Land-atmosphere coupling strength in an AGCM

Global climate models are often employed to investigate whether and how soil moisture anomalies affect weather and climate both in the current and in a potentially different future climate. A recent model intercomparison demonstrated that the degree of land-atmosphere interaction varies widely between current state-of-the-art AGCMs (Koster et al 2002). Models with different inherent coupling strengths can lead to vastly different conclusion about climate sensitivity. In particular, land-atmosphere coupling strength, or the extent to which a precipitation-induced soil moisture anomaly influences the overlying atmosphere and thereby the evolution of weather and the generation of precipitation, is extremely weak in the Hadley Centre modeling system (HadAM3). The current understanding of the soil moisture-precipitation feedback suggests that the feedback, as simulated in a model, will involve a relatively complex interplay between the land surface, boundary layer, and convection schemes. Here, we explore land-atmosphere interaction in HadAM3 in detail and attempt to identify which aspects of the atmospheric and land-surface models (e.g. soil and vegetation parameterization, convection scheme, boundary layer scheme, simulation of diurnal cycle) control the degree of land-atmosphere interaction in HadAM3.