It is well recognized that the land surface state (i.e. soil wetness, temperature, albedo, roughness, overlying vegetation state and thickness) can impact climate at local-to-regional scales via complex physical controls on the partitioning of surface turbulent fluxes that in turn regulate the PBL. However, the degree to which these land-atmosphere feedbacks can influence weather varies regionally, seasonally, even inter-annually due to the affect of other factors that modulate the ability of climate signals to persist and propagate through the hydrologic branch of the coupled land-atmosphere system. Due to the scale at which these processes proceed (4-400 km, days to weeks), on-the-ground observations of land-atmosphere coupling are extremely limited. Some studies suggest that the impact of the land on the coupled system can be quite remarkable-- quantified in terms of changes in rainfall frequency and rainfall accumulations. Yet others propose that the land signal, while not measurable in terms of rainfall, still persists in the local coupling ‘process-chain' in the form of fluxes and PBL evolution. These findings carry important implications about the predictability of rainfall, or for example, the persistence of drought. In fact, the focus of the 2nd phase of the Global Land-Atmosphere Coupling Experiment (GLACE-2) is to quantify the impact of land surface initialization (i.e. soil moisture) on seasonal (or sub-seasonal) precipitation forecast skill. From a long-term perspective, the ability of models to accurately represent present-day coupling is tantamount to being able to project future land hydrology with confidence.

In this study, we investigate the climatology of land-atmosphere coupling strength generated by EC-Earth. EC-Earth is a state-of-the-art earth fully coupled atmosphere-ocean-sea ice model system, based on the European Centre for Medium-Range Weather Forecasts (ECMWF) seasonal forecasting system. Specifically, we consider two 22-year simulations: the first representative of present climate (forcings as in year 2000) and the second representative of a future 4xCO2 concentration. Both runs were produced at T159/L62 resolution, with 6-hourly output of 3-D atmospheric fields and 3-hourly output of surface fields. We apply multiple diagnostics to analyze the near-surface and low-level (to 300mb AGL) atmospheric coupling, with particular attention to the following variables: surface soil moisture, evaporative fraction, lifting condensation level, convective triggering potential, humidity index, convective and stratiform precipitation, and cloud albedo. We use the two time slices to evaluate if (and where) coupling characteristics change in response to climate change. To determine the realism of EC-Earth, results for the present climate were inter-compared with satellite-based (using AIRS, AMSR-E, and CMORPH) findings for the period of 2003 to 2009. This study serves as an initial experiment in a larger multi-model effort that seeks to include many of the Coupled Model Intercomparison Project (CMIP)-5 models.