The impact of new land surface physics on the GCM simulation of climate and climate sensitivity

Recent improvements to the Hadley Centre climate model include the introduction of a new land surface scheme called "MOSES" (Met Office Surface Exchange Scheme). MOSES is built on the previous scheme, but incorporates in addition an interactive plant photosynthesis and conductance module, and a new soil thermodynamics scheme which simulates the freezing and melting of soil water, and takes account of the dependence of soil thermal characteristics on the frozen and unfrozen components. The impact of these new features is demonstrated by comparing 1 x CO2 and 2 x CO2 climate simulations carried out using the old (UKMO) and new (MOSES) land surface schemes. MOSES is found to improve the simulation of current climate. Soil water freezing tends to warm the high-latitude land in the northern Hemisphere during autumn and winter, whilst the increased soil water availability in MOSES alleviates a spurious summer drying in the mid-latitudes. The interactive canopy conductance responds directly to CO2, suppressing transpiration as the concentration increases and producing a significant enhancement of the warming due to the radiative effects of CO2 alone.