regions are too high. One possible reason for this is that the physical properties of organic soil content such as moss and peat are not currently included. This organic matter is abundant at high latitudes and provides thermal insulation to the soil. In this work we include a routine to represent moss cover on the land surface, and find that it significantly improves the modelled soil temperatures. We quantify the effect of this new process on the large-scale representation of permafrost in JULES, including the active layer thickness.