The common land model

C limate and weather forecasting models requirethe energy, water, and momentum fluxes acrossthe land–atmosphere interface to be specified. Various land surface parameterizations (LSPs), rang-ing from the simple bucket-type LSP in the 1960s to the current soil–vegetation–atmosphere interactive LSP, have been developed in the past four decades to calculate these fluxes. The Project for Intercompari-son of Land Surface Parameterization Schemes (PILPS) has demonstrated that, even with the same atmospheric forcing data and similar land surface pa-rameters, different LSPs still give significantly differ-ent surface fluxes and soil wetness, partly because of the differences in the formulations of individual pro-cesses and coding architectures in participant mod-els (Henderson-Sellers et al. 1995). On the other hand, most LSPs share many common components, sug-gesting the need to develop a publicly available com-mon land model with a modular structure that could facilitate the exploration of new issues, less repetition of past efforts, and sharing of improvements and re-finements contributed by different groups. The Common Land Model (CLM) effort dates back to the mid-1990s and has evolved through various work-shops and e-mail correspondence. The initial motiva-tion was to provide a framework for a truly commu-nity-developed land component of the Nationa