resources management and climate change adaptation studies (Leung et al., 2004). Addressing limitations in measurements and data assimilation over mountain regions can provide stronger observational constraints for modeling.

Besides orography, frontal forcing is another precipitation mechanism where increasing model resolution is beneficial. Storm tracks are prominent features of the extratropical regions. A cold front can produce narrow bands of precipitation, sometimes with embedded severe rainstorms or snowstorms, and in the warm sector, squall lines and severe thunderstorms are common. High-spatial-resolution and nonhydrostatic models can better capture the temperature gradients and simulate frontogenesis that produces the upward motion responsible for frontal clouds and precipitation.

The land surface, particularly where there is substantial vegetation, plays a significant role in the global hydrologic cycle, but current estimates of evapotranspiration and precipitation are not sufficiently accurate to close the hydrologic cycle, even on an annual basis over relatively large river basins (Lawford et al., 2007; Roads et al., 2003). There are a variety of challenges associated with simulation of the hydrologic cycle in GCMs, some associated with representation of convection and cloud processes (see above), but some connected with issues of resolution and appropriate representation of land-surface processes (e.g., land-surface cover, soil moisture, vegetation, agriculture, and the associated evapotranspiration), as well as feedbacks between the land surface and the atmosphere (Dirmeyer et al., 2012).

Sophisticated regional- and continental-scale models exist for land-surface hydrology, but these models are only coupled with GCMs, through the grid scale, with subgrid variability of essential land-surface processes being forced by grid mean atmospheric forcing. For realistic routing of surface water and representation of land cover, hydrology models require fine resolution (1 km on continental scales, and considerably less in many regional studies). This resolution is essential to predictions of soil moisture and evapotranspiration fluxes to the atmosphere and is also the scale of information needed by water resource managers. Work to couple land-surface hydrology models with atmospheric models is advancing, through direct coupling approaches and through “tiling”’ or “representative land-surface units” (subgrid representation of the landscape), and more sophisticated, energy- and moisture-conserving schemes are needed.

In addition to precipitation, many other processes involving land surface-atmosphere moisture, energy, and chemical exchange at regional scales are expected to be better represented as coupling schemes and resolution improve, for example, influences of land-use changes on the climate, aerosol sources, crop- and biome-specific evapotranspiration rates, and the influence of built structures (e.g., cities, wind farms) on



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